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EA-I. (Clyde Wahrhaftig)

PHYSIOGRAPHY OF ALASKA

       

CONTENTS

	Page
Introduction	1
Coasts	2
Physiographic Regions	4
Arctic Plains and Foothills Region	4
Arctic Coastal Plain	4
Arctic Foothills Province	5
Brooks Range	6
Central Plateau Region	8
Seward Peninsula	10
Mountains between Norton Sound and the Yukon River	11
Lowlands of the Lower Yukon and Koyukuk	12
Delta of the Yukon and Kuskokwim	13
Yukon and Koyukuk Flats	13
Yukon Plateau	15
Kuskokwim and Kilbuck Mountains	18
Tanana-Kuskokwim Flats	20
Pacific Mountain System	22
Alaskan Range	24
Aleutian Range	27
Talkeetna Mountains	28
Wrangell Mountains	30
Susitna-Cook Inlet Lowland	31
Copper River Plateau	33
Chugach-Kenai Mountains	34
St. Elias Range	37
British Columbia Coast Range (Coast Mountains)	37
Topographic Details in Glaciated and Nonglaciated Areas	39
Physiographic History of Alaska	40
Bibliography	42

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MAPS

	Page
Plate I Physiographic regions and provinces in Alaska	4-a
Plate II Physiographic diagram of Alaska	4-b

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EA-I. (Clyde Wahrhaftig)

       

Author: Where do

references (17) and (24)

go? (new numbers).

       

PHYSIOGRAPHY OF ALASKA

       

INTRODUCTION

        The larger part of the Territory of Alaska is an irregular rectangular

peninsula forming the extreme northwest corner of the North American con–

tinent, approximately between meridians 141° and 168° W. longitude and

parallels 58° and 72° N. latitude (see plate 1.). The natural boundaries

of this peninsula are the Arctic Sea on the north, the Chukotak and Bering

seas on the west, and the Pacific Ocean on the south. The artificial boundary

on the east, along meridian 141° W. longitude for nearly the entire distance

between the Arctic Sea and the Pacific Ocean, is the political boundary between

Alaska and the Yukon Territory of Canada. On the west, the extremity of the

Seward Peninsula is only 56 miles form the Siberian mainland; and the two

Diome t d e Islands, which are situated between the Seward Peninsula and the ✓

Siberian coast of Bering Strait, lie on opposite sides of the international

boundary.

        The projection to the southwest and west along the Alaska Peninsula, 500

miles in length, and the Aleutian Islands are, 1,100 miles in length, extends

the Territory of Alaska nearly to meridian 172° E. longitude.

        In the opposite direction to the southeast, the panhandle of southeastern

Alaska includes a narrow strip along the Pacific coast mainland and the adjacent

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I i slands of the Alexander Archipelago. this panhandle area is approximately ✓

550 miles in length from Mount St. Elias, near meridian 141° W. longitude,

to Dixon Entrance and Portland Canal, approximately at longitude 130° W.

and parallel 55° N.

        Other islands, in addition to the Aleutians and the Alexander Archipelago,

are included within the limits of the Territory and add much to its area and

the length or its coast Line. The largest of the islands are Kodiak, off the

south coast, and Nunivak and St. Lawrence in the Bering Sea.

        The area of Alaska is approximately 586,400 square miles.

       

COASTS

        The north and west coasts are characteristically low, with few hills or

prominent headlands. the arctic coast is notably low and shelving, and commonly

shows an offshore sand bar and lagoon. Southwest of Barrow and in the vicinity

of Wainwright, low cliffs front the ocean, and cliffs and rocky headlands are

present in the vicinity of Cape Lisburne. along Kotzebue found and on the north shore

of Seward Peninsula, low coasts with swampy hinterlands and extensive lagoons

predominate. the south coast of Seward Peninsula and the east coast of Norton

Sound, however, are higher and have a discontinuous coastal shelf, behind which

rise coastal terraces backed by mountains 3,000 feet high. south of Norton

Sound is the great delta of the Yukon and Kuskokwin, a broad depositional plain.

The shores of Bristol Ba r y are low and straight with the exception of the country ✓

at the southwest end of the Kilbuck Mountains, where peaks 1,000 to 2,000 feet

high are near the coast.

        The coast facing the Pacific Ocean presents an entirely different aspect.

It is characterized by deep indentations and numerous islands, and is dominated

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by high and rugged mountains, many of which rise directly from the water’s

edge. the extreme irregularity of this coast line is probably due more to

glacial erosion than to submergence. It is a typical fjord coast. This coast

is best described in four parts: the Alaska Peninsula and Cook Inlet; Kodiak

Island and the coast of Kenai Peninsula and Princes William Sound; Cape St. Elias

to Cape Spencer; and southeastern Alaska.

        The southern coast of the Alaska Peninsula, although rugged and rocky for

the most part, is characterized by numerous indentations and wide bays. north–

eastward in Cook Inlet the coast line is low and comparatively even.

        The coasts of Kodiak Island and Kenai Peninsula, and those bordering Prince

William Sound are extremely irregular, with deep fjords and other indentations,

and many offshore islands. Rugged mountains, up to 12,000 feet in altitude,

dominate the shore. At the head of Prince William Sound, numerous glaciers,

notably the Columbia, Yale, Harvard, and Harriman glaciers, reach tidewater.

        From Cape St. Elian (or, rather from Cape Suckling, on the mainland)

southeastward to Cape Spencer, the coast is a remarkably straight , narrow , coastal ✓

bench, 5 to 20 miles wide, which is commonly terraced and is beaked by a

mountain range of extreme ruggedness, reaching altitudes up to 18,000 feet.

The highest coastal mountains in the world are in this area. This coast is

broken by only three prominent indentations: Icy Bay, Yakutat Bay, and Lituya Bay.

        The coast of southeastern Alaska is extremely irregular. Much of this part

of the Territory consists of the islands of the Alexander Archipelago. The

archipelago is notable for its many straight and deep waterways, the reticulate

pattern of which probably, in part, reflects the underlying structure of the region.

The distal ends of these waterways, and tributaries to them, extend as long,

narrow fjords, some straight and some winking, far into the mainland and into

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the islands. The coast line is dominated by rugged mountains and some of

the fjords have walls a mile high. Across the outer islands of the archi–

pelago, however, there has been cut a narrow strand flat, 5 miles wide and

300 feet high.

       

PHYSIOGRAPHIC REGIONS

        Topographically, Alaska may be compared to a great plain with raised

walls near its north and south sides, tilted slightly to the west and draining

largely into Bering Sea. Along the north side of the northern wall is a shelf

or plain that drains northward to the Arctic Sea. This shelf, the northernmost

of the physiographic regions of Alaska is the arctic Plains and Foothil l s region. ✓

South of it is the Brooks Range or north wall which is referred to as part of

the Rocky Mountains System. The southern wall is a complex of mountains and

intervening plains described as the Pacific Mountain System. The broad and

interior area between the walls is the Central Plateau region. Each of these

major physiographic regions may be further subdivided into provinces or smaller

areas that are distinctive geologically and physiographically (2). The

physiographic character of each of these provinces is described in detail below.

Maps accompanying this report show the boundaries of each physiographic province

(Plate I) and additional detail in some provinces (Plate II). Plate I

Plate II

       

Arctic Plains and Foothills Region

        Arctic Coastal Plain . The northernmost physiographic provinc e of Alaska is ✓

a plain, 10 to 100 miles wide, which rises almost imperceptibly from the Arctic

Sea to a maximum altitude of 600 feet at its inner or southern margin where it

merges into the Arctic Foothills province. Along some parts of the boundary

between these provinces, the Arctic Foothills face the coastal plain as a gentle

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scarp 200 to 300 feet high. The coastal plain extends seaward as the

continental shelf beneath the Arctic Sea. The coast line makes only a slight

break in the profile across the coastal plain and shelf. Generally, the

shore is only 1 to 10 feet above the sea and the highest coastal cliffs are

only 50 feet high.

        The coastal plain, particularly in the western part, is spotted with

closely spaced oval to rectangular lakes, which have a nearly uniform orien–

tation of N. 9° W. to N. 21° W. (1). The average size of the lakes is 4 to

6 miles long and 1/2 to 1 mile wide. These lakes are believed to originate

through thawing of the underlying permafrost; the direction of enlargement

by thawing is believed to be controlled by the prevailing winds.

        Rocks underlying the coastal plain are nearly flat-lying fine-grained

types of sedimente of Cretaceous, Tertiary, and Quaternary age.

        The Arctic Foothills Province (22) is a belt of low rolling country,

local plateaus, and hills of moderate relief, south of the Arctic Coastal Plain

and north of the Brooks Range. The belt extends entirely across Alaska, about

on parallel 69° N. latitude, and ranges in width from 20 miles near the

Canadian border to 80 miles in the vicinity of the Colville River. In alti–

tude the foothills rise from 600 feet along the north border to 3,000 or 4,000

feet along the south border where they abut the northern front of the Brooks

Range, but local relief is rarely more than 1,000 feet. Extensive interstream

plains and flat divides are common.

        The Arctic Foothills province is underlain by Cretaceous and Tertiary

sedimentary rocks, which have been deformed into folds with an easterly trend.

Bands of medium-hard and soft rock are prevailing features of the geology.

Level upland surfaces truncate the folded beds, and locally, as on the Canning

River, these truncated beds are capped with gravel.

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        The Arctic Foothills province is drained largely by rivers that rise in

the Brooks Range on the south and flow north across the foothills belt and

the coastal plain to the Arctic Sea. A few streams, notably the Meade and

the Ikpikpuk, head in the foothills province. The course of the Colville

River, the largest on the arctic slope, is a striking exception to the general

north trend of the drainage. This stream rises within the north border of

the Brooks Range at about longitude 160° W., flows about 20 miles northward,

then turns and flows east and slightly north of east for more than 200 miles

to longitude 151°30′. In this course it receives the waters of several north–

flowing rivers that also rise in the Brooks Range. At longitude 150°30′ W.,

or just before it joins the Anaktuvak River, the Colville River turns abruptly

northward to flow to the Arctic Sea. Several tributaries on the north side

of the Colville River, the headwater tributaries of the Ikpikpuk and Meade,

and some tributaries of the Utukok, have easterly courses. All these streams

appear to be adjusted, more or less, to the regional structure, whereas the

north-flowing streams cross it.

        The border line between the foothills and the Brooks Range is sharp, for

the mountains present a bold north-facing front.

        The Arctic Foothills province was formerly referred to as the Anaktuvuk

Plateau or Arctic Plateau. However, its upper surface it not smooth, and it

is underlain by folded rocks; therefore, it is here referred to as a foothills

belt.

       

Brooks Range

        The Brooks Range is a rugged mountain belt that extends from east of the

Canadian boundary to the west coast of Alaska, south of Point Hops. It is

more than 600 miles long and nearly 100 miles wide, and lies between the Arctic

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Foothills province on the north and the Central Plateau region on the south.

The higher mountain peaks of the Brooks Range commonly reach an altitude of

between 7,000 and 8,000 feet, although at least two rise above 9,000 feet.

Mount Michelson, near the Canning River, is the highest mountain of the

Brooks Range and has an altitude of above 9,200 feet. A remarkable uniformity

of summit altitudes in the Brooks Range has been reported by Schrader (18),

Leffingwell (9), and Smith and Mertie (20); however, Maddren (11), who examined

the south side of the range, discounts the idea of accordance of summit levels,

and reports that the interstream rides gradually decrease in height southward

from the crest of the range.

        The Brooks Range is underlain by deformed and locally metamorphosed sedi–

mentary rocks of Paleozoic and early Mesozoic age. Structural lines generally

trend east, although in the southeastern part of the Brooks Range, around the

headwaters of the Chandalar and Sheenjek rivers, a northeasterly trend of

structural lines remains from an earlier period of orogenic activity.

        East of the 157° meridian, the Brooks Range is a mass of high rugged

mountains, with peaks commonly more than 7,000 feet and few passes loss than

4,000 feet. It was intensely glaciated during the Pleistocene, and a few small

glaciers still exist in the highest parts of the range. In this eastern part

both the north and south borders of the range are distinct and consist of abrupt

escarpments several thousand feet high. West of the 157° meridian, however,

the range is a group of roughly parallel low mountains, few of which are more

than 5,000 feet high, and is broken by many passes lower than 3,000 feet. The

western part of the Brooks Range merges more gradually on its borders into the

adjacent provinces than does the eastern part of the range.

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        The range is separated from the Central Plateau region along much of its

southern edge by a trough, 5 to 10 miles wide, which runs in an almost straight

line for most of the distance between the mouth of the Kobuk and the East Fork

of the Chandalar. East of the Chandalar, it is indistinct. This trough is

floored with Quaternary deposits, even where it crosses low divides that

separate the trough into sections along its length.

        The Brooks Range is drained in large part by rivers that flow either north

or south out of the range. The drainage divide is irregular in ground than,

but, in general, is about 30 miles from the north border of the mountain s . The ✓

Noatak River rises within the range approximately at longitude 155° W., and

flows westward near the axis of the range for about 200 miles then turns south

and finally discharges into the sea. Its entire course is through a succession

of broad alluvial valleys, separated by groups of hills through which the river

passes in narrow canyons. The Kobuk rises near the head of the Noatuk, and,

after leaving the range, flows westward along the south base of the mountains

carrying the water drained from the south flank of the western part of the range.

       

Central Plateau Region

        The Central Plateau region is the extensive low-lying, largely unglaciated,

central band across Alaska, bounded on the north by the Brooks range, and on

the south by the Alaska and Aleutian ranges. It measures nearly 800 miles from

west to east between Cape Prince of Wales and the Canadian border. This region

is about 450 miles wide at the Canadian border; narrows westward to the 150°

meridian where it is only 220 miles wide; and farther westward spreads broadly

until at the Bering Sea coast, between Cape Krusenstern and Ugashik, it is more

than 700 miles wide.

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        Almost the entire area of the Central Plateau region is drained by the

the great river systems of Alaska, the Yukon and the Kuskokwim. The Yukon

River system alone drains about 60 per cent of this area as well as considerable

areas in contiguous physiographic provinces in Alaska and a large area in

Yukon Territory.

        The Central Pl e ateau region has certain physiographic features that ✓

are characteristic of its entire area, but for more detailed description it

is convenient to divide the region into a number of provinces (Plate I).

        The region is an irregular assemblage of intricately dissected, moderately

low, even-topped uplands, and broad, flat, alluvi a u m-floored plains. Surmounting ✓

the uplands are small scattered groups of rugged mountains, which rise from

1,000 to 3,000 feet above the surrounding hilltops, and in most places are

composed of resistant igneous rocks. On the Seward Peninsula and in the

Kuskokwim Mountains, ridge tops average lose than 2,000 ft. In altitude.

Eastward, ridge tops rise until, along the Canadian border between the Yukon

and Tanana rivers, their average height is between 4,000 and 5,000 feet. The

rise in altitudes of the rivers is not as rapid, the Yukon and Porcupine at

the Canadian border being less than 1,000 feet above sea level, and tributaries

at the head of the Tanana near the Canadian border being less than 2,000 feet.

        The uplands in the western part of the Central Plateau region, with the

exceptions noted below, are generally underlain by folded and faulted Meso c z oic ✓

sedimentary rocks, but the isolated high mountains commonly are local igneous

intrusives. Upper Cretaceous rocks are notably widespread. Seward Peninsula

and the Kilbuck Mountains are underlain by metamorphosed Paleozoic sedimentary

rocks and associated intrusives. The highlands of the eastern part of the

Central Plateau region are underlain by tightly folded Paleozoic rocks and

pre-Cambrian schists.

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        Seward Peninsula extends westward for 200 miles and averages 120 miles

in width. Its northernmost point, Cape Espenberg, is on the Arctic Circle,

and its westernmost point, Cape Prince of Wales, is about 56 miles from the

nearest point on the Siberian mainland.

        In general, Seward Peninsula (19) has a topography characterized by

broad, rounded hills and swampy lowlands, surmounted by groups of rugged

mountains, and drained by many meandering streams. The prominent highland

groups are the York, Kigluaik or Sawtooth, Bendeleben, and Darby Mountains.

The York Mountains begin east of Cape Prince of Wales and extend eastward,

but lose much of their identity before reaching the American River, north

of Imuruk Basin. They are made up chiefly of altered limestone and phyllite.

The Kigluaik Mountains, nearer the coast of Norton Sound, are carved from

crystalline limestone, biotite schist, and gneiss, and form a second and

higher east-west range, south of the York Mountains, roughly parallel to the

south coast of the peninsula but from 20 to 30 miles north of it. Both the

Kigluaik and the Bendel e ben Mountains to the northeast are glaciated. The ✓

Kigluaiks are separated from the Bendeleben Mountains by a lowland that is

little more than 200 feet above sea level. The Darby Mountains, still farther

east, trend north-south and, together with the Bendeleben group, form a

crescentic chain open to the south and extending to the coast of Norton Sound.

The highest point of the peninsula is Mount Osborn (4,720 feet) in the Kigluaik

Mountains; few other peaks rise above 3,000 feet, and the general relief of the

peninsula is less than 2,000 feet. An extensive Pleistocene basalt plateau

surrounds Imuruk Lake, in the northeastern part of the peninsula.

        In contrast with the mountain highlands are several brose, low basins

floored with alluvium. The largest of these is the Kuzitrin Flats, the lower

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submerged portion of which is Imuruk Basin. This swampy lowland separates

the southern from the northern mountain group. It extends southeastward

across the southern mountain group into the valley of the Niukluk River and

thence south to Golovnin Bay. The upper Fish River basin is a plain on the

south side of the Bendeleben Mountains, and is completely surrounded by

highlands.

        The north coast of Seward Peninsula is a marshy coastal plain, as much

as 30 miles wide, fringed with a barrier reef and lagoon. Extensive marine

benches along the south coast suggest emergence. However, buried beach

placers (10) well below present sea level indicate that the relation of land

to sea along this coast has been changeable.

        Mountains between Norton Sound and the Yukon River . The mountainous

country between Norton Sound and the Yukon River (19), extending northward

between Seward Peninsula and the Koyukuk River to the Arctic Circle, consists

of low, even-topped ridges, commonly with steep walls and narrow valleys, all

trending in a northeasterly direction. The ridge tops average between 1,500

and 20,000 feet in altitude, but a few conical summits form prominent landmarks

from 500 to 1,000 feet above the general summit level. This region is underlain

by folded and faulted Cretaceous rocks having a pronounced northeasterly structural

trend. Drainage is wall adjusted to the structure; and nearly all the large

streams in this mountain belt flow northeast or southwest in conformity with

the structure. The valleys of these streams are commonly narrow and consist

of alternating gravel-floored flats and short rock-walled canyons generally

incised in a broad valley floor.

        Summit levels of the ridges are all closely accordant and have been

interpreted by some to indicate an uplifted peneplain. This, however, is not

indicated by the drainage, for discordant patterns are absent.

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        Lowlands of the Lower Yukon and Koyukuk . The lowlands of the lower Yukon

and Koyukuk rivers comprise an irregular area, extending 320 miles northeast

from the head of the Yukon Delta at Holy Cross and ranging from 10 to 80 miles

in width. The Kaiyuh Hills, projecting into the flat just south of the

junction of the Yukon and Koyukuk, almost out this province in two. To the

northwest of this lowland province lies the lowland valley of the Kobuk,

about which little is known. The Kobuk lowland is grouped with the lowlands

of the lower Yukon and Koyukuk provinces (Plate I).

        This lowland area exhibits three physiographic units in the vicinity of

Galena, where it has been studied in detail. Along the rivers are flood plains,

10 miles wide, which are commonly covered by spring floods. They are flat

areas underlain by peat, muck, and silt. Polygonal ice lenses indicate the

presence of permafrost. Oxbow and meander lakes are characteristic features

on the flood plains. Slightly higher than the flood plains are low terraces,

similarly underlain by fine sediments and peat, on which are irregular lakes.

The third physiographic unit includes high terraces bordering the plain at the

bass of surrounding bedrock hills. Remnants of the high terraces are scattered

over the lowland in the form of low isolated hills and buttes. These terraces

are 100 to 200 feet high and are composed of sand and silt. A few cave-in or

thaw lakes are present on these surfaces. Eardly (7) interpreted these

terraces as deltas built into a Pleistocene high-level arm of the sea. Elias

and Vosburgh (8) show, however, that the filling of the valley was once con–

tinuous and that the present terraces are remants of a formerly continuous sheet.

        Bedrock hills, belonging to other physiographic provinces, border this

province and a few isolated bedrock hills project above the valley floor, notably

Pilot Mountain and Bishop Rock, near the junction of the Yukon and the Koyukuk.

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        Delta of the Yukon and Kuskokwim . A large somewhat triangular area

extending southwest from the south end of the lower Yukon lowlands — and

bordered on the southwest, west, and northwest by the Boring Sea — is the

compound delta of the Yukon and Kuskokwim rivers. This almost flat area is

occupied by swamps and many lakes, and is crossed in part by the meandering

and anastomosing outlet sloughs of the Yukon. Tidal inlets along abandoned

distributaries indicate that this land, recently built out into the sea, is

slowly sinking. Only in the northwest part, where the Yukon is vigorously

constructing its delta, are tidal estuaries lacking.

        The surface of this otherwise level deltaic lowland is diversified by

a few volcanio mountains, which rise to a maximum height of about 2,500 feet.

Nunivak and Nelson Islands, off the west coast of this area, are largely

coalescing groups of volcanoes, although Tertiary sedimentary rocks are

reported on both islands.

        Yukon and Koyukuk Flats . The Yukon Flats (14) are situated between

parallels 65°50′ and 67°30′ W. latitude and meridians 148° and 150° W. longi–

tude. The extreme east-west dimension of the area is 190 miles, and at the widest

part it extends 65 miles from north to south. This crescent-shaped area has

the same trend as the great bend of the Yukon River which changes in a down–

steam direction on the flats, at Fort Yukon, from a northwesterly to a

southwesterly course. The altitudes of the Yukon River at Circle, in the

upper or southeastern extremity of the flats, is 500 feet; at Fort Hamlin,

where the river leaves the flats, it has been estimated at 300 feet. The rate

of fall of the river across the flats is about one foot per mile.

        The Yukon flows across this tilted alluvial surface at the rate of 3 to 6

miles per hour, through a system of intricately braided channels and meandering

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side sloughs. In meandering, the rivers on the flats are destroying the

deposits which they built by alluviation, and many banks are now above the

present level of alluviation. Furthermore, the exit channel at the lower

and is cutting into bedrock and thus is lowering the base level. Therefore,

the Yukon Flats are considered to be in the stage of destruction rather than

construction. Geomorphically, the Yukon Flats consist of an axial low-lying

flat, 20 to 40 miles wide, bordered by broad belts of marginal terraces, con–

sisting of alluvium or out-in bedrock, 12 to 30 miles wide.

        Along the axial flat is a meander belt of annual flooding and shifting

channels, 5 to 10 miles wide, bordering the Yukon and Porcupine rivers.

Narrower zones extend up other streams.

        Outside of this meander belt, the low-lying flats contain active scalloped

cave-in lakes and are crossed by many sloughs that branch off from the river

and follow sinuous and meandering courses across the flats until they finally

unite again. Soils in the meander zone show no profiles. Soil profiles in

the low-lying flats are only weakly zoned. Permafrost is extensive beneath the

flats and is close to the surface.

        The bordering terraces surrounding the flats range in altitude from a few

feet to about 1,000 feet, but are commonly only 200 to 300 feet high. In part,

terraces are out in alluvium, but many appear to be on bedrock. Relief is

moderate on the terraces. Thaw lakes are present, but do not appear to be as

numerous on the higher terraces and many of them are being drained. Soils

developed on the terraces generally show pronounced zoning.

        In general, the Yukon Flats are largely underlain by Quaternary alluvium

which consists of fine silts, sand, and gravel. The surface deposits are

commonly silt and sand to a depth of several feet, and sand and gravel at greater

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depth. The surrounding hills are, for the most part, made of hard rocks of

early Mesozoic, Paleozoic, or pre-Cambrian age. A few bedrock hills project

through the unconsolidated deposits of the flats, particularly in the north–

west corner. Soft Tertiary coal-bearing rocks have been reported from two

places in the flats, one on the bluff of the Hodzana River and another near the

forks of the Dall River. Aerial photographs, however, suggest that Tertiary

rocks may be extensively developed beneath terraces on the south side of the

flats. A wall drilled at Fort Yukon passed through 237 feet of unconsolidated

deposits and then entered hard bedrock.

        The Koyukuk Flats, another low alluvium-floored plain about 50 miles

across, include part of the drainage basin of the upper Koyukuk River. Little

is known about this plain, but it is probably closely similar [ ?] in physiographic

details to the Yukon Flats.

        Yukon Plateau . A highland province completely surrounds the Yukon Flats.

It is contignous with the Yukon Plateau in Canada. The only part of this

highland that is known in even slight detail is the area between the Yukon

and Tanana rivers, generally called the Yukon-Tanana Plateau (14). This area

is probably very similar to the other highland areas surrounding the Yukon Flats.

The Yukon-Tanana Plateau is the only part of the Yukon Plateau province

described in this report.

        The Yukon-Tanana Plateau as defined above is a triangular area bounded

on the south by the Tanana River, which trends about N. 70° W. from the vicinity

of the Canadian boundary to its junction with the Yukon River, a distance of

about 340 miles; on the north by the Yukon River; and on the east by the inter–

national boundary for 150 miles.

        The plateau area consists for the most part of even-crested, rounded ridges,

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EA-I. Wahrhaftig: Physiography of Alaska

with steep slopes and generally accordan ce t summits, separated by long, narrow ✓

V v alleys. The average altitude of the ridges is from 1,500 to 2,000 feet in ✓

the western part, but isolated peaks rise to 4,800 feet; the altitude of the

ridges gradually increases toward the east. Near the international boundary,

the ridges average between 3,000 and 4,000 feet in altitude and the highest

summits reach 7,000 feet.

        The rocks of the Yukon-Tanana Plateau are, for the most part, tightly

folded and somewhat metamorphosed Paleozoic rocks and pro-Cambrian schist.

Both types of rocks show a pronounced structural trend of about N. 60° E. A

few stocks of Tertiary granite form isolated high mountains in the western

part of the area. In the southeastern highland portion, extensive areas are

underlain by granitic intrusives which are not known to have any pronounced

structural trend. Tertiary volcanic rocks are present between the headwaters

of the Fortymile and the Tanana rivers.

        The Yukon-Tanana Plateau is drained by streams which flow into the Yukon

on the north and into the Tanana on the south. In general, stream courses are

well adjusted to structural lines in the plateau, but adjustment is still going

on as is indicated by wind gaps along some of the highland ridges.

        The Yukon River borders the plateau on the north from the Canadian boundary

to Circle at the southwest end of the Yukon Flats, and flows through a broad

canyon in which are terraces at 75, 500, and 700 to 800 feet above the river

level. The river in this stretch separates the highly deformed rocks of the

Yukon-Tanana Plateau from the slightly deformed rocks of the same age in the

Yukon-Porcupine portion of the Yukon Plateau to the north, and is a structural

boundary of great importance. The Yukon leaves the Yukon Flats through a huge,

broadly winding gorge, which is superposed upon the bedrock structure from

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Fort Hamlin to the mouth of Hess Creek. Downstream from Hess Creek to the

junction with the Tanana, the river follows a large isoclinal syncline of

poorly consolidated Tertiary rocks. This syncline appears to extend east

up the broad valley of Hess Creek. Terraces in the lower canyon are at 60

to 75 feet, 300 to 500 feet, and 900 to 1,000 feet above river level.

        The drainage divides in the Yukon-Tanana Plateau are extremely irregular

in ground plan. For 50 miles along the extreme southeast part of the plateau,

the divide between Tanana and Yukon drainage is only 10 miles from the Tanana

River, but to the west is close to the Yukon River.

        The lower valleys of rivers draining southward to the Tanana, west of

the mouth of the Johnson River, are filled with alluvium as much as 400 feet

deep. Rivers draining north to the Yukon generally have steep courses with

high terraces along the valleys. The valley of the Fortymile River is especially

interesting geomorphically. The headwater branches of this river rise in great

flat alluvium-filled basins, only a few miles north of the Tanana. As they

flow northward they leave those flats, and a prominent bench or terrace appears

on the walls of the canyon. This terrace rises downstream until near the mouth

of the river; it is 500 to 600 feet above the river. These features strongly

suggest tilting to the south along this stream.

        Various drainage features which have been regarded as anomalous in this

plateau, such as the tendency of streams like Birch Creek and Beaver Creek to

follow courses that bring then out of the plateau only a relatively short

distance from their sources, although many miles downstream, are probably the

result of close adjustment of an originally discordant drainage to structural

lines. The Yukon-Tanana Plateau, like the plateaue to the north and west, is

unglaciated for the most part, although small alpine glaciers appear to have

existed in the higher parts of the plateau during some or all of the glacial

stages in the Pleistocene.

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EA-I. Wahrhaftig: Physiography of Alaska

        Kuskokwim and Kilbuck Mountains . The Kuskokwim Mountains (6; B) are a

low mountain chain about 375 miles long and 50 to 100 miles wide. They

extend in a southwest direction from the area southwest of the junction of

the Yukon and Tanana rivers to the headwaters of the Holitna and Aniak rivers,

east of the lower Kuskokwim. They are perhaps extended to the southwest as

the Kilbuck Mountains. This latter group is about 200 miles long and 150

miles wide and reaches the sea coast between Kuskokwim and Bristol bays.

The Kilbuck Mountains differ physiographically from the Kuskokwim Mountains

in being much higher and more rugged and in having been the site of an exten–

sive icecap during the Pleistocene.

        In many ways the Kuskokwim Mountains are an epitome of the specific

characteristics of the highland areas of the Central Plateau region. Accordant

summit levels of round-topped low-lying ridges are extensively developed;

Tertiary granitic intrusives rise conspicuously above the general level of the

country in isolated, nearly circular, mountain masses; and the drainage, with

the exception of a few major streams, is wall adjusted to structural lines.

        The Kuskokwim Mountains consist, for the most part, of Paleozoic and

Mesozoic sedimentary rocks, of which Upper Cretaceous greywacke and argillite

predominate. These sedimentary rock have been intruded by 15 to 20 “granite”

stocks, which range from 8 to 15 miles across the present erosion surface.

Structure lines in the sedimentary rocks trend northeast and probably include

both fault lines and the traces of tilted and folded beds.

        The average height of the even-topped ridges is 1,500 to 2,000 feet. These

ridges are mostly straight and trend northeast-southwest in conformity with the

drainage pattern, but branching ridges are also common. Most of the large

streams rising in the Kuskokwim Mountains flow for the greater part of their

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EA-I. Wahrhaftig: Physiography of Alaska

length in northeast-trending valleys, but turn directly across the ridges

where they leave the range. Several streams cross bedrock hills and ridges

rising out of or bordering flat basins, suggesting that they have been super–

posed on these hills. The Kuskokwim River flows along the south side of the

Kuskokwim Mountains from near Lake Minchumina to Sleetmute, where it turns

sharply west and crosses the mountains in a narrow winding gorge. Most of

the drainage on the south side of the mountains reaches the sea by way of the

Kuskokwim, Drainage at the northeast and reaches the Yukon by way of the

Kantishna, Cosna, and Chitanana rivers. Drainage on the north and west sides

reaches the Yukon by way of the Nowitna, Innoko, and Iditarod rivers.

        Isolated mountains of granitic rocks rise above the general summit level

to altitudes of between 3,000 and 5,000 feet above sea level. Nearly all are

intensely glaciated and have U-shaped valleys and cirques and narrow ar e ê tes. (no accent)

Drainage out of each of these mountain groups is roughly radial, although

north-flowing streams are commonly longer and drain the greater part of each

group.

        J. M. Hoare, in a personal communication, reports evidence of Pleistocene

faulting along the southeast border of the mountains, indicated by drainage

diversions and steep straight scarps. Hoare also reports that the valleys of

most streams show broad alluvium- or bedrock-floored flats in which the present

streams are incised in narrow canyons. These benches are approximately at stream

level near the heads of the streams, but the difference in altitudes of the

benches and streams increases until at the lower ends of the streams the bedrock

bench may be 300 feet above stream level. Hoare also reports that buried

channels, now filled with alluvium, exist beneath these benches and extend in

depth to present stream level or below.

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        Extensive deposits of gray silt are found on the sides of ridges bordering

the Kuskokwim and above Sleetmute up to an altitude of 800 feet.

        The Kilbuck Mountains appear to rise sharply above the general accordant

summit level of the Kuskokwim Mountains, despite the fact that this summit

level rises in the general direction of the Kilbuck Mountains. These mountains

consist of Mesozoic and Paleozoic sedimentary and metamorphic rocks but include

granitic intrusives. They have been extensively glaciated. A notewarthy

features of these mountains is a series of very deep and large glacial rock-basin

lakes, the Tikchik Lakes, which occupy canyons on the east wide of the range and

extend eastward on the bordering flats.

        Tanana-Kuskokwim Flats . The southern border province of the Central Plateau

region (14; 23) is a great gravel apron, flanking the Alaska Range on the north

and west, that extends as an unbroken arcuate lowland for a distance of 500 miles,

from east of the Delta River to the upper Holitna. It ranges from 40 to 60 miles

in width. South of the Holitna, the lowland is separated by a hilly divide from

the headwaters of the Nushagak. The lowest pass on this divide is about 800 feet

in altitude. The broad plain of the Nushagak, at the head of Bristol Bay, in

about 90 miles wide.

        The drainage pattern on the Tanana-Kuskokwim Flats is distinctly asymmetrical,

the axial streams flowing alose to the north side of the flats and grazing pro–

monotories of bedrock hills. From 144° W. longitude to Lake Minchumina at

longitude 152° W., the alluvial basin is drained by the Tanana and its tribu–

taries. From Lake Minchumina [ ?] south to the Holitna-Nushagak divide,

drainage is by headwaters of the Kuskokwim. Farther south, drainage is by the

Nushagak and other rivers into Bristol Bay.

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        The Tanana Flats are gently sloping, nearly featureless plains of deposi–

tion, floored by both fine alluvium and outwash gravel undoubtedly derived

in large part from the south in the Alaska Range and in lesser amounts from

the hills to the north. On the basis of drainage pattern and underlying

sediments, the plains consist of three parallel longitudinal zones.

        The southern zone, 15 to 30 miles wide, is a series of coalescing alluvial

fans from the Alaska Range and is crossed by north-flowing streams, which, in

places, spread into radial distributaries. The gravel composing the fans is

derived from the Alaska Range and is coarse near the mountains but finer north–

ward. The gravel deposit is at least 100 feet thick, and may be much thicker.

The water table in this gravel is deep and permafrost is absent.

        The axial zone of the province includes the modern and former meander

belts of the axial streams, the Kuskokwi n m and Tanana rivers. It is level in ✓

cross-profile. Drainage courses trend west or southwest parallel to the axis

of the flats. The belt consists of an intricate not of crisscrossing active

and aban c d oned stream channels. Thaw or thermokarst lakes are common. This belt ✓

is from 5 to 10 miles wide and impinges upon bedrock spurs on the north. Wells

drilled on the north side of this belt show that the unconsolidated deposits

are at least 400 feet thick and that the floor of the Tanana Flats is at

least at sea level or perhaps below.

        North of the axial zone is a discontinuous zone, present chiefly in the

many alluvial embayments that extend into the hills on the north side of the

flats. This zone is drained by sluggish, widely meandering streams that head

in the Yukon-Tanana Plateau. The alluvial filling of the embayments consists

of as much as 200 feet of black frozen silt with high organic content, containing

wedges, veins, and lenses of clear ice, and is known to the min d ers as “muck.” ✓

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EA-I. Wahrhaftig: Physiography of Alaska

This material rests on a layer of gravel, 10 to 50 feet thick. The gravel floors

the stream valleys and the embayments, and contains the placer gold in the Fair–

banks district.

        The south border of the Tanana Flats, from the Delta River west to the

Toklat River, is a steep northward-dipping monocline of poorly consolidated

Tertiary rocks. It is likely that Tertiary rocks underlie the southern part

of the Tanana-Kow us kokwim Flats, in part at least. No Tertiary rocks have been ✓

reported from the north side of the Tanana Flats.

        A few bedrock hills, notably the Wood River buttes, project through the

alluvium of the flats, and bedrock hills of Birch Creek schist and Mesozoic

sedimentary rocks border the flats on the north. The south-facing sides of

these northern hills are commonly covered with a deposit of tan silt, as much

as 50 feet thick, that is believed to have been derived from the bars of rivers

in glacial times by wind action. It is also believed to be the source, through

mudflow and other processes, of the inorganic part of the muck. The silt cover

is inconspicuously gullied, but gully walls and interfluves are smoothly

rounded and gullying is not now in progress.

        [ ?] Pacific Mountain System

        The mountain ranges that border the Pacific coast of North America extend

from the Sierra-Cascade chains northwestward along the coast of British Columbia,

southeastern Alaska, and the Gulf of Alaska. This Pacific Mountain System, as

named by Brooks, is separated from the inland Rocky Mountain system by a series

of plateaus, known in the United States as the Colorado Plateaus, the Columbia

Plateaus, and the Basin and Range province; in Canada as the British Columbia

Plateau and the Yukon Plateau; and in Alaska as the Central Plateau region.

        The Pacific Mountain system in Alaska comprises the following ranges or

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provinces: British Columbia Coast Range, St. Elias Range, Wrangell Mountains,

Talkeetna Mountains, Chugach-Kenai Mountains, Alaska Range, and Aleutian

Mountains.

        The British Columbia Coast Range includes all of the coast and islands

of southeastern Alaska. Where the panhandle joins the main part of Alaska,

the Pacific Mountain System narrows and becomes of such height as to con–

stitute one of the great mountain groups of the world, the St. Elias Range.

Beyond the western part of the St. Elias Range, the Pacific Mountain System

branches into two distinct subparallel ranges which form roughly concentric

arcs around the Gulf of Alaska.

        The southern range is known as the chugach Mountains from longitude 143° W.

as far west as Turnagain Arm, and as the Kenai Mountains southwest of Turnagain

Arm. These two portions of this branch are separated only fortuitously by

narrow glacial valleys and are otherwise structurally and gaomorphically

continuous. Southwest of the Kenai Peninsula, the continuation of the range

again appears as the Kodiak Mountains of Kodiak Island. The trend of this

chain of mountains is roughly N. 70° W. from longitude 143° W. as far as

longitude 147° W. where it changes sharply and holds about S. 40° W. to the

tip of Kodiak Island.

        The eastern part of the northerly branch is known as the Alaska Range.

It extends toward the west and southwest as an are, with a radius of curvature

of about 300 miles and a center in the north part of the Gulf of Alaska, from

the Canadian border as far west as the 151° meridian. There it is identical

with the Aleutian Range which extends still farther west, first as the mountains

of the Alaska Peninsula and beyond as the Aleutian Islands. The peninsula and

islands form another great are concave to the north with a radius of curvature

of about 1,000 miles.

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        The two branches of the Pacific Mountain System in Alaska are separated

by a predominately lowland belt, 40 to 120 miles wise, above which rise two

large, roughly oval mountain masses, the Wrangell Mountains, east of the Copper

River, and the Talkeetna Mountains, west of the Copper River.

        The Pacific Mountain System is drained by many large rivers which flow

south to the Pacific, north to the Yukon, and north and west to the Kuskokwim.

The most important rivers draining to the Pacific are the Copper and Susitna,

west of Mount St. Elias, and the Alsek, Taku, and Stikine, east of Mount St. Elias.

The rivers draining north are much smaller. A noteworthy feature of the drainage

of the Pacific Mountain System is the large number of streams that head on one

side of a major mountain range but cut through that range to the other side.

This causes the drainage divides to coincide only incidentally with the axes

of the major mountain masses. Five rivers heading in the plateau east of the

British Columbia Coast Range, north of latitude 54°, cross the mountains and

flow into the Pacific Ocean; five rivers which head south of the axis of the

Alaska Range cut through the range and flow into the Tanana River. The St. Elias

Range is crossed by the Alsek, the Chugach Mountains by the Copper River. The

Susitna River crosses the northwestern Talkeetna Mountains by a canyon, although

low structural valleys are present as the north and south sides of the range.

        The Alaska Range (4; 16; 23) is an are, about 600 miles long, having a

radius of curvature of about 300 miles and a center of curvature located just

north of Middleton Island in the Gulf of Alaska. It is about 30 miles wide near

the Canadian boundary but widens gradually westward in the next 470 miles until,

in the vicinity of Mount McKinley, it is 60 miles wide; 130 miles farther south

in the vicinity of Mount Spurr, it is 120 miles wide. Its southern termination

is an arbitrary separation from the Aleutian Mountains. Throughout its length

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EA-I. Wahrhaftig: Physiography of Alaska

the range is rugged and intensely glaciated.

        The highest portion is Mount McKinley (longitude 151° S W ., latitude 63°04′ N. ) ✓ ✓

It is 20,300 feet high and is the highest point on the North American continent.

However, the number of high mountains in the Alaska Range is comparatively

small. Less than 20 peaks in the whole range are higher than 10,000 feet,

and the crest of most of the range averages between 7,000 and 9,000 feet in

in [ ?] altitude. Seen from either south or north, the crest line presents an uneven

aspect. The range appears low on the whole but is dominated by four great

mountain masses: the Mount Spurr-Mount Gerdine group in the extreme southwest,

culminating in Mount Gerdine (12,600 feet); the Mount McKinley group at the

great bend in the range, culminating in Mounts McKinley (20,300 feet) and

Foraker (17,000 ) feet); the Mount Hayes group near longitude 147° W., culminating

in Mount Hayes (13,740 feet); and the mountains are around Mount Kimball (9,680 feet)

at longitude 145° W.

        The rocks of the Alaska Range are schists and gneisses of pre-Cambrian age;

well-consolidated sedimentary and metamorphic rocks, ranging in age from early

Paleozoic to Upper Cretaceous; intrusive and volcanic rocks of various ages;

and local areas of poorly consolidated sedimentary rocks of Tertiary age along

the borders and in some of the valleys. Structural lines and trends of the rocks,

in general, parallel the trend of the range.

        The Alaska Range has been subject to repeated periods of mountain building,

the greatest of which probably took place at the end of the Cretaceous. The

structure the mountain range inherited from that orogeny is that of a great

syncline with Upper Cretaceous rocks in the center and older rocks on the flanks.

However, the present relief of the range is due to structures produced in

Cretaceous rocks in a later orogenic epoch in the middle of the Tertiary.

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        A remarkable feature of the Alaska Range is the presence of great

longitudinal vertical faults of large displacement that extend most of the

length of the range, and which are zones of weakness along which valleys and

Mountain passes have been carved. The longest of the known faults has been

recognized over a distance of 300 miles. West of Mount McKinley, this fault

is near the north side of the range, but is crosses the range east of Mount

McKinley; from the vicinity of Broad Pass to east of the Delta River, it marks

the south boundary of the range. Farther east it again crosses the range and

east of the Nabesna River marks the north side of the range, which is there

known as the Nutzotin Mountains. This and other faults of similar trend have

apparently been intermittently active from pre-Cretaceous into Tertiary time,

and there is some evidence that at least one of them may be active today.

        The Alaska Range is crossed by several low passes occupied, for the most

part, by several rivers which head on the south side of the range and flow

across the range to empty into the Tanana or Tukon. From east to west these

rivers and passes are: the white River just east of the Canadian border,

Beaver Creek at the Canadian border, the Chisana River at longitude 142° W.,

the Nabesna River at longitude 142°30′ W., Mentasta Pass and Suslots Pass

(altitudes about 2,500 and 3,000 feet), the Delta River at longitude 146° W.,

the Nenana River at longitude 149° W., Rainy Pass (2,950 feet) at longitude

153° W., and Merrill Pass (3,000 feet) near the south end of the range.

Elsewhere the range is drained by rivers that rise on its flanks and flow

south into the Copper or Susitna rivers, or north or west into the Tanana or

Kuskokwim. Most of these rivers are short and steep, and nearly all those

rising in the higher portions of the range head in glaciers. In particular,

the mountains around Mounts Spurr, McKinley, and Hayes support enormous glaciers,

some of which extend 20 or 30 miles from their sources and spread out as great

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piedmont lobes at the edge of the plains bordering the range. Glaciers on the

south side of the range are much larger than those on the north side, as that

side receives the most precipitation. On the other hand, ice is found in

north-facing cirques at lower altitudes than in south-facing cirques.

        Bordering the Alaska Range, between Mount McKinley on the west and the

Delta River on the east, is a northern foothill belt, about 20 miles wide, that

consists of parallel east-trending ridges and valleys. The ridges are underlain

by crystalline schists for the most part, and the valleys are commonly developed

on soft Tertiary rocks. The foothills belt is crossed by north-flowing streams

rising in the Alaska Range. These streams are clearly superposed on the structure

and topography of the foothill belt. The same pattern of north-flowing streams

superposed on an east-trending ridge and valley system is likewise shown in the

main range in Mount McKin e ly Park. ✓

        The Aleutian Range Aleutian Range , which forms the crest of the Alaska Peninsula and its ✓ underline

westward extension, the Aleutian Islands, consists, in general, of a strongly

glaciated “fretted” upland, ranging in altitude from 5,000 feet in the eastern

Alaska Peninsula to about 2,000 feet in the western Alaska Peninsula and the

Aleutian Islands. Rising about this upland is a chain of remarkable volcanic

cones, many of which are still active. The northernmost of the volcanoes that

properly belong to this chain is Mount Spurr, which is within the province of

the Alaska Range. The northernmost volcano of the Aleutian Mountains proper,

as used in this report, is Mount Redoubt, 10,200 0 feet high. Other high peaks ✓

are Iliamna, 10,085 feet, Pavlof, 8,900 feet, and Shishaldin, 9,978 feet in

height. (See also “Volcanoes of Alaska.”) The Aleutian Range is about 80 miles

wide at the widest point and 1,500 miles long. Of this length, 500 miles is

mainland (Alaska Peninsula), and 1,000 miles the Aleutian Islands.

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        The mountains consist in part of unmetamorphosed middle Mesozoic

sedimentary rocks, in part of pre-Tertiary metamorphic and igneous rocks, and

in part of Tertiary volcanic and interbedded sedimentary rocks. Structural

lines in the rocks are generally parallel to the range; however, deformation

in the Aleutian Mountains is probably in large part Quaternary, and the com–

binations of recent deformation and intense Quaternary volcanic activity have

produced a pattern of ridges and drainage that is not all regular and certainly

does not reflect the underlying structure. Many streams rise on one side of

the range and cross to the other. Furthermore, the range is broken in many

places by low passes which, west of the tip of the Alaska Peninsula, are all

below sea level.

        Along the north side of the Alaska Peninsula is a plain, from 10 to 50 miles

wide, apparently a prograded shore into Bristol Bay. On the physiographic map,

this is indicated as part of the Central Plateau region, to which province it

belongs physiographically. It apparently bears the same relation to the

Aleutian Mountains as the Tanana Flats bear to the Alaska Range; that is, it

is a basin of sedimentation bordering the range.

        The Talkeetna Mountains (5) are a rudely circular mountain mass, bounded

on the west by the Susitna Lowland and the valley of the Chulitna River, on

the north by Broad Pass and the upper valley of the Nenana River, on the east

by the Copper River plateau, and on the south by the Matanuska valley. They

measure approximately 100 miles in a north-south direction and from 80 to 90

miles in an east-west direction. South of the canyon of the Susitna, the

Talkeetna Mountains are a high, compact, exceedingly rugged mass, 7,000 to 8,000

feet high. North of the Susitna River, the Talkeetna Mountains consist of

separate mountains with intervening alluvial depressions, and few altitudes

exceed 6,000 feet.

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        The bedrock of the Talkeetna Mountains falls into two types roughly

corresponding with the two divisions north and south of the Susitna. The

southern Talkeetna Mountains are, for the most part, a great grenite batho–

lith, although Mesozoic and Tertiary sedimentary rocks occur on the south

and east flanks, and extensive flat-lying Tertiary volcanic deposits are

known in the center of the range. The northern Talkeetna Mountains, while

underlain in part by intrusive rocks, are predominantly areas of Mesozoic

sedimentary and volcanic rocks.

        The northern front of the Talkeetna Mountains is a bold scarp about

4,000 feet high, facing Broad Pass and the upper Nenana Valley. The southern

front is a similar great scarp 5,000 to 6,000 feet high, facing the Matanuska

Valley. To the east, the Talkeetna Mountains merge gradually into the Copper

River plateau. On the west, a great smooth surface, probably an unconformity,

emerges from the Susitna Lowland, the rises to the east where it has been so

reduced largely by dissection that, at its highest position, over the center

of the Talkeetna Mountains, it has been entirely destroyed and the serrate

peaks of the mountains are carved from the rocks beneath it.

        The drainage of the Talkeetna Mountains, while crudely radial, is dis–

tinctly asymmetric and presents many anomalous features. The Susitna River

rises in the slopes of the Alaska Range at the northwest corner of the Copper

River plateau, and flows directly south to latitude 62°40′ N., where it turns

abruptly west and crosses the Talkeetna Mountains in a deep, narrow stream-out

gorge. It enters the Susitna Lowland between Chulitna and Talkeetna. A

prominent drainage divide lies in the high part of the range only 15 to 20

miles north of the Matanuska Valley; from this divide the Talkeetna, Kashwitna,

and other rivers flow north and west in great concentric arcs to reach the

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Susitna Lowland. Farther east, the Oshetna and Little Nelchina rivers

drain the east flank of the mountains. The south flank drains into the

Matanuska; and the extreme northwestern part drains by various small streams

into the Chulitna on the north or into the Susitna.

        The Talkeetna Mountains are strongly glaciated. Extensive glaciers

still exist in the highlands of the southern part of the range.

        The Wrangell Mountains (4; 12; 15; 16) are a compact elliptical mountain

group centered approximately at latitude 62° N., longitude 142°30′ W., and

are distinctly different in structure and topography from any mountain group

so far described, with the possible exception of the Aleutian Mountains. The

Wrangell Mountains group is about 95 miles long and 65 miles wide. Its

longest diameter trends about N. 60° W. On the northwest and west, the

mountains are bordered by the Copper River plateau; on the south, by the

valley of the Chitina River. They are separated from the Alaska Range to the

northeast by a series of passes that lead from the head of the Copper River

to the head of the White River, and from the St. Elias Mountains on the south–

east by the pass at the head of Skolai Creek and the White River. The highest

peak in the Wrangell Mountains is Mount Sanford, 16,208 feet, in the extreme

northwest corner of the group; but several other peaks exceed 10,000 feet,

notably Mount Blackburn (16,140 F f eet), Mount Wrangell (14,005 feet), Regal ✓

Mountain (13,408 feet), and Mount Drum (12,002 feet). The drainage of the

Wrangell Mountains is radial; water from the northwest, west, and south sides

flown into the Copper and its principal tributary, the Chitina. On the

northeast and east sides, the Wrangell Mountains are drained by the Nabesna

and Chisana rivers into the Tanana, and by the white River into the Yukon.

The central part of the Wrangell Mountains is covered by a vast ice field from

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which numerous large glaciers extend to low altitudes and supply water to the

rivers draining the range.

        The Wrangell Mountains are a group of composite lava-cinder and exogenous

lava domes, resting on a platform of folded Paleozoic and Mesozoic rocks,

about 4,000 feet high. The higher mountains have been built up approximately

10,000 to 12,000 feet above the 4,000-foot platform on which they rest. Evi–

dence from fossils indicates that this area has been a center of volcanic

activity since the beginning of Tertiary time. The volcanoes, with the excep–

tion of Mount Wrangell, are now extinct. The present forms of the mountains are a

product of both volcanic activity (which normally produces symmetrical cones and

domes), and glacial sculpture. The form of the individual peak depends in part

on the type of volcanic activity (whether explosive or quiet flows), and the

time elapsed since the volcano was active, and during which glaciers have been

sculpturing it. Thus, Mount Wrangell is a broad, smooth, almost perfectly

preserved dome; Mounts Sanford and Drum, originally composite cones with steep

sides, now have great cirques gnawed out of their sides by glacial action, and

their cones are only partly preserved. Mounts Regal and Blackburn have been

extinct so long that glaciers have largely destroyed their original form.

        The Susitna-Cook Inlet Lowland (5) is a nearly flat, low-lying plain,

about 100 miles long from Homer to Talkeetna, and trends W. 30° E. to N. It

extends inland from the south coast of central Alaska and its southern end is

partly submerged beneath the waters of Cook Inlet. The seaward extension can

be followed for 100 miles southwest as Cook Inlet and Shelikof Strait. To

the north and northeast its structural continuation is the Broad Pass depression,

which can be followed for 60 miles. Branching to the east is another structurally

controlled depression, the Matanuska Valley, which geologically and physiographically

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EA-I. Wahrhaftig: Physiography of Alaska

is closely related to the Susitna-Cook Inlet Lowland. At its widest part, at

Anchorage, the Susitna-Cook Inlet Lowland is 70 miles wide. Farther to the

southwest, however, it is only 40 miles wide and the Broad Pass depression at

the north end averages 10 miles in width. The Susitna-Cook Inlet Lowland is

bordered on the west and north by the Alaska Range, on the east by the Talkeetna

Mountains, and on the south by the Chugach and Kenai Mountains.

        The Susitna-Cook Inlet Lowland is drained by rivers flowing into Cook

Inlet. The largest, the Susitna River, enters the lowland between Talkeetna

and Curry, and flows slightly west of south for 80 miles before reaching Cook

Inlet. A large tributary of the Susitna, the Chulitna, drains part of Broad

Pass. The western part of the lowland is drained by the Yentna, another tributary

of the Susitna. Matanuska valley is drained by the Matanuska, which enters Knik

Arm at the northeast end of Cook Inlet.

        The surface of the Susitna Lowland is largely mantled by glacial and flufvia- ✓

glacial deposits, formed during Pleistocene time when a vast ice sheet covered

this area. Its topography, though minor in relief, is diversified; the northern

part has a great system of parallel north-south ridges, 10 to 50 feet high, and

looks as if it had been furrowed by a great plow. Between the Susitna Valley

and the Matanuska Valley is a broad, low, medial moraine extending southwest from

the corner of the Talkeetna Mountains; this is hilly country, with relief of as

much as 200 feet, and contains numerous lakes. The floor of lower Matanuska

Valley is a complex assemblage of outwash plains, sand plains, crevasse filling,

and terraces, all of glaciofluvial origin. Near the lower Matanuska River and

north of Knik Arm, the surface is mantled with a deposit of wind-blown loess,

which forms the rich agricultural land of the lower Matanuska Valley. The

shores of Cook Inlet are generally outlined by cliffs, 50 to 200 feet high.

The inlet itself is shallow, and two headward branches, Knik Arm and Turnagain

Arm, are almost silted up.

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        A few high hills and low mountains of granite rise in the center of

the Susitna Lo s w land just north of Cook Inlet. The most prominent of these ✓

is Mount Susitna, 4,397 feet high.

        Most of the rocks older than Pleistocene that are exposed in the

Susitna Lowland are poorly consolidated terrestrial Tertiary sedimentary

rocks known variously as the Kenai group, Chickaloon formation, or simply

as the coal-bearing formation. These rocks underlie a large part of the

Kenai Foreland, the low-lying northwestern part of Kenai Peninsula, and

crop out on the northern shore of Cook Inlet near Tyonek, where they are

deformed and dip to the east, away from the Alaska Range. Farther north they

are exposed on the banks of the Beluga, Yentna, Skwentna, and Susitna rivers;

in Peters Hills; and around the flanks of Mount Susitna. Extensive deposits

are well exposed in the Matanuska Valley, and small patches of Tertiary rocks

occur in the floor of Broad Pass. It is believed that the entire Susitna

Lowland is underlain by deformed Tertiary rocks, across which the essentially

plane surface of the lowland has been cut, probably by stream erosion.

        The Copper River Plateau (12) is a roughly equidimensional basin, about

70 miles in diameter, at an average altitude of about 2,000 feet. It is bounded

on the west by the Talkeetna Mountains, on the north by the Alaska Range, on

the east by the Wrangell Mountains, and on the south by the Chugach Mountains.

Two narrow prongs, extending east, wrap partly around the Wrangell Mountains.

        The Copper River plateau is drained by several rivers that rise in the

plateau and leave by great gorges through the surrounding mountains. Conse–

quently, drainage divides between important river systems cross the plateau

as inconspicuous water partings, commonly sinuous in plan, indicating an

accidental origin for a large part of the plateau. The southern and eastern

[ ?]

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EA-I. Wahrhaftig: Physiography of Alaska

three-quarters of the plateau are drained by the Copper River and its tribu–

taries. The Copper River leaves the southeast corner of the plateau and flows

across the Chugach Mountains in a prominent glaciated gorge. The northwest

corner of the plateau is drained by one of the headwater tributaries of the

Susitna River. The Delta River rises on the north side of the plateau and

crosses the Alaska Range to empty into the Tanana. In the extreme southwest

corner of the plateau, the headwater tributaries of the Matanuska River rise

on the plateau side of the Talkeetna Mountains. Many lakes, perched on the

drainage divides, discharge their overflow into two river systems.

        The Copper River plateau is largely floored with Pleistocene glacial and

fluvioglacial deposits. The Copper River has cut 600 feet into these deposits

on the east side of the plateau without disclosing their base. In general,

deposits in the southern and central parts of the plateau, as exposed in the

Copper River valley, appear to be composed of fine gravel, sand, and silt, and

those of the peripheral areas and northern part of the plateau, of co u a rse ✓

gravel and sand. Much of the surface of the plateau is mentled with till;

the western part has many lakes in morainal depressions. In the northern and

western part of the plateau, many bedrock hills, composed of greenstone and

Paleozoic sedimentary rocks, rise above the plateau surface.

        Chugach-Kenai Mountains . The mountain chain that forms the south coast

of Alaska from about meridian 143° W. longitude to Turnagain Arm is known as

the Chugach Mountains. The mountains of Kenai Peninsula, south of Turnagain

Arm, although structurally and physiographically continuous with the Chugach

Mountains, are known as the Kenai Mountains. A southwest extension of the same

mountain chain appears in the mountains of Kodiak Island. This distinct physic–

graphic province is here called Chugach-Kenai Mountains.

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EA-I. Wahrhaftig: Physiography of Alaska

        The Chugach and Kodiak Island physiographic province is about 600 miles

long and ranges in width from 40 miles on Kodiak Island to 100 miles, east of

the Copper River. From 143° to 147° W. longitude its trend is roughly N. 70° W.,

and then changes abruptly to S. 40° W. and continues so throughout Kenai

Peninsula.

        The average height of the higher mountains in the Chugach Range is between

7,000 and 8,000 feet. A few peaks are higher, the highest mountain being Mount

Marous Baker, 13,250 feet high, which stands near the point where the range makes

it sharp bend in trend, and is only 13 miles from tidewater at the head of

College Fjord. Other high peaks of the Chugach Range are Mount Witherspoon,

12,023 feet, and Mount Miller, 11,000 feet, in the east part of the range.

Mountains in the Kenai Range are much lower, averaging between 3,000 and 5,000

feet in altitude. The highest peak (unnamed) in this range in 6,400 feet high.

The Kodiak Mountains average about 4,000 feet in height.

        The Chugach-Kenai Mountains are extremely rugged, the average relief of

valley walls being more than a mile. They have been intensely glaciated, the

serrate peaks of fretted uplands separated by U-shaped valleys. The Kenai and

Chugach Mountains are the loci of some of the greatest remaining ice fields of

the continent. The largest are the Harding icecap on Kenai Peninsula, the

Columbia ice fields in the vicinity of Mount Witherspoon, and the ice fields

that feed the Guyot and Bering glaciers in the eastern Chugach Mountains.

Many of the glaciers rising in these ice fields descend to tidewater.

        A portion of the coastal side of the Chugach Range near the sharp bend of

the mountains is partly submerged and forms the island-studded indentation of

the seacoast known as Prince William Sound. East of Prince William Sound the

coast is fairly straight, and the mountains are separated from the sea by a

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EA-I. Wahrhaftig: Physiography of Alaska

narrow coastal shelf, but in Prince William Sound and west of it, the shores

are deeply frayed by extensive glacial fjords, and islands fringe the coast.

A broad strait separates the Kodiak Island mountains from the mainland.

        The bedrocks of the Chugach-Kenai Mountains consist almost entirely of

Mesozoic greywacke and argillite, but include some interbedded greenstone

and numerous granitic intrusives. These rocks are in most places tightly

folded, and elsewhere are thrown into broad open folds; everywhere, however,

they are affected by a well-developed cleavage which dips steeply and strikes

parallel to the trend of the mountains. Tertiary sedimentary rocks border the

south coast east of Copper River. On the southeast coast of Kodiak Island,

beds of Tertiary age are faulted and closely folded.

        Drainage lines and trends of valleys within the Chugach-Kenai Mountains

tend to follow two directions at right angles to each other. Where glacial

erosion met favorable conditions, as in the vicinity of Copper River and at

the northern end of the Kenai Peninsula, the mountains are broken into many

closely fitting rudely rectangular mountain masses separated by broad glacial

valleys and low passes. One major trend of valleys and drainage lines is normal

to the direction of the range. The other trend is roughly parallel to the axis

of the range, and apparently is controlled by the occurrence of weak or much

fractured rocks. These trends give a conspicuous reticulate pattern to the

drainage, and some of the glacial lakes, such as Kenai Lake, have a zigzag

pattern from following first one drainage direction and then the other.

        The streams that drain the Kenai Chugach Mountains are nearly all short

and swift. The Copper River, which rises in the Wrangell Mountains and Alaska

Range, flows across the Chugach Mountains from the Copper River plateau to the

sea at longitude 145° W.

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EA-I. Wahrhaftig: Physiography of Alaska

        St. Elias Range . Probably the most impressive mountain group on the

North American continent is the St. Elias Range, situated where the larger

part of Alaska joins the panhandle of southeastern Alaska. This range is

approximately 300 miles long and has a maximum width near Mount St. Elias

of about 120 miles. Its trend is about N. 45° W.

        The St. Elias Mountains have been almost inaccessible and their geology,

consequently, in largely unknown, yet it is believed that the core of the

range is made up of Palaeozoic metamorphic rocks and granitic intrusives,

associated with Mesozoic and Tertiary rocks along the south flank. The range

is out by many great faults, and is separated from a low coastal shelf of

Tertiary rocks by a great fault zone which has been a place of notable earth

movement as late as 1899 (21).

        The highest mountain in the St. Elias Range is Mount Logan, 19,850 feet,

in the southwest corner of Yukon Territory. Mount St. Elias, from which the

range takes its name, rises to an altitude of 18,008 feet directly from a

coastal terrace only 2,000 feet high. Twelve other mountains of the range are

higher than 14,500 feet, the highest altitude reached by any mountain in the

so-called continental United States. The southesterly extension of the St. Elias

Range is known as the Fairweather Range and is dominated by Mount Fairweather,

15,300 feet.

        British Columbia Coast Range (Coast Mountains) . The British Columbia

Coast Range province (3) includes all of the southeastern panhandle of Alaska

and extends for to the south and east following the coast of British Columbia.

Included in this province are the mountainous islands of the Alexander Archipelago,

which fringe the mainland coast of southeastern Alaska.

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        In southeastern Alaska the province is characterized by rugged mountains,

deeply indented fjords, and islands separated by many great channels and straits,

some of which are of great depth. The highest point in the Coast Range, in

Alaska, is Kates Needle (10,002 feet).

        Southeastern Alaska can be divided into three parallel longitudinal belts

in which the types of bedrock are reflected in part by the topography.

        The eastern belt, which includes the mainland and some bordering islands,

is a high mountain mass, capped by extensive ice fields. The general upland

altitude is from 6,000 to 8,000 feet. This area supported a much greater ice

field in Pleistocene time. As a consequence, it is deeply scored by tremendous

fjords, as much as 100 miles long. The mountain walls tower as much as 5,000

to 6,000 feet above waters that are themselves in places 1,500 to 2,000 feet

deep. The highest peaks stood above the Pleistocene ice field and are now

ar e ê t é e s and horns, whereas all land that was covered with ice now has rounded delete accent

summits. This eastern belt is underlain by the granodiorities of the Coast Range

batholith, and by a belt of crystalline metamorphic rocks which border the

plateau country on the northeast side of the Coast Range, cross the range in

Alaska to enter the Pacific Ocean. Two others, the Nass and Skeena, cross it

in British Columbia.

        West of the granitic mountain mass is a belt, 25 to 35 miles wide, of

low islands and broad channels that is generally underlain by Mesozoic and

Tertiary sedimentary and volcanic rocks. Few of the mountains in this belt

exceed 2,000 feet in altitude, and extensive low, flat areas are common. This

belt can be traced from the southeast, where it is continuous with Hecate Strait

in Canada, as far northwest as Chatham Strait which truncates the belt.

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EA-I. Wahrhaftig: Physiography of Alaska

        The western islands, including Prince of Wales, Baranof, and Chichagof

Islands, are higher, and are composed largely of Paleozoic and crystalline

rocks, but include some deformed Mesozoic rocks. Known mountain heights

range between 2,500 and 4,000 feet. Some mountains on Baranof Island are

reported to be higher. Mountains on these islands, excepting a few of the

highest, were overridden by a great ice sheet that originated in the higher

mountains to the east on the mainland. The summits consequently are rounded.

These islands also are deeply scored by fjords and channels. Along parts of

the seaward border of Chichagof and Baranof Islands is a strand flat, 5 miles

wide, with altitudes up to 300 feet behind which the higher mountains rise

abruptly.

       

TOPOGRAPHIC DETAILS IN GLACIATED AND NON GLACIATED AREAS ✓

        The Pacific Mountain System and the Brooks Range have been extensively

glaciated, whereas the great interior portion of Alaska has been glaciated

only locally. Topographic details in the two types of regions are remarkably

contrasted and reflect the different dominant cold-climate erosional processes

that have been active in sculpturing the topography: glaciation in one area,

solifluction and creep in the other.

        In the two glaciated mountain regions, valleys are commonly straight and

have U-shaped cross-profiles. Where glaciation has been recent, the valley

walls bear grooves parallel to the valley trend. Mountain tops that projected

above the ice are sharp horns and ar e ê t é e s[?], crowned with numerous pinnacles and ✓ arête

spires. Those that did not project above the ice have rounded and mammillated

surfaces, often complex in detail, that reflect every difference in rock hardness.

        In the nonglaciated mountain regions, valleys are more commonly winding.

The ridge tops are usually broadly rounded, and slopes are steepest toward the

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EA-I. Wahrhaftig: Physiography of Alaska

base. Where flood plains are absent, valleys are V-shaped in cross section.

Striping, if present, trends directly down the slope. Outcrops are scarce,

and surface fragments cannot always be relied upon as an indication of the

type of bedrock because of extensive hillside creep and solifluction.

Locally, steep-sided bizarre rock monuments crown the ridge crests.

        The lowland areas of glaciated regions commonly contain hummocky deposits

of ground moraine, and many lakes, some of which are deep. The lakes are

fixed in position. Anastomosing patterns of gravel and sand plains, which

were glacial moltwater channels, are superposed on the hummocky topography.

        The lowlands of the unglaciated areas in much of Alaska are underlain by

permafrost at shallow depths. Such areas commonly show little relief, except

for occasional sand dunes and frost mounds, and are crossed by rivers which

wind in tortuous meanders or, branching and rebranching, form anastomosing

and meandering streams across the flats. Many flats are dotted with lakes,

some of which originate as oxbows (abandoned meanders) and others through the

thawing of permafrost. The latter are commonly oval in outline, and enlarge

through thawing and undermining of the banks, particularly on their leeward sides.

       

PHYSIOGRAPHIC HISTORY OF ALASKA

        The physiographic history of Alaska, like the geologic history, is known

only sketchily. Practically all present physiographic knowledge is based on

inferences from reconnaissance surveys in which physiography was studied only

incidentally to a study of the mineral deposits in bedrock and placers. These

inferences are not sufficiently widely accepted at the present time (1950) to

warrant a statement on the physiographic history in this article.

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EA-I. Wahrhaftig: Physiography of Alaska

        Alaska present s some physiographic features that have posed problems and ✓

have aroused interest in explanations of the topography and drainage patterns

of Alaska. Among these physiographic problems are: the origin of the great

Yukon River system which crosses ridges and valleys without much regard for

existing topography; the origin of the extensive flat alluvium-covered plains,

surrounded by mountains, that are known in may parts of Alaska; and the origin

of the extensive deposits of silt and muck that mantle the low hills of

interior Alaska.

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---

BIBLIOGRAPHY

1. Black, R.F. and Barkedale, W.L. “Oriented lakes of northern Alaska,”

J.Geol . vol.57, pp.105-118, 1949.

2. Brooks, A.H. The Geography and Geology of Alaska. With a Section on Climate

by Cleveland Abbe, Jr., and a Topographic Map and Description Thereof

by R.U. Goode . Wash., D.C., G.P.O. 1906. U.S.Geol. Surv. Prof. Pap . 45.

3. Buddington, A.F., and Chapin, Theodore. Geology and Mineräl Deposits of

Southeastern Alaska . Wash., D.C., G.P.O., 1929. U.S. Geol.Surv.

Bull . 800.

4. Capps, S.R. The Chisana-White River District, Alaska . Wash., D.C., G.P.O.,

1916. Ibid . 630.

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6. Eakin, H.M. The Cosna-Nowitna Region, Alaska . Wash., D.C., G.P.O., 1918,

pp. 42-50. Ibid . 667.

7. Eardley, A.J. “Unconsolidated sediments and topographic features of the

lower Yukon Valley,” Geol.Soc.Amer. Bull . vol.49, pp. 303-42, 1938.

8. Elias, M.M., and Vosburgh, R.M. Terraine and Permafrost in the Galena Area,

Alaska . Wash.,D.C., G.P.O., 1946. U.S.Geol.Surv. Permafrost Progr o am —

Progress Report No.1.

9. [k?] Leffingwell, E. de K. The Canning River Region, Northern Alaska . Wash., —

D.C., G.P.O., 1919, pp. 166-77. U.S.Geol.Surv. Prof. Pap . 109.

10. MacNeil, F.S., Mertie, Jr., J.B., and Pilsbry, H.A. “Marine invertebrate

faunas of the buried beaches near Nome, Alaska,” J.Paleont . vol.17,

pp.69-96, Jan., 1945.

11. Maddren, A.G. The Koyukuk-Chendaler Region, Alaska . Wash., D.C., G.P.O.,

1913, pp.11-27, 56-67. U.S.Geol.Surv. Bull . 532.

12. Mendenhall, W.C. Geology of the Central Cooper River Region, Alaska . Wash.,

D.C., G.P.O., 1905, pp.16-22, 79-90. U.S.Geol.Surv. Prof. Pap . 41.

13. Martie, Jr., J.B., and Harrington, G.L. The Ruby-Kuskokwim Region, Alaska .

Wash., D.C., G.P.O., 1924, pp. 41-55, 80-109. U.S.Geol.Surv. Bull . 754.

14. Mertie, Jr., J.B. The Yukon-Tanana Region, Alaska . Wash.,D.C., G.P.O., 1937,

pp.12-32, 178-98, 235-37. Ibid . 872.

15. Moffit, F.H. Geology of the Eastern Part of the Alaska Range and Adjacent

Areas. Ibid. (In Press.)

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16. ----. Geology of the Chitina Valley and Adjacent Area, Alaska . Wash.,

D. C., G.P.O., 19 4 3 , pp.9-14, and Pl.2. Ibid . 894. —

17. P e é w e é , T.L. Permefrost Investigations, Fairbanks, Alaska . Wash., D.C.,

G.P.O., 1948. U.S.Geol.Surv. Preliminary Permafrost Report .

(Published by U.S. Army Map Service.)

18. Schrader, F.C. A Reconnaissance in Northern Alaska . Wash., D.C., G.P.O.,

1904, pp.42-45. U.S.Geol. Surv. Prof. Pap . 20.

19. Smith, P.S., and Eakin, H.M. A Geologic Reconnaissance in Southeastern

Seward Peninsula and the Norton Bay-Nul ta at o Region, Alaska . Wash., —

D.C. G.P.O., 1911, pp.20-32. U.S.Geol.Surv. Bull . 449.

20. ----, and Mertie, Jr., J. B. Geology and Mineral Resources of Northwestern Geology and Mineral Resources of Northwestern —

Alaska Alaska . Wash.,D.C., G.P.O., 1930 pp.27-51. Ibid . 815.

21. Tarr, R.S., and Butler, B.S. The Yakutat Bay Region, Alaska. Wash.,D.C.,

G.P.O., 1909. U.S. Geol.Surv. Prof. Pap . 64.

22. U.S. Geological Survey. Oil and Gas Sheet No.106. (In press.)

23. Wahrahaftig, Clyde. “Preliminary report on the physiographic history of part

of the Alaska Range.” Wash.Acad.Sci. J . vol.36, pp.134-35, 1946.

24. Wallece, R.E. Terraine Analysis in the Vicinity of Northway, Alaska, with

Special Reference to Permafrost . Wash.,D.C., G.P.O., 1946.

U.S.Geol.Surv. Permafrost Report . No.3. (Mimeographed.)

       

Clyde Wahrheftig