Frost Action and 

Vegetation Patterns 

on Seward Peninsula 

Alaska

By D. M. HOPKINS and R. S. SIGAFOOS

CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

GEOLOGICAL SURVEY BULLETIN 974-C

A Study of the Geomorphic Significance 

of Vegetation Patterns as Related to 

Frost Action at High Latitudes and in 

Areas of Perennially Frozen Ground

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1951
UNITED STATES DEPARTMENT OF THE INTERIOR 

Oscar L. Chapman, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

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

Washington 25, D. C. - Price 40 cents (paper cover)
CONTENTS

Page

Abstract__ ._-_____---___--_-__---__-_--_--___--_-_-__--__-__-_-_ 51

Introduction. _____________________________________________________ 52

Definitions. _ _ _ _______________________________________________ 53>

Acknowledgments.____________________________________________ 54

Geologic and geographic environment______---_--_-__-_-_-_-___---___ 55-

Location _____________________________________________________ 55

Climate._______________________-_-------__---_--_-_--__-----. 55'

Topography and drainage._____________________________________ 58

Geology.__l_________-__________--._-_______-_-___---^_------- 59

Soils and soil-making processes-_________________________________ 59

Perennially frozen ground and frost action._______________________ 60'

Congeliturbation and instability of the soil surface.________________ 63-

Vegetation ____________________________________________________ 63'

Cryopedologic features.______-___-_______-__-____-_____---___--__-_ 66-

Frost scars._________________________________________________ 66-

Tussocks ____-_-_-____-_____--____-__-________-___--__-_--____ 70'

 ' Peat rings.__-____-___-__-__---____--_-_-___-_-___-----__--_-. 76-

Tussock rings and tussock groups______-_____-_______---_-_--____ 84

Tussock-birch-heath polygons.________-__-____-_____--__-_-_-_-_ 87

Age of the features.___________________________________________ 92

Conclusions. _ ______________________________________________________ 96

Bibliography.__________________-_.______-______--_-_____-___---_-_ 99

Index.__________________________-_--_--------._--_--_---------.-. 101

ILLUSTRATIONS

Page 

PLATE 2. Peat rings on valley slopes.____________--_______-____--._ 78

3. Characteristic terrain of Imuruk Lake area_________________ 86

FIGURE 17. Scope and interrelationships of certain cryopedologic processes 54

18. Index map of Seward Peninsula, Alaska.__________________ 56'

19. Climatic data for Candle, Alaska.________________________ 57

20. Vertical section through seedling tussock of cottongrass____ 71

21. Vertical section through young cottongrass tussock..._______ 72

22. Culm bases of Eriophorum vaginatum subsp. spissum and

stolons of E. russeolum var. leucothrix. __.___--._._______ 73;

23. Relationship shown in 12 cottongrass tussocks between height 

and diameter of tussocks and height of mineral soil mound at 

base__________________________________.___--_______ 74

24. Group of cottongrass tussocks.___._______-___---_____-___ 75-

- 25. Plan and cross section of peat ring________________________ 77

26. Large and small peat rings.______________________________ 78

27. Plan and cross section of elongate peat ring._______________ 801

28. Diagrammatic sketches showing evolution of a peat ring. _____ 83

29. Small tussock ring._-__________---_____-____-__-___----_ 84

30. Cross section through a tussock-birch-heath polygon._______ 89

31. Diagrammatic sketches illustrating the effect of change in climate upon the distribution of cryopedologic features.--..- 95

m

FROST ACTION AND VEGETATION PATTERNS ON 

SEWARD PENINSULA, ALASKA

By D. M. HOPKINS and R. S. SIGAFOOS

ABSTRACT

Cottongrass tussocks, frost scars, peat rings, tussock rings and groups, and 

tussock-birch-heath polygons in the Iniuruk Lake area, Seward Peninsula, Alaska, 

are products of the interaction of congeliturbation (frost churning) and vegetation development. These cryopedologic features are found on poorly drained 

areas on summits, slopes of less than 10°, and lowlands where silty mineral soil 

is present beneath a cover of peat or turf less than 3 feet thick. Perennially 

frozen ground is present at depths of 3 feet or less.

The tussock is a ball-like tufted plant form characteristic of certain grasses 

and sedges found in swampy areas. Differential frost heaving increases the 

height of tussocks of cottongrass (Eriopliorwn vayinatum subsp. spissum) by 

lifting the plant above the soil surface during the autumn freezing cycle. Frost 

scars are areas of bare soil resulting from disruption of the vegetative cover by 

local intense frost heaving.

Peat rings, tussock rings, and tussock groups consist of vegetation patterns 

associated with widely spaced mounds of mineral soil projecting through otherwise continuous layers of peat or turf. Tussock-birch-heath polygons consist of 

closely spaced mounds of mineral soil separated by channels filled with pent. 

The peat rings, tussock rings, tussock groups, and vegetation polygons represent 

stages in several developmental series, all of which start with frost scars as the 

initial form. The final features of these developmental series are the result of 

congeliturbation, a process which is effective chiefly during the fall freezing cycle 

and which is modified by the development of vegetation.

All of the cryopedologic features probably have been present in the area during 

and since the last glaciation on Seward Peninsula. Changes in climate probably 

have been reflected by changes in the local distribution of the features.

The concept of a "climax" vegetation must be modified when applied to tundra 

vegetation. Disturbance of the substratum recurs repeatedly, and all stages 

in the plant succession exist on unstable surfaces. The vegetation in areas of 

frost scars, peat rings, tussock rings and groups, and vegetation polygons represents an equilibrium assemblage adjusted to the climate in which it exists, but 

differs from a "climax" assemblage in that bare areas and areas covered by 

pioneer plants are intimately mixed among areas covered by assemblages representing the highest stage in the succession.

Fossil tussock-birch-heath polygons may eventually be recognized in northern 

United States and Europe. Their presence would indicate the probable former 

presence of perennially frozen ground, and the former depth of summer thaw 

could be inferred from the vertical dimensions of the polygons.

Recognition on aerial photographs of the cryopedologic features described above 

will assist in the interpretation of terrain conditions.

51
52 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

INTRODUCTION

Intensive frost action in areas of perennially frozen ground produces microrelief features in soil, some of which have been grouped 

under the term "structure soil." The interrelationship between developing vegetation and intensive frost action produces patterns in the 

distribution of plants comparable to some of those formed in soil. 

Detailed studies of these features and patterns are essential to an 

understanding of the nature and intensity of geomorphic and biological processes that shape the landscape of tundra regions. Many of 

the small-scale features are described and illustrated by Griggs (1934), 

Polunin (1934, 1935), Porsild (1938), Kaup (1941, 1947), and Sharp 

(1942). Troll (1944) summarizes most of the previous literature.

During the summers of 194T, 1948, and 1949 the writers conducted 

areal geomorphic and botanical studies on Seward Peninsula, Alaska. 

The studies were part of the permafrost program of the United States 

Geological Survey and were intended to lead to a better understanding of perennially frozen ground and of the geomorphic processes 

characteristic of high latitudes. The features described in this paper 

were studied in detail in the Imuruk Lake area (fig. 18) but were 

observed in many other places on Seward Peninsula.

Seward Peninsula is subject to a rigorous, subarctic climate, in 

which geomorphic processes associated with frost action and perennially frozen ground are dominant and geomorphic processes characteristic of more temperate regions are inconspicuous and of minor 

importance. Congelifraction and congeliturbation (Bryan, 1946, pp. 

626, 633) are by far the dominant processes in the shaping and reduction of slopes in the Imuruk Lake area. Frost churning prevents the 

development of a "normal" soil profile. The instability of the soil and 

the presence of frozen ground throughout the summer exclude many 

plant species from the region, affect the form of those that are present, 

and have far-reaching effects upon the development of plant communities (Griggs, 1934; Polunin, 1934, 1935; Porsild, 1938; Kaup, 1941, 

1947 ;Sigaf oos, 1949). 

The present paper discusses cottongrass tussocks, frost scars, peat 

rings, tussock rings, and tussock-birch-heath polygons. All of these 

features result from the interaction of congeliturbation and the growth 

and development of vegetation. Frost scars ("spot medallions" of 

Sochava, 1944; "mud circles" of Washburn, 1947, p. 99) and tussocks 

("niggerheads" of Alaskan parlance) are well known to travelers on 

the tundra and are not limited to arctic and subarctic regions. Frost 

scars and tussocks have been observed by the writers at low altitudes 

in northern New England. Previous descriptions of peat rings, tussock rings, and tussock-birch-heath polygons have not come to the
FROST ACTION ON SEWARD PENINSULA 53

attention of the writers. It is believed, however, that variations of 

these features are widespread in tundra regions, and it is suggested 

that "fossil" examples eventually may be recognized in temperate 

regions adjacent to the southern margins of the ice sheets of Pleistocene age.

DEFINITIONS

Several of the botanical terms used in this paper may be unfamiliar 

to some of our readers, and others have not been clearly defined in 

previous papers. The writers' usage of these terms is given below.

"Heath," in the strict sense, refers to members of the family Ericaceae, but the writers use the term to mean an assemblage of plants 

that includes Ledum palustre subsp. decumbens, Vaccinium uliginosum, V. vitis-idaea subsp. minus, and Empetrwn, nigrum.

"Peat" is a deposit of partly decomposed plant parts.

"Sod" is a tight, interwoven mat of rhizomes (horizontal underground stems) or stolons (horizontal above-ground stems) from which 

jointed aerial stems (culms) of grasses or grasslike plants arise. The 

mat forms a nearly uniform vegetation blanket.

"Turf" includes low vegetation other than sod of both open and 

closed stands of plants.

Processes grouped under the general term "frost action" are dominant factors in the development of soil and vegetation and in the shaping of the landscape in the rigorous climate that prevails on Seward 

Peninsula. Bryan (1946) attempted to clarify the terminology 

of processes associated with intensive frost action. Because the 

processes are discussed at length in this paper, it seems desirable to 

define more clearly their scope and interrelationships according to 

the usage of the writers (fig. 17).

"Cryopedologic processes" include all processes involving intensive 

frost action or perennially frozen ground. "Pergelation" and 

"depergelation" (the formation and decay of perennially frozen 

ground, Bryan, 1949, p. 101) and the cracking of the surface of the 

frozen ground owing to sudden large temperature fluctuations below 

freezing are examples of cryopedologic processes. Microrelief features that result from cryopedologic processes, including "structure 

soils," can be termed "cryopedologic surface features."

"Intensive frost action" is a general term designating all geomorphic 

processes dependent upon repeated cycles of freezing and thawing. 

Two broad categories of processes, "congelifraction" and "congeliturbation," are recognized under intensive frost action. Congelifraction can be defined as frost splitting (Bryan, 1946, pp. 626-627). Congeliturbation, as used by the writers, includes all processes by which 

repeated cycles of freezing and thawing result in churning or stirring
CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

CRYOPEDOLOGIC PROCESSES

|PERGELATION|

INTENSIVE FROST ACTION

CONGELI FRACTION CONGELITURBATION

FROST HEAVING | | FROST THRUSTING

[CREEP! [viscous FLOW!

FIGURE 17. Scope and interrelationships of certain cryopedologic processes.

of the soil, with or without net clown-slope movement. The soil is 

churned in place on horizontal surfaces by small differential movements resulting from frost heaving (vertical expansion), frost thrusting (horizontal expansion) (Eakin, 1916, p. 76), and subsidence during thawing. The soil is moved down slope by creep resulting from 

these processes and by viscous flow if the moisture content is sufficiently 

high. Additional categories of "intensive frost action" could be suggested, but they lie beyond the scope of this discussion.

ACKNOWLEDGMENTS

The study was made in connection with the program of the Alaska 

Terrain and Permafrost Section of the United States Geological 

Survey and was financed in part by the Engineer Intelligence Division, 

Office of the Chief of Engineers, United States Army.

Special appreciation is expressed to A. T. Fernald, J. F. Seitz, 

W. B. Quay, Mary D. Sigafoos, and Joan P. Hopkins for their
FROST ACTION ON SEWARD PENINSULA 55 

f

assistance in conducting the field work and for their interest and con-' 

tributions offered during the formulation of the ideas. Grateful 

acknowledgment is made to W. S. Benninghoff, R. F. Black, Kirk 

Bryan, L. L. Ray, and H. M. Raup, and to the writers' associates in the 

Alaska Terrain and Permafrost Section for their suggestions and 

' criticisms.

GEOLOGIC AND GEOGRAPHIC ENVIRONMENT

T

LOCATION

., Seward Peninsula is in northwestern Alaska between the Bering 

f Sea and the Arctic Ocean (fig. 18). The features discussed in this

paper were studied along a transect extending across the peninsula

from Nome on the southwest to Deering and Candle on the northeast. 

1 Most of the work was concentrated near the center of the peninsula

in the Imuruk Lake area. Imuruk Lake lies at latitude 65°35' N. 

i and longitude 163°10' W., 100 miles northeast of Nome and 60 miles

south of the Arctic Circle.

CLIMATE

Year-round weather observations are lacking in the Imuruk Lake 

area. A weather station was maintained by the llth Weather Squadron at Lava Lake, near the western margin of the area, from April 

through November 1945, and fragmentary weather records were collected by the Geological Survey during the summers of 1947 and 

1948. The following remarks are based upon these records and upon 

Weather Bureau data for Candle, Shishmaref, and Council, Alaska 

(U. S. Weather Bureau, 1943).

The climate is rigorous and continental, characterized by cool summers and very cold winters (fig. 19). The mean annual temperature 

is probably about 20° F. The absolute maximum temperature recorded 

at Candle prior to 1943 is 85°; the absolute minimum is 60°. Subfreezing temperatures predominate from early October to mid-May, 

although occasional thaws probably can be expected during most of 

the winter months. Nocturnal frosts are common during all of the 

summer months. Mean diurnal temperature fluctuations range in 

amplitude from 13° during October to 23° during June. The mean 

annual precipitation is probably between 7 and 8 inches, of which 

about 25 percent falls as snow. More than 50 percent of the annual 

precipitation occurs during a well-defined rainy season extending 

from July through September.

Winters are cold and rather dry. Clear weather predominates, but 

intense storms, accompanied by high winds and precipitation, are 

frequent. The sun rises above the horizon only a short distance and

904812 51 2
168" 167° 166° 165° 164" 

FIGURE 18. Index map of Sewarcl Peninsula, Alaska, showing location of Imur
INCHES OF PRECIPITATION 

= p 

o b o' 

INCHES OF SNOW
58 -'CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

for only a few hours each day during early winter; consequently, 

diurnal fluctuations in temperature due to solar warming are small. 

Longer days and a higher sun during late winter are reflected by 

diurnal fluctuations of 15° to 30°, but the maximum daily temperatures 

remain well below freezing. Diurnal fluctuations are marked throughout the winter, however, by sharp fluctuations accompanying the 

passage of cyclonic storms. Winter thaws generally are confined to 

stormy periods. Storms are less frequent during April, May, and 

June, and diurnal temperature fluctuations due to solar warming 

become conspicuous. Thawing temperatures are common at noon 

during late April and early May, and by late May temperatures 

remain above freezing throughout most of the day.

Summers are cool and moist. Low ceilings and short nights reduce 

the amplitude of diurnal temperature fluctuations, especially during 

July and August. Seventy percent of the annual precipitation is concentrated in the period from May through September, and much of 

it falls as drizzle or as light showers. Few days pass without at least 

a trace of precipitation. Moderate temperatures and prevailing high 

humidities inhibit surface evaporation; consequently, the summer 

climate is wet despite the low total rainfall.

Killing frosts occur in middle or late August, and by early September frosts occur nearly every night. After early October freezing temperatures predominate. Even during September and October the 

duration and intensity of subfreezing temperatures is related more 

closely to cyclonic conditions than to diurnal-nocturnal fluctuations.

TOPOGRAPHY AND DRAINAGE

The Imuruk Lake area is a broadly rolling upland ranging from 

500 to 1,000 feet in altitude. The most conspicuous relief features 

are isolated hills, 300 to 800 feet high, characterized by broad, domeshaped summits and smooth, gentle slopes rarely exceeding 10°. The 

hills are separated by undulating lowlands with a relief of 100 to 300 

feet; distances from divides to the axes of adjacent stream valleys 

commonly are 1 to 3 miles.

Drainage is poor throughout most of the region. An integrated 

drainage pattern is lacking over large areas, and lakes, ponds, and 

swamps are abundant. Luxuriant vegetation, low evaporation, and 

the presence of perennially frozen ground at shallow depths inhibit 

surface runoff and subsurface drainage and maintain moist conditions 

in silty soils on even relatively steep slopes.

Most of the lowland areas are drained by small, sluggish, widely 

spaced watercourses. Drainage lines are completely lacking on some 

slopes, and extremely wet conditions prevail over the entire surface.
FROST ACTION ON SEWARD PENINSULA 59

More commonly, however, the slopes are drained by sets of long, 

shallow, subparallel swales separated by broadly convex interfluves 

on which swampy ground is lacking. Commonly, 3 to 20 swales join 

abruptly near the foot of the slope in a ragged candelabralike pattern. 

Some swales head in swampy oval indentations on the upper slopes; 

others merge with the swampy ground at the hill summits. Definite 

channels containing running water generally are lacking in the swales j 

instead, the floors are swampy and choked with vegetation.

GEOLOGY

Most of the Imuruk Lake area is underlain by basalt and andesite 

lava flows of Quaternary age. Pre-Quaternary bedrock consists of 

granite, marble, and schist (Moffit, 1905). Terraces composed of 

unconsolidated sediments are present along the shores of Imuruk Lake 

and in the valleys of some of the larger streams.

No part of the Imuruk Lake area has been glaciated, but small 

glaciers were present to the south in the Bendeleben Mountains. At 

least two distinct glacial advances are recorded.

SOILS AND SOIL-MAKING PROCESSES

The existing soils of the Imuruk Lake area originated in a climate 

similar to or more rigorous than the present one. Congelifraction 

(frost riving, Bryan, 1946, pp. 626-627) is the dominant process of 

rock weathering in such a climate and ultimately produces, under ideal 

conditions, a well-sorted silty soil containing few large rocks and very 

few particles in the clay-size range (Taber, 1943, pp. 1449-1450; Troll, 

1944, pp. 573-575; Hopkins, 1949, p. 121). The degree to which this 

end is approached depends upon the age of the soil, the topography and 

drainage of the site, and the texture and mineral composition of the 

parent rock.

Bare bedrock or coarse rubble of pre-Quaternary age is exposed 

throughout the area of the lava flows of Quaternary age and on the 

upper slopes of hills underlain by marble or granite. The lower slopes 

of granite hills consist generally of sandy soil with abundant rocks. 

Areas underlain by schist or by older Quaternary flows and the lower 

slopes of hills underlain by marble generally bear a mantle of uniform 

silty soil 5 to 20 feet thick. Particles between 0.01 and 0.1 millimeter 

in size make up 70 to 95 percent of these soils.

A mantle of peat of variable thickness is present on almost all surfaces where bedrock is covered by silty soil. Luxuriant vegetation, 

poor drainage, and low evaporation favor the accumulation of peat, 

not only on lowlands but also on hilltops and on slopes of less than 

10°. The peat mantle is 5 to 20 feet thick in swampy lowlands and
60 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

swales and 1.5 to 3.0 feet thick 011 wet summits and wet slopes. Even 

in the "drier" sites a discontinuous layer of peat or turf as much as 1.5 

feet thick mantles the silty mineral soil.

Peat in the Imuruk Lake area is coarsely fibrous and little decomposed. It consists of recognizable fragments of willows, sedges, 

and mosses in a sooty matrix of black finely comminuted organic 

matter. Living roots and stems are an important constituent of the 

peat within 6 inches of the surface. In some of the wetter sites the 

black peat is overlain by a reddish peatlike mat of living mosses 0.5 

to 1.5 feet thick.

Bock and sandy soils generally are dry and well-drained on horizontal surfaces as well as on slopes. Subsurface drainage is good, because perennially frozen ground lies at depths exceeding 3 feet in these 

materials.

The moisture content of silty soils varies, but even the "driest" soils 

are extremely wet compared with soils having a similar texture in 

temperate regions. Surface runoff is impeded by the vegetation; surface drainage is retarded by the impermeability of the soils and by the 

presence of perennially frozen ground at shallow depths. Low evaporation and the melting of frozen ground helps to maintain the high 

moisture content. The wettest soils will not stand in the walls of 

excavations, and artificial holes quickly fill by floAvage from the walls. 

Drier soils will stand in the walls of excavations; but, when samples 

of these "dry" soils are sacked and hung to dry, water drains from 

them.

Areal variations in soil moisture in silty soils depend upon the depth 

of thaw, the degree of slope at the site, the position of the site hrrelation to drainage lines, and the character of the parent material. . In 

general, the moisture content varies inversely with the depth of thaw 

and the degree of slope. Soils on slopes where definite drainage lines 

are lacking are much wetter than the interfluvial soils on slopes with 

well-defined swales. Soils derived from marble are generally wetter 

than soils derived from other rock.

Peat is completely saturated with moisture in almost all areas, The 

peat is coarse, open-textured, and more permeable than silty mineral 

soil, and in some of the features described below ground-water circulation above the frost table is concentrated in the peat.

PERENNIALLY FROZEN GROUND AND FROST ACTION

Perennially frozen ground is present throughout northern Seward 

Peninsula, except near lakes, large streams, and warm springs. Drillhole data and comparisons with other areas indicate that f ro'zen ground 

extends to depths ranging from 50 to at least 300 feet. The character 

of the perennially frozen zone is described by Hopkins (1949, pp.
FROST ACT-ION ON SEWARD PENINSULA 61

121-122). The surface layers thaw during the summer to depths 

ranging from 1 to 10 feet, depending upon the character of the soil, 

the vegetative cover, topography, exposure, and drainage conditions. 

The relationships between these factors and depth of thaw are described in greater detail in the discussion of the individual cryopedologic features.

Frost scars, peat rings, tussock rings and groups, and tussock-birchheath polygons in the Imuruk Lake area are underlain by perennially 

frozen ground and are formed by frost heaving, frost thrusting, and 

creep in the upper layers of the soil which are subject to annual cycles 

of freezing and thawing.

Frost heaving and frost thrusting result from volume increase in 

masses of soil due to the segregation of masses of clear ice during 

freezing. Taber (1930) has shown that when silty soils are subjected 

to subf reezing temperatures in the presence of abundant moisture they 

absorb a quantity of water greatly in excess of the natural porosity of 

the unfrozen material. The excess water is present as segregations of 

clear ice, which commonly comprise 50 to 80 percent of the volume of 

the frozen material (Taber, 1943, pp. 1517-1518). The water forming 

these clear ice segregations can be derived from any nearby source and 

is drawn upward or laterally to the growing ice crystals through capillary interspaces in the soil.

If clear ice lenses formed in equal abundance in all of the materials 

at the surface, frost heaving would consist of a uniform dilation of 

the entire surface. The surface of the tundra is characterized, however, by a multitude of small-scale inhomogeneities, which result in 

large local differences in the intensity of frost heaving. Among the 

principal causes of differential frost heaving are variations in the 

thicknesses of peat and mineral soil, in the character of the vegetation 

cover, in the amount of moisture present, and in the amount of snow 

cover during the critical autumn freezing period.

The intensity of frost heaving varies inversely with the thickness of 

peat or turf covering the mineral soil. Two factors are involved in this 

relationship: (1) Ice lenses form more abundantly in silty mineral soil 

than in peat, probably because water is drawn less readily through the 

large openings that predominate in the peat and because the tough, 

fibrous structure of the peat resists disruption by growing ice crystals 

and (2) silt is a much better heat conductor than peat or turf. Summer 

thaw penetrates much deeper into bare mineral soil than into peat, and 

severe autumn frosts produce several inches of frozen ground in bare 

areas, whereas only the subaerial portions of vascular plants are frozen 

in dry turf a few inches away. Thick wet mats of living sphagnum 

are poorer insulators than dead peat, and frost penetrates into them 

almost as rapidly as into mineral soil. At Nome, following several
(32 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

days with minimum temperatures of 10° to 27.5° F., ice an inch thick 

formed on open puddles. At the same time the ground froze to a 

depth of more than 6 inches in bare soil, 3 to 5 inches in living sphagnum, 1 to 4 inches in wet peat, and 0 to 2 inches in dry peat and turf.

If the entire soil surface freezes at a uniform rate, then, according to 

Taber (1943, p. 1523), the segregation of ice lenses is favored by slow 

freezing. When the rate of freezing varies widely within a radius of a 

few feet, however, ice lenses are formed most abundantly in the material that freezes first, because water is drawn laterally to growing ice 

lenses in the freezing soil from adjoining unfrozen areas. Thus, areas 

of bare soil or areas covered by only a thin mantle of peat will freeze 

rapidly and will expand at the expense of areas nearby that freeze more 

slowly because they are mantled by a thick layer of peat (fig. 28, -A, B).

Frost heaving in bare areas or areas covered with isolated plants 

is more intense than in areas covered with dense vegetation. The 

tightly interlocked roots and stems form a dense mat of vegetation 

that resists expansion at the surface and an insulating blanket that 

retards freezing of the underlying soil.

Differences in snow cover during early fall play an important role 

in differential frost heaving. Intense winds are effective close to the 

ground surface in exposed places; an early snowfall followed by high 

winds results in a pattern of drifts and bare patches. Distribution 

of drifts is determined in part by the vegetation; drifts accumulate in 

willow thickets and in local areas of tall plants in the cottongrass 

tussock-birch-heath community. Snow-covered areas are insulated 

from the cold air, while freezing temperatures penetrate rapidly into 

nearby snow-free spots. Intense dilation of the snow-free areas and 

relative stability in the snow-covered areas can be expected.

If other conditions are uniform, the surface layer of the wettest 

spots can be expected to distend more than dryer spots nearby, because 

a large supply of water is available for the formation of clear ice 

lenses. Frost heaving and other cryopedologic processes proceed with 

maximum intensity in areas adjacent to persistent snow banks, because 

abundant meltwater is present during the fall freezing cycle.

Repeated freeze-thaw cycles in soils on slopes result in net downslope movement. Two components, creep and viscous flow, contribute 

to this movement. Creep results from the expansion of the soil normal 

to the slope during freezing followed by vertical subsidence during 

thawing. Viscous flow occurs in thawed soils that contain excessive 

water owing to the thawing of ice lenses. In many localities additional water is provided by seepages or by melting snow banks. It is 

commonly impossible to distinguish between down-slope movements 

due to creep and those due to flow.
FROST ACTION ON SEWARD' PENINSULA 63 

CONGELITUBBATION AND INSTABILITY OF THE SOIL SURFACE

Cryopeclologic processes affect to some degree all parts of the soil 

surface layers. Each winter the surface is subject to general upward 

expansion throughout moist areas underlain by peat or silty mineral 

soil. The dilation is not uniform, however, and differential frost 

heaving is a general phenomenon, not an exceptional one. Small 

mounds and hummocks are raised where differential heaving is slight, 

and the turf is broken where it is intense. Slight differential heaving 

is favored by thick turf, intense heaving by thin turf.

The character of the soil is determined and modified by cryopedologic processes. The parent rock is reduced to small-grade sizes 

by congelifraction operating over a long period of time. That part 

of the resulting silty soil that lies above the frost table is churned by 

congeliturbation. In the course of long-continued cycles of freezing 

and thawing, soil particles at the surface are moved to a considerable 

depth within the zone of annual thaw, and new particles that lie 

within this zone are brought to the surface and exposed to subaerial 

weathering processes. Peat and dead plants are churned beneath the 

surface and increase the amount of organic matter in the soil. In cut 

banks of streams the zone subject to annual thawing can readily be 

distinguished from the underlying perennially frozen soil by color 

and structure.

On horizontal surfaces perennially frozen soil is light blue-gray and 

more or less structureless in the Imuruk Lake area. The annually 

thawed zone is yellow-gray when dried and characterized by contorted streaks of reddish limonite-stained silts and dark grayish-brown 

amorphous organic material. Where the parent material consists of 

water-laid silt, stratification is completely destroyed in the zone subject to annual thawing.

Intensive congeliturbation limits the vegetation to species adapted 

to existence upon an unstable substratum. The pioneer plants are 

those able to survive intense disturbance. Concepts of "pioneer" 

and "climax" communities lose much of their significance, however, in 

Avet tundra, because even the late stages in the succession must be 

regarded as existing in unstable equilibrium with an 'environment 

subject to sudden, severe change. Bare areas are colonized by pioneers, 

followed by species later in the succession; however, these later species 

eventually are destroyed when local intense congeliturbation creates 

a new bare area and the cycle is repeated.

VEGETATION

The Imuruk Lake area lies beyond the arctic timber line within 

the tundra region (Griggs, 1934; Raup, 1941; Palmer and Rouse,

904S12 51 3
64 'CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

1945). Spruce occurs 30 to 50 miles south and east of Imuruk Lake, 

and isolated stands of small cotton wood (Populus tacamahacca) 

trees occur in the dissected upland at the north and southwest margins 

of the area. The vegetation of the Imuruk Lake area consists of 

dwarf shrubs, sedges, grasses, other herbaceous plants, and mosses on 

the uplands with large, shrubby willows confined, for the most part, 

to watercourses. Alder thickets are scattered on steeper betterdrained slopes.

The large number of vegetation types in the tundra makes their 

classification difficult, but the broad categories suggested by Middendorf in 1864 stand as the most useful distinctions yet proposed. In 

the Siberian arctic, he recognized dry or high tundra and wet or low 

tundra. On the Seward Peninsula, dry tundra is characterized by 

the predominance of low matted woody plants and grasses, which 

are the tallest plants, reaching 6 to 10 inches in height. It is confined 

to areas in which the soil is relatively dry and well-drained and of 

sandy or rocky texture. Wet tundra is characterized by the predominance of species of the Cyperaceae family, mostly species of the genera 

Eriophorum and Carex, and is confined to areas in which the soil is 

wet, poorly drained, and composed largely of silt and peat. It is in 

this wet tundra that an organization and pattern of the vegetation 

can be distinguished. The patterns observed in the two following 

vegetation types are discussed in later sections.

Tussock-birch-heath vegetation is dominant on fairly well drained 

slopes and uplands away from the drainage lines. The following 

species are found in this community. The nomenclature used follows 

that of Hulten (1941-1948).

Primary species: Eriophorum vaginatum subsp. spissum,

Betula nana subsp. exilis, Empetrum nigrum, Ledum

palustre subsp. decumbens, Vactinium uliginosum, and

V. vitis-idaea subsp. minus. 

Secondary species: Arctagrostis latifolia, POOL arctica,

Carex bigelowii, Caress lugens, and Rubus chamae- 

" V ' morus.

The Eriophorum * grows only as tussocks, 6 to 15 inches in diameter 

and 2 to 12 inches high. The tussocks are more or less evenly spaced 

at intervals of a few inches to 2 feet and are rooted in the mineral soil. 

Woody shrubs grow in the moss peat and Sphagnum mosses in the 

spaces surrounding the tussocks. The soil is silty and moist, and 

depths to the perennially frozen ground range from 15 to 36 inches.

1 Eriophorum vaginatum subsp. spissum is one of several species of this genus that forms 

tussocks but is the.only one identified by the writers in the Imuruk Lake area. In later 

sections it is referred to as cottongrass and tussocks unless otherwise noted. To avoid 

confusion, Eriophorum angustifolium, also commonly known as cottongrass, is referred 

to by its Latin name.
FROST ACTION ON SEWARD' PENINSULA 65

The form and distribution of the plants in this community, as affected 

by congeliturbation, is discussed later. The birch and heath shrubs 

tend to grow with the accumulation of Sphagrmtn moss peat; the 

apexes continue to elongate while the lower portions of the plants 

become buried and subsequently die. The birch and heath shrubs are 

not rooted in the mineral soil, but adventitious roots extend into the 

moss peat from the dense tangle of branches and stems, forming a 

tight mat. The stems also grow around the tussocks that are adjacent to the birch-heath community, so that the tight mat is firmly 

anchored to the mineral soil. A mat of this type cannot be removed 

without tearing it unless the stems are cut around a small section.

A sedge sod occurs in poorly drained sites where water occurs at the 

surface and its lateral movement is slow. Such places are found on 

broad hill summits and gentle slopes, in drainage lines, and around 

the margins of many ponds. The following list of species has been 

compiled from these habitats.

Primary species: '\Eriophortvurin angustifolium and Garex

agit/itilis.

Secondary species: Eriophormn scheuchzeri, Scirpus 

cespitosus, Car ex rotundata, G. membranacea, and 

Salix cf. flagellaris.

The sod is an interwoven mass, 6 to 10 inches thick, of rhizomes 

(underground horizontal stems) of Carex aguatilis firmly rooted in a 

thick layer of fibrous peat. The peat consists of an accumulation of 

dead parts of Carex, Eriophorum, and mosses. This type of sod is 

extremely tenacious, and only with difficulty can it be cut with a 

shovel. Because of its tough structure, the sod reacts to pressure as a 

tight-layered mass rather than as an inert blanket or as discrete plants. 

It modifies and restrains the forces of congeliturbation. The peat 

mantle ranges in thickness from 1 to 10 feet but is interrupted locally 

by areas of bare soil. Depths to perennially frozen ground range from 

15 to 40 inches, depending upon the thickness of the peat, drainage 

conditions, and texture of the underlying soil.

Willow and birch thickets also are prominent in the Imuruk Lake 

area. Willow (Salix pulchra) thickets 4 to 10 feet high occur in the 

lower reaches of the swales draining higher slopes, on steep lake banks, 

a,nd on flood plains of the larger streams. Depth to perennially frozen 

ground is more than 6 feet, and frozen ground may be absent locally 

under flood plains. Definite channels of running water are present 

under the willows in the swales; these channels are among the few 

sites where erosion by running water is occurring in the Imuruk Lake 

area. Thickets of birch (Betula nana subsp. exilis) occur on the better-drained, higher parts of steep banks and -well-drained slopes. 

Depth to perennially frozen ground in these thickets is not known to
66 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

the writers but it is probably more than 6 feet, or comparable to that 

under the willow thickets.

These plant communities are clear-cut and can be denned in space 

and correlated with factors in the environment. The three factors 

most closely correlated with the distribution of the communities are 

drainage conditions, depth to perennially frozen ground, and the intensity and character of congeliturbation. These in turn are determined, in part, by topography, type of soil, and the character of the 

vegetation. Each type of vegetation, however, is found locally within 

the area dominated by another type.

CRYOPEDOLOGIC FEATURES

Frost scars, peat rings, tussock rings, tussock groups, and tussockbirch-heath polygons are characteristic features of certain widespread 

environments in northern Seward Peninsula and elsewhere in tundra 

regions in Alaska. These features represent stages in several developmental series, in each of which the frost scar is the initial form. The 

interrelationships between different features are discussed under the 

heading "Conclusions."

Peat rings, tussock rings and groups, and tussock-birch-heath 

polygons are best developed in areas underlain by silty mineral soil 

mantled with peat less than 3 feet thick. Frost scars are found on all 

types of soil, but the description presented here applies chiefly to frost 

scars in silty mineral soil.

The descriptions of individual features are based upon the writers' 

observations in northern Seward Peninsula. Similar features in 

other parts of Alaska differ considerably in certain details. Discussion of some of these variations, however, lie beyond the scope of this 

paper.

FBOST SCABS

Tundra vegetation is rarely complete and unbroken. Small patches 

of bare soil, are characteristic and conspicuous features scattered 

through the vegetation. As frost action is a major factor in the formation of these bare patches, the name "frost scar" is proposed. The 

"spot medallions" of Sochava (1944) and the "mud spots" of Washburn (1947, p. 99) are types of frost scars.

Frost scars in various environments range from a.few inches to 

several tens of feet in maximum diameter. In areas of sedge sod or 

cottongrass tussock-birch-heath vegetation the frost scars are larger 

than intertussock areas and range from 1.5 to 10 feet in diameter. 

Some of the scars are irregular, and others are circular or oval. They 

are conspicuously convex from late autumn through spring and-are 

flat during middle and late summer. A network of small cracks (mini-*
FROST ACTION ON SEWARD PENINSULA 67

ature zellenboden, Troll, 1944, p. 619) resembling drying cracks 

generally is present on the surface of the larger scars. Moisture conditions vary widely, but the soil of the frost scars generally appears 

drier during the summer months than the surrounding turf or peat. 

The bare soil thaws more rapidly and to greater depth during summer 

than the adjoining turf and peat, so that the larger frost scars are 

underlain by depressions in the frost table (fig. 28, C). Conversely, 

during at least the early part of the autumn freezing cycle the bare 

soil freezes more rapidly than soil having a partial or complete cover 

of turf or peat.

Bare areas in the tundra originate in a variety of ways. BurroAving 

mammals can destroy the stems and roots of plants and probably are 

effective in initiating a break in the turf, which can be enlarged by 

congeliturbation. Close grazing by reindeer is an important factou.' 

in breaking turf in some localities (Palmer and Rouse, 1945, p. 26)). 

From the air, reindeer corrals near Teller, Alaska, can be distinguished 

from the surrounding tundra by the presence of closely spaced frost 

scars in the corrals as contrasted with their absence or relative scarcity 

in adjoining ungrazed tundra. Cottongrass tussocks overturned by 

frost heaving are found locally; the area can become the nucleus of a 

growing frost scar. In areas mantled with a continuotis layer of peat 

or turf, the surface cover can be broken during the autumn freezing 

cycle by intense dilation of the underlying mineral soil in spots Avhere 

the peat is relatively thin (fig. 28, A, B) , in exceptionally wet spots, or 

in spots swept bare of snow by the Avind folloAving an early snoAvfall. 

El ton (1927, p. 171) observed small patches of fresh soil Avhere "the 

plant-covered surface layer had been rent, giving the appearance of 

three slits meeting in a central point."

Once formed, frost scars tend to be self-perpetuating. Because bare 

soil thaAvs to a greater depth than soil covered Avith peat or plants, 

a thicker zone is available in Avhich clear ice can form during the 

autumn freezing cycle. This condition is augmented by the tendency 

for bare soil to freeze much more rapidly than turf- or peat-coA^ered 

soil. The supply of moisture available at depth for the formation of 

ice lenses is limited by the frost table, Avhich lies 18 to 40 inches beloAV 

the surface. During rapid freezing of the soil in the frost scar, Avater 

is draAvn laterally to the groAving ice crystals from the unfrozen soil 

and peat under adjoining turf-covered areas. Thus, some of the Avater 

which might otherwise have contributed to the formation of clear ice 

lenses beneath the vegetated areas augment, instead, the dilation of the 

bare areas.

During the first stages of freezing, expansion of the soil is relieved 

by uparching of the surface. The drying cracklike miniature zellenboden probably are dilation cracks, Avhich result from continued
68 -CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

arching and expansion after freezing has progressed deep enough to 

create a rigid cover over the scar (fig. 28, C}. Upward dilation is 

greatly hindered, however, after a hard crust of frozen soil several 

inches thick has formed at the surface of the bare patch, and most of 

the' later expansion is relieved by lateral thrusting into the adjacent 

unfrozen layer of peat.

Despite continuing frost heaving, many frost scars eventually become partly or completely colonized by vegetation. The interaction of 

vegetative growth with continuing congeliturbation results in the evolution of neAV forms. In wet areas mantled with a considerable thickness of peat, the frost scars evolve into peat rings, tussock rings, or 

tussock groups. In drier areas dominated by cottongrass-birch-heath 

vegetation, they develop into tussock-birch-heath polygons. In sandy 

or rocky soils frost scars evolve into still other forms, the discussion of 

which is outside the scope of this paper. In certain environments the 

scars are colonized by structureless tundra indistinguishable from the 

surrounding vegetation.

Colonization of the scars is accomplished by the seeding of new 

plants within the bare patches and by encroachment of runners of old 

plants from the margins. These two methods of colonization proceed 

at different rates in various environments. Species that migrate into 

a bare area are limited to those in the surrounding plant population 

(Gleason, 1917), so that the rate of stabilization is, in turn, dependent 

upon the adjacent species. Some of these, however, may not be able 

to survive in the new habitat until a "seed bed" has been prepared for 

their migration. In a sedge marsh, for example, dispersal by seed 

seemingly is most effective in plant migration, and observations indicate that the soil must be somewhat stabilized by Eriophoiwii angustifolium before Carices become established. E. angustifoliujrm invariably is the first plant found on otherwise bare soil in this environment. 

In areas dominated by cottongrass-birch-heath vegetation, migration of plants occurs by seed dispersal and germination, and by vegetative growth of stems and rhizomes. Cottongrass tussocks become 

established on new sites only by germination of seed and only on soils 

where the intensity of congeliturbation has been reduced by the 

growth of other species or by the presence of small, thin fragments 

of peat. Birch and heath, in this habitat, grow only in moss or moss 

peat; it is necessary, therefore, that species of Sphagnum, Calliergon, 

and Hypnum become established prior to invasion by birch and heath. 

These mosses grow only in moist areas, such as the spaces between 

cottongrass tussocks. Considerable development of the tussocks, 

therefore, is necessary before migration of the mosses can begin. Birch
FROST ACTION ON SEWARD PENINSULA 69

and heath, which migrate mostly by growth of stems, follow when 

peat has accumulated beneath the mosses. It can be seen that the rate 

of stabilization of a frost scar will be faster if the vegetation development from bare soil to the "mature" stage is accomplished in a few 

steps, as in the sequence Eriophorum angustifolium through Oarex spp. 

In some environments new frost scars are formed before sufficient time 

has elapsed to permit the maturation of vegetation that requires many 

steps.

In other types of environments frost scars occur in° areas dominated 

by Dryas. Here, encroachment by runners at the margin of the scar 

becomes an important factor in stabilization.

Growth of new plants at the center.of a frost scar is inhibited by 

renewed dilation of the scar during the autumn freezing cycle and by 

the formation of miniature zellenboden, which are capable of breaking the roots of young plants. Needle ice ("kammeis" or "pipkrake" 

of various authors) is the chief agent hindering encroachment of vegetation at the margins of the scar. Ice needles form beneath the thin 

marginal turf during sharp nocturnal frosts and uproot and overturn 

the advancing vegetation. The frost scar can be enlarged under favorable conditions by this process.

It can be seen from the foregoing statements that colonization of 

the scars is a slow process, which is hindered by continued congeliturbation within the scars. Shifts in the balance of plant growth versus 

congeliturbation result in net advances or net losses in the progress of 

colonization. Predominance of one process over the other appears to 

be a cyclic "good year-bad year" phenomenon in some localities. On 

the Arctic coastal plain of Alaska observations over a period of several 

years indicate that during some years many frost-formed and windformed scars are in the process of stabilization by vegetation, but during other years most of the scars appear active and many new scars are 

formed (Black, R. F., personal communication). In many areas on 

the Seward Peninsula, however, newly formed frost scars, older welldeveloped frost scars, and old, nearly stabilized frost scars can be 

found within a few hundred feet of one another.

Variations from year to year in the number and sharpness of diurnal 

freeze-thaAv cycles probably are reflected by general regional growth 

or decay of frost scars. During years when short-period freezing 

cycles are numerous and sharp, repeated cycles of needle-ice growth 

should result in general disruption of marginal vegetation, with a net 

growth of many scars. In years of few nocturnal frosts and mild 

temperatures, the marginal vegetation probably makes a net gain on 

most bare-soil areas. The favorability of the growing season is an-
70 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

other important factor governing encroachment of vegetation upon 

the scars. Drying winds are important in inhibiting colonization 

of frost scars in northern Alaska (Black, R. F., personal communication) .

The distribution of snow cover in time and space also is important. 

The flat surfaces of the frost scars lie somewhat below the level of the 

surrounding vegetation during early autumn. If an early snowfall 

precedes the freezing of the soil, snow is likely to accumulate to greater 

depths on the surfaces of the frost scars than on the surrounding 

vegetation. Such an occurrence retards the freezing of the soil in 

the scar and should reduce the amount of dilation of the center. Later 

short-period cycles of freezing and thawing in the air would be less 

effective beneath the snow cover, and thus disruption of marginal 

vegetation by needle ice would be at a minimum. If the first snowfall 

is delayed until freezing temperatures have lasted long enough for 

dilation of the scars to take place, the snow will collect ih the vegetated 

areas, and disruption of marginal and surface vegetation on the scars 

will be increased.

In summary, colonization of frost scars by vegetation appears 

favored by an early sudden freeze-up with few previous nocturna] 

frosts and by an early snowfall, especially if it occurs before final 

freeze-up.

TUSSOCKS

The tussock is a tufted plant form characteristic of certain grasses 

and sedges growing in areas in which congeliturbation is active. It 

consists of a ball-like mass of living and dead plant parts that stands 

as a small mound or hummock above the ground surface in areas where 

the water table and silty mineral soil are close to the surface. Tussocks 

are a characteristic plant form in tundra regions, but they also are 

common in swamps at least as far south as Baltimore, Md. (Black, R. 

F., personal communication). The important role of congeliturbation in the origin of tussocks near Boston, Mass., will be discussed in 

a future publication.

Many tussocks are formed by the interaction of plant growth and 

differential frost heaving in wet mineral soil. Although several species occur in tussock form, the developmental processes described below 

pertain to Eriophorum) vaginatum subsp. spissum, the cottongrass 

tussock or "niggerhead" of the Alaskan layman. Cottongrass tussocks and "earth hummocks" ("palsen" or "torfhtigeln" of German 

writers) described by Sharp (1942, pp. 297-299) are formed in a 

similar manner, but the tussocks are quite different.

The tussock is a single plant; its leaves, culms (jointed stems), and 

roots give it characteristic form. Small year-old tussocks consist of
FROST ACTION ON SEWARD PENINSULA 71

small tufts without winterkilled leaves (fig. 20). Culm bases rest 

directly on mineral soil. The older, well-developed tussocks are 

mounds nearly spherical to vertically elongate in shape and are '2 to 1'2 

inches high (fig. 21). Dead and living culms and leaves cover the 

surface and give the tussock its ball-like form. The culm bases form 

a tight mat in the upper 1 to 2 inches of the tussocks. A zone of 

tightly packed roots lies immediately below the culms and accounts 

for most of the height of the tussock. The distal ends of the roots 

grow in a low mound of mineral soil, which projects 1 to 6 inches 

above the level of the soil in inter-tussock areas.

FIGURE 20. Vertical section through seedling tussock of cottongrass. Note rhizomes 

resting directly in mineral soil and lack of winterkilled leaves.

The thickened culm bases form a tight mat, because new upright 

culm branches form from buds beneath the leaf sheaths surrounding 

older culm bases. The upright culms die each year, leaving only the 

thickened culm base, which elongates but 1 to 2 millimeters each year. 

Leaves form as the upright culm is elongating, and when the leaves 

are mature new buds form on the thickened culm bases (fig. 22, A, G}.2

2 Some species of Eriophurum, Cm-ex, and grass reproduce vegetatively by runners or 

stolons (fig. 22, B). Stolons arise from buds on the culm bases, but annual growth varies 

from 1 to 2 inches or more. From scattered nodes grow aerial stems and new leaves. 

This type of growth produces a loose mat or sod. -

904812 51-
72 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

0 I 2 

INCHES

A STEM BASES 

B ROOTS

PEAT AND MOSS

REDDISH- BROWN 

SOIL

BLUE-GRAY 

SOIL

FIGURE 21. \rertical section through young cottongrass tussock. Most of height of 

tussock consists of roots in nipund of mineral soil.
FROST ACTION ON SEWARD PENINSULA 73

0.5r

0 0.5 

Inches

Sheathing leaf base

Culm base 

] Buds 

Root

FIGURE 22. Culm bases of Eriophorum vayinattim subsp. spissum and stolons of E. 

ritsseolum var. leucothrix. A. Thickened culm base and upright culms of E. vayinatum 

subsp. spissum. Narrow part of culm is the annual growth that dies each year. The 

thickened part below represents growth that is perennial, and from it grow roots and 

upright culms. B, Stolons and upright culms of E. russeolum var. leucothris. Horizontal branch is stolon that grew in one year ; upright culms grow from distal ends of 

stolons. C, Drawing of thickened culm base and upright culms of A.

As the number of new stems increases at the center of the tussock, old 

stems are crowded aside and die, leaving a mass of dead rhizomes and 

leaf bases fringing the sides. The root zone consists of a tightly 

packed mass of contorted living and dead annual roots. Individual
74 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

roots, which are long and slender, can be traced to their point oic 

attachment on the rhizome.

The height of the mound of mineral soil at the base of the tussocks 

appears to vary directly with the height of the tussock (fig. 23). Mineral soil probably occurs higher within the zone of roots than could be 

detected by field observation.

The larger tussocks are spaced at intervals of a few inches to 2 feet

Roots

Mineral soil and roots

II

1 23456 7- 89 10 

DIAMETER IN INCHES

FIGURE 23. Relationship shown in 12 cottongrass tussocks.between height and diameter 

of tussock and height of mineral soil mound at base. Total length of each bar represents height of single tussock ; diameter of tussock is shown by position of bar on horizontal line. Stippled portion of bar represents height of mineral soil mound at base of 

tussock ; black portion represents height of roots extending above soil mound.
FROST ACTION ON SEWARD PENINSULA 75

FIGURE 24.- Group of cottongrass tussocks.

and are surrounded by a thick mat of mosses (species of Sphagnum, 

Colliery on, and Hypnum) or by low areas in which; vegetation of 

equivalent size is lacking (fig. 24). If thick mats of mdss are lacking, 

the soil surrounding the tussocks is bare or is covered with humus and 

low mosses forming a layer less than an inch thick.

The culms, which are the only parts that can increase the height 

of the plant by growth, form only a small fraction of the height of the 

 tussock. The height, therefore, is due to some cause other than the 

growth of the culms. Two hypotheses might explain this height: 

The culms started at the level at which they are found and the surrounding soil was removed by erosion or the culms started at a lower 

level and were pushed upward to their present position by frost action.

In a few exceptional localities the relief of the tussocks has been 

increased by rill erosion in the intertussock areas. These tussocks are 

exceptionally high and stand 30 to 36 inches above the surrounding 

soil. Kill markings between the tussocks and small silt fans farther 

down the slope testify to soil removal by running water. ' Most tussocks, 

however, do not owe their height to soil erosion, for rill markings and 

stratified sediments are lacking. The possibility of soil removal by 

creep or viscous flow of the soil also can be disregarded, because the 

tussocks commonly occur in soil areas completely enclosed by peat. No 

outlet exists through which the soil could have been removed. Most
76 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

tussocks appear to have been pushed to their present height by vertical 

heaving beneath the plants.

During the autumn freezing cycle, the sides of the tussocks and the 

Sphagnum mosses and mineral soil surrounding the tussocks freeze 

more rapidty than the matted culm bases. The writers have observed 

that the culm bases were still unfrozen after the mosses had frozen to 

a depth of 4 inches and the mineral soil and sides of the tussocks had 

frozen 1 to 2 inches. Lower moisture content and better insulation due 

to. the presence of dead air spaces account for the slow rate of freezing 

of the culms and roots. The freezing mineral soil between the tussocks 

expands and moves laterally into the thawed zone beneath the tussock, 

forcing it upward (fig. 28, G}. Repeated cycles of frost thrusting 

from the intertussock areas raise a mound of mineral soil beneath the 

tussock and force the proximal portions of the roots vertically out 

of the soil.

Tussocks involving species other than Eriophorum vaginaturn 

subsp. spissum are formed by other processes in addition to congeliturbation. Tussocks of the sedge, Carex aguatalis, are present 

but uncommon in the Imuruk Lake area. Much of the height of these 

tussocks is due to the growth of culms, but frost heaving is a contributing factor. Tussocks of C'alamagrostis spp., occurring in Massachusetts and on the Seward Peninsula are due largely to the growth 

of culms but can start on tussocks of other species formed and enlarged by congeliturbation in the manner previously described. Frost 

heaving is responsible for much of the height of tussocks of Carex 

spp. found in marshes near Concord, Mass.

PEAT RINGS

Peat rings are low ridges of peat surrounding circular or oval 

patches of mineral soil (figs. 25 and 26). The rings are found 

on poorly drained uplands and on long smooth, wet slopes of 10° 

or less. They are most common on southwestward-, southward-, or 

southeastward-facing slopes. Areas where peat rings occur are 

mantled by 1 to 3 feet of peat overlying mineral soil, on which is 

growing a tight sedge sod. Water stands about the lower parts 

of the plants and in all minor depressions.

On nearly horizontal surfaces the rings are more or less circular 

and are 4 to 12 feet in diameter. The centers of some rings in the 

Imuruk Lake area are bare but more commonly support a sparse and 

broken cover of vegetation. Isolated fragments of peat and torn 

sedge sod (Carex aquatilis] are scattered over the surface. Seedlings 

of Eriophorum angustifolium, E. vaginatum subsp. spissum, and 

Scirpus cespitosus grow in the wetter areas. On the slightly drier
FROST ACTION ON SEWARD PENINSULA 77

SEDGE SOD

PLAN

 SEDGE SOD -- -- 

U-PEAT RIDGE

-----FROST SCAR-- -

-- - PE AT RIDGE---

BLUE-GRAY SOIL

FROZEN GROUND

CROSS SECTION

FIGURE 25. Plan and cross section of peat ring.
78 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

FIGURE 26. Large and small peat rings. In right foreground small, young peat ring 

consists of bare soil partly covered with standing water and surrounded by low ridge 

of peat. In background 10-inch white square rests against peat ridge of large, old peat 

ring the center of which is overgrown by Ei'iopliorum anyustifolium.

spots small tussocks of Eriophorum vaginatum subsp. spissum, as well 

as plants of Empetrum nigmm and Vactinium uliginosum, cover small 

areas. Most of the surface of the soil, however, is bare. In most rings 

the soil area is slightly convex, with the center standing a few inches 

higher than the margins. Miniature zellenboden generally are 

present.

The marginal peat ridges range from 8 to 36 inches in height and 

2 to 4 feet in width. For the most part, the vegetation on the ridge 

is similar to that in the surrounding marsh and consists of a dense, 

tight mat of sedge sod composed of a tangle of stolons, rhizomes, and 

roots of Carex aquatilis in which the following secondary species 

grow: Eriophorum angustifolium,, Salix reticulata, S. pulchra, Betula 

nana subsp. exilis, Rubus chamaemorus, Empetrum nigrum, Ledum 

palimtre subsp. deewmbens, and Vaccinium uliginosum. In limestone 

areas Silene acaulis, Saxifraga hirculus, Dryas integrifolia, and 

Cassiope tetragona also are present. Nearly all of the secondary 

species occur on the top and inner face of the ridge. The vegetation 

has retained the form it had before the formation of the ridge, but 

additional species have become established both on the ridge and 

around it. In some rings a few well-formed tussocks of Eriophorum
GEOLOGICAL SURVKV 

PEAT KINGS ON VALLEY SL

Oblique aerial photograph showing distribution of peat rings on slopes of a sh

comes from the west (left).
FROST ACTION ON SEWARD PENINSULA 79

vaginatum subsp. spissum are rooted in the mineral soil at the inner 

margin of the ridge.

In cross section the rings consist of local mounds of mineral soil 

projecting through an otherwise continuous layer of peat a foot or 

more thick (fig. 25). On Seward Peninsula the marginal ridge generally consists of peat, but some rings examined by the writers consisted of a ridge of mineral soil about a foot high, capped on the 

outer slope by a layer of peat a foot thick. The peat of the ridges 

is intruded by involutions of mineral soil from the centers. Roots 

of sedge and stems and roots of the other species present are interwoven with the peat in the ridges. The roots of cottongrass tussocks 

extend through the thin marginal peat into the mineral soil. During 

middle and late summer the frost table under the soil mound is a 

basinlike depression 1 to 2 feet deep, whereas under the peat ridge 

there is a ridge in the frost table a few inches high.

Moisture conditions in the soil centers of peat rings vary widely 

from time to time and from place to place during the summer. During 

the spring thaw and the late summer rainy season the centers consist 

of fluid, saturated silt. During summer dry periods the centers 

remain saturated in some areas; in others the centers are moist, but 

not saturated, and will stand in vertical walls for several days in 

artificial excavations. The peat surrounding the rings, however, is 

saturated, and AAT ater draining from it quickly fills an excavation.

On gentle slopes the peat ridges are slightly elongate (-fig. 27), the 

down-slope axis reaching 20 to 100 feet on slopes of 7° to 10°. Closely 

spaced rings on these slopes commonly are arranged in trains extending down slope (pi. 2). The soil centers of elongate rings are less 

conspicuously convex than those of circular rings. The soil center 

lies 6 to 12 inches below the general level of the slope at the up-slope 

end of the ring and a few inches above the general level at the lower 

end. The marginal peat ridge is highest at the down-slope end of the 

ring, where it acts as a dam, holding in the mineral soil. At the 

up-slope end, the ridge generally is missing, and in some localities 

narrow gullies 6 to 18 inches deep extend 5 to 20 feet up slope from the 

heads of rings.

Elongate peat rings differ from "solifluction lobes" (Washburn, 

1947, pp. 90-92) in several respects. The lobes lack a marginal peat 

ridge, and typical lobes are larger in all dimensions than peat rings. 

Solifluction lobes generally consist of unsorted silt. sand, and gravel 

with many rocks more than 6 inches in diameter; the soil in peat rings 

examined by the writers consisted of silt with a little sand and a few 

pebbles but contained no rock fragments more than 3 inches in longest 

dimension. Forms transitional between solifluction lobes and elongate peat rings probably exist.
80 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

PLAN

Contour interval I foot 

EXPLANATION

5 Feel

Bare soil Sedge sod Willows

505

500

495

Peat or turf

CROSS SECTION 

EXPLANATION

Silty mineral soil Sedge sod

&& IM1 . Willows Perennially frozen

peat or soil

FIGURE 27. Plan and cross section of elongate peat ring.

Elongate rings with completely bare soil centers were observed 

in only one locality. More commonly, the elongate rings bear a 

nearly complete cover of sedge (Carex aquatilis) and cottongrass 

(Eriophorum angustifoliwn) and a few herbaceous flowering plants.
FROST ACTION ON SEWARD PENINSULA 81

Peat rings originate as frost scars in swampy areas covered by a 

layer of peat and a tight, nearly homogeneous sod, the combined thickness of which is approximately equal to the depth of annual thaw. 

Intense frost heaving disrupts the sod cover and forces mineral soil 

to the surface in scattered areas where summer thaw extends below 

the peat into the mineral soil (fig. 28, A). The scars are few in number and widely spaced in the marsh because of the relatively great 

thickness of the peat cover and because of the great strength of the 

fibrous sod when frozen. Once the surface cover is breached and the 

mineral soil is heaved to the surface, differential frost heaving reaches 

maximum intensity in the bare-soil areas. Extreme dilation of the 

scars upon freezing is favored by the presence of abundant moisture 

in the adjoining swamp, while dilation in the inter-ring areas is at a 

minimum because summer thaw does not extend through the peat into 

the mineral soil.

Frost thrusting from the soil centers during later freezing cycles 

pushes aside the surrounding peat and sod and warps it into a circular 

ridge surrounding the scar (fig. 28, D}. As the frozen layer thickens 

beneath the bare area, involutions of mineral soil are thrust into the 

lower parts of the marginal peat (fig. 28, E] . Outward displacement 

of the peat at depth is prevented by the presence of an incompressible 

ridge of frozen ground; part of the displaced peat must be moved, 

therefore, toward the center of the scar at depth through still unfrozen 

soil. The increment of movement during each freezing cycle is very 

small, and a great many cycles must be required to produce a ring 

as well-developed as the one shown in figure 25.

Part of the relief of the marginal ridges results from the accumulation of peat after the ridge is formed. The dense tangle of 

living roots enmeshed in the peat protects the ridge from later destruction by congeliturbation.

Elongate peat rings originate as frost scars on swampy slopes 

where the combined thickness of peat and sod is approximately equal 

to the depth of thaw. During the early stages of development viscous 

flow in the saturated soil centers and creep due to dilation and subsidence of the centers are effective in elongating the frost scars. During the raising of the marginal peat ridges differential frost thrusting 

also elongates the soil centers. Frost thrusting is most effective down 

slope and relatively ineffective upslope. As a result, the peat ridges 

are raised highest at the down-slope ends of the rings and commonly 

are missing at the up-slope ends. After a well-developed peat ridge 

has been formed, down-slope movement in the soil centers is retarded 

by the peat itself and by the presence of shallow frozen ground within 

the ridge. Creep and viscous flow reduce the slope of the soil area 

within the peat ridge to a lower angle than that of the inter-ring
CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

STAGE B

STAGE C

STAGE D

IM 

STAGE E.
FROST ACTION ON SEWARD PENINSULA

<-

-JL.

STAGE G

EXPLANATION

Peat or turf

Perennially frozen 

peat or soil

Seasonally frozen 

peat or soil

Cotton gross tussocks I 0

i i i , i

Silty mineral soil

Sedge sod

Birch or willow

5 Feet

FIGURE 28. Diagrammatic sketches showing evolution of a peal; ring. Stage A, Unusually warm summer. In most areas depth of thaw does not extend below base of peat,, 

but mineral soil is thawed in a few areas where peat is exceptionally thin. Stage B,. 

Late autumn, same year. Thawed mineral soil expands upon freezing and breaks overlying thin cover of peat, initiating a frost scar. Stage G, Early autumn, several years, 

later. Frost scar has been enlarged by shoving aside of peat at margins of bare soil 

area. Surface of scar expands upward as surface layers of soil freeze and expand. 

Dilation cracks (miniature zellenboden) are formed. Stage D, Late autumn, several 

years later. After surface layers of soil have frozen, a hard cover of frozen soil resists 

further uparching. Continued expansion resulting from freezing of deeper layers of 

soil is relieved by lateral thrusting. Ridge of peat is raised around bare soil center. 

Stage E, Late autumn, several years later. Ridge of perennially frozen ground beneath 

the peat ridge and thick layer of seasonally frozen ground in peat at the surface prevent further widening of the bare soil area by lateral thrusting. Instead, lateral expansion of soil center results in intrusion of masses of silt into peat of the ridges, forcing 

some of the peat to move in the opposite direction along the surface of the frozen 

ground beneath the frost scar. Stage F, Summer, several years later. Seedlings of 

cottongrass take root at inner edge of peat ridge. Stage G, Early autumn, same year. 

Bare soil has frozen to a depth of several inches, but soil beneath tussocks is still; 

thawed. Lateral expansion forces freezing soil beneath tussocks, heaving them upward.. 

Peat ridge no longer affected by lateral thrusting from soil center.
84 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

areas, but are no longer effective in increasing the length of the 

feature. Further elongation can result only from displacement of 

the peat ridge by down-slope frost thrusting within the soil center. 

Movement in solifluction lobes consists chiefly of creep and viscous 

flow, but frost thrusting may be a minor factor. The origin and 

development of solifluction lobes will be discussed in a future paper.

TUSSOCK KINGS AND TUSSOCK GROUPS

A complete gradation exists between peat rings and tussock rings. 

Tussock rings consist of closely spaced plants of Eriophorum vaginaFIGURE 29. Small tussock ring. Scale is indicated by jackknife thrust into center of 

bare soil area. Note miniature zellenboden.

turn subsp. spissum surrounding a low mound of silty mineral soil, 

which, as in peat rings, is bare or covered with various amounts of 

Eriophorum angustifolium (fig. 29). Tussock rings occur in habitats 

similar to those in which peat rings are found, such as poorly drained 

lowlands, terraces, and hill summits where peat has accumulated to 

a depth of a few feet. No tussock rings were seen on slopes steeper 

than 1°.

The rings range from 4 to 12 feet in diameter and are spaced at 

intervals of 30 to 100 feet in the Imuruk Lake area. The surface of 

the mineral soil is distinctly convex and at the center stands 3 to 6 

inches above the soil at the margin. The entire soil center extends
FROST ACTION ON SEtWARD PENINSULA 85

slightly above the general level of the marsh. A network of miniature zellenboden invariably is present on the surface of the soil.

The low, relatively broad mound of mineral soil is surrounded by 

a ring of tussocks which grow on the inner margin of a low, inconspicuous ridge of peat. The peat ridge stands 4 to 8 inches above 

the general level of the surrounding marsh but only slightly above the 

soil at the center of the ring. Species of 8pha<]nwn moss grow on 

the ridge and constitute part of its height. The ring of tussocks is 

1 to 2 feet wide and stands 8 to 12 inches above the marsh and 4 to 6 

inches above the soil center. Dwarf birch and heath grow in the 

Sphagnwn and around the outside of the ring and around the individual tussocks.

In cross section the tussock rings are similar to peat rings. They 

occur on local mounds of silty mineral soil extending to the surface 

through the otherwise continuous mantle of peat. The soil centers 

are underlain by a 1-foot to 2-foot depression in the frost table, as 

in the peat rings. Fingerlike projections of silt extend into the peat 

at the margins, and contorted stringers of peat are strung out through 

the silt beneath the central part of the rings.

Recently initiated tussock rings were observed in 1948 on a broad 

hill summit in the Imuruk Lake area from which the vegetation had 

been cleared in 1945 with a bulldozer. All of the vegetation, as well as 

6 to 12 inches of peat and mineral soil, was removed during the clearing. The hill summit apparently was rather well drained and supported a dense cover of tussock-birch-heath vegetation before clearing. 

Accelerated thaw followed removal of the original vegetation. The 

area became poorly drained, and in 1948 there were several small 

ponds.

Frost scars, peat rings, or tussock rings were present before the area 

was cleared, and the surface of the soil centers had undergone only 

slight modification 3 years later. Tractor tracks left in 1945 were 

still visible on the soil centers in 1948. New zellenboden had formed, 

adapting their pattern in part to the rectilinear indentations left by 

the cleats of the tractor treads. Young tussock rings were forming, and 

vegetation had begun to colonize the centers.

.The rings consisted of widely spaced young tufts of cottongrass 

that were less than 1 year old when observed. The centers of most 

of the rings were bare, but a few supported small stands of Eriophorum 

angustifolium. Peat surrounded the rings and extended to the bare 

soil, in the spaces between tufts of cottongrass. The age of the tufts 

was substantiated by the fact that no dead leaves were present at the 

base of the stems. Moreover, the plants had not developed the typical 

tussock form, and the stems rested on the mineral soil.
86 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

The ringlike pattern of the tussocks in the cleared area is due to the 

growth of tussocks on a frost scar formed by congeliturbation. At 

first it was thought that the tufts had been pushed or moved from 

random distribution on the centers of the scars, to their marginal 

positions by frost heaving somewhat in the manner that large stones 

migrate to the margins of stone polygons. The well-preserved tractor 

tracks and the undisturbed roots of the plants indicate, however, that 

the surface of the rings has not been sufficiently disturbed by frost to 

cause such a migration. Instead, conditions for the growth of Eriophorum vaginatum subsp. spisswm, are more favorable at the margins 

of the rings than at the centers. Drying winds, formation of needle 

ice, and heaving of seedlings from the soil are effective in preventing 

young plants from becoming established in the centers of the rings. 

At the margins, however, the small amount of peat on the surface 

may insulate the soil sufficiently to inhibit frost action and may lessen 

the water loss from the soil, so that seedlings can develop into mature 

tussocks.

Development of tussock rings on frost scars or peat rings in areas 

undisturbed by human activities probably proceeds in similar fashion. 

At the inside edge of the peat ring, where thin peat overlies the bare 

soil, a seed bed exists similar to that in the "artificial" frost scar described above. Scattered small tussocks have been observed on this 

inner margin in some rings; with the growth of more cotton grass 

plants, a complete ring of tussocks will be formed (fig. 28. F).

Once a complete circle of tussocks has developed, autumn frost 

thrusting ceases to affect the peat ridges, because a zone of thawed 

soil beneath the young tussocks intervenes between the ridge and the 

soil center. Instead, frost thrusting forces soil from the freezing 

centers into the thawed areas beneath the tussocks, and the tussocks are 

heaved upward (fig 28, G}. Repeated cycles of heaving eventually 

raise the tussocks higher than the adjoining peat ridge, and a typical 

tussock ring is formed. Few tilted or overturned plants are seen, 

which indicates that the tussocks themselves are not moved outward 

but only upward. Outward movement would not be expected, because 

the compact tufted part of the tussock stands entirely above the soil, 

and the thrusting goes on several inches below the soil surface.

Certain tussock rings are two tussocks wide, and in those rings the 

outer tussocks are higher than the inner ones. The "two-storied" rings 

represent a ]ater stage in the interaction of congeliturbation and plant 

growth. After a complete ring has developed, or perhaps while it-is 

still developing, additional seedlings start on the mineral soil just 

inside the older rings. The relief between the two rings is maintained, 

because both rings are forced upward by frost thrusting and heaving 

continued over several years.
GEOLOGICAL SURVEY BULLETIN 974 PLATE 3

CHARACTERISTIC TERRAIN OF THE IMURl'K LAKE AREA.

Vertical aerial photograph showing distribution of tussock-birch-heath polygons. 

These polygons are represented by the stipples and short dashes in white. 

Larger polygons arc ice wedge polygons (lower left and upper right). Note 

partly drained thaw lake at right margin, small stream with angular course 

determined by trenches of ice wedge polygons at hot ton margin, and swale 

overgrown with sedge sod at right center. Photograph by U. S. Air Force.
FROST ACTION ON SEIWARD PENINSULA 87

Isolated circular groups of tussocks are present in some sedge 

marshes and seem to be related to tussock rings. All characteristics of 

site and soil are similar in both features, and the tussock groups differ 

from tussock rings only in the nature of the vegetative cover of the 

soil centers. In the tussock groups the centers are entirely covered 

with tussocks, whereas in the tussock rings the centers are bare or 

covered with Eriophorum angustifoUum.

The development of two banks of tussocks in some sites and of 

circular clusters of tussocks in others suggests the mechanism by 

which some tussock rings become colonized by Eriophorum vaginatum 

subsp. spisswn and by which the intensity of congeliturbation is reduced within the soil centers. The bare centers of some rings may 

become covered with tussocks by inward advance. The addition of 

successive rows of tussocks at the inner edge of the rings eventually 

may result in covering the surface of the low mound of mineral soil. 

Each new row of tussocks reduces the central area subject to early 

freezing (fig. 28, G}. As the bare soil area becomes smaller, frost 

thrusting at the margins becomes less intense, and with the addition 

of the final tussocks at the center, congeliturbation reaches a minimum. 

Even in this comparatively stable state, however, congeliturbation is 

more active beneath the tussocks than in the adjacent areas covered 

with a thick peat layer.

Tussock rings with centers initially covered with Eriophorum 

angustifoUum may become covered with tussocks by the growth of 

seedlings in random distribution throughout the centers. Scattered 

cottongrass tussocks are found among the E. angustifoUum in some 

rings; with continued growth and development of the tussocks, E. 

angustifoUum eventually could be replaced. It should be emphasized 

that not all peat and tussock rings pass through the sequences outlined above. On some sites, peat or tussock rings surrounding areas 

of E. angustifoUum or bare soil appear to represent the vegetation of 

the feature in equilibrium with the present climate. Even when the 

cover of cottongrass tussocks is complete, congeliturbation is more 

intense in the mineral soil than in the adjoining peat. During a 

severe winter isolated tussocks can be overturned and new frost scars 

initiated on the site of the old peat rings.

TUSSOCK-BIECH-HEATH POLYGONS

Tussock-birch-heath polygons in the Imuruk Lake area consist of 

dwarf, shrubby, woody plants growing in a network of peat-filled 

channels. The channels enclose areas of mineral soil covered by 

closely spaced cottongrass tussocks. The tops of most of the tussocks 

are level with the peat in the channels, so that the features have no 

obvious relief. During the summer the pattern in the vegetation and
88 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

soil is striking from the air (pi. 3) but is not immediately obvious to 

the casual observer on the ground. In autumn, however, the contrast 

between the rows of red birch and heath and the intervening areas of 

yellow-green cottongrass becomes striking.

Tussock-birch-heath polygons occur on sites that are better drained 

than those on which peat rings, tussock rings, and tussock groups are 

found. The polygons occur on relatively dry uplands and on modT 

erately well drained intern1 uves on slopes. On slopes of less than 3° 

the polygons are equidimensional and range in diameter from 7 to 15 

feet. On slopes of 4° to 5° the nets become elongate, and the down- j 

slope axes are as much as 100 feet in length. On still steeper slopes 

the peat-filled channels extending across the slope disappear completely, and the pattern consists of continuous alternating stripes of 

birch-heath and cottongrass tussocks extending down slope. Tussockbirch-heath nets commonly are present throughout broad dome-shaped 

hills with a thick mantle of soil. The lineation of the elongate polygons resembles a flow pattern in aerial photographs (pi. 3). The 

direction and degree of maximum slope on any part of the hill can 

be estimated from the pattern of the polygons.

In cross section the polygons consist of closely spaced dome-shaped 

mounds in the mineral soil, each underlain by a discrete basin in the 

frost table (fig. 30). Thick layers of peat are present in the gullies 

between the domes in the mineral soil and are underlain by ridges in 

the frost table that commonly rise high enough to intersect the base 

of the peat. Thus each polygon is a closed cell of mineral soil, at 

least until late August or early September, when the entire partition -: 

of peat may become thawed. Involutions of mineral soil extend into l~" 

the peat, and stringers of peat extend into the mineral soil along the 

base of the annually thawed zone.

The soil centers are composed of blue-gray and reddish-brown silty % 

soil (fig. 30). The reddish-broAvn soil is found in a well-defined zone . 

an inch or less thick beneath the hnmic materials at the surface and 

adjacent to the peat at the margins and in a poorly defined, ragged 

zone a foot or more thick extending along the frost table at the base 

of the soil center. The basal zone consists of mixed and involuted 

masses of reddish-brown soil, blue-gray soil, humic material, and peat. 

Stringers of reddish-brown soil extend upward into the blue-gray 

soil, which composes the remainder of the center. The reddish-brown 

color is most intense in the surface zone, in the zone adjacent to the 

marginal peat, and at depth in zones adjacent to masses of peat and 

living and dead cottongrass roots.

In the absence of chemical analyses the writers assume that iron 

is present in the blue-gray soil as FeO and in the reddish-brown soil 

as Fe2O3 . An alternative possibility would be that iron is present as
i i

COT TON-GRASS-^---BIRCH-HEATH -- -»U-- -- COTTON-GRASS TUSSOCKS--- 

TUSSOCKS

SPHAGNUM MOSS

PEAT

REDDISH-BROWN SOIL

BLUE-GRAY SOIL

CLEAR ICE

FROZEN GROUND

TIGDRE 30. Cross section through a tussock-birch-he
90 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

Fe2O3 in both soil types but that most of the iron has been leached 

from the blue-gray soil and redepositecl in the reddish-brown soil. 

Reddish humic acids also may contribute to the pigmentation. In any 

case, the close association of the reddish color of soil with living and 

dead vegetation suggests a genetic relationship. The writers believe 

that the reddish color of the soil originates adjacent to masses of organic matter, and that congeliturbation later mixes the reddish soil 

with the surrounding blue-gray soil.

Tussock-birch-heath polygons probably originate and develop much 

as tussock rings do. The mounds of mineral soil represent frost scars 

that have been colonized and partly stabilized by cottongrass tussocks, and the peat in the surrounding channels represents the remnants of a once fairly continuous thin layer of turf, which now constitutes a favorable site for the growth of birch,, heath, and Sphagnum.

In most areas the polygonal or striped pattern in cottongrass-birchheath vegetation appears to be very old and in equilibrium with the 

present environment. The polygons cannot be regarded as "inactive," 

however, because from time to time the pattern is altered in detail by 

the development of new frost scars, which later become colonized by 

vegetation and once more become an integral part of the reticulate 

pattern. The features are in equilibrium, nevertheless, because both 

the initial and the final pattern in this developmental sequence is the 

tussock-birch-heath polygon. In most areas the polygonal pattern 

originated in the past in response to small environmental changes.

It is possible that new polygons can originate in areas of structureless cottongrass-birch-heath vegetation growing on fairly well 

drained slopes covered with a nearly continuous mantle of turf. 

Where this takes place, polygon development is believed to involve 

the following sequence of events. Scattered frost scars disrupt the 

thin turf cover. In the early stages the scars are closely spaced but 

not continuous. The isolated frost scars acquire rings of cottongrass 

tussocks about their margins much as the tussock rings described above 

do. By growth of additional tussocks around the inner margin of 

the rings the surface of the frost scar becomes covered. Eriophorum 

angustifolium invades many of the centers. The insulating effect of 

the plants and dead leaves and the stabilizing effect of the roots lessens the intensity of congeliturbation. This plant cover then is invaded by E. vaginatum subsp. spissum, and a cluster of tussocks is 

formed.

In the meantime new frost scars form in areas between the original 

scars. When the scars become closely spaced, drainage is concentrated 

in the network of marginal channels. Peat shoved from the centers of 

the scars fills many of the channels, and on the peat Sphagnum, mosses 

grow, adding to the thickness of the peat and forming a habitat in
FROST ACTION ON SEWARD PENINSULA 91

which dwarf birch and heaths can grow. The internal structure of 

the peat in the birch-heath areas indicates that the peat ridges owe 

their relief almost entirely to differential peat accumulation. The 

involutions in the peat ridges reflect lateral thrusting from the soil 

centers, but the effect of the thrusting seems to be to fold the peac 

deeper into the soil rather than to raise it above the surface.

The distribution of decomposed fragments of peat in the mineral 

soil of the polygons along the base of the annually thawed layer 

and upward at the center (fig. 30) suggests a sort of connective circulation of material within individual polygons. A similar pattern 

was noted within peat rings and tussock rings (fig. 28, B). For the 

origin of stone polygons, Low (1925) and Gripp and Simon (1933, 

1934a, 1934b) proposed a theory of convection currents in a suspension of soil in water at temperatures near the freezing point. The 

Low-Gripp theory is generally discredited by geologists for a variety 

of reasons (Sharp, 1942, pp. 286-287) and can readily be ruled out 

here because of the undistributed state of the tussocks and because the 

soil centers of the tussock-birch-heath polygons always are relatively 

dry and Avell-drained. The distribution of the peat in the mineral 

soil suggests, however, that material is moved from the peat ridges 

at the margins, along the frost table, to the centers of the polygons, 

and at the centers, from the frost table toward the surface.

The movement of peat toAvard the centers of the polygons probably 

represents a reactive movement in response to the intrusion of mineral 

soil into the peat-filled channels. After several autumn frosts a thick 

surface layer of frozen ground is present throughout the soil centers 

except beneath the tussocks. The surrounding peat consists of a 

rigid, frozen surface layer, strengthened and anchored to the adjoining mineral soil be a dense tangle of stems and roots of birch and heath, 

and an underlying layer of thawed peat. As freezing progresses 

downward the mineral soil expands laterally into the peat; the channels are compressed by lateral thrusting from the freezing soil centers 

on both sides, and part of the peat is squeezed outward and downward 

into the thawed ground underlying the soil areas. Heaving of the 

peat between the two mounds is not accomplished because the rigid 

layer of frozen peat, reinforced by living stems, is sufficiently strong 

to resists upward movement. With repeated cycles of thrusting the 

peat moves by small annual increments to the center of the soil areas. 

Each increment of movement must be accompanied by a slight uparching of the surface of the mound. The upward projecting stringers 

probably originate after the peat has become widely distributed 

beneath the soil centers. During the early part of the freezing cycle, 

while cottongrass is being heaved, peat becomes involved in upward 

movements in the zones beneath the tussocks.
92 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

On slopes the development of the polygons is affected by the downslope movement of soil, elongate polygons being formed on gentle 

slopes and stripes on steeper slopes. Frost thrusting and creep due 

to dilation and subsidence are the chief factors in down-slope movement. The observations 'of the writers indicate that the moisture 

present is insufficient to permit viscous flow in tussock-birch-heath 

polygons or stripes during summer and autumn. Meltwater from 

snow furnishes adequate moisture during early spring, but only a 

thin layer of soil is affected during that period because of the shallow 

depth of thaw, which ranges from 1 to 6 inches at that time of year.

AGE OF THE FEATURES

The cryopedologic features described above are active at the 

present time, and the discussion of their origin is actually a discussion 

of their perpetuation. New frost scars form and old frost scars are 

stabilized, but the total number that are present in a given site remains about the same as long as environmental conditions do not 

change. Local changes in drainage conditions related to lateral shifts 

in the positions of major streams on their flood plains or to the formation or destruction by thaw lakes (Hopkins, 1949) are reflected by small 

changes in the distribution of frost scars, peat rings, tussock rings, 

and tussock-birch-heath polygons. In general, however, the distribution of these features has remained nearly constant as long as 

the present climate has prevailed. Small changes in the climate probably have resulted in the initiation of new areas, variations in the 

degree of activity in some preexisting areas, and the destruction of 

other preexisting areas. Greater changes in the climate may have resulted in the complete elimination of some of the features.

There is evidence of major climatic fluctuations within Quaternary 

time in many parts of Seward Peninsula. At least two stages of 

glaciation can be distinguished in the Kigluaik and Bendeleben Mountains. The younger stage is subdivided tentatively into several substages. Present knowledge in Alaska is inadequate for an attempt to 

correlate the glacial stages on Seward Peninsula with the chronologies 

established elsewhere, but the age of the younger glaciation is thought 

to be about the same as that of the Wisconsin glaciation in the United 

States. The climate of Seward Peninsula was warmer than at present 

during some parts of Quaternary time. Spruce and birch logs and 

bones of extinct mammals are found together in deposits of Quaternary 

age as much as 50 miles west of the present timber line.

Widespread stabilized rubble fields, stone polygons, and stone stripes 

testify to the existence of a past climate even more rigorous than the 

present in the Imuruk Lake area. Most of these features are believed to have been active during the younger glaciation and during 

earlier cold periods.
FROST ACTION ON SEIWARD PENINSULA 93

Evidence concerning the character of "postglacial" 3 climates is 

scanty. Low terraces and beach lines at Imuruk Lake and several 

smaller lakes suggest the following: Lake levels lower than the present 

levels (subaerial peat is found several feet below the present lake level 

in some terraces); a slow rise in water levels to an elevation 3 to 7 

feet above the present lake levels; and a slow drop to the present level. 

The fluctuations in lake levels may have taken place in response to 

fluctuations in the balance of precipitation, runoff, and evaporation. 

If this is true, the low lake levels represent a period warmer than 

the present during which the rate of evaporation was high relative to 

precipitation and surface runoff, and the later high lake levels represent a period slightly colder than the present during which the evaporation rate was low. Lowering of the lakes to their present levels would 

have taken place in response to a gradual warming of the climate 

with a consequent rise in the rate of evaporation relative to precipitation and runoff. Bones of extinct mammals are absent in the low 

terraces, although they are abundant in older higher terraces; hence, 

the sequence of events outlined above is believed to have taken place 

after the extinction of those mammals and after the last local 

glaciation.

Other evidence supports the view that recently the climate has been 

growing warmer on Seward Peninsula. Small actively growing gullies in some swales suggest that in favorable localities soil movements 

due to congeliturbation have been retailed or have ceased, and that 

stream erosion has become important. Areas of alder are expanding 

in the region northeast of Imuruk Lake. The area of spruce is expanding on the Fox River, southwest of Council, and spruce is growing 

vigorously on all favorable sites in that area.

Under ordinary conditions later congeliturbation rapidly destroys 

evidence of past generations of frost scars, peat rings, tussock rings, 

and tussock-birch-heath polygons; consequently, speculations on the 

past distribution of these features is, at best, an intelligent guess. 

Present knowledge does permit speculation upon the effects of past 

climates upon depth of thaw and drainage conditions .in various 

sites, however, and the resulting distribution of cryopedologic features 

can be inferred.

All four features, as well as most of the same plant species, probably 

have been present in the Imuruk Lake area throughout postglacial 

time and perhaps throughout the period of the last, glaciation on 

Seward Peninsula. The distribution, abundance, and degree of activity of the features probably has varied, however, with varying climate.

3 "Postglacial" is used here in an informal sense to indicate the period of time which 

has elapsed since the younger glaciation on Seward Peninsula.
94 CONTRIBUTIONS TO GE'NERAL GEOLOGY, 1950

STAGE A

STAGE B
FROST ACTION ON SEWARD PENINSULA 95

Peot rings and isolated tussock rings

Elongate peat rings

STAGE C

EXPLANATION

Cotton grass tussock-birch-heath polygons

Elongate polygons Willow shrubs

FIGURE 31. Diagrammatic sketches illustrating the effects of change in climate upon the 

distribution of cryopedologic features. Stage A, Distribution at the present time. Peat 

mounds are largely confined to low-lying swampy areas mantled by a thick layer of 

peat. Peat rings are present on poorly drained uplands and summits. Tussock-birchheath polygons are present on slightly drained summits and slopes. Large willows 

and birch shrubs are confined to swales containing well-defined streams. Stage B, 

Distribution of features if the climate were warmer than the present time. Drainage 

is much better because summer thaw extends to greater depths and because gullies 

are incised into swales and drainage lines. Birch and willow shrubs cover drainage 

courses and lower parts of slopes. Vegetation polygons are restricted to poorly drained 

summits, and peat rings are almost completely eliminated. Stage G, Distribution if 

the climate were colder than at present. Drainage is much poorer because summer 

thaw extends to shallower depth and because definite stream channels are lacking in 

swales and drainage lines. Peat mounds are present on poorly drained lowlands and 

summits. Vegetation polygons are restricted to steeper exceptionally well drained 

slopes (not shown). Large willow and birch shrubs are restricted to the flood plains 

of major streams (not shown).

During the altithermal period ("postglacial optimum," Antevs, 

1948), the depth of summer thaw probably was greater and drainage 

better than at present at nearly all sites. Frost action was less intense 

throughout most of the areas but more intense in certain sites. Depergelation caused the summer frost table to retreat into the underlying 

mineral soil in areas mantled by several feet of peat. Peat rings probably were eliminated from most of their present sites and may have 

disappeared from the region (fig. 31, B) .
96 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

Frost scars become more closely spaced on areas mantled by a 

thinner layer of peat, and tussock-birch-heath polygons developed 

where peat rings or tussock rings had existed before. Slopes at 

present covered with vegetation polygons or stripes may have become 

so well-drained that congeliturbation ceased, and cryopedologic features were entirely lacking or completely stabilized. The vegetation 

on these sites probably Avas dominated by birch and willoAv shrubs 

similar to the vegetation now growing on steep banks and exceptionally Avell drained slopes.

During the following cold cycle the depth of thaw was notably 

shallower than at present, and the drainage was poorer in most sites 

(fig. 31, 0). Congeliturbation and congelifraction were more active 

throughout most of the area but less intense in certain sites. In 

areas mantled by 2 to 3 feet of peat, the frost table rose above the 

base of the peat and no new frost scars could be initiated, although 

preexisting peat rings and tussock rings in these areas probably continued to be active. A shallower depth of thaw produced conditions that favored the growth of sedge sod on areas that had been 

covered with tussock-birch-heath polygons; peat rings and tussock 

rings may have been the only cryopedologic features present on these 

surfaces. Vegetation polygons probably could exist on only a few 

of the most favorable, best-drained surfaces.

The distribution of active cryopedologic features approached the 

present pattern with renewed Avarming of the climate. NeAV frost 

scars formed in areas mantled Avith 2 to 3 feet of peat. Cottongrass 

tussocks colonized the surfaces of peat and tussock rings on areas 

mantled Avith a thinner layer of peat (fig. 31, A ). Continued vigorous 

congeliturbation on these surfaces resulted in the formation of neAV 

closely spaced frost scars, which later Avere colonized by tussocks. 

Eventually the sedge sod was destroyed, and the present polygons became dominant on the sites on which they are found today.

CONCLUSIONS

Frost scars, peat rings, tussock rings, tussock groups, and tussockbirch-heath polygons are characteristic features in areas where silty 

mineral soil is present beneath a mantle of peat or turf less than 3 feet 

thick. These features represent stages in several developmental series, 

all of which start Avith the frost scar as the initial form. Each feature 

represents the equilibrium feature for certain environments and supports the vegetation which is in equilibrium Avith that environment.

Closely spaced frost scars in relatively Avell drained areas where the 

soil is mantled by a thin, discontinuous layer of peat or turf evolve 

into tussock-birth-heath polygons. A different developmental series 

is encountered in poorly drained areas Avhere the peat mantle is approximately equal in thickness to the depth of annual thaAv. Here,
FROST ACTION ON SEWARD PENINSULA 97

frost scars are initiated only at the widely separated points where 

summer thaw extends through the peat into the underlying mineral 

soil. Such frost scars evolve into peat rings, and in certain marshes the 

peat rings evolve further into tussock rings. The tussock rings in a 

few sites ultimately become completely colonized by cottongrass, and 

tussock groups are formed. In many marshes, however, tussock rings 

or peat rings appear to represent the ultimate, equilibrium forms, with 

which the developmental sequence ends.

Frost scars initiated in rocky soils evolve into forms that lie beyond 

the scope of this discussion. If rocks are sufficiently abundant, segregation of coarse and fine material forms stone polygons or with the 

growth of vegetation on slopes forms turf-banked terraces.

Frost scars are lacking in areas mantled by a layer of peat that is 

thicker than the depth of annual thaw. Frost heaving in these areas 

occurs entirely within the peat and results in the formation of peat 

mounds of irregular shape.

It is apparent that even the most liberal concept of "climax vegetation" (Cain, 1947, p. 193) must be modified when applied to much of 

the vegetation in regions of intensive frost action (Sigafoos, 1949). 

Disturbance of the substratum is an environmental factor that generally is disregarded in theoretical discussions of the development of 

A^egetation into climax communities. In temperate regions most plant 

succession proceeds upon an essentially stable substratum, and disturbances are rare in the time required to attain the climax vegetation. Repeated disturbance of the substratum is an important environmental factor, however, in most plant communities in tundra regions. 

Plant succession on frost scars proceeds upon a substratum that rarely 

becomes completely immobile, and the succession may be arrested at 

any point by continued mild heaving, or it may be interrupted by renewed heaving sufficiently violent to create a new bare spot. The final 

stage in the succession on many frost scars consists of a complete cover 

of cottongrass tussocks, but even this final stage exists on a mobile 

surface the instability of which contributes to the exclusion of possible 

later successional stages. The vegetation in areas of frost scars, peat 

rings, tussock rings, and tussfcck-birch-heath polygons can be regarded 

as an equilibrium assemblage adjusted to the environment in which it 

exists but differs from a climax assemblage because areas of bare soil 

and areas being actively colonized by pioneer plants always constitute 

important elements in the vegetation, interspersed among larger 

areas covered by vegetation which represents the highest stage in the 

succession.

Palmer and Rouse (1945) believe that a plant cover consisting 

predominantly of lichens is a later developmental stage in areas characterized at present by tussock-birch-heath vegetation. They state 

that reindeer are effective in destroying the lichen-dominated vegeta-
98 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

tion, which then is replaced by cottongrass tussocks. If the tussock 

vegetation is proteced from grazing, however, it will be replaced 

again by lichens, according to Palmer and Rouse. We have seen 

polygons of birch and heath within a lichen-dominated vegetation, 

but the area in which these polygons were found was better drained 

than most areas in which tussock-birch-heath vegetation is present. 

Therefore, it is believed that continued congeliturbation prevents 

tussock-birch-heath polygons from evolving into lichen-dominated 

vegetation, and thus the polygons can be regarded as the equilibrium 

feature covered by the equilibrium vegetation in most areas in which 

the polygons are found. During periods of Avarmer climate, when 

better-drained soils were more widespread, lichen-dominated vegetation probably occurred within areas where tussock-birch-heath 

polygons now exist. Lichen-dominated vegetation may have been 

an intermediate stage in the development of willow-birch-heath shrub 

vegetation on better-drained soils and probably persisted on a few 

sites throughout the warm period.

Some of the cryopedologic features described in this paper eventually may be recognized in "fossil" form in northern United States 

or Europe. Peat rings and vegetation polygons are the features most 

likely to be preserved in recognizable and distinctive forms; even 

these are likely to be recognized only where exposed in cross section. 

Chances of preservation would be best if the features were buried 

beneath a mantle of loess or alluvium. Some of the "involutions" and 

"plications" described by previous authors may represent ancient 

peat rings or tussock-birch-heath polygons. In "fossil" form peat 

rings would consist of isolated stubby columns of silt in a layer of 

peat. Tussock-birch-heath polygons would consist of a crudely polygonal network of peat masses surrounding stubby columns of mineral 

soil. They could be distinguished from ancient ice wedge 'polygons 

("Taimyr polygons") by their small vertical and horizontal dimensions. Ice wedge polygons generally are 25 to 100 feet in diameter 

(Leffingwell, 1919, pp. 205-212), although polygons as small as 15 

feet in diameter are not uncommon (Black, R. F., personal communication) . Tussock-birch-heath polygon* generally are less than 15 

feet in diameter. Ice wedges of ice wedge polygons generally have 

a vertical extent in excess of 5 feet. Structureless masses of peat, soil, 

or till occupy the former sites of the ice wedges, and these also generally have vertical dimensions in excess of 5 feet (Schafer, 1949, pp. 

165-169; Horberg, 1949, pp. 132-134). The peat masses of tussockbirch-heath polygons, on the other hand, range from 1 to 3 feet in 

vertical dimension.

The peat rings, tussock rings, and tussock-birch-heath polygons 

studied by the writers were found in areas where perennially frozen 

ground lies at shallow depth; the presence of perennially frozen
FROST ACTION ON SEWARD PENINSULA 99

ground appears to be an essential factor in their origin. It can be 

assumed that "fossil" forms of these features indicate the former 

presence of perennially frozen ground. In modern rings and polygons, perennially frozen ground lies a few inches above the base of 

the peat at the margins of the soil centers. Stringers and lenses of 

humic material are concentrated at the base of the annually thawed 

zone within the soil centers. The vertical position of these features in 

fossil rings and polygons should afford a reliable measure of the 

former depth of summer thaw.

Kecognition of the cryopedologic features described here is useful 

to the photointerpreter in predicting trafficability and foundation 

problems. Peat rings, tussock rings, tussock groups, and tussockbirch-heath polygons can be recognized on aerial photographs with 

a scale of 1:10,000 or larger. Peat rings, tussock rings, and tussock 

groups generally cannot be distinguished from one another on these 

photographs, but these three features can be distinguished from 

tussock-birch-heath polygons or stripes. The presence of any of 

these features indicates the probable presence of fine-grained soil 

in which perennially frozen ground is present at shallow depth. 

The thickness of peat at the surface and the moisture conditions of 

the surface can be estimated if the polygons can be distinguished 

from the rings. Maximum degree of slope can be estimated from 

the degree of elongation of the features.

BIBLIOGRAPHY

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times; III, Climatic changes and pre-white man: Utah Univ. Bull., vol. 38, 

no. 20, pp. 168-191.

BRYAN, KIKK, 1946, Cryopedology the study of frozen ground and frost action, 

with suggestions on nomenclature: Am. Jour. ScL, vol. 214, pp. 622-642.

 1949, The geologic implications of Cryopedology: Jour. Geology, vol. 57,

pp. 101-104.

CAIN, S. A., 1047, Characteristics of natural areas and factors in their development : Ecol. Mon., vol. 17, pp. 185-200. 

EAKIN, H. M., 1916, The Yukon-Koyukuk region, Alaska: U. S. Geol. Survey Bull.

631. 

ELTON, C. S., 1927, The nature and origin of soil polygons in Spitsbergen: Geol.

Soc. London Quart. Jour., vol. 83, pp. 163-194. 

GLEASON, H. A., 1917, The structure and development of the plant association:

Torrey Bot. Club Bull., vol. 44, pp. 463-481. 

GUIGGS, R. F., 1934, The problem of arctic vegetation: Washington Acad. Set.

Jour., vol. 24, pp. 153-175. 

GRIPP, KARL, and SIMON, W. G., 1933, Experimente zum Brodelbodenproblem:

Centralb. f. Mineral., Geol., Paliiontol., Abt. B, pp. 433-440.

 1934a, Die experimentelle Darstellung des Brodelbodens: Neues Jahrb. f. 

Mineral., Geol., Paliiontol., Referate II, p. 601.

 1934b, Nochmals zum Problem des Brodelbodens: Centralb. f. Mineral.,

Geol., PaUiontol., Abt. B, pp. 283-286.
100 CONTRIBUTIONS TO GENERAL GEOLOGY, 1950

HOGBOM, BKKTIL, 1914, liber die geologische Bedeutung der Frostes: Upsala Univ.,

Geol. Inst. Bull., vol. 12, pp. 257-389. 

HOPKINS, D. M., 1949, Thaw lakes and thaw sinks in the Imuruk Lake area,

Seward Peninsula, Alaska: Jour. Geology, vol. 57, pp. 119-131. 

HORBERG, LELAND, 1949, A possible fossil ice wedge in Bureau County, Illinois:

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Band 44, no. 1, pp. 1203-1341. 

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INDEX

Page 

Acknowledgments for aid._-.....-----.-.... 54-55

Bendeleben Mountains, glaciation in__..... 59,92

Candle, temperature at..._............_..

Climate, in Quaternary time...-.-----.-.----.

of the area.................... 55,57-58,02-

postglacial._...................-.-...-....

Congelifraction, definition....................

soil produced by......_..--.--.-.--..-..

Congeliturbation, definition..................

effect on soil and vegetation..............

Cryopedologic foaturcs, age..........._.....

character and formation............... 61,

fossil, possible recognition of..............

interrelationships..-.----..----.-.-.---...

significance of, in predicting t.rafficability

and foundation problems...----..

Cryopcdologic processes, definition...........

effect on soil surface.---.....---...---....

factors affeetin g_...----..-.-------.--...

Cryopedologic surface features, definition ....

Definitions.-.-....---.--. 

Depergelation, definition. 

Drainage in the area......

Frost heaving and thrusting, factors affecting. 

Frost scars, character and formation-..-..-...

Geology of the area.... 

Glaciation, occurrence.

Heath, definition......

Intensive frost action, definition, 

effects of--.......-...-.-.....

Kigluaik Mountains, glaciation in. 

Location of area studied._......

55, 57 

92

 93,95 

93,95 

53 

59

53-54 

52, 63 

92-96 

66-92 

52. 98 

96-99

99 

53

52,63 

60-62 

53

53-54

53

58-59

61-62 

66-70

59

59,92

53

53-54 

52

92

55,56

Peat, character and occurrence.........

definition..-.-.-..........----...-........

relation to frost heaving..................

Peat rings, character and formation...........

Perennially frown ground, character and 

depth............................

relation of, tocryopedologicfeatures.......

to frost action._._______._._____..._.._

Pergelation, definition.........................

Plant communities, distribution of...........

Precipitation in the area................... 55,

Purpose of the study.........................

Quaternary rocks, occurrence..... 

Quaternary time, climate during.

Page

59-60

53

61-62 

76-84

60-61 

9S-99 

61-62

53

64-66 

57, 58

52

59

92

Sedge sod, plant species in.................... 65

Sod, definition............__................ 53

Soil, effect of cryopedologic processes on...... 52,63

origin and character...................... 50-60

Solifluction lobes, features.....--.----------'-. 79,84

Temperature of the, area..-...---..-.-.-... 55,57.58 

Terraces, occurrence.-----.-.-..-.-----.-.---. 59

Topography of the area_.................... 58

Tundra, vegetation of.........--..--..-----.. 64-6f>

Turf, definition............................... 53

Tussock-birch-heath community, plant species

Of ............._..--.-. ..-... 64-65

Tussock-hirch-he.ath polygons, character and

formation-..... '.................... 87-92

Tussock rings and groups, character and formation.--.............-----..-..-.-- 84-87

Tussocks, character and formation............ 70-76

Vegetation, effect of congeliturbation on....... 52,63

factors affecting distribution.............. 66

of the area....-...............-...---.-.--- 63-66

relation to frost heaving.................. 61-62-

o