Permafrost and 

Ground Water in 

Alaska

By DAVID M. HOPKINS, THOR N. V. KARLSTROM, and OTHERS

A SHORTER CONTRIBUTION TO GENERAL GEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 264-F

A study of the interrelations of permafrost and 

ground water^ and a discussion of the role of 

aerial photographs in the mapping and 

evaluation of permafrost conditions. Includes 

descriptions of areas by Robert F. Black^John R. 

Williams^ Troy L. Pewe, Arthur T. Fernold^ and 

Ernest H. Muller.

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1955
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price $1 (paper cover)
CONTENTS

Abstract---.---- ___________________________________

Introduction _______________________________________

Definitions.. _ __________________________________

Acknowledgments __ __ _ ________________________

Regional zonation of permafrost- _____________________

Factors affecting the distribution of permafrost__----_-_

Past and present climates _ _____________________

Local environmental factors. _____________________

Factors affecting the distribution of ground water _ ____

Permafrost and ground water in some representative

Arctic slope, by R. F. Black______________________

Northern Seward Peninsula, by D. M. Hopkins__ .__ 

Southern Seward Peninsula, by D. M. Hopkins_____

Yukon Flats, by J. R. Williams ___ ______________

Middle Tanana Valley, by T. L. Pe"we"___ ___ _____

Upper Kuskokwim Valley, by A. T. Fernald- ______

Bristol Bay region, by E. H. Muller __ ___________

Kenai lowland, by T. N. V. Karlstrom. ___________

Photointerpretation of permafrost conditions___________

Mapping permafrost from aerial photographs. ______

Permafrost indicators ___________________________

Vegetation. ________________________________

Tundra and forest associations. __________

Distribution . patterns of trees and large 

shrubs _ _ ___________________________

Page 

113

113

114

114

115

115

115

117

118

118

118

120

122

124

126

130

131

133

134

134

135

135

135

135

Page

Photointerpretation of permafrost conditions—Con. 

Permafrost indicators—Continued

Polygonal microrelief patterns-___--_-________ 137

Frost-stirred polygons.__________________ 138

Contractional polygons._________________ 138

Low-center ice-wedge polygons _______ 138

High-center ice-wedge polygons-______ 138

Frost-crack polygons._______________ 139

Recognition of contractional polygons

on aerial photographs _____________ 139

Pingos__--__-__-------_------------------- 139

Features resulting from thawing ______________ 140

Thaw lakes._------__---_________ 140

Beaded drainage-_-_-__-_-_--------__-_- 141

Collapse along trails, near installations, and 

in cultivated fields.___________________ 141

Hydrologic phenomena_________--__-_-_____- 141

Limitations ____________________________________ 142

Photointerpretation keys ________________________ 142

Conclusions.___________________________________ 143

Literature cited-__-_______-__-___-____---_--------_- 143

Index.__..___-_-.-___-_-__---__---_-_----------_-- 145

ILLUSTRATIONS

[Plates 30-44 follow page 146]

PLATE 30. Typical upland terrain, northern Seward Peninsula, with diagrammatic cross section.

31. Basaltic lava flows, northern Seward Peninsula, with diagrammatic cross section.

32. Bog flats, oblique view, upper Kuskokwim Valley.

33. Bog flats, vertical view, upper Kuskokwim Valley.

34. Bog flats, Kenai lowland.

35. Mixed forest and tundra, northern Seward Peninsula.

36. Stone stripes and vegetation stripes, Bristol Bay region.

37. Ice-wedge polygons and thaw lakes, northern Seward Peninsula.

38. Ice-wedge polygons, Arctic slope.

39. Frost-crack polygons, Bristol Bay region.

40. Stereopairs showing effect of scale on appearance of permafrost indicators, northern Seward Peninsula.

41. Pingos and modified oxbow lakes, northern Seward Peninsula.

42. Accentuation of microrelief due to thawing in cleared fields, middle Tanana Valley.

43. Flood-plain icings, northern Seward Peninsula.

44. Permafrost that cannot be identified by Photointerpretation, Bristol Bay region.

FIGURE 5. Index map of Alaska showing permafrost zones______________________-________-------------------

6. Diagrammatic cross section showing distribution of permafrost in coastal plain at Nome. _____________

7. Diagrammatic cross section showing distribution of permafrost in the Yukon Flats__-________________

8. Diagrammatic sketch showing character and distribution of permafrost in the Fairbanks area——______

9. Diagrammatic cross section showing distribution of permafrost and ground water in the Big Delta area.

10. Diagrammatic cross section showing artesian water confined by permafrost in the Fairbanks area__-__- 

11. Distribution of forested and treeless areas in Alaska and part of Canada.___________--___----._-----

Page 

116 

123 

125 

128

128

129

136

HI

A SHORTER CONTRIBUTION TO GENERAL GEOLOGY

PERMAFROST AND GROUND WATER IN ALASKA

BY DAVID M. HOPKINS, THOR N. V. KARLSTROM, AND OTHERS*

ABSTRACT

The distribution of ground water in Alaska affects and is 

affected by the distribution of permafrost. Present knowledge 

of permafrost and ground-water conditions is summarized for 

the following representative areas of Alaska: The Arctic slope 

and northern Seward Peninsula in the continuous-permafrost 

zone; southern Seward Peninsula, the Yukon- Flats, the middle 

Tanana Valley, and the upper Kuskokwim Valley in the discontinuous-permafrost zone; the Bristol Bay region in the sporadicpermafrost zone; and the Kenai lowland in the no-permafrost 

zone. The application and limitations of aerial-photograph 

interpretation in permafrost studies also are discussed.

Regional climatic differences result in a transition from 

thick, continuous permafrost in northern Alaska to permafrostfree terrain in southern Alaska. However, local differences in 

topography, lithology, and drainage result in sharp local differences in the character and distribution of permafrost that 

tend to obscure the regional zonation. Moreover, frozen ground 

formed during past cold periods persists to the present time in 

many areas, so that the distribution pattern is not exclusively 

the product of present-day climates.

Much of the ancient frozen ground is differentially thawing 

today. Recently formed frozen ground also thaws locally where 

natural or artificial alteration of the landscape alters the thermal regimen of the ground. Thus, both recent and ancient 

permafrost are interrupted horizontally and vertically by 

thawed zones through which ground water may circulate. Conditions that favor active circulation of water, both at the surface and at depth, promote thawing of permafrost and retard 

formation of new permafrost. Consequently, potential aquifers 

are similar in character but more restricted in size and abundance in permafrost areas than in areas of no permafrost.

Aerial photographs are almost indispensable to geomorphic 

studies in Arctic and subartic regions. Their use permits recognition of many features and geomorphic relationships difficult 

to discern on the ground and allows controlled extrapolation 

of field data into nearby areas. However, many geomorphic 

interpretations based solely on a study of aerial photographs 

are inadequate; to be valid, photointerpretation must be supported by field observations.

Photointerpretation is also of potential assistance in the 

evaluation of permafrost conditions, but the limitations of the 

n/ethod are even more stringent here than in more general 

geomorphic studies. The detailed boundaries of permafrost

*Robert F. Black, John R. Williams, Troy L. Pe"w4, Arthur T. Fernald, 

and Ernest H. Muller.

cannot be delineated by photointerpretation alone, but a generalized estimate can be developed by establishing, preferably by 

field studies, the regional distribution pattern of frozen and 

unfrozen zones in various types of terrain. These terrain 

types then can commonly be mapped from aerial photographs.

Assessment of permafrost conditions is aided by recognition 

on aerial photographs of positive and negative indicators such 

as certain plant associations, polygonal microrelief patterns, 

surface features produced by local thawing of permafrost, pingos, 

and hydrologic phenomena such as flood-plain icings. Individual indicators are found to be ambiguous to varying extents, 

erratically distributed, and generally difficult to recognize even 

on large-scale photographs. With rare exceptions, no single 

feature recognizable on aerial photographs can be taken as an 

infallible indication of the presence or absence of permafrost. 

When considered collectively in the context of the geologic and 

climatic setting, the significance of the indicators increases, but 

the limitations remain severe.

Comprehensive photointerpretation keys for the photointerpretation of permafrost are extremely difficult to construct and 

are practically useless as a tool for the interpretation of permafrost conditions because of the poor relationship between surface manifestations and underlying permafrost.

INTRODUCTION

An adequate supply of water is essential for intensive land utilization throughout the world. The 

needs of pioneer communities commonly can be satisfied by small amounts obtained from minor streams, 

caught in cisterns, or melted from snowdrifts that persist during most of the summer. The establishment 

of large towns, industrial centers, agricultural communities, or military bases, however, results in an 

increased demand for water and an increased danger 

of contamination of local surface supplies. The distribution system necessary to import water from distant 

sources is costly to construct, is subject to sabotage or 

military attack, and, in cold regions, must be protected 

from freezing. (See Alter, 1950a, 1950b, and unpublished report.1 ) Development of local ground-water

1 Alter, A. J., 1950, Water-supply problems in low-temperature areas: 

Mimeo, rept. distributed at the 1st Alaska Sci. Conf., Washington, 

D. C., 13 p.

113
114 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

resources, therefore, can aid materially in the growth 

of population centers in northern regions.

Water supply is one of the factors limiting settlement of many parts of Alaska and restricting the expansion of some existing communities. Surface water 

in Alaska commonly is poor in quality owing to a 

heavy silt load in some streams and a high organic 

content in others. Many surface streams in the colder 

parts of Alaska dry up during winter and thus do not 

offer a dependable supply of water. A few coastal 

settlements, such as Kotzebue, have been established in 

areas without surface streams. Obviously, groundwater supplies are increasingly needed for the further 

development of Alaska.

The distribution of ground water in many parts of 

Alaska affects and is affected by the distribution of 

permafrost. General geological and geographical investigations by the Geological Survey have resulted in 

the accumulation of considerable information on 

ground-water and permafrost conditions in some representative areas. This report is a summary of data 

that have been collected in these areas (pp. 118-134) and 

of generalizations formulated (pp. 115-118) concerning the distribution and interaction of permafrost and 

ground water in Alaska. Aerial photographs are useful aids in the search for ground water and the exploration of permafrost, but their strictly supplementary 

role has been misunderstood by several nongeological 

authors. The use of aerial photographs in permafrost 

studies and the significance of certain patterns recognizable on aerial photographs for the interpretation of 

permafrost conditions are discussed on pages 134-143.

DEFINITIONS

Several of the geologic and botanical terms used 

in this paper may not be well known; some terms have 

been used by other authors in different ways. The 

writers' usage of these terms is given below.

Permafrost is defined by Muller (1947, p. 219) as 

"a thickness of soil or other surficial deposit or even 

of bedrock, at a variable depth beneath the surface 

of the earth in which a temperature below freezing 

has existed continuously for a long time (from two to 

tens of thousands of years)." Perennially frozen 

ground (Taber, 1943, p. 1436) and pergelisol (Bryan, 

1946) are equivalent terms. The character of permafrost in various sediments, soils, and rock types is discussed by Muller (1947), Taber (1943), Black (1951), 

and Ray (1951).

The active layer is the thin surface layer of the 

ground that freezes in winter and thaws in summer 

(Muller, 1947, p. 213). The base of the active layer 

commonly but not invariably coincides with the top

of permafrost. Seasonally frozen layer is an equivalent term.

A pingo is an isolated steep-sided hillock 10 to 50 

feet high that is composed of a core of massive ice 

overlain by a few feet of silt, sand, and peat (Porsild, 

1938). Pingos generally appear suddenly during the 

winter, persist for a few years, and ultimately collapse 

when the ice core melts. Hydrolaccolith is an equivalent term (Muller, 1947, p. 217).

Flood-plain icings, also called aufeis or "glaciers"— 

are thick masses of ice that cover large areas on valley 

floors of nearly every large Arctic and subarctic stream. 

They form in reaches where drainage is blocked by ice 

freezing to the bottom of the channel during early winter. Water breaks through to the surface near the upstream end of the constriction, flows in thin sheets over 

the flood plain, and freezes. Successive sheets of overflow ice build up icings as much as 10 feet thick. Most 

of them on Seward Peninsula melt during late summer, 

but many icings on the Arctic slope persist several 

years without melting.

Tundra in this report refers to a wide variety of 

vegetation types that are found above or beyond the 

latitudinal and altitudinal limits of trees in central and 

northern Alaska (fig. 11). Muskeg is a tundralike 

vegetation occupying swampy areas scattered in the 

forests of interior Alaska; however, muskeg may contain scattered black or white spruce trees of poor 

growth form.

Bog flats* consist of irregular areas overgrown by 

black spruce and Sphagnum mosses interspersed 

among slightly lower marshy areas in which the vegetation consists chiefly of sedges and Sphagnum mosses. 

The bog-flat association is a characteristic cover on 

poorly drained, gently sloping or flat surfaces within 

the interior forest.

Cottongrass is the common name applied in Alaska 

to plants of the genus Eriophorum. The grasslike 

plant bears a white, cottony flower during its blooming

season.

ACKNOWLEDGMENTS

This report is the product of the experience of many 

members of the Geological Survey. D. B. Krinsley, 

D. R. Nichols, and L. L. Ray, geologists, and W. S. Benninghoff and R. S. Sigafoos, botanists, and the individual authors of parts of this report contributed many 

of the ideas and generalizations presented throughout 

the report. The group is not in complete agreement 

concerning some of the generalizations presented; for

8 Drury, W. H., Jr., 1951, Formation and destruction of bog flats in 

some areas of perennially frozen ground: Paper presented in Section G 

(Botanical Sciences) of 118th Annual Meeting of Am. Assoc. for Advancement of Science.
PERMAFROST AND GROUND WATER IN ALASKA 115

these the senior authors, Hopkins and Karlstrom, accept complete responsibility.

D. J. Cederstrom, of the Geological Survey, conducted test-drilling projects in search of ground water 

in several parts of Alaska and collected data on existing 

wells (Cederstrom, 1952). A later, more general study 

of ground-water occurrences and development in 

permafrost regions (Cederstrom, Johnston, and Subitzky, 1953) was made available in manuscript form to 

the authors of the present report. The United States 

Smelting, Eefining and Mining Co. and its subsidiary, 

the Fairbanks Exploration Co., generously made unpublished records of extensive prospect drilling near 

Nome and Fairbanks available. Many other mining 

companies, salmon canneries, government agencies, and 

private individuals contributed data that aided this 

work significantly.

The descriptions of bedrock geology are largely generalized from Geological Survey bulletins and professional papers too numerous to cite individually. Smith 

(1939) summarizes the bedrock geology in more detail 

than is possible here and gives an extensive bibliography 

for most of the areas discussed. Most of the climatic 

data were compiled by the U. S. Weather Bureau 

(1-943).

The work has been supported largely by funds transferred by the Engineer Intelligence Division, Office of 

the Chief of Engineers, U. S. Army. This report was 

prepared originally at the request of and with financial 

support from the Engineer Eesearch and Development 

Laboratory, Office of the Chief of Engineers, U. S. 

Army.

REGIONAL ZONATION OF PERMAFROST

On the Arctic slope of Alaska (fig. 5), soils, sediments, and bedrock are frozen nearly everywhere to 

depths of about a thousand feet. Southward, perennially frozen ground is progressively thinner, and permafrost-free areas are progressively more abundant. The 

Pacific coastal region of Alaska is nearly free of permafrost. Local differences in topography, lithology, and 

drainage result in sharp local differences in the character and distribution of permafrost that tend to obscure the regional zonation. For example, permafrost 

conditions in bog-flat areas in central Alaska differ 

more sharply from conditions in nearby flood-plain 

areas than they do from bog-flat areas along the coast 

of Cook Inlet in southern Alaska.

For purposes of description, four permafrost zones 

are delineated on figures 5 and 11, although they are not 

rigidly defined, and their boundaries must be somewhat 

arbitrarily drawn. The ground is perennially frozen

nearly everywhere in the continuous-permafrost zone; 

unfrozen ground is found only at a few widely scattered 

sites. Perennially frozen ground is less widely distributed in the discontinuous-permafrost zone, and 

areas of unfrozen ground predominate in the southern 

part. In the sporadic-permafrost zone perennially 

frozen ground is confined to isolated sites where vegetation, topography, soil, and drainage permit its continued existence or its formation. Permafrost also is 

encountered locally in the no-permafrost zone, but is so 

rare as to have little influence on the landscape or human 

activities.

FACTORS AFFECTING THE DISTRIBUTION OF 

PERMAFROST

PAST AND PRESENT CLIMATES

Permafrost in Alaska is a product of both the present 

climate and of colder climates that have prevailed at 

interrupted intervals during the past hundred thousand years (Pewe and others, 1953). The regional 

transition from thick, continuous permafrost in northern Alaska to the permafrost-free terrain of southern 

Alaska reflects present and past regional climatic 

differences.

New permafrost forms today in all three permafrost 

zones and possibly in a few exceptional sites in the 

no-permafrost zone. If the present climates persist 

unchanged for an indefinitely long time, materials at 

the base of the zone affected by annual temperature 

fluctuations ultimately will have temperatures approximately equal to the local mean air temperatures. 

Linear temperature gradients will be established at 

greater depths, and heat loss to the air will be balanced 

by heat flow from the interior of the earth. Permafrost 

will reach thicknesses determined by the equilibrium 

between local mean air temperatures and heat flow from 

the earth's interior, and all of the permafrost can be 

said to be in equilibrium with the climate of that time 

(Birch, 1948; Eeduzobov, 1954).

Heat loss at the surface was greater during past cold 

periods, and consequently permafrost extended to 

greater depths and developed in places where it cannot 

form today. Much of the ancient frozen ground is 

slowly thawing as geothermal gradients approach equilibrium with the present climate, but much has persisted to the present time. Relict masses of frozen 

ground are found chiefly in impermeable soils and 

rocks, commonly adjoined or overlain by permeable 

layers and zones that have been thawed by circulating 

ground water. Recently formed permafrost also thaws 

in areas where artificial or natural alteration of the 

landscape alters the thermal regimen of the ground. 

Thus, permafrost of both ages is interrupted hori-
116 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

160' 156° 152°

FIGURE 5.—Index map of Alaska showing permafrost zones.
PERMAFROST AND GROUND WATER IN ALASKA 117

zontally and vertically by thawed zones in which 

ground water can circulate.

LOCAL ENVIRONMENTAL FACTORS

The local distribution of permafrost is determined 

largely by subsurface drainage and surface insulation. 

In the rigorous climate of northern Alaska, local differences in these conditions generally result only in differences in the depth to permafrost. Farther south, differences in drainage and insulation can determine the 

presence or absence of permafrost, and in southern 

Alaska permafrost is found only in materials and areas 

where the combination of these factors is extremely 

favorable.

The role of insulating surface materials upon the 

formation and preservation of permafrost is well 

known (Muller, 1945, p. 6-10, 33-34; Black, 1951, p. 

278-279). In general it may be said that permafrost 

lies at shallowest depth in areas mantled with peat, 

organic silt, or a dense mat of living vegetation, and it 

lies at greatest depth beneath bare gravel or in exposed 

bedrock. Kecently formed permafrost in southern 

Alaska is found only in areas mantled with a thick 

layer of peat or silt; the rare permafrost found in areas 

of bare gravel lies well below the zone of annual freezing and thawing and probably is relict.

Permafrost is absent or lies at great depths beneath 

lakes and ponds throughout Alaska. Bodies of standing water readily absorb the sun's heat during summer; 

agitation caused by waves carries the heated water to 

the bottom and sides of shallow lakes and ponds. Heat 

loss during winter is largely absorbed in converting the 

water in the ponds into surface ice. Thus, new frozen 

ground is not formed.

Heat is distributed through earth materials more 

rapidly by circulating water than by direct conduction. Thus, active circulation of water, both at the 

surface and at depth, promotes thawing of permafrost and retards formation of new permafrost. Owing 

to more active water movements, streams generally are 

underlain by deeper and wider unfrozen areas than 

are lakes, and coarse, permeable sand or gravel is more 

likely to be free of permafrost than is impermeable silt. 

An abundant source of ground water, steep slopes, and 

coarse, permeable sediments or fractured bedrock favor 

the presence of unfrozen zones in all of the permafrost 

regions of Alaska. Because of these relationships, unfrozen zones in the continuous- and discontinuous-permafrost zones generally are good aquifers. All potential aquifers in the sporadic zone are unfrozen, and 

frozen ground occurs only where the material is relatively impermeable.

319004—55———2

Differences in the factors affecting drainage, insulation, and exposure are sufficient in adjacent areas of 

contrasting topography to produce appreciable differences in the relative proportions and systematic distribution of permafrost. For example, permafrost commonly is more widely and continuously distributed at 

shallow depth in extensive lowlands than in adjoining areas of more rugged relief, because near-surface 

circulation of ground water generally is restricted in 

the lowlands by fine-grained soils and gentle slopes. 

Surface drainage also is impeded, and consequently 

a thick, insulating layer of peat or turf is likely to 

develop over much of the lowland surface. Thick peat 

and shallow permafrost are self-perpetuating because 

they further impede surface and subsurface drainage.

Kelatively little information is available concerning 

the distribution of permafrost in mountainous areas. 

It can be argued on theoretical grounds, however, that 

unfrozen zones at shallow depth are likely to be more 

common in mountains than in adjoining areas of low 

relief, despite the colder climates characteristic of high 

altitudes. A thick snow cover insulates the ground 

from deep freezing during winter and furnishes abundant melt water in spring to promote thawing of the 

seasonally frozen ground. Surface drainage is good 

on steep mountain slopes. Discontinuous coarse-grained 

soils overlying frost-shattered bedrock on ridges, 

and coarse-grained till and outwash in mountain valleys, permit active near-surface circulation of ground 

water and result in good subsurface drainage. Vegetation commonly is sparse, and any thick turf or peat 

mats present are likely to be confined to broad interfluves and to a few poorly drained areas in valley 

bottoms. These factors permit surf ace'water to infiltrate during summer and to carry heat to considerable 

depths beneath the surface. Consequently, abundant 

unfrozen zones at shallow depth can be expected in 

mountainous areas, especially on south slopes. The 

most favorable sites for formation or preservation of 

permafrost in mountain areas are on north slopes and 

beneath poorly drained surfaces on broad interfluves 

and valley bottoms.

Past glaciations and erogenic events also are factors 

affecting the distribution of perennially frozen ground 

in areas of high relief. Most mountain ranges in 

Alaska were occupied by valley glaciers, and several 

ranges were completely buried beneath ice during cold 

periods of Quaternary age (Pewe and others, 1953). 

The large areas insulated by ice were protected from 

low air temperatures at the same time that perennially 

frozen ground was forming to great depths in adjoining ice-free lowlands. Permafrost beneath glaciated 

surfaces has formed largely in periods of less cold
118 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

climate during and since deglaciation and thus probably 

is limited to relatively shallow depths.

Many mountain ranges in Alaska have been actively 

rising during Quaternary time. Steepened thermal 

gradients due to erogenic heat or cooling granite bodies 

in these ranges probably have limited the depths to 

which freezing temperatures can penetrate during both 

glacial and interglacial periods. Thawing of relict 

permafrost also probably proceeds more rapidly in 

mountain areas of orogenically steepened thermal gradients than in adjoining lowlands where there has been 

no recent orogenic activity.

Permafrost conditions in upland areas of rolling 

topography generally are intermediate between those 

of lowlands and mountains in the same region.

FACTORS AFFECTING THE DISTRIBUTION OF GROUND

WATER

Supplies of ground water in central and northern 

Alaska are limited by the perennially frozen conditions 

of many potential water-bearing soils and rocks. It 

was shown earlier, however, that the few unfrozen 

zones found in the continuous-permafrost zone generally are good aquifers, and that many potential aquifers in the discontinuous-permafrost zone and nearly 

all the potential aquifers in the sporadic zone are unfrozen. Thus, the ground-water distribution pattern in 

permafrost regions is similar to but more restricted than 

the pattern of ground-water distribution in geologically 

analogous regions that are free of permafrost.

Low precipitation is a factor potentially limiting 

ground-water supplies throughout much of Alaska away 

from the Pacific coast. Anchorage, about 70 miles inland from the open ocean, receives a mean annual precipitation of 15 inches; most of interior and northern 

Alaska receives less. Even the relatively humid coast of 

the Bering Sea north of Bristol Bay has a mean annual 

precipitation of less than 20 inches. However, low temperatures, high humidities, and prevailingly cloudy 

skies minimize the rate of evaporation and tend to 

conserve the small total precipitation for surface and 

ground-water supplies.

Low infiltration rates reduce the rate of groundwater recharge in many parts of Alaska. Thick layers 

of eolian, fluvial, and residual silt mantle most areas 

of low to moderate relief and retard downward percolation of surface water. Peat-forming vegetation 

grows on these poorly drained, flat and gently sloping 

surfaces and further diminishes subsurface drainage. 

Transpiration returns some of the moisture to the atmosphere. In cold areas shallow permafrost forms 

readily beneath the poorly drained surfaces and tends 

to seal off deeper aquifers. In slightly warmer areas

the deep layer of frozen ground formed each winter 

persists through most of the summer, so that surface 

water can infiltrate only during a few weeks preceding 

autumn frosts. When no frozen ground is present, 

water slowly filters through the fine-grained surface 

materials and carries particles of silt and clay downward to be deposited in underlying coarser sediments. 

Permeability in potential aquifers at shallow depth is 

thus reduced, and percolation to deeper aquifers is 

further restricted.

PERMAFROST AND GROUND WATER IN SOME 

REPRESENTATIVE AREAS

ARCTIC SLOPE 

By R. F. BLACK

PHYSIOGRAPHY, CLIMATE, AND VEGETATION

The Arctic slope lies entirely within the continuouspermafrost zone (fig. 5). It is bounded on the south 

by the crest of the Brooks Range and on the north by 

the Arctic Ocean. It consists of three main provinces: 

the Brooks Range, the Arctic foothills, and the Arctic 

coastal plain (Payne and others, 1951). The Brooks 

Range, the Alaskan counterpart'of the Rocky Mountains, comprises several groups of rugged, glaciated 

mountains having a relief of 3,000 to 6,000 feet and 

maximum altitudes of 3,600 to 9,200 feet. The Arctic 

foothills province is a hilly region between the mountains and the coastal plain. It is divided topographically and geologically into the southern foothills, 

characterized by irregular topography with isolated 

hills and ridges that rise 500 to 2,000 feet above areas 

of low relief, and the northern foothills, characterized 

by persistent ridges, mesas, and hills generally of 500 

to 1,000 feet relief and having approximately accordant 

summits. The Arctic coastal plain is characterized by 

abundant lakes and swampy areas, wet tundra, and 

meandering streams. Local relief rarely exceeds 100 

feet.

The climate on the Arctic slope is cloudy, cold, and 

windy. The annual mean temperature is 10° F at 

Barrow on the Arctic coast and 10.8° F at Umiat, 175 

miles to the southeast. The recorded extreme temperatures are 79° and -57° F at Umiat and 78° and 

— 56° F at Barrow. July, the warmest month, has an 

average of 13 days of freezing temperatures at Barrow. Mean annual precipitation is 4.2 inches at 

Barrow and 5.4 inches at Umiat. Half the precipitation is represented by rainfall during the months of 

July, August, and September. Annual snowfall is 

about 29 inches at Barrow and 33 inches at Umiat.

The Arctic slope lies beyond the north limit of 

spruce. Tall willows, alders, and scattered poplars
PERMAFROST AND GROUND WATER IN ALASKA 119

grow along the channels of large streams in the southern part of the region. Tundra vegetation consisting 

primarily of dwarf shrubs, grasses, sedges, lichens, and 

herbaceous plants cover all other areas.

GEOLOGY

The Brooks Range consists of limestone, schist, phyllite, and quartzite in the southern part and complexly 

folded and faulted sandstone and limestone in the 

northern part. Moraines and gravel outwash terraces 

are common in most valleys. Most slopes are partly 

covered with frost-rived rubble, but bedrock outcrops 

are abundant.

The Arctic foothills consist of shale, sandstone, conglomerate, limestone, and chert. The rocks of the 

southern foothills are tightly folded along east-west 

axes; the rocks of the northern foothills are more 

broadly folded and have a regional northward dip. 

Moraines are conspicuous along the south border of 

the Arctic foothills province near the mouths of large 

valleys of the Brooks Range. Gravel outwash terraces 

are common along parts of most major streams. Windblown silt is widespread but generally is thin and mixed 

with frost-stirred soil derived from underlying bedrock.

Unconsolidated silt and sand of the Gubik formation 

covers the Arctic coastal plain. Siltstone and sandstone underlie the Unconsolidated mantle at depths of a 

few tens of feet. Regional dips are low and variable 

in direction but are generally northward.

PERMAFROST

Permafrost is present nearly everywhere beneath 

the Arctic slope of Alaska. Frozen ground generally 

extends to a depth of at least 1,000 feet; thicknesses 

ranging from 600 to 1,300 feet are recorded in wells near 

Point Barrow on the Arctic coast. The soil thaws in 

the summer to depths of % to 4 feet, depending upon 

the surface material, drainage, vegetation cover, and 

exposure.

Permafrost lies at considerable depth or may b© 

absent beneath major rivers, such as the Colville, which 

flow throughout the winter and have flood plains several thousand feet wide. Permafrost also lies at considerable depth or is absent beneath most lakes deeper 

than 8 feet and wider than 2,000 feet. It is absent in 

the immediate vicinity of hot springs and may be lacking in a few other areas in which geothermal gradients 

are steepened by recent igneous activity.

GROUND WATER

Potable ground water in large quantities is known 

only in deep lakes and major perennial streams scattered throughout the Arctic slope and in hot-spring 

areas in the Brooks Range. The low ground temperatures measured in many drill holes indicate that there 

is little possibility of finding water-bearing thawed 

zones within permafrost. Potable subpermafrost 

ground water possibly is available in several untested 

bedrock formations of favorable lithology and structure in the Brooks Range and the Arctic foothills. 

Subpermafrost water encountered in wells in the Arctic 

coastal plain, however, has a salinity of several thousand parts per million.

Several hot springs are known in the Brooks Range. 

The largest are the Shublik Springs on the Canning 

River (Leffingwell, 1919, p. 58-59). The springs 

emerge at the contact of the Lisburne limestone and 

the overlying Sadlerochit sandstone, and they discharge an estimated 1,000 gallons per minute (Marvin 

Mangus, U. S. Geological Survey, oral communication). 

Summer water temperatures of 43° and 48° F are reported. The springs flow all winter and keep the Canning River open in their vicinity. The Sadlerochit 

Springs on the Sadlerochit River are about the same 

size but may be slightly warmer. Other, much smaller, 

springs are reported on the Sagavanirktok River, the 

Shaviovik River, and the Okpilak River, but it is not 

known whether these springs flow throughout the 

winter. Other perennial springs may b© present.

Flood-plain icings are present on nearly every trunk 

stream on the Arctic slope and indicate that some flow 

of water continues after the fall freezeup. The water 

may originate within the stream channel, within the 

bed of the stream, in terrace gravel, in local gravel fill 

associated with lakes, or from springs in bedrock. 

Areas upstream from icings thus are relatively favorable sites for near-surface ground-water prospecting.

Ground water may be available in unfrozen zones 

beneath some lakes more than 8 feet deep and 2,000 

feet wide. A few such deep lakes are scattered over 

the Arctic coastal plain; they are less common within 

the Brooks Range and are rare in the Arctic foothills. 

Their beds commonly consist of Unconsolidated silt or 

clay resting on bedrock of various types. Each lake 

presents individual problems in determining amount 

and quality of ground water available.

The Lisburne limestone and the overlying Sadlerochit sandstone are among the most widespread formations in the Brooks Range, and because of favorable 

structure and lithology they are the most likely reservoirs of ground water below permafrost. No wells 

have been drilled in them, and no geologic investigations for ground water have been made.
120 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

NORTHERN SEWARD PENINSULA

By D. M. HOPKINS 

PHYSIOGRAPHY, CLIMATE, ANI> VBOETATION

Seward Peninsula north of the Kigluaik Mountains 

and east of Cape Darby lies in the continuous-permafrost zone (fig. 5). Lowland areas with local relief of 

generally less than 100 feet are interspersed in rolling 

uplands with relief of up to 2,000 feet and mountains 

locally reaching altitudes of 4,000 feet.

Northern Seward Peninsula has a cold, rigorous climate strongly continental in the eastern part but moderated somewhat by oceanic influences in the western 

part. The mean annual temperature ranges from 20.3° 

F at Candle to 26.4° F at Shishmaref. Eecorded extreme temperatures are 85° and — 60° F at Candle and 

73° and — 45° F at Shishmaref. Mean annual precipitation ranges from about 7 inches at Candle and Shishmaref to 14.9 inches at Wales. More than half the annual precipitation occurs during a well-defined rainy 

season from July through September. Annual snowfall 

ranges from 30 inches at Candle to 34 inches at Shishmaref.

Winters are cold and relatively dry; clear weather 

predominates, but intense storms accompanied by high 

winds and precipitation are frequent. Summers are 

cool and rnoist, especially in the western part of the 

peninsula; a low overcast and drizzle are common, and 

few days pass without at least a trace of precipitation.

Vegetation throughout most of northern Seward Peninsula is tundra consisting chiefly of dwarf shrubs, 

sedges, grasses, other herbaceous plants, lichens, and 

mosses. Large shrubby willows are found along watercourses. Alder thickets are scattered on the steep, welldrained slopes of the mountains and hills in the east. 

Spruce trees are confined to hill slopes and river flood 

plains of the eastern part of the area (fig. 11). Isolated stands of cottonwood trees occur locally in the 

east and southwest.

The vegetation on well-drained slopes mantled with 

muck or windblown silt consists primarily of cottongrass tussocks, dwarf birch, and heath shrubs. A sod of 

dense sedges (Carex) grows on poorly drained hill summits and slopes, in minor drainage courses, and around 

the margins of many ponds.

Terraces and upland surfaces having gravelly or 

rocky soils and lacking a mantle of windblown silt support an inconspicuous cover of low, matted woody 

plants and dwarf grasses generally less than 12 inches 

high. Rocky or gravelly soils derived from siliceous 

bedrock bear dwarf birch and heath shrubs commonly 

separated by patches of bare ground. Mats of Dryas

and other dwarf tufted plants grow in isolated patches 

or form a complete turf cover on calcareous rocks.

GEOLOGY

Bedrock throughout most of the area consists of 

metamorphic rocks; schist predominates, but extensive 

outcrops of marble are present. Granite stocks are 

present at several localities. Folded unmetamorphosed 

limestone predominates in the York Mountains. Belts 

of folded sandstone and shale extend across the peninsula from the east edge of Kotzebue Sound to Norton 

Bay. Quaternary lava flows cover large areas near 

Imuruk Lake and in the valley of the Koyuk River.

Lowland areas are underlain by unconsolidated sediments as much as several hundred feet thick. The fill 

generally consists of 10 to 20 feet of silt, muck, and 

peat over thick layers of gravel. In a few areas, however, the lowland fill consists almost entirely of finegrained sediments and includes only scattered lenses of 

gravel. Aprons of alluvial gravel mantled with only a 

few feet of silt and peat occur along the coast north 

of Teller and at the margins of some lowland areas 

adjoining the Bendeleben Mountains. Volcanic ash 

mantled with 10 to 20 feet of silt, muck, and peat underlies a large area south of Cape Espenberg.

Most upland ridges (cross section, pi. 30) are 

mantled by a few feet of windblown silt beneath which 

lies frost-rived bedrock rubble; on some ridges the 

rubble is exposed. The fine-grained mantle thickens 

in saddles and on slopes, reaching a thickness of several 

tens of feet on the lower slopes. Swales and minor 

valleys generally are underlain by V-shaped bedrock 

gullies filled with 20 to 50 feet of unconsolidated sediments. This fill generally consists of 2 to 6 feet of 

gravel overlain by silt, muck (silt rich in organic 

debris), and peat. The flood plains of larger valleys 

generally are underlain by 5 to 30 feet of gravel and a 

few feet of silt.

The bedrock of the upper slopes and summits of 

mountain ridges is mantled by 5 to 10 feet of coarse 

rubble. Gravelly glacial till as much as 50 feet thick 

is present on lower valley walls. Valley floors are 

covered with alluvial and outwash gravel several 

tens of feet thick. A young fault scarp extends for 20 

miles along the south front of the Bendeleben Mountains. Glacial moraines and alluvial fans are displaced 

locally.

Bare rock is exposed throughout areas occupied by 

the youngest Quaternary lava flows. Coarse, openwork 

rubble as much as 10 feet thick mantles areas underlain 

by slightly older flows. Still older flows (cross section, 

pi. 31) are covered with silty soil 1 to 20 feet thick.
PERMAFROST AND GROUND WATER IN ALASKA 121

Permeable beds of gravel or flow breccia are interstratified with lava flows at depths of 50 to 200 feet.

PEBMAJPKOST

Subsurface materials are perennially frozen nearly 

everywhere in northern Seward Peninsula except beneath and near lakes, channels of perennial streams, 

some ocean beaches, and hot springs. The frozen layer 

ranges in thickness from 15 to more than 260 feet; in 

most places subsurface materials probably are perennially frozen to depths of at least 200 feet.

The surface layers of the soil thaw to depths of 1 to 

10 feet, depending on the surface material, vegetation 

cover, and exposure. In general the depth of summer 

thaw increases with increase in coarseness of surface 

material and sparseness of vegetation. Depth of thaw 

is less on steep north slopes than on steep south slopes ; 

the difference is imperceptible, however, on slopes of 

less than 15°.

Unfrozen zones within permafrost are scarce and generally localized by conditions that promote groundwater circulation. Thus, the bed gravels of the larger 

streams generally are unfrozen, and unfrozen zones are 

common in weathered bedrock and basal gravel beneath 

flood plains and low terraces (cross section, pi. 30). 

Improved ground-water circulation in the vicinity of 

abandoned underground mine workings commonly results in maintenance of thawed zones in their vicinity. 

Unfrozen zones, probably in flow breccias, are indicated 

in areas of Quaternary lava flows by springs and closed 

depressions having interior drainage (cross section, pi. 

31). The presence of springs in many places indicates 

that unfrozen fracture zones are common in marble or 

limestone bedrock. Unfrozen zones are scarce in terrain 

where other rock types are at the surface and extremely 

scarce in areas mantled by 5 feet or more of silt and 

peat.

Unfrozen zones probably are more abundant in mountains than in other terrain types because of past glaciation, steepened thermal gradients due to erogenic heating of rocks at depth, steep slopes, coarse-grained soils 

and sediments, and an abundant supply of surface water from precipitation and melting snow. Unfrozen 

zones are extremely scarce in lowlands and generally 

can be found only along the channels of large streams.

GROUND WATER

Unfrozen gravel near and beneath channels of large 

streams (cross section, pi. 30) constitutes the most dependable and widely available source of ground water 

in the continuous-permafrost zone in northern Seward 

Peninsula. Water probably can be obtained from this

source in all types of terrain. Surface flow is greatly 

reduced during the winter and ceases entirely in many 

streams, but considerable underflow probably is maintained in the channf i gravels of the larger streams.

Flood-plain icing s are useful guides to ground-water 

supplies in stream valleys on Seward Peninsula. 

Many icings are closely associated with perennial 

springs; others form in reaches where bedrock underlies the stream bed at shallow depth, forcing to the 

surface water that had been moving through deep 

channel gravel farther upstream. Thus, perennial 

supplies of ground water can be expected in streambed gravels of stream segments upstream from icings. 

Some flow probably also is maintained downstream 

from icings which overlie fairly deep channel gravels. 

Field measurement of the amounts of ground water 

available near typical icings would be an important contribution to information concerning water resources 

in the continuous-permafrost zone and in the northern 

part of the discontinuous-permafrost zone.

Limited supplies of ground water are available in 

upland and mountain valleys in unfrozen zones in terrace gravel and in flood-plain gravel around and beneath stream channels (cross section, pi. 30). Waterbearing unfrozen zones are most common at the contact 

between gravel and bedrock. Ground-water supplies 

in terraces may diminish sharply during the winter 

because of lack of recharge.

Unfrozen zones in alluvial fans in the mountains and 

in alluvial aprons at the margins of some mountain 

ranges offer large supplies of ground water during 

summer and limited supplies during winter. Streams 

crossing the fans lose much or all of their surface flow 

by percolation into the underlying coarse gravel. The 

water emerges again as springs near the bases of the 

fans. Wells should be located along the projected 

course of the stream, upslope from the springs. Several exploratory wells may have to be driven before an 

unfrozen zone is encountered.

Limited supplies of ground water may be obtained 

from thawed zones beneath some lakes. Many lakes, 

however, are underlain by layers of silt and peat that 

are incapable of transmitting appreciable quantities of 

ground water to replace that withdrawn during pumping. Moreover, the thawed zones beneath lakes are 

likely to be isolated from any source of water except 

the lake itself (fig. 6).

Locally, large springs are present near the young 

fault scarp at the south front of the Bendeleben Mountains. It is likely that the fault provides channels 

along which ground water from the adjoining highlands rises to the surface. Large perennial supplies of 

ground water probably can be obtained in wells that
122 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

intersect the fault at depth. Water in the fault zone 

may be highly mineralized.

Marble, limestone, and basaltic lava flows are promising sources of ground water in bedrock in upland and 

mountain areas. Unfrozen crevices, lava tubes, and 

interstratified flow breccias and gravel beds are potential aquifers in basaltic lava (cross sections, pis. 30, 31). 

Unfrozen fracture zones and solution cavities in limestone and marble are ground-water channels. Wells in 

rock of these types should be drilled on the lower slopes 

of hills or in valley bottoms, near known springs, or 

upstream from large flood-plain icings. Contact zones 

between marble and schist are especially favorable sites. 

Several test wells may have to be driven before a productive unfrozen zone is encountered. Natural springs 

could be developed as sources of ground water at many 

localities.

Limited ground-water supplies are available during 

summer and fall above permafrost in the active layer 

in areas where coarse materials lie at the surface. Summer thaw extends to depths of 6 to 10 feet in coarse 

gravel or rubble. Water from precipitation and melting snow accumulates in the active layer, where it is 

retained by irregularities in the surface of the perennially frozen layer. A small airbase at Point Spencer, 15 miles west of Teller, derived its entire water 

supply from suprapermafrost water in beach gravels 

during operations which extended through summer and 

late fall.3 Suprapermafrost water is not available during the winter, however, unless storage is provided in 

an artificial lake 10 feet or more in depth. Suprapermafrost water is subject to contamination almost as 

readily as is surface water.

In summary, ground water is abundant throughout 

the year in the mountainous regions where it can be 

obtained beneath stream channels, in alluvial fans, in 

unfrozen zones in coarse-grained glacial deposits on 

valley walls, and in limestone or marble bedrock. 

Ground water is much less abundant in the uplands, but 

adequate supplies can be obtained locally beneath the 

flood plains of large streams and in basalt, limestone, 

or marble bedrock. Ground water is extremely scarce 

in the lowland areas and generally can be obtained 

only beneath the channels of a few large streams.

SOUTHERN SEWARD PENINSULA

By D. M. HOPKINS 

PHYSIOGRAPHY, CLIMATE, AND VEGETATION

Seward Peninsula south of the north front of the 

Kigluaik Mountains and west of Cape Darby lies in

8 Black, R. F., 1946, Permafrost investigations at Point Spencer, 

Alaska: U. S. Geol. Survey Permafrost Program Prog. Kept. no. 2, 

20 p. (Unpublished report.)

the discontinuous-permafrost zone (fig. 5). Most of 

this area consists of rugged uplands. Lowlands extend 

along much of the coast and inland along several large 

streams. The topography is generally similar to that 

of northern Seward Peninsula but the uplands are more 

rugged and have steeper slopes and wider valley 

bottoms.

The climate is cold and rigorous but less strongly 

continental than that of northern Seward Peninsula. 

The mean annual temperature is 26.0° F at Nome and 

27.1° F at White Mountain, 33 miles northeast of Cape 

Darby. Recorded extreme temperatures are 89° and 

-55° F at White Mountain and 84° and -47° F at 

Nome. Mean annual precipitation ranges from 14.5 

inches at the town of White Mountain to 17.7 inches 

at Nome. Stream-gage records indicate about 50 inches 

of annual precipitation in the Kigluaik Mountains 

(Henshaw and Parker, 1913, p. 31). About half the 

annual precipitation falls during the rainy season, from 

July through September. Snowfall is notably heavier 

than in northern Seward Peninsula and ranges from 

49 inches at White Mountain to 67 inches at Nome.

Winters are cold, but the temperatures generally are 

less extreme than those of northern Seward Peninsula. 

The predominantly clear winter weather is interrupted 

more frequently by intense storms accompanied by high 

winds and precipitation. Summers are cool and moist; 

clear days are unusual at the coast but are more frequent a few miles inland.

The vegetation consists of tundra in the western part 

and mixed tundra and white spruce forest in the eastern part. A complex assemblage of Dryas and heath 

mats forms a nearly complete cover on moderately well 

drained slopes west of the partly forested region. Low 

thickets of birch and heath shrubs grow on river terraces and the best drained hill slopes. Sedge sod that 

is similar to that which grows on marshy sites in northern Seward Peninsula occurs where the drainage is 

poor. Willow thickets are more common and more 

extensive than in northern Seward Peninsula; they 

grow on river flood plains, along streams, and at sites 

of ground-water seeps. Dense alder thickets grow on 

well-drained slopes in the mountains and locally in the 

uplands.

Spruce forest grows on well-drained soils on the 

middle slopes of hills, on river flood plains, and along 

minor drainage channels in the eastern part of the peninsula. The largest trees range from 20 to 50 feet in 

height, and their trunks attain a maximum diameter of 

18 inches. The canopy is open so that sunlight penetrates to the forest floor or to the tops of the underbrush. Dense underbrush of willows and heath shrubs 

forms thickets under the trees on the upper slopes.
PERMAFROST AND GROUND WATER IN ALASKA 123

Balsam poplar trees grow in the wetter parts of the 

forest where some of the spruce trees have been cut 

for lumber.

GEOLOGY

Bedrock throughout southern Seward Peninsula consists of metamorphic and granitic rocks. Quartz-micacalcite schist and chlorite schist predominate; marble 

is abundant. Brittle graphitic slate and graphitic 

quartzite are present locally.

Most lowland areas are underlain by 100 feet or more 

of unconsolidated glacial outwash and gravelly till with 

some marine gravel and sand. A layer of silt, generally 

less than 6 feet thick, covers the coarse sediments. Lagoons-deposited silt and peat underlie parts of some 

lowlands.

Most upland and mountain ridges are mantled with 

frost-rived bedrock rubble 2 to 20 feet thick. Upper 

slopes are mantled with rocky soil which merges at 

lower elevations with glacial till 10 to 100 feet thick. 

The larger valleys of perennial streams are underlain 

by a few feet of silt and as much as 100 feet of gravel.

A young fault scarp extends several miles along the 

north front of the Kigluaik Mountains; end moraines 

and alluvial fans of late Quaternary age are displaced 

locally. Similar escarpments are present at boundaries 

between uplands and lowlands in several other localities.

PERMAFROST

Much of the subsurface material of southern Seward 

Peninsula is perenially frozen, but thawed zones" are 

considerably more abundant than in the northern part

SOUTH

of the peninsula (fig. 6). Measured thicknesses of the 

perennially frozen ground range from a few feet to 

more than 350 feet; in most places it probably is 100 

to 200 feet thick. The surface layers of the soil thaw 

each summer to depths of 3 to 10 feet.

As in northern Seward Peninsula, thawed zones are 

localized by conditions that favor ground-water circulation and surface heat absorption. Upland and 

mountain ridges, generally underlain by frozen 

ground in northern Seward Peninsula, commonly are 

unfrozen in southern Seward Peninsula (fig. 6). 

Slopes of ridges generally are underlain by frozen 

ground, but unfrozen zones are found locally on south 

slopes where soil and vegetation are thin. Ground 

water that percolates into bedrock in the ridge crests 

commonly emerges as springs and seeps within the till 

or gravel that mantles the lower slopes. Unfrozen 

zones are common at the spring line, a few hundred 

yards below the break in slope that marks the upper 

edge of these deposits.

Unfrozen zones along perennial streams are more 

extensive than in northern Seward Peninsula, and they 

are also found along smaller streams. They are present not only beneath channels and gravel bars but also 

beneath much of the flood plain. Alluvial fans, common in upland valleys as well as in mountain areas, 

generally contain extensive unfrozen zones. Ancient 

beach deposits interbedded with coastal-plain sediments 

("second" and "third" beaches, fig. 6; also, see Moffit, 

1913, pp. 40-49) generally are unfrozen. Spits, bay 

bars, young raised-beach deposits, and older gravels

NORTH

- Sea level

EXPLANATION

By D. M. Hopkins, 1951

Peat, silt, and muck Gravel and sand

1000 0

Horizontal scale

Bedrock

5000 Feet

Permafrost

Vertical exaggeration approximately 5.5 times 

FIGURE 6.—Diagrammatic cross section showing distribution of permafrost in coastal plain at Nome.
124 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

within a few hundred feet of the coast generally are 

unfrozen.

Topographic and stratigraphic characteristics combine with climatic differences to make the thawed zones 

in each physiographic province more widespread in 

southern Seward Peninsula than in equivalent terrain 

in northern Seward Peninsula. As in the northern 

region, unfrozen zones are most abundant in the mountains, less abundant in the uplands, and relatively 

scarce in the lowlands. Steeper slopes, wider distribution of coarse-grained unconsolidated sediments, and 

higher precipitation promote ground-water movement 

and make unfrozen zones more common in each of these 

areas, however, than in equivalent areas in northern 

Seward Peninsula.

GROUND WATER

Ground water is more widely available in the discontinuous-permafrost zone of southern Seward Peninsula than in the continuous-permafrost zone of northern 

Seward Peninsula, primarily because unfrozen zones 

are more extensive and widely distributed and secondarily because precipitation is heavier. Large perennial supplies of ground water can be obtained from 

gravel beneath the flood plains of streams as well as 

beneath the channels. Streams along which icings 

occur are particularly promising sources of supply.

Unfrozen zones in alluvial fans in large mountain 

and upland valleys and at the edges of lowlands contain large supplies of ground water during the summer 

and smaller supplies during the winter. Wells should 

be located upslope from springs where ground water 

emerges at the edges of the fans.

Unfrozen zones beneath lakes in uplands and mountains may offer considerable perennial ground-water 

supplies. Many of these lakes occupy depressions in 

morainal topography and are underlain by coarsegrained, relatively permeable sediments. Most of the 

lowland lakes, however, are underlain by fine-grained 

impermeable sediments in which ground-water movement is restricted by the same factors that limit the 

water supply beneath lakes in northern Seward 

Peninsula.

Large springs are present near young fault scarps. 

Large perennial supplies of ground water probably can 

be obtained in wells that intersect these faults, but the 

water may be highly mineralized. Fractured or cavernous zones in marble commonly are unfrozen and 

carry considerable supplies of ground water. Large 

springs are common near contacts between marble and 

schist. The brittle graphitic quartzite and graphitic 

slate contain thawed fracture zones in which ground 

water circulates. Mine shafts in rocks of these types

have encountered large flows of ground water. Wells in 

marble, slate, or quartzite should be drilled on the lower 

slopes of hills, near known springs, or upstream from 

flood-plain icings. Several test wells may have to be 

drilled before a productive thawed zone is encountered. 

Natural springs could be developed as sources of ground 

water at many localities.

Marine gravel and sand in spits, barrier bars, raised 

beaches, and some deltas contain small supplies of 

ground water and are used as a source of water for 

households near Nome. Heavy pumping would soon 

result in contamination by salt water.

YUKON FLATS

By J. R. WILLIAMS

PHYSIOGRAPHY, CLIMATE, AN1> VEGETATION

The Yukon Flats lie in the discontinuous-permafrost 

zone and consist of a broad lowland extending along 

the Yukon Eiver from Circle to Stevens (fig. 5). Upstream and downstream from the flats the Yukon is confined to a relatively narrow valley. The flats are about 

10,000 square miles in area and are located approximately between 144° and 149° west longitude and 66° 

and 69° north latitude. Most of the area consists of a 

complex series of low alluvial terraces that slope gently 

toward the Yukon River. About a third of the area 

consists of the flood plain of the Yukon River and its 

tributaries. The Yukon and several other large streams 

have built broad gravel fans along the eastern and 

northern margins of the flats. Bedrock escarpments 

100 to 500 feet high, capped with gravel, separate the 

flats from a rolling upland to the north. A 200-foot 

escarpment in which bedrock generally is not exposed 

extends along the south edge of the flats. A gently 

rolling upland with a relief of approximately 150 feet 

grades into the flats on the east.

The Yukon Flats are characterized by a cold, continental climate. Fort Yukon, near the center, has a 

mean annual temperature of about 20° F; recorded 

extreme temperatures are 100° and —76° F. The 

mean annual precipitation is 6.67 inches, but annual 

precipitation ranges from 3 to 10 inches. August is 

the rainiest month, having a mean rainfall of 1.23 

inches. Average snowfall is 42 inches per year.

Summer, considered to begin in May at the breakup 

of the river ice and to end in September when the first 

snow falls, is brief and warm. Summer temperatures 

generally are between 50° and 75° F, but occasionally 

are as high as 80° to 100° F. Thundershowers are common along tributary valleys and marginal escarpments 

but rare in the central part of the flats.

Winter is characterized by one to three 2-week periods of extremely low temperatures and calm, clear
PERMAFROST AND GROUND WATER IN ALASKA 125

weather. During the remainder of the winter, temperatures range from 10° to —40° F, and winds are from 

the north or northeast. Midwinter thaws occur at 

intervals of several years at the foot of the White 

Mountains along the south boundary.

Vegetation on the Yukon Flats consists of spruce 

forest; mixed spruce and birch forest; tall, dense 

brush; and muskeg vegetation. Mature forests composed dominantly of pure stands of white spruce and 

secondarily of mixed stands of white spruce and white 

birch and of white spruce and balsam poplar cover 

much of the flood plain. The forest on the low terraces 

has been modified by fires and consists of patches of 

mature white spruce and white birch mixed with a 

mosaic of brush types, including stands of young spruce, 

mixtures of aspen and willow, and thickets of young 

white birch. Poorly drained areas, especially common 

on the low terraces, are occupied by muskeg vegetation 

consisting chiefly of black spruce, sedge tussocks, and 

Sphagnum mosses. Uplands bordering the flats are 

covered with dense forests of mixed white and black 

spruce, pure stands of birch, and scattered muskegs. 

Fire-swept areas in the uplands, like those on the low 

terraces, are covered with a willow, aspen, and birch 

scrub.

GEOLOGY

Silt, organic silt, and peat, as much as 25 feet thick, 

are the principal surficial materials on flood plains, low

terraces, and outwash fans. Gravel is present at the 

surface in river beds, on some low terraces, and on a 

few of the outwash or alluvial fans (fig. 7).

Alluvial or glacial-outwash gravel and sand, as much 

as 200 feet in thickness, is present beneath the silt. Bedrock is reported at a depth of 237 feet in a well at Fort 

Yukon (Mertie, 1937, p. 16). No other information 

is available on the depth and nature of the bedrock 

surface beneath the Yukon Flats. Extrapolation from 

marginal exposures indicates that bedrock probably 

consists of greenstone and associated sediments of 

early Mississippian age. Tertiary deposits also may be 

present beneath the Quaternary alluvium in parts of 

the basin.

Windblown silt, 4 to 100 feet thick, overlies alluvial 

gravel on the high terrace south of the Yukon Flats. 

The silt cover probably thins southward toward the 

White Mountains. Bedrock lies at depths of more than 

150 feet at the northern, frontal edge of the terrace and 

at shallower depths near the White Mountains.

Sand and gravel, generally less than 30 feet thick and 

mantled with 1 to 2 feet of silt, covers the bedrock in 

the gently rolling country beyond the north margin of 

the Yukon Flats. Bubble occurs at the surface where 

bedrock is at shallow depth. Creek valleys contain a 

thick fill of organic silt.

The gently rolling country along the eastern margin 

of the flats is believed to be underlain by thick deposits

SOUTH NORTH

ByJ. R.Williams, 1948

EXPLANATION

Silt and peat Sand and gravel Permafrost

Horizontal scale

Bedrock

10 Miles

Vertical exaggeration approximately 4 times 

FIGURE 7.—Diagrammatic cross section showing distribution of permafrost in the Yukon Flats.
126 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

of windblown silt. The presence or absence of gravel 

between bedrock and the silt mantle is not known.

PERMAJFROST

Subsurface materials are perennially frozen at depths 

of iy2 to 10 feet in most environments in the Yukon 

Flats. Test pits 6 to 8 feet deep at several sites failed to 

encounter frozen material, but frozen ground may be 

present at greater depth. The thickness of the frozen 

layer is not known. Drill holes in the low terrace at 

Fort Yukon penetrated 25 to 89 feet into frozen sand 

and gravel without passing into unfrozen material.

Unfrozen zones probably are present beneath some 

large lakes and most rivers (fig. 7). Permafrost is present, however, in some of the higher islands in the Yukon 

River (Purington, 1905, p. 158). Some streams that 

cross alluvial fans percolate into the gravel, suggesting 

the presence of permeable unfrozen zones. Unfrozen 

zones may be present elsewhere but can be located only 

by deep drill holes.

Most of the rivers are eroding frozen banks. As the 

banks are cut back, new deposits of unfrozen sand and 

gravel are formed on the slip-off slopes of the meanders. 

These deposits gradually freeze, the frozen layer thickening at an unknown rate. Unfrozen ground thus is 

most likely to be found in the most recently deposited 

part of the flood plain (fig. 7).

GROUND WATER

The Yukon Flats are believed to lie in the discontinuous-permafrost zone and thus probably contain permeable unfrozen zones of possible importance for 

ground-water supplies. The best and most dependable 

source of ground water consists of thawed gravel beneath beds and slip-off slopes of major streams. Local 

residents obtain water by driving pipes equipped with 

sand points into unfrozen gravel along the Yukon River 

margin. Water filtering through the gravel from the 

river is silt-free; however, wells driven in these sites are 

subject to seasonal flooding. Ground water probably 

can be obtained beneath some large lakes and in local 

unfrozen zones in gravel fans. A water-bearing unfrozen zone was found beneath permafrost at a depth of 

40 feet in a high island in the Yukon River near Eagle 

(Purington, 1905, p. 158). Water-bearing unfrozen 

zones may be present elsewhere but cannot be located 

by ground observation nor by photointerpretation.

MIDDLE TANANA VALLEY 

By T. L. PEWE

PHYSIOGRAPHY, CLIMATE, AND VEGETATION

The middle Tanana Valley is that part of the valley 

of the Tanana River between Big Delta on the east and

Nenana on the west (fig. 5). The valley is 50 miles wide 

and includes an area of about 5,000 square miles. The 

northern part lies about lt)0 miles south of the Arctic 

Circle.

The valle}' is bordered on the south by the rugged 

Alaska Range, in which many peaks rise to altitudes of 

more than 10,000 feet and in which local relief is as much 

as 4,000 feet. Uplands bordering the north side of the, 

valley consist of rounded hills and ridges about 2,500 

feet in altitude, separated by broad valleys containing 

small meandering streams and many lakes; local relief 

ranges from 100 to 1,500 feet.

The Tanana River lies near the north side of the 

valley and is adjoined by a low plain 5 to 25 miles wide 

that is subject to annual floods. The flood plain, 

marked by many oxbow lakes, lies at an altitude of 400 

to 800 feet. A sloping alluvial plain composed of 

coalesced silt fans leads northward from the river to 

the upland. A much broader alluvial plain composed 

of piedmont gravel fans, 40 to 50 miles wide, slopes 

upward from the flood plain to the front of the Alaska 

Range, where it reaches an altitude of about 1,500 feet. 

Glacial moraines and outwash plains occupy small fanshaped areas on the piedmont at the mouths of several 

major canyons in the Alaska Range. The morainal 

areas consist of complex systems of ridges, hillocks, and 

small lakes; the outwash plains are smooth, gently 

sloping surfaces. In several places isolated bedrock 

hills project 50 to 400 feet above the surface of the 

Tanana Valley.

The Tanana Valley has a continental climate characterized by short, warm summers and long, cold winters. 

The mean annual temperature is 26.1° F at Fairbanks 

and 26.7 F at Nenana. Extreme temperatures of 99° 

and —66° F are recorded at Fairbanks. Mean annual 

precipitation ranges from 11.7 inches at Fairbanks to 

10.6 inches at Nenana. More than 60 percent of the 

precipitation falls during the period from May through 

September. Mean annual snowfall ranges from 67 

inches at Fairbanks to 52 inches at Nenana.

The Tanana Valley supports mixed forest, tundra, 

and muskeg vegetation. White spruce, birch, willow, 

and balsam poplar grow in pure and mixed stands 

along rivers, sloughs, lakes, and minor drainage courses. 

Birch, aspen, and white spruce grow on bedrock slopes, 

morainal knobs, and hillocks, and burned-over areas on 

glacial-outwash plains. Dense black spruce forest 

covers undisturbed areas on outwash plains. Tundra 

vegetation consisting of sedges, grasses, dwarf birch, 

small heaths, mosses, and lichens is the predominant 

vegetation on the lower parts of alluvial silt fans north 

of the Tanana River. Valleys in the upland are cov-
PERMAFROST AND GROUND WATER IN ALASKA 127

ered by tundra or stands of stunted black spruce, larch, 

and willow.

GEOLOGY

Bedrock in the upland consists of well-jointed micaceous schist and isolated masses of granite and basalt. 

Similar rocks are exposed in the isolated bedrock hills 

that project above the alluvium of the Tanana Valley. 

Hills in the upland are generally mantled with windblown silt as much as 100 feet thick on the lower, gentle 

slopes (Pewe, 1950). Valleys in the upland are filled 

to depths of 50 to 300 feet with silt, muck (organic silt), 

and peat, beneath which lies 10 to 200 feet of coarse 

creek gravel.

The flood plain of the Tanana Eiver is underlain by 

silt, sand, and fine gravel at least 400 feet thick near 

Fairbanks and at least 500 feet thick near Nenana. 

Micaceous silt 1 to 15 feet thick mantles the surface; 

clean, sandy gravel is present just beneath the silt. The 

narrow, sloping, alluvial plain north of the Tanana 

Eiver flood plain is underlain by silt as much as 200 

feet thick that overlies coarser sediments. Silt also 

predominates in the northern part of the sloping piedmont fans south of the flood plain, but the sediments are 

progressively coarser farther south, and gravel predominates near the mountain front. The glacial-outwash 

plains are underlain by medium- to coarse-grained, 

clean, sandy gravel locally mantled by 5 to 10 feet of 

windblown silt. The morainal ridges are underlain by 

an unsorted aggregate of sand, gravel, and boulders.

PERMAFROST

Areas of unfrozen ground are distributed much more 

widely in the Tanana Valley than on southern Seward 

Peninsula; they probably are comparable in distribution to unfrozen zones in the Yukon Flats but are more 

abundant.

In the upland north of the Tanana Valley, silt and 

bedrock are frozen to unknown depths beneath north 

slopes. South slopes, however, are unfrozen except near 

valley bottoms where slope gradients are low and drainage is sluggish (fig. 8). The flat, swampy valley floors 

are underlain by frozen ground to depths of 50 to 200 

feet. Muck, gravel, and, in places, bedrock are frozen.

Unfrozen areas are lacking in the narrow alluvial 

plain north of the Tanana Eiver; sediments in this 

zone are frozen from the mouths of the silt-filled creek 

valleys in the upland to the flood plain of the Tanana 

Eiver. Frozen ground is thickest near the flood plain 

and at the mouths of the creek valleys and becomes 

thinner toward the base of the permafrost-free south 

slopes. The ground is frozen to depths of about 175 

feet at the edge of the flood plain. Summer thaw extends to depths of 1 to 3 feet adjacent to the flood plain

and to depths of 3 to 4 feet in upland creek valleys. 

Permafrost lies at depths of 5 to 20 feet at the inner, 

north edge of the alluvial plain, near the contact with 

the unfrozen, silt-covered bedrock slopes.

Large masses of clear ice occur as horizontal and 

vertical sheets, wedges, and saucer-shaped or irregular 

masses in the frozen silt of the alluvial plain and the 

creek valley bottoms north of the Tanana Eiver. Much 

of the ice forms a polygonal or honeycomblike network 

that encloses silt polygons as much as 40 feet in 

diameter.

The thickness of perennially frozen sediments in the 

Tanana flood plain varies widely and irregularly; 

depths as great as 242 feet are reported.4 No large 

ice masses are present; unfrozen areas and layers are 

common. Frozen ground is lacking beneath present or 

former river channels, sloughs, and lakes. Elsewhere, 

frozen and unfrozen layers are intercalated. In general, silt and sand layers are frozen and gravel layers 

are unfrozen. Gravel in the flood plain occurs in lenses 

for the most part; thus, no single broad unfrozen layer 

exists. Connections between unfrozen areas and unfrozen layers, however, form a network of irregular 

thawed passages throughout the perennially frozen 

sediments of the Tanana Eiver flood plain.

Little information is available concerning the distribution of frozen sediments in the broad piedmont 

fans south of the Tanana Eiver. The lower part 

nearest the river is poorly drained and probably frozen 

to depths of 20 to 130 feet. Large ice masses probably are not present. The higher, southern part of the 

plain is underlain by coarser sediments, and drainage 

is relatively good; unfrozen zones may be widespread.

Permafrost is sparsely distributed in the outwash 

fans on the piedmont slope and has no surface expression (fig. 9). Frozen ground is lacking in many places, 

especially where ground-water circulation is active. 

Elsewhere, layers of frozen ground 20 to 130 feet thick 

lie within 10 to 30 feet of the surface. Thick layers of 

windblown silt commonly are perennially frozen below 

depths of 4 to 5 feet. In places a layer of frozen silt 

5 feet thick overlies unfrozen gravel. Large ice masses 

probably are not present.

No subsurface data are available concerning the distribution of permafrost in the hununocky topography of 

the glacial moraines. Permafrost is probably absent 

in the well-drained till hillocks. Fine-grained sediments in depressions are frozen below depths of 1 to 

3 feet. The thickness of the frozen layer probably is at 

least equal to the thickness of the fine-grained fill.

* Pewe, T. L., 1948, Permafrost investigations, Fairbanks area, Alaska : 

U. S. Geol. Survey Permafrost Program Prelim. Kept., 16 p. (Unpublished report.)
128 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

S3AIS VNVNVJ,,

8
PERMAFROST AND GROUND WATER IN ALASKA 129

NW.

900-1

700-

500"-

BIRCH HILL

SE.

V'ZlWs ////,' S/.'

* ^ _, ^ -- J^ f^ -" r ^

'/?/Ss'/'/S//

'•'/S///S/'/,

S Ss's SS s','S

'sSS/s'SS/'S\ •>^-'^r^'x ' X -^^-^ ^ y j s- s 

. s .^ x- *s _•* ** .' S

EXPLANATION

Inferred boundary

"Doubtful boundary

Permafrost

Horizontal scale

600 0 1200 Feet

Vertical scale exaggerated 2 times 

FIGURE 10.—Diagrammatic cross section showing artesian water confln ed by permafrost in the Fairbanks area.

GROUND WATER

Ground water in the Tanana Eiver flood plain has 

been exploited extensively in the Fairbanks area. Water circulates freely below permafrost, through unfrozen channels within permafrost, and above permafrost where the top of frozen ground lies below the 

water table. Water is supplied to residents of the Fairbanks area by hundreds of small-diameter private wells. 

Many are only 15 to 30 feet deep and take water from 

above permafrost or from unfrozen zones within permafrost. Others, 100 to 250 feet deep, take water from 

unfrozen sediments below the frozen layer. Large 

yields are available both above and below the permafrost. Some 2-inch wells yield as much as 40 gallons 

per minute, and records indicate that yields as high as 

3,000 gallons per minute with a drawdown of 10 to 15 

feet are available to large-diameter wells less than 100 

feet deep.

Ground water is found only below permafrost in the 

alluvial plain north of the Tanana Eiver and in valleys 

of the upland in the Fairbanks area; wells on the lower 

slopes must penetrate 100 to 200 feet of frozen ground 

to obtain water. Some wells drilled on the lower slopes

have encountered artesian water in sandy or gravelly 

layers that are overlain by permafrost (fig. 10). A 

flowing artesian well drilled in 1946 yields more than 

40 gallons per minute.5

Permafrost is thinner on the upper slopes of the 

alluvial-silt apron; however, the water table is at considerable depth and here, too, wells must extend to 

depths of 100 to 200 feet. Because permeable beds are 

few, ground water is scarce even below permafrost; 

some wells must penetrate bedrock to obtain water.

The quality of ground water generally is poor in the 

flood plain, in the northern alluvial apron, and in 

valleys in the upland because much of it percolates 

through organic sandy silt, which adds organic matter 

and iron that imparts a dark color and a bad taste.

The water table is low in interstream areas in the 

upland. The silt mantle yields water so slowly that 

wells must be continued into the underlying schist bedrock. Deep wells are necessary, and the yield generally 

is small and of poor quality.

Ground water of good quality is abundant in the 

gravel of outwash plains south of the Tanana River.

5 Pewe, T. L., op. cit.
130 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Much of the water is derived from mountain streams 

that recharge the gravel in the higher, southern parts 

of the plains. Ground water issues from springs at 

the north end of the outwash plain near Big Delta. 

Depth to water increases upslope at a rate of 15 feet per 

mile to as much as 200 feet, halfway up the outwash 

plain (fig. 9).

No data are available concerning ground-water conditions in the alluvial plains south of the Tanana River. 

Conditions probably are similar to those in the glacial 

outwash plains.

UPPER KUSKOKWIM VALLEY 

By A. T. FERNALD

PHYSIOGRAPHY, CLIMATE, AND VEGETATION

The upper Kuskokwim. Valley is in the discontinuouspermafrost zone along the northwest flank of the 

Alaska Range between 62° and 64° north latitude and 

152° and 156° west longitude (fig. 5). The region is 

divided into lowlands along the Kuskokwim and Nixon 

Rivers, and a piedmont slope, which extends northward 

from an altitude of about 2,000 feet at the base of the 

Alaska Range to an altitude of about 500 feet where it 

merges with the Kuskokwim River lowland.

The climate of the upper Kuskokwim Valley is characterized by cool, wet summers and cold, relatively dry 

winters. At McGrath the mean annual temperature 

is 25.5° F, the extremes being 89° and -64° F. The 

January mean temperature is —8.7° F and the July 

mean temperature, 58.7° F. The mean annual precipitation is 19.1 inches, of which 10.4 inches falls during 

the 4 months from June through September. Variability of precipitation is quite high; the annual total 

ranges from 15 to 25 inches. The average snowfall is 

92 inches; the greatest annual depth of snow on the 

ground is approximately 4 feet. Clear weather prevails during the spring months, the driest of the year.

Farewell, near the base of the Alaska Range and 

about 1,000 feet higher than McGrath, has a mean annual temperature of 25.7° F. The January mean temperature is 2.0° F; the July mean temperature is 54.9° 

F. Strong and frequent foehn winds in the winter 

months are a major factor in producing the warmer 

temperatures at Farewell than at McGrath. Mean annual precipitation at Farewell is 16.3 inches.

The upper Kuskokwim Valley supports forests of 

mixed white spruce and white birch, a black spruce and 

Sphagnum association, and tundra. Tundra is confined to the tops of the higher mountains and the crests 

of many interfluves on the piedmont slope. At lower 

altitudes mixed white spruce and white birch cover hill 

summits, steep or southward-facing slopes, and the

youngest parts of flood plains. The black spruce and 

Sphagnum association is widespread on the older parts 

of flood plains, other alluvial plains and terraces, and on 

gentle or northward-facing bedrock slopes. Patches of 

the black spruce and Sphagnum association also are integral parts of the bog flats.

GEOLOGY

The piedmont slope is a huge compound fan of unconsolidated alluvial, eolian, glacial, and organic 

deposits. A series of end moraines is present on the 

piedmont in front of major valleys in the Alaska Range. 

Two distinct glacial episodes are recognized and are 

represented by two sets of end moraines and associated 

outwash aprons at different levels. The present-day 

rivers dissect the piedmont surface most deeply near 

the range, where they flow northward in braided channels confined in valleys up to 200 feet deep. The confining banks disappear 5 to 25 miles north of the range, 

and the rivers spread out to form alluvial fans.

The piedmont slope is underlain by till, gravel, sand, 

silt, and organic material. Organic material and windblown silt, ranging in thickness from an inch to many 

feet, covers much of the surface. Few data are available concerning the total thickness of the unconsolidated sediments. As much as 200 feet is exposed in the 

valley walls of the two largest rivers. A well at Farewell near the north front of the Alaska Range penetrates 360 feet of unconsolidated material.

The two outwash aprons formed on the piedmont during the two glacial episodes can be traced into two surfaces at different levels in the lowland. The older and 

higher surface, a greatly dissected plain, has a widespread surficial cover of organic material, and windblown sand and silt of varied thickness. Large parts of 

the younger surface are covered with sand dunes, which 

reach a maximum height of 50 feet. A surficial cover 

of windblown silt and organic material exists locally.

The rivers traversing the piedmont join in the lowland to form the main Kuskokwim River. This river 

has dissected the two older surfaces and has formed a 

third, the Kuskokwim alluvial plain, consisting of the 

flood plain and a marginal area, of bog flats away from 

the river. The Kuskokwim alluvial plain is underlain 

mostly by sand, silt, and organic material reaching a 

thickness of 260 feet at McGrath. Lenses of gravel are 

present at depth and locally near the surface.

In the Nixon lowland, an alluviated slope leads down 

from the surrounding bedrock uplands to a broad alluvial plain, which also consists of the flood plain of 

the river and a bordering area of bog flats. Sand, silt, 

and organic material underlie the alluvial plain.
PERMAFROST AND GROUND WATER IN ALASKA 131

PERMAFRO ST

Subsurface data are scarce in the upper Kuskokwim 

Valley. The presence or absence of frozen ground at 

depths greater than 8 feet is established only at 

McGrath and Farewell.

Permafrost is known to be present at shallow depths 

in three types of terrain in the upper Kuskokwim 

Valley. In the alluvial plains and slopes of the lowlands, permafrost occurs at depths of \% to 3 feet beneath black spruce and Sphagnum, moss growing as 

a nearly continuous cover in peat a foot or more thick. 

Permafrost is lacking at such shallow depths where 

the peat is less than a foot thick. Six wells in an area 

of black spruce and Sphagnum moss on the Kuskokwim 

River flood plain at McGrath passed through the bottom of frozen ground at depths of 40 to 50 feet. Permafrost also is present in the black spruce and Sphagnum islands of the bog flats. It is lacking within 6 

feet of the surface in the adjoining quaking bogs, but 

may be present at greater depth. Tundra areas on the 

interfluves of the piedmont slope are underlain by 

permafrost at depths of 1 to 3 feet. A well 360 feet 

deep in the tundra area at Farewell apparently passed 

through the bottom of permafrost at 12 feet, and no 

frozen ground was encountered at greater depth.6

GROUND WATER

Information on ground-water supplies is available at 

only two localities in the upper Kuskokwim Valley— 

at McGrath on the flood plain of the Kuskokwim River 

and at Farewell on the piedmont slope.

Shallow wells supply households in the McGrath 

area with water from sand or gravel lenses in the floodplain alluvium. Wells in the older part of the flood 

plain penetrate water-bearing sediments below permafrost at depths of 40 to 50 feet. Wells in the younger 

part penetrate no permafrost and generally range in 

depth from 12 to 40 feet. A well drilled for the Civil 

Aeronautics Administration reached water at a depth 

of a few tens of feet but was continued until bedrock 

was penetrated at a depth of 262 feet. The water level 

in wells in the unfrozen sediments fluctuates from a few 

feet to 30 feet beneath the surface; the fluctuations are 

synchronous with and evidently due to fluctuations in 

the level of the Kuskokwim River. The well at Farewell extends through 360 feet of unconsolidated sediments, and the water level was 338 feet below the land 

surface in 1943.

6 The thickness of permafrost encountered in the well at Farewell is 

uncertain. Cederstrom (1952, p. 29) reports that permafrost was found 

throughout the upper 125 feet of the well at Farewell. However, a 

summary of the well log inspected by the writer in the office of the Civil 

Aeronautics Administration at Anchorage, Alaska, recorded "Boulders, 

gravel, and frost" in the upper 12 feet and "Boulders, gravel" between 

depths of 12 and 125 feet.

Ground water probably is available at unknown 

depth beneath the flood plains of braided streams, in 

alluvial fans, and in outwash aprons on the piedmont 

slopes.

BRISTOL BAY REGION

By E. H. MULLEE 

PHYSIOGRAPHY, CLIMATE, AND VEGETATION

The Bristol Bay region lies principally in the zone 

of sporadic permafrost but extends southwestward into 

the no-permafrost zone (fig. 5). This area of about 

16,000 square miles extends from 155° 30' to 159° west 

longitude and from the axis of the Alaska Peninsula 

northward to 60° north latitude. It includes the lowlands of the Nushagak River drainage basin, the lowland along the north side of the Alaska Peninsula, and 

portions of the adjacent mountains. Glaciated bedrock 

foothills form a piedmont zone between the Alaska 

Peninsula lowland and the Aleutian Range.

The Alaska Peninsula lowland, along the southeast 

coast of Bristol Bay, is 50 miles wide at its northeast 

end but narrows southwestward to 10 miles in the vicinity of Port Heiden. The land surface rises gently from 

sea level to an altitude of a few hundred feet at the 

south edge of the piedmont. End moraines of glaciers 

from the Aleutian Range form arcuate belts of low 

gravel ridges and knobs on the nearly level lowland. 

Large lakes occupy glacier-scoured basins in the 

piedmont.

The Nushagak River drains another broad lowland 

about 75 miles wide that is enclosed by mountains on 

the east, north, and west. Isolated bedrock hills rise 

1,000 to 2,000 feet above the surrounding plain.

The Bristol Bay region is cool, humid, and windy. 

Its climate is intermediate between the constant storminess and moderate temperatures of the Aleutian Islands and the extreme temperatures of interior Alaska. 

The mean annual temperature at King Salmon and 

Dillingham is about 35° F; extreme temperatures recorded at Dillingham are 89° and -41° F. Although 

total annual precipitation is about 25 inches at both 

towns, snowfall at King Salmon averages 37.5 inches 

as compared with 62.4 inches at Dillingham.

Airflow from the south dominates the summer climate. Temperatures are mild, averaging 50° to 55° 

F. Fog is common, particularly along the coast. About 

40 percent of the annual precipitation falls as light 

showers during July, August, and September.

In winter the dominant airflow is from the north, 

resulting in continental modification of the climate 

with fair weather and temperatures that average near 

0° F. The snow cover may be 1 or 2 feet deep, but 

exposed surfaces are often blown free, especially on
132 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

the lowlands of the Alaska Peninsula. Southerly airflow in advance of migrating low-pressure systems may 

produce freezing rain and thawing temperatures during any month of the winter.

North of 58°30/ north latitude the Bristol Bay region 

is covered with intermixed forest and tundra vegetation, but tundra covers all the southwestern part. White 

spruce grows in sheltered areas at low altitudes in the 

northeast, either in pure stands or associated with 

white birch, poplar, aspen, and willow. The best development of woodland is along the major rivers and 

near the western (lowland) ends of the piedmont lakes 

east and northeast of Naknek.

Elsewhere in the lowland areas, tundra vegetation 

dominated by sedges, cottongrass, and Sphagnum, 

mosses occupies wet areas; heaths and mosses predominate in slightly better drained areas. Above altitudes 

of 800 to 1,000 feet and in exposed locations nearer 

sea level, a sparse cover of dwarf heaths and alpine 

meadow plants is characteristic.

GEOLOGY

The Nushagak River lowland is a broad bedrock 

basin partly filled with unconsolidated glacial and glactiofluvial materials derived from adjacent uplands. 

Isolated bedrock hills on the floor of the basin consist 

of granitic rocks and impure quartzite. The mountains west of the lowland are composed of tightly folded 

impure quartzite, metamorphosed sandy argillite, and 

minor intrusive granitic masses. Intrusive and extrusive igneous rocks predominate in the mountains east 

of the lowland.

The Alaska Peninsula consists of a broad geanticline of gently folded and faulted sediments intruded 

by both fine- and coarse-textured igneous rocks; the 

Quaternary volcanic rocks of the Aleutian Range are 

superposed upon an ancient erosion surface that bevels 

the older rocks. The lowlands are underlain by dominantly coarse-textured, unconsolidated materials, including glacial till and outwash sand, gravel, and silt 

reaching thicknesses of more than 600 feet near Naknek. 

A few thin beds of peat and organic silt are interbedded with the coarser sediments. Windblown silt 

a few feet thick mantles much of the surface of the 

Alaska Peninsula and Nushagak River lowlands.

PERMAFROST

The climate of the Bristol Bay region is marginal 

for the formation or preservation of frozen ground. 

The probability of encountering microclimates or soil 

climates favorable for the development of permafrost 

decreases southwestward down the Alaska Peninsula. 

Areas beyond Egegik are considered to be in the 

no-permafrost zone.

Frozen ground in lowlands of the Bristol Bay region 

is found chiefly in silt, especially in silt having a high 

organic content, but it also occurs in fine sand and 

glacial till. Shallow permafrost is found most commonly beneath swampy lowlands. Perennially frozen 

ground has not been observed in the till of terminal 

moraines, but irregular pockets may be present in favorable sites. Layers of frozen fine-grained sediments 

more than 10 feet thick, interbedded with unfrozen 

sand and gravel, have been found in wells at depths 

as great as 175 feet. Permafrost generally is absent 

in coarse-textured materials and in areas of moderate 

relief; thus, shallow permafrost is lacking in sanddune areas, in gravelly outwash, in sediments beneath 

flood plains, and in river and lake terraces composed 

of coarse sediments.

The rigor of the environment in the higher part of 

the piedmont and in the mountains indicates that areas 

of frozen ground are probably present. However, the 

coarse texture of the soil materials and the availability 

of abundant water from melting snow favor a deep 

zone of summer thaw. Permafrost bodies, if present, 

probably are irregular in form and cannot be recognized by surface criteria.

Much of the perennially frozen ground of the Bristol Bay area probably is relict from past periods of 

colder climate. Permafrost can form to depths of a 

few tens of feet at the present time, however, under 

favorable conditions. Restricted ground-water percolation, a thick layer of insulating- organic material, 

and a predominance of silt in the surface soil favor 

formation of perennially frozen ground. Ice in perennially frozen sediments formed under present climatic 

conditions is chiefly interstitial, but small veins and 

irregular thin ice laminae also are present. Ice in 

relict permafrost occurs as laminae as much as an inch 

or two in thickness interlayered with frozen layers a 

few inches thick. No larger ice segregations have been 

observed.

GROUND WATER

Ground-water supplies adequate for large communities can be obtained throughout the lowlands of the 

Bristol Bay region from wells 100 to 600 feet deep. 

Several wells may have to be driven in order to obtain a 

sufficiently large yield. Supplies for single dwellings 

can be obtained from shallower wells 15 to 50 feet deep.

Most wells in the lowlands penetrate several waterbearing zones at depths of 30 to 600 feet. In general, 

the yield of wells near the piedmont is greater than the 

yield of those near the coast. Water in the lowland 

sediments generally is under hydrostatic pressure; 

artesian flows of from 6 to 138 gallons per minute have 

been obtained from depths of 90 to 600 feet.
PERMAFROST AND GROUND WATER IN ALASKA 133

The quality of the water in most wells is good, but 

water associated with peat or muck layers has a high 

organic content and a bad taste. An artesian flow of 

unpalatable water containing 5,800 ppm of dissolved 

solids was encountered at 600 feet in a well at Koggiung, 

20 miles north of Naknek. Water from depths of less 

than 300 feet in the same area has a lower mineral content and is potable.

Slight difficulties may be encountered in drilling 

wells through the discontinuous lenses of permafrost in 

fine sediments in the lowlands of the Bristol Bay region, 

in bypassing the flow of unpotable ground water in 

organic layers, and in flushing out flowing quicksand in 

certain water-bearing horizons. Excessive pumping in 

wells near the coast may result in encroachment of salt 

water.

KENAI LOWLAND

By T. N. V. KAKLSTROM

PHYSIOGRAPHY, CLIMATE, AND VEGETATION

The Kenai lowland lies in south-central Alaska in 

the no-permafrost zone. The area is a broad, low shelf, 

20 to 60 miles wide, lying between the Kenai Mountain 

Eange and Cook Inlet (fig. 5). The lowland covers 

an area of about 3,600 square miles between 150° and 

152° west longitude and 59° and 61° north latitude.

The Kenai lowland is a product of repeated episodes 

of glacial erosion and deposition within a bedrock 

trough, with subsequent modification by wind, stream, 

tidal, and frost processes. The topography exhibits a 

wide range of glacial and associated glaciofluvial forms 

such as moraines, outwash aprons and plains, kames, 

and eskers. Most of the lowland lies within 400 feet 

of sea level, surfaces are flat to undulating, and local 

relief varies from a few feet to more than 200 feet. The 

Caribou Hills, a broad upland northeast of Homer, rise 

abruptly 1,000 to 2,000 feet above the general lowland 

level.

The Kenai lowland has a cool, moderately maritime 

climate; the warmest temperatures and greatest precipitation are in the southern part. The mean annual 

temperature ranges from 33.3° F at Kenai in the north 

to 38.3° F at Homer in the south. Extreme recorded 

temperatures are 89° F and -46° F at Kenai and 79° F 

and —17° F at Homer. The mean annual precipitation 

is 19 inches at Kenai and 32 inches at Homer; most of 

the precipitation falls as rain during the months of 

July and August. The average annual snowfall ranges 

from 55 inches at Kenai to 82 inches at Homer. The 

average number of days with freezing temperatures is 

219 at Kenai and 189 at Homer.

The vegetation of the Kenai lowland consists of intermixed forest, muskeg, grassland, and subalpine shrub

tundra. Forests cover about half the area and consist 

of pure stands of black, white, or Sitka spruce and 

mixed stands of spruce, birch, aspen, and balsam poplar. The mixed forests of spruce and birch and of 

spruce and aspen are limited to the better drained portions of the lowland where seasonal frost thaws relatively early. Pure stands of stunted white or black 

spruce grow in areas of more restricted drainage and 

lingering seasonal frost.

Muskeg vegetation—mosses, grasses, sedges, heath 

shrubs, and scattered black spruce—covers more than 

a third of the Kenai lowland. This vegetation is found 

in broad, shallow drainage basins and abandoned glacial drainage channels, where subsurface drainage is 

poor and frost persists at shallow depth during most 

of the summer.

Grasslands predominate in the rest of the Kenai lowland. They are found chiefly on slopes and in depressions on the broad interfluves of the Caribou Hills. Soil 

in the grasslands generally consists of a thick layer of 

silt that is subject to intense frost action during winter 

but which thaws early and becomes well drained during summer. Grasslands also cover the better drained 

abandoned drainage channels in the lowland.

Subalpine shrub tundra is restricted to coarse rubble 

and fractured bedrock on slopes above 1,500 feet in 

altitude.

GEOLOGY

Bedrock in the Kenai lowland and Caribou Hills consists of poorly consolidated siltstone, sandstone, and 

'lignite. It is mantled with glacial till, outwash, and 

alluvial silt, sand, and gravel. The thickness of the unconsolidated mantle varies widely. In the Caribou 

Hills, bedrock is exposed at the surface or is mantled 

by only a thin veneer of till and gravel. Depth to bedrock increases northward, in general, and the northern 

part of the lowland is underlain locally by as much as 

several hundred feet of unconsolidated deposits.

Topographic depressions are partly or completely 

filled with peat and organic silt. Knobs, ridges, and 

terraces are covered with 2 to 5 feet of windblown silt.

PERMAFROST

It is believed that permafrost cannot form in the 

present climate of the Kenai lowland. Permafrost that 

probably formed under a more rigorous climate in the 

past is preserved locally, however, in areas where conditions are exceptionally favorable. Known occurrences of permafrost are restricted to black spruce 

islands in bog flats in the northern interior part of the 

area where a dense forest cover, a thick insulating mat 

of moss, and tightly compacted peat soils favor its 

preservation (pi. 34). The mean annual temperature
134 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

of the northern part of the lowland is so close to freezing that areas having microclimates favorable for preservation of permafrost may be found in other types of 

terrain. No information is available concerning the 

maximum depths at which these small bodies of frozen 

sediment may be found.

GROUND WATER

An estimate of ground-water supplies is hindered by 

the lack of wells or other subsurface data throughout 

most of the Kenai lowland. Present knowledge of the 

geology, however, permits speculation as to the probability of obtaining adequate supplies of ground water 

in various terrains. Permafrost is so rare that it has 

little effect on the distribution of ground water. Persistent seasonal frost reduces infiltration over wide 

areas but does not affect circulation of ground water at 

depth.

Shallow ground-water supplies probably are available in the lowland in permeable materials of surface 

outwash plains, stream terraces, and alluvial fans. In 

upland valleys, shallow ground water is likely to be 

found in granular material lying on the buried bedrock 

valley floor.

Ground-water supplies probably are available at 

depth throughout the lowland in sand and gravel lenses 

or beds in buried outwash or alluvium. Granular sediments in buried bedrock valleys and porous bedrock 

sandstone layers also may be sources of ground water 

at considerable depth. Ground water probably is most 

abundant in the lowland adjacent to the mountain front 

and in terraces and bed gravels of major streams that 

head in the mountains. Elsewhere the ground-water 

supplies are restricted by a predominance of finegrained, tight sediments and by limited surface recharge. In these areas the best aquifers are likely to be 

found just above the bedrock floor.

Ground water generally is of fair to good quality but 

locally may have a high organic and iron content. 

Wells drilled near the coast probably would be subject 

to encroachment of sea water if they were heavily 

pumped.

PHOTOINTERPRETATION OF PERMAFROST CONDITIONS

Aerial photographs are useful aids in mapping geology, soils, and vegetation. In addition to serving as 

base maps on which to plot field observations, aerial 

photographs may be used to extrapolate the results of 

field mapping into nearby areas by photointerpretation. 

Many geomorphic features and relationships are more 

readily recognized when seen on aerial photographs 

than when viewed on the ground; moreover, many inaccessible localities are brought into view. Thus, aerial

photographs expand considerably the understanding of 

the area portrayed. They are an almost indispensable 

supplement to regional reconnaissance studies of natural phenomena in Arctic and subarctic regions.

The interpretation of aerial photographs alone, however, is a poor substitute for field observations in terrain 

studies of any kind. The topographer does not attempt 

to construct an accurate topographic map without 

ground control; neither should the earth scientist expect to prepare accurate maps of vegetation, geology, 

soils, or permafrost without the control of field 

observations.

Permafrost is an especially difficult phenomenon to 

map by aerial photography alone, for perennially frozen 

ground rarely is exposed at the surface and thus seldom can be observed directly from the air. Field studies lacking subsurface data from drill holes, wells, or 

mines fail to delineate with much precision the horizontal and vertical boundaries of frozen ground. Still 

less precision is possible in a study that depends mostly 

upon the interpretation of aerial photographs.

Some recent publications indicate a misunderstanding 

of the strictly supplementary role of photointerpretation in mapping permafrost. Several authors have implied that permafrost conditions can be interpreted in 

considerable detail by photointerpretation alone (Sager, 

1951, p. 551-571; Frost, 1951, p. 223-246; Wilson, 1948, 

p. 164). However, examination of aerial photographs 

of various regions in Alaska where ground conditions 

are known indicates that, except for rare actual exposures of frozen ground containing large masses of 

clear ice, no single feature recognizable on aerial photographs can be taken as unequivocal evidence of the 

presence of permafrost. Thus, the detailed boundaries 

of permafrost in most parts of Alaska cannot be delineated solely from aerial photographs.

A similar misunderstanding of the supplementary 

role of photointerpretation is implicit in proposals 

for the establishment of general aerial-photograph 

keys for the interpretation of permafrost conditions 

throughout Arctic and subarctic regions. Keys can be 

prepared for the generalized interpretation of permafrost conditions in small areas, but they lose validity 

rapidly with increasing distance from the area where 

the criteria employed were first established by means 

of detailed ground studies.

MAPPING PERMAFROST FROM AERIAL 

PHOTOGRAPHS

Although the detailed boundaries of permafrost generally cannot be mapped from aerial photographs, the 

photographs can be the source of considerable information concerning the distribution of landforms and
PERMAFROST AND GROUND WATER IN ALASKA 135

terrain units in which the probable permafrost conditions can be estimated. The first step in such a study 

consists of establishing, preferably by field studies, the 

regional distribution pattern—the prevailing distribution of frozen and unfrozen zones in various types of 

terrain. If the area is inaccessible to field studies, the 

pattern commonly can be estimated, though with much 

less accuracy, by study of available information concerning the climate, geology, vegetation, and topography of the region to be mapped and by comparison 

with an analogous region in which the permafrost distribution pattern is known.

Having established the regional pattern, the investigator begins intensive study of the photographs and 

delineates terrain units such as flood plains, poorly 

drained lowlands, glacial moraines, and mountain valleys. Permafrost conditions in these are assumed to 

be generally similar to conditions at ground-control 

points in similar terrain units or at comparable sites 

in an analogous area.

PERMAFROST INDICATORS

Recognition on aerial photographs of certain indicators helps the investigator to estimate permafrost 

conditions in areas in which they occur if he understands their significance and limitations. Useful indicators include certain plant species and assemblages 

and inicrorelief features of several types.

VEGETATION

TUNDRA AND FOREST ASSOCIATIONS

It is occasionally suggested that tundra vegetation 

indicates the presence or absence of permafrost in 

Alaska (for example, Woods, Hittle, and Frost, 1948, 

p. 498). Detailed studies indicate, however, that the 

distinction of tundra and forest vegetation has only 

limited value in the photointerpretation of permafrost 

conditions.

The term "tundra" is applied to a wide variety of 

vegetation types including sparse covers of matted 

dwarf shrubs, herbs, and grasslike plants separated 

from one another by large areas of bare ground; dense, 

continuous covers of cottongrass, dwarf birch, heaths, 

mosses, and lichens; and rank growths of willow, alder, 

or dwarf birch shrubs. Each type of tundra in a 

given region reflects different soil, moisture, and permafrost conditions. On Seward Peninsula, for example, 

dry tundra composed of matted dwarf plants separated 

by areas of bare ground occupies areas of rocky soil in 

which permafrost either lies at depths of 5 to 10 feet 

or is absent. Tundra composed of cottongrass, birch 

shrubs, and heaths, on the other hand, is found in areas 

of silty soil in which permafrost lies at depths of 2 or

3 feet. Dense growths of willows or alders are confined to areas where permafrost either lies at depths of 

more than 5 feet or is lacking.

Similarly, the term "forest" applies to widely differing vegetation types, including forests of aspen beneath 

which permafrost generally is lacking and stands of 

black or white spruce growing in peat in which permafrost lies at depths of only 2 or 3 feet. Thus, permafrost conditions differ as widely beneath different types 

of tundra or forest vegetation as they do between these 

two major types of vegetation.

Areas of tundra vegetation are interspersed in forested areas throughout most of Alaska (fig. 11), and 

both forest and tundra occur in all of the permafrost 

zones as well as the no-permafrost zone. The commingling is even more complex than is suggested in 

figure 11. Throughout much of Alaska, forests are 

limited to slopes at low altitudes and to the better 

drained parts of the valley bottoms. Extensive areas 

of tundra cover high ridges and plateaus, while smaller 

tundra areas are found on poorly drained sites in the 

lowlands and valleys. An almost uninterrupted belt 

of tundra extends along the coasts of western and 

northern Alaska from the Alaska Peninsula to the Canadian boundary. Thus, the presence of tundra does 

not necessarily indicate the presence of permafrost, nor 

does the presence of forest necessarily indicate the 

absence of permafrost.

DISTRIBUTION PATTERNS OF TREES AND LARGE SHRUBS

The distribution patterns of trees and tall shrubs 

supplement topographic expression in the recognition 

of landforms such as end moraines (pi. 34), outwash 

plains, and abandoned stream channels. Thus they 

contribute indirectly to the interpretation of probable 

permafrost conditions based upon inferences concerning the age of the land surface, the climate, and the 

character of the underlying material.

Observation of the kinds of trees and shrubs and 

their manners of growth and association also permits 

inferences concerning the depth to permafrost in local 

areas (Stoeckeler, 1948,1952). The exact significance 

of these vegetation phenomena varies from one region 

to another, however, and they must be used with caution 

and full understanding of their variability.

The distribution patterns of tree and shrub vegetation are functions of several variables including climate, 

character and stability of the soil, drainage conditions, 

topographic position, age of the land surface, plant 

succession, and disruptive factors such as fire and disease. Permafrost is a controlling factor only when 

it occurs at a depth shallow enough to influence subsurface drainage, soil stability, and soil temperature
FIGURE 11.—Distribution of forested and treeless areas in Alaska and part of Canada. Treeless areas are predominantly tundra but also include exp

and grasslands. Compiled by R. S. Sigafoos.
PERMAFROST AND GROUND WATER IN ALASKA 137

within the zone of root growth. Some trees, such, as 

larch and black spruce, have shallow root systems and 

manage to grow in wet areas where permafrost lies at 

shallow depth. Others, such as white spruce, aspen, 

balsam poplar, white birch, alder, and several kinds of 

willows, have deeper roots and grow best in relatively 

well drained areas where the ground thaws to depths 

of at least several feet in midsummer. Stunted specimens of some of the usually deeper rooted trees, however, grow on sites where permafrost lies at shallow 

depth. Eecognition of the shallow-rooted species on 

aerial photographs aids in delineating the areas where 

permafrost is most likely to occur at shallow depth, 

and recognition of deep-rooted species, especially those 

of good growth, helps delineate areas least favorable 

for the formation or preservation of permafrost. The 

tree- and shrub-distribution patterns are significant, 

however, only when considered in the light of other 

information concerning permafrost distribution in the 

region under study.

Information concerning permafrost conditions under 

various types of trees and shrubs can be systematized 

into vegetation keys for local areas. The keys indicate only the probable minimum depth to permafrost; 

they do not indicate the maximum possible depth, nor 

do they permit distinction between forests growing on 

perenially frozen ground and those growing on unfrozen ground. They decrease in usefulness rapidly 

with increasing distance from the area of the geobotanical field work upon which they were based.

The following tabulation indicates the minimum 

depth to permafrost, if it is present, beneath some vegetation assemblages that are widespread in Alaska:

Feet

Tall willows on flood plains__________________ 8 

Pure stands of mature aspen or white birch_________ 4 

Mixed willow, alder, and white birch____________ 3-4 

Pure stands of balsam poplar__________________ 3-4 

Mixed white spruce and balsam poplar____________ 3 

Pure stands of white spruce___________________ 2-3 

Mixed stands of white birch and white spruce—_______ 2-3 

Mixed white and black spruce__________________ 1-2 

Black spruce in wet tundra or muskeg_____________ 1

Areas overgrown by black spruce are among those 

most likely to be underlain by permafrost at shallow 

depth in Alaska. In the interior of Alaska, open uniform stands of black spruce growing on flat or gently 

sloping moss-covered ground are found mainly in areas 

that have peaty soil and permafrost at shallow depth 

(foreground, pi. 32). Permafrost generally is lacking 

beneath simil-ar forest in southern Alaska (c, pi. 34), 

but seasonal frost frequently persists during most of 

the summer.

Bog flats are especially useful permafrost indicators 

in the discontinuous, sporadic, and no-permafrost 

zones. These distinctive patchworks of black spruce 

forest and sedge marsh have a ragged, mottled appearance that is readily recognized on aerial photographs 

(pis. 32-34). Permafrost is absent or lies at depths of 

more than 6 feet beneath the marshy areas, but the black 

spruce islands are favorable sites for the formation and 

preservation of permafrost. Black spruce islands underlain by permafrost are generally distinguished by 

the presence of steep banks, a marginal moat or trench 

of standing water, and an abrupt contact between the 

dense spruce vegetation of the island and the tundra 

vegetation of the surrounding marsh. Spruce islands 

that lack permafrost generally have less distinct margins which grade into the adjoining open marsh 

(e, pi. 34).

Tall willow shrubs and isolated pure stands of balsam 

poplar on river flood plains generally indicate the presence of unfrozen ground (a, pi. 33). The lateral limits 

of the patches of tall shrubs in tundra regions commonly 

indicate closely the lateral limits of the unfrozen zones 

(d, pi. 35). Tall shrubs are particularly useful indicators in the continuous-permafrost zone and in the 

northern part of the discontinuous-permafrost zone, 

where thawed zones are rare and difficult to locate.

POLYGONAL MICRORELIEF PATTERNS

Intensive frost action produces distinctive microrelief 

patterns which aid in interpreting aerial photographs 

of many areas in Alaska. Two broad categories of 

polygonal patterns may be distinguished: frost-stirred 

or frost-sorted polygons, including stone polygons, vegetation polygons, and related striped features; and contractional or tensional polygons, including ice-wedge 

and frost-crack polygons. The two types of polygons 

generally differ in size and shape as well as in origin 

(contrast stone polygons at 5 and ice-wedge polygons at 

/, pi. 31; contrast vegetation polygons and ice-wedge 

polygons at m and frost-crack polygons at 0, pi. 43 A). 

Individual frost-stirred polygons generally are rounded 

and range in diameter from a few inches to 25 feet. Individual contractional polygons are sharply angular, 

have 4 to 8 sides, and generally are 25 to 100 feet in diameter, although ice-wedge polygons 5 to 400 feet in 

diameter are present on the Arctic coastal plain near 

Point Barrow (E. F. Black, personal communication). 

The pattern of ice-wedge and frost-crack polygons is 

closely similar to the pattern of desiccation polygons, 

formed by mud cracks, but desiccation polygons generally are much smaller.
138 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

FROST-STIRRED POLYGONS

Frost-stirred polygons and stripes are products of 

processes in the surface zone of annual freeze and thaw 

(Washburn, 1950). On flat surfaces these processes 

result in heaving and thrusting which operate with 

equal force in all directions to give rise to roughly 

equidimensional polygonal forms. On slopes they act 

most strongly in the direction of maximum slope, giving rise to elongate polygons or stripes. The formation of frost-stirred polygons and stripes is favored by 

the presence of permafrost at shallow depth, but they 

are common in many areas where permafrost lies at 

great depth or is absent.

Stone polygons and stripes (e, pi. 36) form in initially coarse-grained soils (Sharp, 1942, p. 275-277). 

The largest rock fragments are segregated in marginal 

ridges or channels that adjoin central areas of finegrained or unsorted soil. Active stone polygons and 

stripes are common in areas where the ground is not 

perennially frozen, and thus they are not reliable indicators of permafrost. Moreover, they are persistent 

features of the landscape; relict polygons formed during past periods of more rigorous climate are common 

(&, pi. 31).

Most types of vegetation polygons and stripes result from frost-stirring in fine-grained soils. Certain 

types on Seward Peninsula (tiny stipples superimposed on ice-wedge polygons at s and w, pi. 40 J., and 

on slopes in pi. 43 A ; also see Hopkins and Sigaf oos, 

1951, p. 98-99) and in the Bristol Bay region (d, pis. 

36 and 39) have been found only in areas underlain 

by permafrost, but other types not easily differentiated 

on aerial photographs bear a much less consistent relationship to perennially frozen ground. Vegetation 

polygons probably are not persistent features of the 

landscape, and relict forms are uncommon. Unfortunately, relict stone polygons overgrown with tundra 

vegetation commonly are indistinguishable on aerial 

photographs from active vegetation .polygons.

Widely distributed frost-stirred polygons and stripes 

found both on well-drained and poorly drained sites 

suggest the presence of permafrost. However, the presence locally of frost-stirred polygons on such favorable 

sites as lake margins, bottoms of ephemeral lakes, and 

borders of late snowbanks has no significance in regard 

to the presence or absence of permafrost.

Most frost-stirred polygonal and striped patterns can 

be identified readily on aerial photographs of scale 

1:10,000 and under favorable conditions on photographs of scale 1: 20,000. They generally cannot be 

distinguished on photographs of scale 1: 40,000.

CONTRACTIONAL POLYGONS

Three interrelated types of contractional polygons 

are recognized: frost-crack polygons, high-center icewedge polygons, and low-center ice-wedge polygons. 

Contractional polygons result from the cracking of icecemented soils and sediments during sharp declines in 

winter air temperatures. Ice forms in the resulting 

fractures. If the ice masses persist through the following summer and become enlarged during subsequent 

winter freezing cycles, ice-wedge polygons are formed. 

If the ice masses melt each summer, frost-crack polygons are formed. The size and shape of contractional 

polygons are dominantly functions of winter temperatures and the physical character of the soil materials; 

thus frost-crack and ice-wedge polygons differ only 

slightly in size and shape (compare frost-crack polygons at 2 and ice-wedge polygons at w\, pi. 43 A), and 

both are much larger than frost-stirred polygons. The 

origin of contractional polygons is discussed in detail 

by Leffingwell (1919, p. 205-214) and Black (1952, p. 

129-131).

LOW-CENTER ICE-WEDGE POLYGONS

Low-center polygons are common in the continuouspermafrost zone, rare in the discontinuous-permafrost 

zone, and lacking in the sporadic-permafrost zone. 

They are among the most dependable indicators of 

permafrost. They are limited to wet tundra areas and 

are found chiefly in basins of drained lakes and lagoons 

and abandoned river channels (pis. 35, 37, 40, 41). The 

polygons consist of networks of broad marginal ridges 

surrounding low central areas that commonly contain 

standing water. The marginal ridges of adjoining 

polygons are separated by troughs or crevices which 

often are the site of active frost cracking (a, pi. 38). 

The polygons are underlain by perennially frozen 

ground at depths of less than 2 feet. Upon thawing of 

the perennially frozen ground, the form of the lowcenter polygon is modified to that of the high-center 

polygon, but it may also evolve into a high-center polygon without thawing of permafrost.

HIGH-CENTEE ICE-WEDGE POLYGONS

High-center polygons are abundant in the continuous-permafrost zone, common in the northern part of 

the discontinuous zone, and rare in the southern part of 

the discontinuous zone and in the sporadic zone! They 

form, for the most part, in silty and peaty soils on hilltops, on gentle slopes, in depressions, and along minor 

drainage courses, although high-center polygons are also 

found locally in sand, gravel, or bedrock on the Arctic 

slope. High-center polygons are found in slightly better drained areas than those in which low-center poly-
PERMAFROST AND GROUND WATER IN ALASKA 139

gons are found (pis. 38,40). Generally, the centers are 

relatively level; however, extremely active high-center 

polygons on the Arctic slope have domed centers, and 

deeply thawed and eroded ice-wedge polygons throughout Alaska have rounded, moundlike profiles (<?, pi. 42; 

y, 43 A). The marginal trenches extend a few inches 

to a few feet lower than the centers and are underlain 

by large masses of clear ice. Drainage is concentrated 

in the trenches, and pools of standing water are common at trench intersections (c, pi. 38). Areas of highcenter polygons locally contain small "thaw gullies," 

which are angular drainage courses that have worked 

headward along interconnecting ice wedges (A, pi. 31; #, 

pi. 37; SB, pi. 43 A).

High-center polygons that have formed in the present 

climate are underlain by perennially frozen ground at 

depths of 1.1/2 to 3 feet, but relict polygons are common in which perennially frozen ground lies at much 

greater depth or is absent. High-center ice-wedge 

polygons are less dependable than low-center polygons 

as criteria for the recognition of areas underlain by 

permafrost, because active polygons underlain by 

permafrost are not readily distinguished from relict 

ice-wedge polygons or frost-crack polygons in which 

permafrost may be lacking.

FEOST-CEACK POLYGONS

Frost-crack polygons are common in all permafrost 

zones and may be found locally in the permafrost-free 

zones of Alaska. They generally form in coarse sand 

or gravel and are largely confined to well-drained areas 

such as stream terraces, alluvial fans, spits, and bay 

bars. In form they are similar to high-center icewedge polygons, but the marginal troughs commonly 

are shallower and less sharply defined (compare frostcrack polygons at 0 and ice-wedge polygons at w, pi. 

43 A). Frost-crack polygons commonly are less regular, less complete, and slightly larger than ice-wedge 

polygons occurring in the same areas. Perennially 

frozen ground is present beneath most frost-crack polygons in the continuous-permafrost zone; in other zones, 

frost-crack polygons commonly are found in areas free 

of permafrost. In the Bristol Bay region they indicate absence of permafrost (e, pi. 39).

EECOGNITION OF CONTEACTIONAL POLYGONS ON AEEIAL PHOTOGEAPHS

Low-center ice-wedge polygons are generally easily 

recognizable on photographs of scale 1:20,000 and less 

easily so on photographs of scale 1:40,000 (a?, pi. 40). 

A location in poorly drained depressions and the presence of pools of standing water in the centers of some 

polygons are diagnostic of these features. The trenches 

between the marginal ridges appear as thin dark lines

on photographs of scale 1:20,000 but are rarely recognizable on photographs of scale 1:40,000. High-center polygons, pingos, thaw lakes, and beaded drainage 

commonly can be recognized in areas adjoining lowcenter polygons.

Generally, high-center ice-wedge polygons and frostcrack polygons are easily seen on photographs of scale 

1:20,000. On photographs of scale 1:40,000, icewedge polygons are fairly easily observed (10, pi. 40) 

but frost-crack polygons commonly are invisible or only 

barely discernible. The following features aid in making the difficult distinction between them:

1. The slightly larger dimensions and less pronounced regularity of frost-crack polygons cause them to contrast with 

adjoining high-center ice-wedge polygons.

2. Frost-crack polygons generally are found in relatively well 

drained sites underlain by coarse, granular soils; ice-wedge 

polygons generally are confined to sites underlain by finegrained, impermeable soils.

3. Commonly, vegetation is sparse on frost-crack polygons, 

and the characteristic light tones of underlying sand and gravel 

may contrast sharply with the darker tones of nearby areas in 

which fine soils bear a more complete cover of vegetation. 

Frost-crack polygons formed in gravel mantled with a few feet 

of silt, however, generally support a complete cover of vegetation and cannot be distinguished by color tone from nearby icewedge polygons.

4. The presence of small pools of water at channel intersections suggests high-center ice-wedge polygons. Frost-crack 

polygons, however, may contain similar pools for brief periods 

during spring thaw or after rainy weather.

5. Active high-center ice-wedge polygons commonly are closely 

associated with pingos, beaded drainage, thaw gullies with 

angular courses, thaw lakes, and low-center polygons.

PINGOS

Pingos are reliable indicators of the presence of 

permafrost. They are common in the continuous-permafrost zone but are rare in other zones. They consist 

of isolated steep-sided hills, generally circular to oval 

in ground plan, that range from 10 to 100 feet or more 

in height. Well-developed pingos commonly bear a 

summit crack or craterlike depression several feet deep 

that extends along the long axis of the hill. Some 

pingos also are cut by radial cracks or by several short 

cracks normal to the long crack. Pingos are confined 

to nearly level, poorly drained areas generally in basins 

of former lakes (pi. 40). They are composed of a core 

of massive ice overlain by a few feet of silt, sand, and 

peat (Porsild, 1938). Potable water can be obtained 

in small springs near the summits of some pingos.

Pingos can be seen on aerial photographs of scale 

1:40,000 or larger but commonly can be identified definitely only by viewing stereoscopic pairs (pi. 40). 

Relatively few landforms simulate the characteristic 

pingo form, but cones of mud or travertine surrounding 

large springs, small basalt cinder cones, and conical
140 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

erosional remnants can be mistaken for pingos that lack 

the summit crack (A, pi. 37).

FEATURES RESULTING FROM THAWING

THAW LAKES

Recognition of thaw lakes aids in the interpretation 

of permafrost conditions. Thaw lakes occupy shallow 

depressions caused by local thawing of perennially 

frozen ground on flat surfaces and gentle slopes underlain by 10 feet or more of peat and silt. They are 

common in the continuous- and discontinuous-permafrost zones and are rare in the sporadic zone. Their 

use as permafrost indicators is severely limited by the 

difficulty of distinguishing on aerial photographs between active thaw lakes, relict thaw lakes, and similarappearing lakes that are in no way related to permafrost.

The origin of thaw lakes is discussed by Wallace

(1948), Black and Barksdale (1949), and Hopkins

(1949). Frozen fine-grained sediments generally contain a volume of ice in the form of clear ice lenses and 

masses greatly in excess of the porosity of the unfrozen 

material. Upon melting of the ice the ground subsides, and a basin is formed in which a thaw lake may 

accumulate. The lake increases in size by thawing and 

wave erosion of its banks and is eventually drained 

when the retreating banks intersect a stream valley or 

other low ground (&, pi. 31).

The well-known oriented lakes of the Arctic coastal 

plain (pi. 38; Black and Barksdale, 1949, p. 113-115), 

formed largely by thawing of perennially frozen 

ground, are as much as several miles in length and several tens of feet in depth. Lakes elsewhere in Alaska 

that have originated chiefly by thawing rarely exceed 

1,000 feet in length and 10 feet in depth. The banks 

generally are 5 to 10 feet high but locally reach heights 

approaching 100 feet. Simple lakes are round, oval, 

or roughly rectangular in outline, but groups of lakes 

commonly coalesce to form compound water bodies with 

scalloped outlines (lakes in pi. 37 and shores of Imuruk 

Lake, pi. 31). The outlines of drained thaw lakes 

commonly can be discerned in the vicinity of existing 

thaw lakes (&, pi. 31; g, pi. 35; dd, pi. 37; pi. 38; pi. 41).

Actively caving banks are ragged and steep or overhanging. Tilted trees leaning over the water along 

the banks afford evidence of active caving in forested 

areas; dead trees with bases submerged commonly can 

be seen adjacent to the active banks. In treeless areas, 

active caving is indicated by crevices parallel to the 

banks (2, pi. 40), marking the inner edge of undercutting and slumping. Thawing proceeds most rapidly 

along ice wedges on shores retreating into areas of 

high-center ice-wedge polygons, and the banks are

etched by closely spaced thaw gullies developed along 

the thawing ice wedges (A, pi. 31 and shores of large 

lake in pi. 37). Where low-center polygons adjoin the 

lake, however, thawing commonly proceeds more rapidly into the frozen ground of the polygon centers, and 

the ice wedges project into the water as miniature peninsulas (&, pi. 38; y, pi. 40).

Oxbow lakes on flood plains in areas of perennially 

frozen ground commonly are enlarged by thawing and 

caving of the banks.7 Series of abandoned meanders 

show progressive modification with increasing distance 

from the river and range from the characteristic arcuate or U-shaped outline of the recently formed oxbow 

lakes to the oval, rectangular, or scalloped form typical 

of thaw lakes (pi. 41).

Thaw lakes probably persist in recognizable form in 

areas in which no perennially frozen ground remains; 

lakes similar in outline but lacking evidence of active 

caving are common in areas in Alaska where little or no 

permafrost exists today (pi. 44). Several types of 

lakes in whose development thaw-collapse plays no 

part can be confused with thaw lakes. The distinctive 

landforms characteristic of young, unmodified glacial 

and volcanic deposits prevent confusion of the associated lakes with thaw lakes, but older, modified glacial 

deposits and lava flows commonly contain rounded, 

oval, and rectangular lakes (pi. 39) that are difficult to 

distinguish from thaw lakes. Lakes resembling thaw 

lakes are common in the basins of former lagoons and 

estuaries on the recently uplifted coastal-plain areas 

that border much of the west and north coasts of 

Alaska.

Thaw lakes can be seen easily on aerial photographs 

of all commonly used scales (pi. 40), but evidence of 

active caving, required for absolute identification, is 

difficult to detect on a scale smaller than 1:20,000. 

Many thaw lakes lack discernible evidence of active 

caving on any aerial photographs. The following 

characteristics may permit identification of thaw lakes 

when evidence of active caving cannot be recognized:

1. Steep banks are present along part of the perimeter.

2. Short, sharp-walled gullies with angular courses are incised in the steep lake banks; the shores are minutely irregular, 

and evenly spaced minor peninsulas project into the water 

where the lake banks are low.

3. Pingos, ice-wedge polygons, beaded drainage, and circular, 

oval, or rectangular depressions, which appear to represent 

the basins of drained lakes, are present nearby.

4. A gradational series of lakes that ranges from those 

formed by some means other than thawing, such as oxbows, to 

typical thaw lakes, can be discerned.

* P6w6, T. L., 1948, Terrain and permafrost, Galena area, Alaska: 

U. S. Geol. Survey Permafrost Program Prog. Kept. no. 7, 52 p. 

(Unpublished report.)
PERMAFROST AND GROUND WATER IN ALASKA 141

BEADED DRAINAGE

The term "beaded drainage" is applied to a characteristic pattern of minor streams in areas underlain 

by perennially frozen peat and silt containing ice 

wedges. Beaded drainage is common in the continuous- and discontinuous-permafrost zones and rare in 

the sporadic zone. The pattern consists of series of 

small pools connected by short watercourses (<7, pi. 31; 

A, pi. 35; &, pi. 41). The pools form in areas of collapse due to the thawing of large ice masses and consequently have steep banks; they range in depth from 

2 to 10 feet and in diameter from a few to a hundred 

feet. The connecting watercourses commonly follow ice 

wedges and thus are straight or sharply angular. The 

watercourses generally but not everywhere are sharply 

incised a few feet below their banks.

A drainage pattern of similar appearance, unrelated 

to permafrost, occurs in swampy areas in many parts 

of Alaska. The pattern consists of shallow vegetation-choked pools with sloping banks, separated by 

straight or curving watercourses (near /, pis. 39 and 

44). The pools generally are less than 3 feet deep.

Beaded drainage can be seen on photographs of scale 

1:40,000 or larger. Care must be exercised to avoid 

confusion between beaded drainage, whose pattern is 

due to the thawing of permafrost, and swamp drainage 

in which permafrost plays no role. The following 

criteria assist in recognizing beaded drainage:

1. Beaded-drainage pools and channels are sharply defined; 

swamp-drainage courses unrelated to permafrost have indistinct, gradational borders.

2. Beaded-drainage channels generally are straight or consist of series of straight segments separated by angular bends; 

swamp-drainage courses are straight or smoothly curved.

3. Beaded-drainage courses generally are associated with 

ice-wedge polygons and locally with pingos and thaw lakes.

COLLAPSE ALONG TRAILS, NEAR INSTALLATIONS, AND IN 

CULTIVATED FIELDS

Human activities in areas of perennially frozen peat 

and silt frequently disrupt the insulating vegetation 

cover and expose the underlying dark soil to the summer sun, so that rapid thawing ensues. This results in 

thermokarst topography, local subsidence of the surface, 

tilting of small buildings, or accentuation of microrelief 

of ice-wedge polygons. Small thaw lakes and beaded 

drainage may develop along tractor trails. Care must 

be exercised, however, in distinguishing between trails 

along which thaw features have formed and similarappearing trails where water stands in original depressions crossed by the tractor. Conspicuous accentuation 

of the microrelief of ice-wedge polygons is noted in cultivated fields in the Fairbanks area (pi. 42; Pewe, 1948, 

1954). Generally, thaw features resulting from human

319004—55——3

activities can be discerned easily on aerial photographs 

of scale 1:10,000 and with difficulty on smaller scales.

HYDBOLOGIC PHENOMENA

Subterranean drainage, springs, and flood-plain 

icings offer indirect evidence of the presence of unfrozen 

zones in areas where permafrost is widespread.

Subterranean drainage—the disappearance of 

streams in the bottoms of closed depressions or by percolation into a gravel-covered surface—commonly can be 

observed directly on aerial photographs (a, pi. 31). 

The following phenomena suggest subterranean drainage through unfrozen zones:

1. Abrupt diminution of the apparent volume of streams crossing alluvial fans, gravel plains, or limestone areas as suggested 

by decrease in width or complete disappearance of streams.

2. The presence of dry depressions without surface drainage.

3. Streams draining into small lakes having no visible outlet.

4. Significant changes in the levels of small lakes when observed on aerial photographs taken at different times.

Springs rarely can be observed directly on aerial 

photographs. Differences in vegetation near springs 

commonly aid in their detection (a?, pi. 30; d, pi. 31; s, 

pi. 43 6/), but the character of these differences varies 

from one area to another, so that interpretation requires 

that the observer be familiar with the area. The following phenomena are suggestive of the presence of 

springs:

1. Streams heading in clusters of small lakes, especially at the 

toes of alluvial fans or near contacts between bedrock slopes and 

unconsolidated valley or lowland deposits.

2. Patches of luxuriant vegetation, commonly expressed on 

aerial photographs as local areas of dark tones extending downslope from the base of a conspicuous bedrock escarpment.

Flood-plain icings (pi. 43) on small streams are 

indirect evidence of the presence of perennial springs. 

Surface water freezes in the winter in the continuouspermafrost zone and in the northern part of the discontinuous zone in Alaska, and slope runoff ceases. 

Small streams whose discharge consists entirely of surface runoff generally dry up, but spring-fed streams 

continue to flow throughout the winter. The presence 

of icings on small streams in the continuous- and discontinuous-permafrost zones indicates perennial flow, 

probably due to perennial springs, and thus suggests 

the presence of water-bearing thawed zones in the drainage basin.

The sites of flood-plain icings can be recognized readily on aerial photographs of scale 1: 40,000 and larger. 

The flat, white surface of the ice deposit itself can be 

observed in early summer; dissected remnants of large 

icings persist in some areas throughout the year (pi. 

43 C) . After the ice has melted, the icing site is distinguishable as a braided segment of a stream that
142 SHOETEE CONTRIBUTIONS TO GENEEAL GEOLOGY

elsewhere is confined to a single channel. Vegetation, 

if present, consists for the most part of patches of 

grasses, sedges, and heaths; trees are lacking, but a 

few 'ragged patches of low willows may be present. 

More commonly the icing site is bare and free of vegetation. The ground surface generally is a rough, patternless jumble of channels, gravel bars, and conical 

gravel piles and ridges having a local relief of 1 to 4 

feet.

LIMITATIONS

Several basic factors limit the usefulness of aerial 

photographs in the interpretation of permafrost conditions. Many aerial photographs are unsatisfactory 

because of small scale or poor quality. Most of the 

suggested indicators are ambiguous; some are found 

in relict form today in poorly drained soils in which 

permafrost is lacking but seasonal frost persists 

throughout most of the summer. Finally, distinctive 

surface indicators are lacking in many areas in which 

permafrost is present (pi. 44).

Most of the photographs used in this report were 

selected because the permafrost indicators are depicted 

with especial clarity. Probably 90 percent of the available aerial photographs of Arctic and subarctic regions 

are less satisfactory for the photointerpretation of 

permafrost conditions because of small scale, poor 

light conditions, improper exposure, poor printing, or 

because they were taken at a season when snow or 

foliage concealed many features of the terrain.

Permafrost plays an essential role in the development 

of only a few microrelief features; most of these are 

difficult to distinguish from similar-appearing features 

in which permafrost plays no role. Most microrelief 

features proposed as permafrost indicators actually 

form in the active layer of seasonally frozen and thawed 

ground. Perennially frozen ground at shallow depth 

restricts drainage and offers favorable conditions for 

intense frost action and for the formation of these microrelief features in the overlying active layer. Almost 

equally favorable conditions are offered, however, by the 

presence of a layer of seasonally frozen ground which 

persists during most of the summer or by a substratum 

of hardpan, dense bedrock, or other impermeable material. Thus, many of the microrelief features and vegetation patterns present in permafrost areas are duplicated elsewhere by frost action independent of permafrost.

Many distinctive microrelief patterns are persistent 

and exist as relict forms long after permafrost has 

thawed. Positive distinction of relict and active features is difficult on the ground and generally impossible 

on aerial photographs (for example, see stabilized stone

polygons at &, pi. 31, and probable relict thaw lakes, pi. 

44).

The importance of permafrost in controlling surface vegetation and microrelief decreases with increasing depth to permafrost. If permafrost lies below a 

depth of several feet, its effect upon surface phenomena 

is negligible. The critical depth is variable, depending 

upon interrelated factors of surface insulation, type of 

vegetation, soil permeability and stratigraphy, and 

slope. No surface indicators of the presence or absence 

of permafrost are known in areas where bedrock is exposed at the surface or lies beneath a few feet of rubble. 

Thus, the absence of microrelief features and vegetation patterns that are elsewhere associated with permafrost does not necessarily mean the absence of permafrost in a given area.

The photographs used as illustrations in this report 

were selected after examining several thousand aerial 

photographs of regions where permafrost is known to 

be present. Most of the aerial photographs showing 

lowland and upland areas in the continuous-permafrost 

zone contained recognizable surface features that suggest the presence of permafrost, but less than half of the 

photographs depicting mountainous areas contained 

such evidence. In the discontinuous- and sporadicpermafrost zones, only a small percentage of the photographs examined contained strong evidence of the 

presence or absence of permafrost.

Analysis of these basic factors emphasizes the following general limitations in the use of indicators for the 

recognition of permafrost:

1. Most indicators merely suggest the presence of permafrost; 

the more that can be recognized in a given area, the more 

likely is the presence of permafrost.

2. Most indicators are significant only when considered in 

relation to the climate, topography, and geology of the region 

in which they occur.

3. Most indicators reflect near-surface soil conditions and 

offer little or no information concerning the thickness, extent, 

and character of permafrost below depths of a few feet.

4. Absence of indicators does not constitute evidence that 

permafrost is not present.

PHOTOINTERPRETATION KEYS

Several categories of information can be obtained 

from aerial photographs. First-order information, 

such as topography, microrelief, drainage patterns, and 

form and distribution of vegetation, can be observed or 

measured directly on the photographs. Analysis of the 

primary information yields second-order data concerning, for example, the species of plants represented and 

the probable geologic and geomorphic origin of landforms and microrelief. This in turn permits thirdorder inferences concerning probable composition and 

texture of subsurface materials and surface and sub-
PERMAFROST AND GROUND WATER IN ALASKA 143

surface drainage conditions. The interpretation of 

permafrost and ground-water conditions is still further 

removed from the original observational data and depends largely upon fourth-order inferences based on 

the conclusions concerning regional climate, the insulating properties of the vegetation, the character of the 

substratum, drainage conditions, and the geomorphic 

history.

^Reasonably accurate photointerpretation keys are 

readily constructed for obtaining second-order information from primary observational data; less accurate 

keys can be established for the derivation of third-order 

information. Keys for fourth-order interpretations— 

in determining the presence or absence of permafrost, 

for example—become so subjective and so encumbered 

with lists of modifications and exceptions that they are 

practically useless.

A complete key for the evaluation of permafrost conditions in a given region must consist of several partial 

keys leading to correct analysis of second- and thirdorder information. However, the partial keys, at best, 

lead only to the evaluation of various sites in relative 

terms of favorability or unfavorability for the formation of permafrost, rather than to evaluation in absolute terms of presence or absence of permafrost. Moreover, the answers given by the partial keys will be conflicting and will have little significance unless they are 

integrated by an interpreter having specialized experience in the study of permafrost. An individual 

trained only in the reading of aerial photographs cannot 

be expected to produce dependable fourth-order interpretations from second- and third-order conclusions 

that are speculative at best.

A comprehensive photointerpretation key for permafrost conditions in all of Alaska would consist of an encyclopedic collection of local keys. Compilation of such 

a comprehensive key is hardly justified in view of the 

low degree of reliability to be expected when the key is 

used by an interpreter lacking specialized field experience. The permafrost specialist, on the other hand, 

finds little need for such a comprehensive key, but needs 

instead & general understanding of the basic geologic, 

geomorphic, botanical, and climatic factors that determine the distribution of permafrost. He will be aided 

by partial keys for vegetation and microrelief features.

CONCLUSIONS

The discussion of permafrost indicators illustrates 

some of the problems of applying photointerpretation 

techniques to the study of permafrost. Indicators taken 

individually are ambiguous, erratically distributed, and 

generally difficult to recognize. They are significant 

only when considered as a group in the context of their

geographic and climatic setting. Photointerpretation 

of permafrost requires a synthesis of all the features of 

the landscape.

Eeliable interpretation of permafrost conditions by 

use of aerial photographs can be achieved only by individuals trained in the natural sciences and armed with 

field experience in regions where permafrost occurs. 

However, the limitations of the method are severe, even 

when employed by well-trained and experienced individuals. The detailed boundaries of permafrost at shallow depth are difficult to establish on the ground and 

impossible to delineate on aerial photographs, even by 

experienced interpreters. The presence or absence of 

relict permafrost at depths of more than 10 feet and 

the depth to which any permafrost extends cannot be 

established by use of aerial photographs. The reliability of photointerpretation in any area is greatly increased by ground studies at representative sites within 

the area.

LITERATURE CITED

Alter, A. J., 1950a, Arctic sanitary engineering: Federal Housing Adm., 104 p.

———— 1950b, Water supply in Alaska: Jour. Am. Water Works 

Assoc., p. 519-532.

Birch, Francis, 1948, The effects of Pleistocene climatic variations upon geothermal gradients: Am. Jour. Sci., v. 246, p. 

729-760.

Black, R. F., 1951, Permafrost: Smithsonian Inst. Ann. Kept., 

p. 273-302.

———— 1952, Polygonal patterns and ground conditions from 

aerial photographs: Photogrammetric Eng., v. 18, p. 123-124.

Black, R. F., and Barksdale, W. L., 1949, Oriented lakes of 

northern Alaska: Jour. Geology, v. 57, p. 105-118.

Bryan, Kirk, 1946, Cryopedology—the study of frozen ground 

and intensive frost action with suggestions on nomenclature : Am. Jour. Sci., v. 244, p. 622-642.

Cederstrom, D. J., 1952, Summary of ground-water development 

in Alaska, 1950: U. S. Geol. Survey Circ. 169, 37 p.

Cederstrom, D. J., Johnston, P. M., and Subitzky, Seymour, 

1953, Occurrence and development of ground water in permafrost regions: U. S. Geol. Survey Circ. 275, 30 p.

Frost, R. E., 1951, Interpretation of permafrost from airphotos: 

Frost action in soils: Highway Research Board, special 

rept. no. 2, Natl. Acad. of Sci., Natl. Research Council Pub. 

213, p. 223-246.

Henshaw, F. F., and Parker, G. L., 1913, Surface water supply 

on Seward Peninsula, Alaska: U. S. Geol. Survey, W. S. P. 

314, 317 p.

Hopkins, D. M., 1949, Thaw lakes and thaw sinks in the Imuruk 

Lake area, Seward Peninsula, Alaska: Jour. Geology, v. 57, 

p. 119-131.

Hopkins, D. M., and Sigafoos, R. S., 1951, Frost action and vegetation patterns! on Seward Peninsula, Alaska: U. S. Geol. 

Survey Bull. 974-C, p. 51-100.

Leffingwell, E. de K., 1919, The Canning River region: U. S. 

Geol. Survey Prof. Paper 109, 251 p.

Mertie, J. B., 1937, The Yukon-Tanana region, Alaska: U. S. 

Geol. Survey Bull. 872, 276 p.
144 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Moffit, F. EL, 1913, Geology of the Nome and Grand Central

quadrangles, Alaska: U. S. Geol. Survey Bull. 533, 140 p. 

Muller, S. W., 1947, Permafrost or permanently frozen ground

and related engineering problems: J. W. Edwards, Inc.,

231 p. 

Payne, T. G., and others, 1952, Geology of the Arctic slope of

Alaska: U. S. Geol. Survey Oil and Gas Inv. Map OM 126. 

P4w<§, T. L., 1948, Origin of the Mima Mounds: Sci. Monthly,

v. 66, p. 293-296. 

————1950, Origin of the upland silt in the Fairbanks area,

Alaska, [abs.] : Geol. Soc. America Bull., v. 61, p. 1493.

1954, Effect of permafrost on cultivated fields, Fairbanks

area, Alaska: U. S. Geol. Survey Bull. 989-F, p. 315-349. 

Pewe, T. L., and others, 1953, Multiple glaciation in Alaska—a

progress report: U. S. Geol. Survey Circ. 289,13 p. 

Porsild, A. E., 1938, Earth mounds in unglaciated Arctic northwestern America : Geog. Rev., v. 28, p. 46-58. 

Purington, C. W., 1905, Methods and costs of gravel and placer

mining in Alaska : U. S. Geol. Survey Bull. 263, 273 p. 

Ray, L. L., 1951, Permafrost: Arctic, v. 4, p. 196-203. 

Reduzobov, D. B., 1954, Thermal field laws of the permafrost in

the Vorkuta region: Snow, Ice, and Permafrost Research

Establishment, Translation 17, 20 p. 

Sager, R. C., 1951, Aerial analyses of permanently frozen

ground: Photogrammetric Eng., v. 17, no. 4, p. 551-571. 

Sharp, R. P., 1942, Soil structures in the St. Elias Range, Yukon

Territory: Jour. Geomorphology, v. 5, p. 274-301.

Smith, P. S., 1939, Areal geology of Alaska: U. S. Geol. Survey 

Prof. Paper 192, 100 p.

Stoeckeler, E. G., 1948, Identification and evaluation of Alaskan 

vegetation from airphotos with reference to soil, moisture, 

and permafrost conditions—a preliminary paper: Inv. of 

Mil. Construction in Arctic and subarctic regions, U. S. 

Corps of Engineers, St. Paul Dist., 103 p.

————1952, Trees of interior Alaska, their significance as soil 

and permafrost indicators: Inv. of Mil. Construction in 

Arctic and subarctic regions, U. S. Corps of Engineers, St. 

Paul Dist., 25 p.

Taber, Stephen, 1943, Perennially frozen ground in Alaska; its 

origin and history: Geol. Soc. America Bull., v. 54, p. 

1433-1548.

United States Weather Bureau, 1943, Climatic atlas for Alaska: 

Rept. 444 (229 p.) and supplement (72 p.) of the Weather 

Inf. Br., Headquarters, U. S. Army Air Force.

Wallace, R. E., 1948, Cave-in lakes in the Nabesna, Chisana, and 

Tanana River valleys, eastern Alaska: Jour. Geology, v. 

56, p. 171-181.

Washburn, A. L., 1950, Patterned ground: Rev. Canadienne de 

GSographie, v. 4, p. 1-55.

Wilson, W. K., Jr., 1948, The problem of permafrost: Mil. Engineer, v. 40, p. 162-164.

Woods, K. B., Hittle, J. E., and Frost, R. E., 1948, Use of aerial 

photographs in the correlation between permafrost and 

soils: Mil. Engineer, v. 40, p. 497-499.
INDEX

A Page

Acknowledgments_______._——_. 114-115 

Active layer, definition_________ ___ 114 

Aerial photographs, conclusions concerning

use.—.————.-- — . — - 142-143

effect of scale......_._ 138,139,140,141; pi. 40

limitations of interpretation_______ 142 

role in interpretation of permafrost _ ___ 134 

Alaska Peninsula, physiography_____. 131,132 

Alaska Range, physiography..._______ 126,130 

Aleutian Islands, temperatures ___ - _ __ 131 

Aleutian Range, geology__________ 131,132 

Aquifers, relation to unfrozen zones______ 118 

Arctic coastal plain, physiography______ 118 

Arctic fo9thills, geology.—....__—- -— 118,119

physiography_______..._.___. 118 

Arctic slope. See Physiography, Climate, 

Vegetation, Geology, Permafrost, 

and Ground water. 

Aufeis. See Flood-plain icings.

B

Barrow, climate________. _. _ _. _ 118

Bendeleben Mountains, geology__...__ 120

springs___________________ 121

Big Delta, ground water.——————— 128-130

permafrost___________-.---.. 127-128 

Black spruce, as a permafrost indicator_. _ 137;

pis. 32,34,39

Bog flats, as a permafrost indicator. ____. 137;

pis. 32-34 

defiiniton...___._..__......__.... 114

Bristol Bay region. See Physiography, Climate, Vegetation, Geology, Permafrost, and Ground water.

Brooks Range, geology____________ 119 

physiography____________ 119

springs.___......-_-------------..... 119

Candle, climate--——— — —— —— — — 120

Canning River, springs...——...._-____ 119

Carex Creek_________________ pi. 43

Caribou Hills, physiography _ ________ 133

vegetation-______._________ 133

Climate, Arctic slope___.__. _ ____ 118

Bristol Bay region_.__.-.-.-.----. 131-132

effect on permafrost.__________ 117

Kenai lowland_______________ 133 

middle Tanana VaUey....——— — .-.. 126

northern Seward Peninsula_..._..... 120

southern Seward Peninsula_____._ 122 

upper Kuskokwim Valley________ 130 

Yukon Flats._—...—————...... 124-125

ttongrass, definition.-—————_.____ 114

D

Definitions___________..___.. 114

Dillingham, climate_______________ 131

Drainage, beaded.. 139,141; pis. 31,35-37,39,41,44

effect on permafrost...--———————_-. 117

subterranean______________ 141; pi. 31

E

Eagle, ground water________ ______ 126 

Exposure, effect on permafrost__——_.-.-. 117

Fairbanks, climate_______________ 126 

ground water_____._.-._.____ 129

Farewell, climate_______________ 130 

ground water____________..__ 131

Flood-plain icings, definition..——__...... 114

location___....__————————.„ 119,121

recognition on aerial photographs. 141-142; pi 43

Forest, as a permafrost indicator. _______ 135 

description_____.._________ 135 

distribution__........__........— 135-136

Forest and tundra mixed, as a permafrost indicator____________________ 137; pi. 35

Fort Yukon, climate______________ 124 

ground water_____——___.„.... 125

G

Geology, Arctic slope__ __-._... _____ 119 

Bristol Bay region.. —.- —————— 132 

Kenai lowland___________ .___ 133 

middle Tanana Valley __________ 127 

northern Seward Peninsula_______ 120-121 

southern Seward Peninsula. ____ _ _ 123 

upper Kuskokwim Valley________ 130 

Yukon Flats....———.............. .„ 125-126

Page

Geothermal gradient, effect on permafrost... _. 115 

Glaciation, effect on permafrost_____ 117-118,121 

'' Glaciers". See Flood-plain icings. 

Ground ice.......__.——._... 127,132,139,140; pi. 42

Ground water, alluvial fans and aprons__ __ 121,

122,123,124,126,127,131

Arctic slope__._______________ 119

basal gravel____._ _...____ ___ 121

beach gravels-- ___.... _ _ _. ____ 122,124

bedrock__————— ———— ——— 121,122,124

beneathlakes—-------------- 119,121,124,126

Bristol Bay region....__-—_______ 132-133

contactzones____..._____ 121,122,124 

deltas__._———.—— —— — ....__ 124

effect of circulation on permafrost__ _ 117,121 

factors affecting distribution ____.-__ 118 

flood plains______________ 124; pi. 30 

fractured bedrock__._-_-—--._____ 117 

glacial deposits___-----__-___ 122,127

to active layer__......._.....____ 122

Kenai lowland__.. —_. _..._....__ 134

lavaflows.____„ —— —... 121,122; pi. 31 

Lisburne limestone.-.----_-______ 119

middle Tanana Valley. — „....———— 129-130

mountains-________.______ 121,123 

near flood-plain icings-.- — _______ 119 

northern Seward Peninsula,__........ 121-122

ocean beach deposits...___-_-____ 123-124 

Sadlerochit sandstone_..._._...__ 119 

south slopes--_—————...——— 123,127 

southern Seward Peninsula_________. 124

spring line 123 

stream gravels————— 121,122,123,126,127,131 

weathered bedrock. ____________ 121 

Yukon Flats.—-._—————___— 126 

Gubik formation.._.. —————_...... 119

H

Heat, distribution through earth materials_ 

Homer, climate________________.. 

Hot springs. See Springs. 

Hydrolaccolith. See Pingo.

117

133

Icings. See Flood-plain icings. 

Imuruk Lake, Quaternary lava flows.... 120; pi. 31 

Inmachuk River, ground water___ ____ pi. 30 

Insulating materials, effect on permafrost—— 117

K

Kenai, climate.—______________... 133 

Kenai lowland. See Physiography, Climate, 

Vegetation, Geology, Permafrost, 

and Ground water.

Kenai Mountains.________________ pi. 34 

Kigluaik Mountains, geology——————.— 123 

King Salmon, climate....——————————— 131 

Koggiung, ground water..—-——————.— 133 

Koyuk River, Quaternary lava flows....._. 120 

Kuskokwim Mountains.--——-—.-....-— pi. 32 

Kuskokwim Valley, upper. See Physiography, Climate, Vegetation, Geology, Permafrost, and Ground 

water. 

KuzitrinFlats————-- ——————— pis. 40,41

Lakes, oriented______...___„.„ 140; pi. 38

thaw, origin_____——————————— 140

recognition on aerial photographs—..- 140;

pis. 31,35,37,38,40,41,44

Lisburne limestone____—— ——————— 119 

Literature cited... —————————— 143-144

M

McGrath, climate-.-——.------------ 130

ground water______.____------ 131

Mulchatna Hills—....—————————— pi. 36 

Mulchatna River.____——„.—.—.— pl.36 

Muskeg, definition..__—————————— 114

N

Naknek, geology....__—————————_._. 132

Nenana, climate_____„——--.—__-... 126

Nixon River lowland, vegetation____ 130; pi. 32

Nome, climate.—— —————— —————— 122

ground water__________--__ 124

Noxapaga River.____-_._-.——-—.— pi.43 

Nushagak River lowland, geology..____. 132

O Page

Okpilak River, springs. ____________ 119 

Orogeny, effect on permafrost—___ 117-118,121

Patterned ground. See Polygons.

Perennially frozen ground. See Permafrost, 

definition.

Pergelisol. See Permafrost, definition.

Permafrost, Arctic slope____________ 119 

beneath lakes and ponds___ _____— 117 

beneath rivers_____________.. 119

Bristol Bay region.._____.______ 132 

definition..______.___________ 114 

depth of thawing.____.______.-. 119,122 

depth to.—— 126,131,137; pis. 37,38,40,41,43,44 

effect of past and present climates.. __ 115-117 

effect of snow._______________ 117 

effect of steep slopes.....-____-..___. 121

effect of subsurface drainage..____.... 117

effect of surface insulation— -______ 117 

in islands of Yukon River. _________ 126 

Kenai lowland——.——————-. 133-134

local distribution___________.. 117-118

lowlands.-.______.______...... 117

middle Tanana Valley-——----------- 127-129

mountainous areas.____. _______ 117 

new.._________________.__ 115 

northern Seward Peninsula....____. 121

regional zonation.__.._._______ 115,116 

relict.._.___.....———— 115,117,118,132

thickness-.——.... 119,121,123,126,127,129,131

upper Kuskokwim Valley.________ 131 

Yukon Flats...._..——„——_.... 126

zones, definitions.__..--.---_____.. 115

location, continuous-__-_____ 115,116,

118,119,121; pis. 30,31,35,37-41,43

discontinuous.---_________ 115-

117,122,124,126,130; pis. 32,33,42

no-permafrost.___ 115-116,133; pi. 34

sporadic—._—-— 115,116,131; pis. 36,44

Photointerpretation keys, beaded drainage... 141 

contractional polygons__ — _______ 139 

limitations in use of indicators______ 142 

springs..________——._.______ 141 

subterranean drainage—__________ 141 

thaw lakes.-_______________ 140 

vegetation.______---------_____ 137

Physiography, Arctic slope___________ 118 

Bristol Bay region___________ 131 

Kenai lowland-. ______________ 133 

middle Tanana Valley.-__________ 126 

northern Seward Peninsula.._______ 120 

southern Seward Peninsula.. _______ 122 

upper Kuskokwim Valley_..._____ 130 

Yukon Flats—————————— — —— 124

Pingo, definition.__._....————.——— 114

recognition on aerial photographs. __ 139-140:

pis. 37, 40, 41

Polygons, contractional_——————___ 137,139;

pis. 35, 37, 38, 39, 40, 41, 43^4

desiccation..____.——------___ 137

frost-crack..__.....—— 137, 139; pis. 39, 43^4

frost-stirred. See Polygons, stone and 

vegetation; Stripes, stone and vegetation, 

ice-wedge, high-center_.._._____ 138-139; 

pis. 31, 35, 37, 38, 40, 41, 42, 43^4 

low-center-...——. 138; pis. 35, 37, 38, 40, 41 

stone.-.---—--._..„———_ 137; pi. 31 

tensional. See Polygons, contractional. 

vegetation.._________ 137; pis. 404, 43^4

S

Sadlerochit River, springs——_______.. 119 

Sadlerochit sandstone.—————_——.—__ 119

Sagavanirktok River, springs..________ 119 

Seasonally frozen layer. See Active layer. 

Seward Peninsula. See Physiography, Climate, Vegetation, Geology, Permafrost, and 

Ground water.

Shaviovik River, springs.—————— .„„. 119 

Shishmaref, climate.___________.__ 120 

Shublik Springs————————————— 119

Springs, alluvial fans.__———„_____ 121 

Brooks Range____.„..———__- 119 

contact zones.—————————— ———— _- 124 

fault scarps.——————————. 121-122,124

Okpilak River--—--—___...._._ 119

permafrost in vicinity of—————————— 119

recognition on aerial photographs———— 141;

pis. 30, 31, 43C

Sadlerochit_———_.————__._ 119 

Sagavanirktok River.——————————— 119 

Shaviovik River.-.-------—-—— 119

145
146 INDEX

Springs—Continued Page 

Shublik.-.___________..„__„.- iiy

Stone nets. See Polygons, stone. 

Stripes, stone.___________.„___ 138; pi. 36 

vegetation-__________ 138; pis. 35, 36, 39 

Surface water, quality_____________ 114

T

Tall shrubs, as permafrost indicators.._ 137; pi. 35 

Tanana River flood plain, ground water__ 129-130 

Tanana Valley, middle. See Physiography, 

Climate, Vegetation, Geology, Permafrost, 

and Ground water.

Thaw features, gullies.-..___._ 139; pis. 31, 43

result of human activities_____ _ 141; pi. 42

Thermal gradients, effect on permafrost- ___ 118

Thermokarst topography.____.-.--..--.- 141

Tundra, as a permafrost indicator.. _____ 135

definition....——————.——————— 114

description.......——..._.............. 135

Tundra—Continued Page 

distribution..._.___...-—.-—.„ 135 

relation to depth of permafrost ----------- 135

II

Umiat, climate..——————.——-— 118

Unfrozen ground, indicators..— — —— ---— 137

Vegetation, Arctic slope..__._—-—— 118-119 

Bristol Bay region -_-_———————— 132 

Kenai lowland.--.—....-—- ——— 133 

key to depth of permafrost-..-. —— ——— 137 

middle Tanana Valley..-————— 126-127 

muskeg...————-.—--- — --— 114,133 

northern Seward Peninsula__-----—— 120 

southern Seward Peninsula-....-_ —.— 122-123 

tundra._____——.--_— — ————.. 114 

upper Kuskokwim Valley-.--,..————— 130 

Yukon Flats....—-——-.————- 125

W Page

Wales, precipitation...___________._ 120

Wells, Arctic coastal plain.__.._.......... 119

Bristol Bay region-.--.---..-----..---- 132,133

Fairbanks________—_~_----- 129

Farewell..-__.__.__.—_.— — -.. 131

Fort Yukon...———.—.———--- 125

McGrath-....——— — -.----.-.-- 131

Nome.___________--.-_.-....--- 124

Point Barrow-----—-----.------.------- 119

southern Seward Peninsula_------------ 124

upper Kuskokwim Valley.._-_._-_--. 131

Yukon Flats.___.____ ———_„—— 126

White Mountain, climate._.....—. — ---— 122

White Mountains, geology.____.... — — 125

Willows, as permafrost indicators_---------- 137

York Mountains, geology.-______——— 120 

Yukon Flats. See Physiography, Climate,

Vegetation, Geology, Permafrost, and

Ground water.

U. S. GOVERNMENT PRINTING OFFICE: 1955
PLATES 30-44
PLATE 30

[Scale at lower edge of photograph 1:20,000 (north arrow=about 1,000 feet). U. S. Army Air Force, Oct. 2,1946]

Location.—Valley of Inmachuk River, 20 miles south of Deering, Seward Pensinsula, in the continuous-permafrost zone.

Geology.—Bedrock mostly schist. Dissected basaltic lava flow (t) forms cliffs along valley walls. Gold-bearing gravel, mantled 

with silt and peat, underlies valley floor and lower valley walls and is mined in open pits (v) and by dredges (u).

Permafrost.—Permafrost underlies most of area but is lacking beneath river channel and adjoining gravel bars and in mined-out 

areas. Springs at base of basalt cliff indicate that unfrozen zones also are present beneath the lava flow. Location of other unfrozen 

zones is shown in cross section along line ZZ'.

Photointerprelation.—Stream gravel bars (w), which are unfrozen, can be recognized by light tone, arcuate bar-and-swale pattern, 

and location adjacent and parallel to stream channel. Mined-out areas, also unfrozen, can be recognized by light areas of flat or 

hummocky relief and irregular outline (u), by rectangular dredge ponds, and by deep pits in valley walls (v). Springs are indicated 

by patches of willows (dark tongues and bands extending downslope from base of escarpment at x) and by gully heading at escarpment (y).

NW. SE.

By D. M. Hopkins, 1951

Peat, silt, and muck Gravel Basaltic lava flow

Vertical and horizontal scale

Schist

100

I_

500 Feet 

I

DIAGRAMMATIC CROSS SECTION ALONG LINE ZZ'

Permafrost; letters indicate 

thawed zones

Large perennial water supply can be obtained in wells in unfrozen gravel beneath and adjoining river channel (A). Small, 

fluctuating supply can be obtained in wells penetrating unfrozen zones near abandoned underground mine (C), in local permeable 

unfrozen zones near contact between gravel and bedrock (B), and in unfrozen gravel beneath lava flow on west wall of valley (D)..
GEOLOGICAL SURVEY PROFESSIONAL PAPER 264 PLATE 30

TYPICAL UPLAND TERRAIN, NORTHERN SEWARD PENINSULA, ALASKA
GEOLOGICAL SURVEY 

•fi— .-^ ,,-_..

X *

PROFESSIONAL PAPER 264 PLATE 81

\

BASALTIC LAVA FLOWS, NORTHERN SEWARD PENINSULA, ALASKA
PLATE 31

[Scale of photograph 1:20,000 (north arrow=about 1,000 feet). U. S. Army Air Force, Oct. 2,1946]

Location.—Near Imuruk Lake, 35 miles south of Deering, Seward Peninsula, 

in the continuous-permafrost zone.

Geology.—Basaltic lava flows containing thin, interbedded gravel layers and 

overlain by 5 to 20 feet of silt everywhere except in flow-front escarpments (6).

Permafrost.—Permafrost underlies most of area, but unfrozen galleries and 

layers are present locally. See cross section along line EE''.

Photointerpretation.—Drainage courses are dark lines arranged in linear or 

dendritic patterns (c).

Extensive permafrost is indicated by high-c

beaded drainage (g), scalloped shore of Imuruk L

at h, thaw lakes (j), and basins of drained thaw la

Presence of local thawed zones is shown by spri

of tall willows, d, along flow front) and by subterr

as c' which empty into closed depressions (a).

Tiny stipples at b are stabilized stone polygons fo

:^%%^^

By D. M

Direction of groundwater movement

Perennially frozen ground

Basaltic lava flow

Vertical and horizontal scale

50 0

i i___i_______i_____

100 Feet

DIAGRAMMATIC CROSS SECTION ALONG LINE EE'

Small stream draining into depression percolates into rubble at lowest point, enters thawed permeable zone i

basaltic lava flows.
PLATE 32

[Scale at lower edge of photograph 1:20,000 (north arrow=about 1,000 feet). U. S. Army Air Force, Feb. 16,1949]

Location.—Nixon River lowland, upper Kuskokwim Valley, in the discontinuous-permafrost zone. Kuskokwim Mountains in 

the background.

Geology.—Entire lowland underlain by organic silt, sand, and peat.

Vegetation.—Open black spruce forest with mossy floor covers rolling terrain in foreground (densely stippled areas at a). Forest 

fire probably burned off spruce in lightly stippled areas at d. Bog flats, consisting of black spruce islands (ft) separated by lower sedge 

marshes (c), extend from forest in foreground to the Nixon River (dark strip extending across photograph near base of mountains).

Permafrost.—Permafrost lies at a depth of a few feet beneath black spruce forest and black spruce islands in bog flats. Permafrost is lacking or lies at depths greater than 6 feet beneath sedge marshes.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 264 PLATE 32

BOG FLATS, OBLIQUE VIEW, LPPER KUSKOKWIM VALLEY, ALASKA
GEOLOGICAL SURVEY PROFESSIONAL PAPER 264 PLATE 33

BOG FLATS, VERTICAL VIEW, UPPER KUSKOKWIM VALLEY, ALASKA
PLATE 33

[Scale 1:20,000 (north arrow=about 1,000 feet). U. S. Air Force, Feb. 16,1949]

Location.—Twenty-two miles southeast of McGrath, upper Kuskokwim Valley, in the discontinuous-permafrost zone.

Geology.—Present-day flood plain, enclosed by the Big River in the lower left corner, is underlain by sand, silt, and peat, and at 

greater depth by gravel and sand. Surface relief consists of bar-and-swale topography.

Low terrace extending from upper left to lower right corner is an older, elevated flood plain of silt, sand, and organic debris, 

mantled in places by peat several feet thick. Surface relief consists of bog flats.

High terrace extending across top of photograph is an ancient glacial-outwash plain of sand covered by a discontinuous blanket 

of peat. Surface relief consists of ancient dunes (e) surrounded by areas of muskeg (/).

Vegetation.—Slip-off slopes on inside of meanders are covered with young willows (a); slightly older parts of present flood plain 

support dense tall white spruce forest (&). Bog flats consist of high areas overgrown by black spruce and Sphagnum (c) surrounded 

by sedge marsh (d). Dunes are'covered with white spruce and birch forest (e) and intervening muskeg supports black spruce and 

Sphagnum (/).

Permafrost.—Permafrost lies at depths of 1 to 3 feet beneath muskeg on high terrace and beneath islands of black spruce in bog 

flats on low terrace. Permafrost is lacking at depths less than 6 feet but may be present at greater depth in dunes and in sedge 

marsh of bog flats. There is no permafrost in the present flood plain.
PLATE 34

[Scale at lower edge of photograph 1:20,000 (north arrow=about 1,000 feet). U. S. Army Air Force, Sept. 27,1946]

Location.—At east edge of Kenai lowland, 20 miles east of Kenai in the no-permafrost zone. Kenai Mountains in the distance.

Geology.—End-moraine complex in right middle ground; outwash plain in foreground and left middle ground. End moraine 

is underlain by till in ridges and peat in depressions. Older parts of outwash plain (c) are underlain by stratified, predominantly 

fine-grained gravel, sand, and silt, mantled by a few feet of windblown silt. Bog flats (a, b) are developing along drainage lines in 

the outwash plain at expense of older surfaces and are underlain by peat and organic silt.

Vegetation.—End moraine is covered with mixed aspen, birch, and white spruce (/) on ridges underlain by gravelly till and with 

black spruce (g) in depressions and on ridges underlain by silty till. Older surfaces on outwash plain are covered with black spruce 

(c); bog flats consist of islands of black spruce (b) surrounded by sedge marsh (a).

Permafrost.—Permafrost is present beneath some of the black spruce islands (see Photointerpretation). Permafrost is absent 

elsewhere, but seasonally frozen ground persists most of the summer beneath some other black spruce islands and beneath older 

surfaces of the outwash plain and parts of the end moraine that are covered with black spruce.

Photointerpretation.—In this area, morainal topography is distinguished from bog-flat topography by higher relief, arcuate 

pattern of ridges that extend transverse to the trunk streams, and presence of deciduous trees on many ridges and knobs, indicating 

a coarse-grained substratum. Bog-flat islands are lower than the morainal ridges, irregular in pattern, extend parallel to drainage 

lines, and are covered with black spruce.

Bog-flat islands underlain by permafrost are characterized by sharply defined scalloped edges (b), marginal moats (narrow, 

light-toned zone at right edge of island d), and by circular pockmarklike bogs on their summits. Black spruce islands that lack 

permafrost are characterized by gradational margins with adjoining sedge marsh (e).
GEOLOGICAL SURVEY PROFESSIONAL PAPER 264 PLATE 34

BOG FLATS, KENAI LOWLAND, ALASKA
GEOLOGICAL SURVEY PROFESSIONAL PAPER 264 PLATE 35

MIXED FOREST AND TUNDRA, NORTHERN SEWARD PENINSULA, ALASKA
PLATE 35

[Scale 1:40,000 (north arrow=about 1,000 feet). IT. S. Navy, Aug. 15,1950]

Location.—Pargon River, 65 miles northeast of Nome, in the continuous-permafrost zone.

Geology.—Hills in southern third of area are underlain by marble mantled with thin rocky soil. Rest of area underlain by 

glacial till mantled by a few feet of silt and peat.

Vegetation.—White spruce forest (dark-gray, finely stippled areas at a) or shrub tundra (light-gray areas of cloudlike texture 

at c) covers most of the hill slopes. Dark-gray areas on flood plain (d) are tall willows; rest of area is sedge tundra.

Permafrost.—Both forest and tundra are underlain by permafrost. Thawed zones are found only beneath tall willows on flood 

plain (d) and beneath lakes.

Photointerprelation.—Presence of permafrost at shallow depth is suggested by low-center ice-wedge polygons (f), beaded 

drainage (K), thaw lakes (gr), peat-mound stripes in shrub tundra (e), vegetation stripes in forest (f). Note characteristic scalloped 

outline, steep banks of thaw lakes, and occurrence in a larger, older lake basin. Low-center ice-wedge polygons are expressed by 

dark lines surrounding light centers on this photograph; these relationships are reversed in the low-center polygons shown on 

plate 37. High-center polygons also are present but cannot be recognized on this photograph.
PLATE 36

[Scale 1:40,000 (north arrow =about 1,000 feet). U. S. Army Air Force, June 8,1943]

Location.—Lowland between Mulchatna Hills (lower right) and 

Mulchatna River (upper left) in the sporadic-permafrost zone.

Geology.—Lowland underlain predominantly by glacial till. Kettlepocked morainal ridges appear mottled and dominantly light toned (a). 

Broad depressions, mantled by a few feet of peat, are expressed in uniform, 

slightly darker tones (b, d, /). Granite bedrock is near surface in hills at 

lower right (e).

Vegetation.—Sparse subalpine tundra and a few scattered spruce trees 

cover light ridges (a). Sedge and heath tundra grows in the broad depressions (b, d, /). Dark, stippled areas (c) are spruce forest growing along 

major drainage lines.

Permafrost.—Permafrost is present at shallow 

and heath tundra growing in peat soil (b, d, /). 

light morainal ridges (a), in dense spruce forest (

Photointerpretation.—Vegetation stripes, exp

perpendicular to small stream at d, are good in

Bristol Bay region but are difficult to distingu

which here are found in unfrozen ground.

Small pools in drainage lines on each side of 

caused by thawing of permafrost or may be caus

by frost-heaved mounds; thus they are not 

permafrost in this area.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 264 PLATE 36

STONE STRIPES AND VEGETATION STRIPES, BRISTOL BAY REGION, ALASKA
GEOLOGICAL SURVEY PROFESSIONAL PAPER 264 PLATE 37

ICE-WEDGE POLYGONS AND THAW LAKES, NORTHERN SEWARD PENINSULA, ALASKA
PLATE 37

[Scale 1:40,000 (north arrow = about 1,000 feet). U. S. Navy, July 27, 1949]

Location.—Coastal plain near Shishmaref Inlet in the continuous-permafrost zone.

Geology.—Entire area underlain by silt and peat.

Permafrost.—Perennially frozen ground present at depth of \/z l to 3 feet everywhere except beneath rivers and lakes.

Photointerpretation.—Indicators of permafrost can be recognized over most of photograph. Thaw lakes (d) and basins of 

drained thaw lakes (dd) cover about three quarters of photograph. Following features suggest active thawing and caving at lake 

shores: scalloped margins of lakes, steep banks and thaw gullies (a), lack of beaches, and lack of aquatic vegetation in shallow water 

near shores.

Other evidence of permafrost includes beaded drainage (6), low-center ice-wedge polygons (light lines surrounding dark centers 

at e) in most poorly drained areas, and high-center ice-wedge polygons (dark lines surrounding light centers at /) in slightly better 

drained sites. Note moundlike form of deeply thawed polygons on hilltop at c and small ponds at junctions of ice wedges in polygons 

at g.

Mound at h may be an erosional remnant of dissected higher surface near a or it may be a pingo. Compare with known pingos 

in plates 40 and 41.
PLATE 38

[Scale 1:20,000 (north arrow = about 1,000 feet;. U. S. Navy, Aug. 1948]

Location.—Arctic coastal plain near Point Barrow, in the continuous-permafrost zone.

Geology.—Entire area underlain by stratified silt and sand.

Permafrost.—Permafrost present at shallow depth throughout area, including beneath, lake.

Photointerpretation.—Large thaw lake is one of the well-known oriented lakes of the Arctic coastal plain. It occupies a larger, 

older lake basin and is adjoined by several other basins of former lakes.

Low-center ice-wedge polygons (a) have dark centers and light marginal ridges in this photograph and are confined to lake 

basins. Trenches separating marginal ridges of adjoining polygons are expressed by thin dark lines. Note the saw-toothed shoreline 

at 6, where some ice wedges, thawing less rapidly than the frozen centers of the polygons, extend into lake as a series of minute 

peninsulas.

High-center polygons (c) are expressed by light centers and dark marginal trenches and are confined mostly to high, relatively 

well drained areas between lake basins. Black spots near c. are small ponds at intersections of ice wedges. Polygons are obscured 

by rapid solifluction in striped slopes between c and the lake.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 264 PLATE 38

^ A •-• :-A

a ..- '.--•:•. • -• - E -«. ?n. v

Si 

•^ . t- -"'. "- 'j 

' 

^' • - ""• 

- 

" * 

.A^-1

'? * •"*• ^itv * "* ' * " '

fc*^i *-'".-'' * "\. • " •• '.•»?"" »" • ^ -Jk!*-

i- 

-. 

- ,.^- 

' -. - 

.iV.TV 

.*&~lWU«

ICE-WEDGE POLYGONS, ARCTIC SLOPE, ALASKA
GEOLOGICAL SURVEY PROFESSIONAL PAPER 264 PLATE 39

FROST-CRACK POLYGONS, BRISTOL BAY REGION, ALASKA
PLATE 39

[Scale 1:20,000 (north arrow=about 1,000 feet). TJ. S. Army Air Force, Sept. 26, 1947]

Location.—Lowland near King Salmon Airfield, Bristol Bay region, in the 

continuous-permafrost zone.

Geology.—Most of area underlain by sand covered by a few inches of turf. 

Area of lakes (d, /) in lower right corner underlain by silt mantled with peat 

several feet thick.

Vegetation.—Most of sand plain and river flood plain covered with open, 

parklike black spruce forest (area of widely spaced dark stipples near a) or with 

white spruce and white birch forest (areas stippled with very light gray flecks 

at b). Area of peaty soils is covered with sedge and heath tundra (uniform 

gray areas at d, f).

Permafrost.—Permafrost present locally in tundra areas at d, f; absent elsewhere.

Photointerpretation.—Well-developed frost-cr

sand plain. Note that they are less well defi

wedge polygons shown in plate 38. Frost-cra

region are considered good evidence of unfrozen 

(very light-gray patches near c) also indicate s

Well-developed vegetation stripes (d) are c

Bristol Bay region of peaty or silty soils and f

ment of permafrost. Small ponds along stream 

or vegetation; they are not beaded drainage 

permafrost. Large black area at upper left is 
PLATE 40

[A, scale 1:8,400 (north arrow=about 1,000 feet). U. S. Army Air Force, Oct. 1,1946. B, scale 1:20,000 (north arrow=about 1,000 feet). U. S. Army Air Force, Oct. 1, 1946

C, scale 1:40,000 (north arrow=about 1,000 feet). U. S. Navy, June 20, 1951]

Location.—Kuzitrin Flats, 55 miles east of Teller, Seward Peninsula, in the continuous-permafrost zone. A, northern part of 

flats; B, southwestern part of flats; C, same locality as B.

Geology.—Entire area underlain by silt and peat as much as 20 feet thick.

Permafrost.—Permafrost present 1% to 3 feet beneath surface everywhere except beneath lakes and streams.

Photointerpretation.—Circular pingos (f) can be distinguished in drained lake basins in all three figures. B and C also show a 

linear, eskerlike pingo (v) with a longitudinal crevice at its summit.

Ice-wedge polygons can be distinguished easily at large scales of A and B but are discernible only with difficulty at small scale 

of C. High-center polygons (w) have light centers and dark marginal trenches; low-center polygons (x) have dark centers and light 

marginal ridges. Narrow trenches separating marginal ridges of adjoining low-center polygons appear as thin or beaded dark lines 

in A but cannot be seen on small scale of B and C.

Small frost-stirred vegetation polygons are expressed as tiny, light stipples on a slightly darker background and are superimposed upon the much larger ice-wedge polygons at s and w in A. These tiny polygons cannot be seen at the smaller scales of 

B and C.

Thaw lakes having characteristic rounded or scalloped outlines appear in all three figures. The detailed irregularities of the 

lake shores at y in A represent ice-wedges projecting into the water along banks actively retreating into an area of low-center polygons. These minor irregularities are too small to be recognized on B and C.

Dark line near to and parallel to lake shore at z on A and C marks the edges of large blocks of frozen ground that have been 

undercut at the water's edge and that have begun to slump into the lake.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 264 PLATE 40

••^r£- ', •. V' 

.""•*••*•,; * k-j

.vtt'^ 

;-&4eU

J^fT-iA ..AVr* ••••--•••."** . "«(fv-v ^ xi^ -. *'. 'v - „•» '..<i..Vi* •"

^^r^f - .* • ** L

STEREOPAIRS SHOWING EFFECT OF SCALE ON APPEARANCE OF PERMAFROST INDICATORS, NORTHERN

SEWARD PENINSULA, ALASKA
GEOLOGICAL SURVEY PROFESSIONAL PAPER 264 PLATE 41

PINGOS AND MODIFIED OXBOW LAKES, NORTHERN SEWARD PENINSULA, ALASKA
PLATE 41

[Scale 1:40,000 (north arrow=about 1,000 feet). TJ. S. Navy, June 20,1950]

Location.—Kuzitrin Flats, 55 miles east of Teller, northern Seward Peninsula, in the continuous-permafrost zone.

Geology.—Northern three-quarters of area underlain by 20 to 25 feet of peat and silt. Southern quarter underlain by fluvial 

and glaciofluvial sand and gravel mantled by only a few feet of peat and silt.

Permafrost.—Permafrost lies 1^ to 5 feet beneath surface everywhere except beneath rivers and lakes.

Photointerpretation.—Widespread shallow permafrost is indicated by thaw lakes, drained thaw-lake basins, modification of 

oxbow lakes by thawing, pingos (p), beaded drainage (6), and high- and low-center ice-wedge polygons.

All of the lakes away from the flood plain are thaw lakes having characteristic rounded or scalloped outlines. Abandoned 

meanders (oxbow lakes) on the flood plain also are undergoing modification by thawing and caving of banks, ti is very recently 

abandoned and has not yet been modified, h is older and its shores are scalloped in a few places, and ts is completely modified in shape 

and can be recognized as an oxbow only by its general arcuate shape and its position on the flood plain.

Pingos are found in almost every lake basin. Only a few are indicated by p. Note craterlike depression at summits of some 

pingos.

Low-center ice-wedge polygons are confined to lake basins and areas of bar-and-swale topography on the flood plain. They 

are expressed on these photographs as dark polygonal flecks on a honeycombed light-gray background. High-center polygons are 

outlined by discontinuous dark lines near a that represent pools of water in the polygon trenches. High-center polygons are present in other areas but cannot be seen on a scale this small.
PLATE 42

[Scale 1:12,000 (north arrow=about 1,000 feet). U. S. Army Air Force, Mar. 24,1948]

Location.—U. S. Department of Agriculture Experimental Farm near Fairbanks, in the discontinuous-permafrost zone.

Geology.—Entire area underlain by muck and silt.

Permafrost.—Entire area shown here underlain by permafrost containing ice-wedge polygons. Adjoining southward-facing 

slopes, not shown in this picture, underlain by unfrozen ground.

Photointerpretation.—Ice-wedge polygons cannot be recognized where the forest is undisturbed, but wherever vegetation has 

been cleared, ground has thawed and ice wedges have melted down, leaving shallow trenches (a), deep, sharp gullies (6), or closely 

spaced mounds up to 8 feet high (c) depending upon depth and distribution of ground ice. Mounds and trenches begin to develop 

3 to 10 years after clearing.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 264 PLATE 42

ACCENTUATION OF MICRORELIEF DUE TO THAWING IN CLEARED FIELDS, MIDDLE TAN ANA

VALLEY, ALASKA
GEOLOGICAL SURVEY PROFESSIONAL PAPER 264 PLATE 43

FLOOD-PLAIN ICINGS, NORTHERN SEXVARD PE]\INSULA, ALASKA
PLATE 43

[A, scale 1:8,400 (north arrow=about 1,000 feet). B, scale 1:20,000 (north arrow=about 1,000 feet). C, scale 1:20,000 (north arrow=about 1,000 feet). Photographs by TJ. S.

Army Air Force, Oct. 1, 1946]

Location.—Noxapaga River, 15 miles west of Imuruk Lake, northern Seward Peninsula, in the continuous-permafrost zone. 

A, vertical stereopair of confluence of Noxapaga River and Carex Creek; B, vertical photograph of same area; C, oblique photograph 

looking east toward head of Noxapaga River from above area depicted in A and B.

Geology.—Valley bottoms underlain by silt, sand, and gravel up to 10 feet thick. Slopes underlain by basaltic lava flows covered 

with silt as much as 20 feet thick.

Permafrost.—Permafrost underlies slopes \Yz to 3 feet beneath surface. Flood plains of streams probably underlain by permafrost at depth of less than 10 feet.

Photointerpretation.—Noxapaga River heads in hot springs (s on (7) issuing near base of volcanic cone. Site of springs is marked 

by dense growth of tall willows expressed as dark patch at s.

Continuous flow of river, maintained through winter by springs, results in development of thick perennial icing near head of 

river (white band along course of river on C). Note sinuous dark line marking channel that river has carved through ice that formed 

during previous winter.

Site of thinner icing that melts each summer is shown on A and B. Narrow bands of white along margins of stream channel 

are new ice that had just begun to accumulate when photograph was taken. The wide, flat, steep-walled, and sparsely vegetated 

flood plain and the irregular, locally braided channel are typical of sites of flood-plain icings and contrast sharply with narrow, V-- 

shaped valley of tributary at lower right on which icings do not form.

Frost-stirred vegetation polygons are expressed by tiny light-gray stipples in a slightly darker background on slopes at upper 

left in A but are too small to be seen on B. . Frost-stirred vegetation polygons are superimposed upon much larger high-center icewedge polygons at w (A). Thaw gullies are developed along ice wedges at x (A). Nearby, deeply thawed and eroded ice-wedge 

polygons at y have developed a moundlike form.

Frost-crack polygons (z on A) cover much of icing site. Note that they are similar in size but less continuous and sharply defined 

than nearby high-center ice-wedge polygons at w.
PLATE 44

[Scale 1:40,000 (north arrow=about 1,000 feet). U. S. Army Air Force, Sept. 15, 1942]

Location.—Lowland 45 miles east of Dillingham, Bristol Bay region, in the 

sporadic-permafrost zone.

Geology.—Entire area underlain by ground moraine, blanketed in most areas 

by several feet of peat. Till and sand exposed locally in small hillocks expressed 

as light-gray patches (a).

Vegetation.—Medium-gray areas (6) covering most of photograph are sedge 

and heath tundra. Floating-bog vegetation grows in mottled dark-gray areas 

adjoining lake shores (c), in drained lake basins (d), and along drainage lines (e).

Permafrost.—Thin permafrost present locally at depths of 2 to 3 feet in areas

of peat soils covered with sedge and heath tundra (

depths greater than 8 feet elsewhere.

Photointerpretation.—No dependable indicators 

nized on this photograph. Lakes resemble thaw la

in the past by thaw-collapse, but they are not acti

present. Pools along stream at / suggest beaded 

are due to disruption of drainage by growth of pea

formation of frost-heaved mounds along water

conditions in this area can be evaluated only by f
GEOLOGICAL SURVEY PROFESSIONAL PAPER 264 PLATE 44

PERMAFROST THAT CANNOT BE IDENTIFIED BY PHOTOINTERPRETATION, BRISTOL BAY REGION, ALASKA