Pingos in 

Central Alaska

By G. WILLIAM HOLMES, DAVID M. HOPKINS, and HELEN L. FOSTER

CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY' BULLETIN 1241-H

A study of the distribution, form, 

vegetation, hydrology, microrelief, 

volution, and age of several hundred 

wnall ice-cored hills, or hydrolaccoliths, 

In subarctic interior Alaska

JNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1968
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecoro, Director

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

Washington, D.C. 20402 - Price 25 cents (paper cover)
CONTENTS

Page

Abstract. _ ______________.___._._.___________..__._________....__.. HI

Introduction. _____________________________________________________ 2

Previous studies.__-___--_-______-_-_________--______________--____ 5

Environmental setting of central Alaska_____..________.__._.__...___. 7

Distribution of pingos..._____--___-_-__-___-__---__--__----__--____ 8

Regional distribution and density_________-___--___-___---_---_-- 8

Topographic distribution_______________________________________ 12

Geologic distribution,..._-_______-___-_-___-_______--___-__--__ 15

General description______..__________._.____.__.._______.__...._.._ 16

Representative pingos__ - _______________________________________ 16

Fairbanks Creek pingo (19)_________________________________ 16

McKinley Creek pingo (21)_-_______-_-_-_-_._--__._--_____-_ 24

Pioneer Creek pingo (23)___________________________________ 25

Discovery pingo (18)____--_-_-__-_--_--_---_--__-_--___-__- 27

Pingo 12______. _____.__..._.__...._..'..._..___.._.__..... 27

Pingo 7._..__._._..._-.__..__-_---_.__._--_____-_-_____.. 28

Size and shape.___-___-_____-___-________-__________-_-----_-- 30

Hydrology._______________-_--__-__-______-_-______--___-__-_- 30

Microrelief___ ---__--__-__________________--._-_-_---_--.---- 32

Discussion._ ______________________________________________________ 33

Speculations on the origin of trench networks _____________________ 33

Age...... __..........___..._...._._-___--._-__.-______--__-_- 33

Conclusions.._ ________--__--_____-__---_-__-____--___-_-__-____-_- 37

References cited.._________________________________________________ 39

in
IV CONTENTS

ILLUSTRATIONS

Page 

FIGURE 1. Index map showing areas of report in relation to Alaskan

permafrost zones. ____________________________________ H3

2. Map showing quadrangle location and vegetation of central

Alaska._____--__--___--_____-____--_----__--____-__- 4

3. Graph showing mean monthly temperatures for selected

stations _____________________________________________ 9

4. Map showing distribution of pingos in the Mount Hayes and

Tanacross quadrangles._-----______-_-__--.____.__.___ 10

5. Map showing distribution of pingos and their geologic setting

in the Manley Hot Springs area._______________________ 11

6. Diagram showing downslope direction measurements of pingo

sites in central Alaska___---_-_________________________ 12

7. Pingos in the upper Tanana area (stereoscopic pair). _______ 13

8. Photograph showing a pingo in the upper Tanana area...... 14

9. Diagram showing typical locations of pingos in the Manley

Hot Springs area.____________________________________ 16

10. Photograph showing section of the Fairbanks Creek pingo... 24

11. Diagram showing section of the Fairbanks Creek pingo...... 25

12. Profile of the McKinley Creek pingo...------....-.--._-_. 26

13. Map and profile of the Pioneer Creek pingo____._____.___-- 26

14. Photograph showing the Discovery pingo..._______________ 28

15. Sections of the Discovery pingo and its area.___________ ___ 29

16. Profile of a very old pingo.______________________________ 29

17. Diagrams showing postulated origin of trench networks.____ 34

TABLE

Page 

TABLE 1. Descriptive summary of pingos investigated in the field. ._.. H17
CONTRIBUTIONS TO GENERAL GEOLOGY

PINGOS IN CENTRAL ALASKA

By G. WILLIAM HOLMES, DAVID M. HOPKINS, and HELEN L. FOSTER

ABSTRACT

Scores of pingos have recently 'been discovered in interior Alaska, mostly in 

forested valleys of the Yukon-Tanana Upland in the zone of discontinuous permafrost. Heretofore, pingos were believed to be characteristic of, if not confined to, 

the tundra of the Arctic where permafrost is continuous, especially in the Mackenzie delta area of northwestern Canada, on the Arctic Coastal Plain and on 

Seward Peninsula in Alaska, and in Greenland.

Pingos of interior Alaska are circular, elliptical, or irregular in plan and range 

from 50 to 1,450 feet in width and 10 to 100 feet in height; none however, attain 

the size of the largest pingos of the arctic tundra region. Most are marked by 

fissures, slump blocks, craters, and disturbed vegetation, and many have flowing 

springs or ponds near their summits. Of about 270 pingos observed, most lie on 

gentle south-facing slopes near the sides of alluvium-filled valleys; none occur in 

old lake basins. Many pingos are alined along contours at the base of the slopes 

and near the transition between the slope mantle and valley-floor sediments. 

Some are clearly related to fracture zones where ground water can freely move. 

All are of the open-system type requiring a restricted quantity of subpermafrost 

or intrapermafrost water under artesian pressure. The pingos are composed of 

stratified alluvial silt; poorly stratified organic, eolian, or colluvial silt; or mixtures of arkosic sand and boulders derived from weathered bedrock. A few have 

clear, coarse- to fine-grained ice exposed.

Pingos appear in several stages of development that suggest a sequence of 

growth, decay, and rejuvenation. Some pingos have low, smooth summits and a 

vegetation cover not adjusted to their slopes and appear to be a few decades old. 

Others, having mature first- or second-generation forests adjusted to slope and 

soil-moisture conditions, seem to be older and show more signs of disturbance 

such as the fractures, steps, craters, ponds, springs, and trench networks on their 

summits. The mantle on pingos at this stage has formed poorly developed subarctic Brown forest soils that indicate an age of several centuries but probably 

less than about 4,000 years. Pingos in an even more advanced stage of development are represented only by ponds or marshes rimmed by low uneven ridges of 

tilted sediments; many of these pingos are the sites of second- and third-generation mounds forming within or adjacent to the initial pingo. One mature cratered 

steep-sided mound has a maximum radiocarbon age of about 7,000 years, a date 

that agrees with other radiocarbon dates in Alaska and Canada and indicates 

that pingo growth is a product of the Recent climate.

HI
H2 CONTRIBUTIONS TO GENERAL GEOLOGY

Most pingos form where subpermafrost or intrapermafrost water is likely 

to attain maximum hydrostatic head and where blockage and localization of 

artesian flow may occur. However, theoretical calculations and limited groundwater data suggest that hydrostatic pressure alone is insufficient to overcome 

the tensile strength of the permafrost layer and the static load of the overburden. 

Freezing pressures probably are also required to lift open-system pingos.

Trench networks on mature-appearing pingos probably result from thawing 

of intersecting vertical ice wedges that transect the overburden and the pingo 

ice. Although modern frost wedges are rare in central Alaska, pingos may provide 

ideal thermal conditions for thedr growth under the present climate.

Present knowledge of the distribution of pingos in northwestern North 

America suggests that open-system pingos are abundant in subarctic regions of 

discontinuous permafrost and are rare in arctic regions of continuous permafrost, 

whereas closed-system pingos are mostly confined to arctic regions of continuous 

permafrost. Canadian geologists have suggested that the closed-system pingos 

on the arctic tundra of northwestern Canada formed at the onset of a cooler 

climate following the Hypsithermal interval. Although all the pingos that we have 

observed in interior Alaska are of Recent age, they cannot be clearly related to 

any single climatic fluctuation. Instead, the pingos of diverse ages there suggest 

that they have been forming continuously during Recent time.

INTRODUCTION

The term "pingo" in Eskimo means "small hill" and was first used in 

a precise scientific sense by Porsild (1938) to denote conical ice-cored 

hills that he found in abundance in unglaciated parts of northern 

Alaska and northwestern Canada. For several decades it has been 

known that pingos are conspicuous and abundant in many parts of 

Arctic America and Arctic Siberia; only recently has it been recognized 

that they are common, though less conspicuous, features of some 

forested regions of the subarctic.

This paper is an extension of a short publication (Holmes and 

others, 1966) which demonstrated that pingos are common features 

in the boreal forest region of central Alaska (fig. 1). The present paper 

describes several dozen pingos visited by the writers and speculates on 

the origin of their microrelief and on their age, evolution, and climatic 

significance.

The paper is based mostly upon studies by G. W. Holmes and H. L. 

Foster in the Tanacross quadrangle and in the eastern part of the 

Mount Hayes quadrangle and by D. M. Hopkins, assisted by K. L. 

Pierce, in the Manley Hot Springs area (fig. 2). We have been helped 

by supplementary information contributed by George Corchary, A. T. 

Fernald, and H. R. Schmoll on the locations of pingos in the Tanacross 

quadrangle; Florence Weber, T. L. Pewe, and R. M. Chapman on the 

locations of pingos in the Fairbanks, Big Delta, Livengood, Kantishna 

River, and Tanana quadrangles; R. E. Isto and Arthur Gervais on the 

locations of pingos in widely scattered parts of central Alaska; and 

D. B. Krinsley and W. E. Davies on the description of pingos near
PINGOS IN CENTRAL ALASKA H3

Circle. T. L. Pewe reviewed the manuscript and assisted us greatly 

with his comments.

FIGURE 1. Permafrost zones in Alaska. (From Ferrians, 1965.) Area of present 

report is shaded. Heavy line encloses area of figure 2.
156° 

FIGURE 2. Quadrangle boundaries, relevant place names, and generalize

from Sigafoos, 1958.)
PINGOS IN CENTRAL ALASKA H5 

PREVIOUS STUDIES

Pingos are the most conspicuous relief features on many low-lying 

tundra areas, and it is not surprising that they were noted by some of 

the first geologists in arctic Alaska. They were observed in western 

Alaska by Mendenhall (1901, p. 207) and in northern Alaska by 

Schrader (1904, p. 94). Leffingweil'l (1919, p. 54) speculated briefly on 

their origin.

Porsild (1938, p. 47-56) first proposed the currently accepted 

hypotheses on the mechanics of pingo formation. He noticed that 

pingos of northern Alaska and north-western Canada occur in two 

environments: (1) On sloping ground, in sandy or other permeable 

soil, 'and (2) on level ground, generally in old lake basins. Mounds 

on sloping ground are small, variously shaped, commonly ruptured at 

the top, and are presumed to have formed by hydraulic pressure. 

Mounds on level ground are commonly very large, as much as 230 feet 

high, and are presumed to have formed by local upheaval of the overburden during progressive downfreezing, such as might occur when 

a lake becomes too shallow to prevent formation of permafrost.

Interest in arctic pingos has been stimulated by quantitative studies 

in Canada and Greenland. A major published result is the treatise by 

Miiller (1959), which supports the conclusions of Porsild on the basis 

of thermal, geochemical, and geological evidence. In Miiller's classification, Porsild's two kinds of pingos are as follows:

1. The Greenland, or open-system type, which according to Miiller 

develops when subpermaf rost or intrapermaf rost water penetrates 

the permafrost under hydrostatic pressure, where there is a small 

temperature difference between the water and the frozen ground, 

and where the rate of flow is low. A hydrolaccolith then forms in 

the upper part of the permafrost and freezes, and hydrostatic 

pressure combined with crystallization pressure heaves the overlying frozen and unfrozen surficial material or, rarely, bedrock 

to produce a mound.

2. The Mackenzie, or closed-system type, which forms when unfrozen 

ground water in a talik becomes confined on all sides by permafrost. This occurs most commonly beneath the basins of shallow 

ponds that have been made shallower by the development of an 

outlet or by the accumulation of sediment or peat. Permafrost 

encroaches upon the talik as heat is lost, and the water in the talik 

becomes increasingly confined, migrates upward, freezes, and 

heaves the frozen overburden to form a mound (Miiller, 1959, 

p. 115-118). Washburn (1950, p. 47) notes that this Mackenzie 

type may also form in recently exposed marine till, which had

275-340 O 67 2
H6 CONTRIBUTIONS TO GENERAL GEOLOGY

previously been free of permafrost, after the till becomes exposed 

to low atmospheric temperatures; he cites pingos on Wollaston 

Peninsula of Victoria Island in the Northwest Territories as 

possibly having formed in this way.

Kecently Mackay (1962) has substantiated the observations of A. E. 

Porsild, Fritz Miiller, and others in the Mackenzie delta area. He 

notes that of 1,350 to 1,400 pingos observed, most occur in areas of 

drained lakes, are of the closed-system type, and are probably several 

hundred to several thousand years old. The greatest number occur in 

Pleistocene deposits, but a few young pingos have formed in the 

Recent sediments of the Mackenzie River delta.

Most previous studies of pingos have been concerned with those 

found in the tundra areas of the far north areas that have thick, 

horizontally continuous, and cold (several degrees below freezing) 

permafrost. However, Porsild (1938) based his description of pingos 

partly upon examples on northern Seward Peninsula; Hopkins and 

others (1955, pis. 40, 41) illustrate closed-system pingos in the tundracovered lowlands of central Seward Peninsula as far south as lat 65° 

N. and near the southern edge of the zone of continuous permafrost. 

Closed-system pingos occur even farther south in western Alaska 

where Burns (1964, p. 205) reported small pingos of this type in the 

Yukon-Kuskokwim delta area at approximately lat 61° N. The delta 

is in the area of discontinuous permafrost, but this fact apparently 

does not preclude the formation of closed-system pingos if the required 

hydrological and thermal conditions prevail.

Reference to pingos in the forested regions of the subarctic are 

rare. (See Miiller's (1959, p. 107-111) comprehensive bibliography 

on pingos.) Leffingwell (1919, p. 153) cities information obtained from 

G. L. Harrington that clearly established the existence of a pingo in 

the Ruby mining district in the lower Yukon River valley of central 

Alaska. Later, Mertie and Harrington (1924, p. 8) described in the 

same district several pingolike mounds that are similar to those described in our paper. A few references appear in the Soviet literature 

to pingos (bulgunnyakhi) from subarctic Siberia. Although precise 

locations are rarely given, pingos are reported in Yakutia (Kosmachev, 

1953), between Yakutsk and Okhotsk (Anger, 193Y, p. 191), in the 

Lower Tunguska River area (Kushev, 1934), in Zabaikal (Tolstikhin, 

1932; Andreev, 1936), and, in general, along the southern border of the 

permafrost zone (Tikhomirov, 1948). These localities are within the 

zone of discontinuous permafrost, according to Tumel' (1946), and 

where permafrost is much thinner than beneath the arctic coastal 

tundra (Baranov, in Tsytovich, 1958). A pingo shown by Kosmachev 

(1953) is in forested country and hence is probably in the zone of 

discontinuous permafrost.
PINGOS IN CENTRAL ALASKA H7

The probable reasons that pingos have largely escaped attention in 

the subarctic are as follows: (1) They are mostly in unglaciated 

regions and in heavily forested, silt-covered lowlands of little interest 

to geologists studying either the bedrock or the Pleistocene, (2) they 

may have been mistaken for landslide blocks, erosional remnants, or 

circular clumps of tall trees, (3) they commonly occur in areas remote 

from roads and trails, (4) they are sometime difficult to see from the 

air, and (5) they are easily overlooked in aerial photographs.

ENVIRONMENTAL SETTING OF CENTRAL ALASKA

The climate of central Alaska differs from that of the arctic coastal 

tundra where pingos have been studied previously. Chief among these 

differences are (1) higher mean annual temperatures, (2) warmer, 

longer summers, and (3) higher mean winter temperatures, but lower 

winter extremes. Precipitation in central Alaska is also somewhat 

lower than on the Arctic coast, but probably this is not significant. 

Mean annual temperatures for typical stations in central Alaska range 

from 22° to 28° F, wheras those stations near pingos on the Arctic 

coast range from 14° to about 21° F (fig. 3). Annual precipitation in 

central Alaska ranges from 10 to 15 inches, most of it falling as rain 

during the summer. Precipitation at stations on the Arctic coast near 

pingos ranges from about Y to 9 inches (U.S. Weather Bureau, 1958, 

1959).

Most of the pingos described by previous authors and including those 

of the Mackenzie delta and eastern Greenland lie in the zone of continuous permafrost, but the pingos of central Alaska lie in the zone of 

discontinuous permafrost. Openings in the permafrost here occur 

beneath some south-facing slopes, river flood plains, alluvial and outwash fans of gravel, and ponds and lakes. Where present, permafrost 

typically extends to depths of 100 to 150 feet; the depth of annual thaw 

differs from place to place, but it is as little as 1.5 feet beneath bogs and 

muskegs and some north-facing slopes.

The vegetation of the boreal forest of central Alaska is described 

by Sigafoos (1958). Areas below altitudes of 2,000 to 3,000 feet are 

covered by a mosaic of forest types composed of white spruce, black 

spruce, white birch, aspen, balsam poplar, larch, willow, and alder trees 

in various combinations and proportions and interspersed with open 

bogs and muskegs. Areas above altitudes of 2,000 to 3,000 feet are covered with tundra of several types. Most of the pingos we have seen lie in 

areas having scrubby forest and mossy ground cover, or in bogs and 

muskegs; none have been seen on the upland tundra.

The valley slopes and valley floors of areas containing pingos are 

covered with various Pleistocene and Recent unconsolidated deposits, 

including colluvium, flood-plain alluvium, alluvial-fan sediments, loess,
H8 CONTRIBUTIONS TO GENERAL GEOLOGY

organic silt, and weathered to slightly weathered bedrock. Pingos have 

not been found in areas of glacial drift in central Alaska, but there is 

no obvious reason why they might not be composed of glacial deposits. 

Elsewhere they also are composed of a wide range of materials, including bedrock (Miiller, 1959, p. 18-23) and drift (Mackay, 1962, p. 29). 

Ground-water hydrology has received relatively little attention in 

most parts of central Alaska, but the pattern is probably similar to 

that of the Fairbanks area described by T. L. Pewe (in Hopkins and 

others, 1955, p. 129). Surface water infiltrates into aquifers on permafrost-free upper slopes and flows downslope beneath permafrost, commonly developing artesian pressure, in permeable unconsolidated material or in fractured bedrock. Intrapermafrost water, derived from 

meteoric sources or from local thawing of permafrost, flows through 

unfrozen layers in the permafrost, but intrapermafrost water is much 

less abundant than subpermaf rost water. The pingos in central Alaska 

appear to have formed in places where subpermaf rost or intrapermafrost water has risen toward the surface and then become frozen.

REGIONAL DISTRIBUTION AND DENSITY

Pingos are known to be abundant in central Alaska from the 

Canadian border at least as far west as the lower Yukon Kiver valley 

and from the Alaska Eange northward to the southern foothills of the 

Brooks Range (Holmes and others, 1966). Very few have been reported from the Copper Eiver basin, which is in the zone of discontinuous permafrost south of the Alaska Eange and which has been 

relatively well studied. Geographical coordinates of most of the pingos 

we observed are listed by Holmes (1966); doubtless many more could 

be found by careful scrutiny of aerial photographs. The abundance of 

pingos in east-central Alaska (fig. 4) suggests that they continue into 

the Yukon Territory to the east; at least a few have been observed 

there (J. E. Mackay, written commun., 1963).

Eeported and probably actual distribution is uneven. Systematic 

study of the Tanacross and the eastern third of the Mount Hayes 

quadrangles during five field seasons by G. W. Holmes, H. L. Foster, 

and A. T. Fernald resulted in the identification of more than 150 

pingos in these 8,300 square miles (fig. 4). More detailed study near 

Manley Hot Springs (fig. 5) by D. M. Hopkins showed that there are 

50 to 100 pingos in the 500 square miles encompassed by the mapped 

area. Eough calculations show that the maximum density of opensystem pingos in central Alaska is about 10 per 100 square miles, a 

density of about one-tenth that of closed-system pingos in the Mackenzie River delta area (Mackay, 1962).
PINGOS IN CENTRAL ALASKA H9

- 15

-25

- -30

FIGURE 3. Mean monthly temperatures for stations in interior and northwestern 

Alaska, on the Mackenzie delta, and in eastern Greenland. Kotzebue lies about 

100 miles north of the pingos of Seward Peninsula (Porsild, 1938; Hopkins and 

others, 1955) ; Tuktoyaktuk, Northwest Territories, is near Fritz Miiller's 

typical closed-system pingos; Myggbukta is near Miiller's typical open-system 

pingos.
H10 CONTRIBUTIONS TO GENERAL GEOLOGY
EXPLANATION

Younger alluvium

cn

Loess

Includes colluvial 

silt composed 

largely of reworked 

loess

Older alluvium

Bedrock

o

Pingo or pingolike 

mound identified on aerial photographs, or by 

aerial observation

.25

Pingo, field checked 

Numbers refer to table 1

FIGURE 5. Geologic setting of pingos in the Manley Hot Springs area. D. M. Hopkins estimates that the pingos and pingolike 

mounds recognized here constitute about half the total number of pingos in the studied area.

o

oo

H

02

W
H12 CONTRIBUTIONS TO GENERAL GEOLOGY

TOPOGRAPHIC DISTRIBUTION

Virtually all of the pingos observed in central Alaska lie on gentle 

to moderate slopes or on very gently sloping valley floors. None 

were observed in drained or shallow lake basins, although such basins 

are abundant on the floor of the upper Tanana Eiver valley. Measurement of the downslope direction of the sites of 270 pingos shows 

a slightly preferred orientation toward the south and southeast (fig. 6). 

We believe this preferred orientation is the result of larger and more 

frequent openings in the permafrost on south-facing slopes that allow 

infiltration of meteoric water and snowmelt.

The stereoscopic pair of a group of pingos in the upper Tanana 

area (approximately lat. 63°28' N., long. 141°58' W.) illustrates a 

typical topographic setting (fig. 7). Most pingos are near but not at 

the bottom of minor valleys, although several lie about halfway up 

still smaller tributaries and nearly astride the drainageway. Several

300

W 270°-

240'

FIGURE 6. Slope orientation of the sites of 270 pingos in central Alaska showing 

a preference for south- to southeast-facing slopes. Measured to nearest 5°.
o

CO

M

a i-3 a 

> F

1 MILE

FIGURE 7. Pingos in the upper Tanana area, in typical topographic settings, and mounds in several stages of development. ft*

(Stereoscopic pair.) J^
H14 CONTRIBUTIONS TO GENERAL GEOLOGY

stages of development are evident: uncratered mounds, mounds with 

ponds at their summits evidently representing a more advanced stage 

of development, and some mounds with breached craters. The photographs were taken in the autumn, after the leaves had turned. Note 

that some mounds support light-colored deciduous vegetation, probably birch or aspen, and that others appear to have the same darkcolored vegetation, probably scrub spruce, as that growing around 

them.

Figure 8 is a closer view of pingo 13 (lat. 63°45' N., long. 143°05' 

W.). This pingo is typically located near the break in slope at the base 

of a moderately high hill, is about average in size, and supports a mixed 

forest vegetation, which contrasts with the black spruce muskeg and 

bog vegetation in the surrounding lowlands. The vegetation, a small 

crater, and a pond suggest that the pingo can be classed as mature.

FIOUEE 8. A pingo in the upper Tanana area, in a typical topographic location, 

near the break in slope of a moderately high hill.
PINGOS IN CENTRAL ALASKA H15 

GEOLOGIC DISTEIBUTION

Where detailed surficial mapping has been undertaken, we observed 

that pingos near the base of slopes lie in clusters, approximately on 

contours, and near or in the transition zone between material mantling the slope and the valley fill. In the Manley Hot Springs area 

(fig. 5), pingos commonly occur near the base of gentle slip-off slopes 

facing east, south, or west and near the boundary between loess or 

older alluvium on the slopes and younger alluvium in the valleys (fig. 

9). In the Tanacross C-3 quadrangle, H. L. Foster found that pingos 

are concentrated near the zone between the mixed colluvium and alluvium on the slopes and the valley-floor alluvium. Similarly, D. B. 

Krinsley (written commun., 1965) found the prominent pingo in the 

Circle C-l quadrangle is formed of valley-fill material near the 

boundary with slope colluvium.

Pingos in the Manley Hot Springs area on the steep north-facing 

slopes (figs. 5, 9) rarely occur singly; more commonly they occur as 

clusters of mutually interfering pingos of different ages. All the 

examples in the Manley Hot Springs area occur in places where the 

master stream that originally carved the bluffs has meandered away 

during a recent cycle of alluviation and where, in consequence, the bluffs 

have been regraded by gully erosion, talus fall, and creep. The pingos 

are found at varying distances below the base of the bedrock bluffs in 

colluvial fans and aprons that are mantled with a thick sphagnum mat 

and that are perennially frozen. All the pingos on north-facing slopes 

recognized in this area lie below bluffs carved in gray wacke and slate of 

Cretaceous age. This may be coincidence, but, alternately, it may reflect 

the relatively poor resistance to weathering of the slate and gray wacke 

and the tendency of these rocks to break down rapidly to silt and clay 

sizes. Most of the pingos on the north-facing slopes lie on the projections of ravines that are believed to have resulted from erosion along 

north-trending shear zones in the bedrock; faults having this orientation are spaced at intervals of about 1 mile throughout the Manley Hot 

Springs area. The positions of the pingos on north-facing slopes suggest that they are localized by ground water emerging from the fault 

zones.

A. T. Fernald (oral commun., 1965) observed that some of the pingos 

in the southeastern part of the Tanacross quadrangle (fig. 4) occur in 

valley-fill material immediately below bedrock slopes, regardless of the 

slope orientation. This also suggests that the ground water first travels 

through fractures or passages in bedrock before it emerges to form a 

pingo.
CONTRIBUTIONS TO GENERAL GEOLOGY

1000 FEET 

__I

VERTICAL AND HORIZONTAL SCALE

EXPLANATION

Silt, frozen Alluvium, 

unfrozen

Talus and colluvium, 

frozen

Pingo ice Bedrock

Direction of ground-water movement

FIGURE 9. Pingos in the Manley Hot Springs area, in typical location, on gentle 

slip-off slopes and at the bases of steep north-facing bluffs.

GENERAL DESCRIPTION

REPRESENTATIVE PINGOS

A summary of all the pingos examined on the ground by the writers 

appears in table 1. Although these pingos do not represent a randomly 

chosen sample, they seem to be typical of the scores of other pingos 

observed from the air or on aerial photographs. Numbers in the text 

refer to pingos in table 1 and in figures 4 and 5.

Certain individual pingos deserve additional comment because they 

seem to illustrate a series of progressive stages of growth and decay.

FAIRBANKS CREEK PINGO (19)

The Fairbanks Creek pingo was transected by a mining excavation, 

and its internal structure was well exposed in 1957. (It is not shown on 

any map in this report, but is at lat 65°04' N., long 147° 12' W. in the 

Livengood A-l quadrangle map, scale 1: 63,360.) Because it is a subtle 

relief feature, it might not have been recognized as a pingo if its ice core 

had not been exposed. We consider it to be a true pingo because the 

grain size and character of its ice were comparable to that in the pingos 

described by Fritz Miiller, because no other proposed origin for the 

large mass> seems plausible, and because it underlay a small and otherwise unexplained convexity in the slope.
TABLE 1. Descriptive summary of pingos inves

[Asterisk indicates pingo is discussed in t

Reference Size (feet) Direction of

figs. 4 and 5 Length Width Height

1__ __ ___ 1.200 600 100 North ____ Pond, slumped banks, Birch, aspen, b

600+ 500+ 50+ South900 600 50+ North. 

400 300 30 West500 400

600 300

400 300

900 500

35 -do.

65 South.

40 -do.

40 West.

fractured rims; disturbed trees; flanks are 

hummocked. 

Three depressions, two 

water-filled; active 

slumping and fractured 

soil.

Fractures; polygons, 3-4 

ft diam; hummocks.

Small amphitheater 

depression on south 

side; smooth surface.

Uneven surface, but no 

cracks or evidence of 

collapse.

Slump blocks and amphitheater; small brook 

draining from slumped 

areas.

Lake 200 by 250 ft, south 

bank 30 ft high; no 

slumping.

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spruce; black 

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pingo.

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as 2.5 ft in di

on inside slop

craters and o

floors.

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in diam; mos

hummocks.

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more than 1 

ground cover 

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ground cover 

plants; spruc

as 1.5 ft in di

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2 ft in diam, 

shrub; veget

turbed or rec

killed.

Spruce as much 

diam; birch s

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much as 1.4 f

willow, birch

shrub; rose a

plants; moss

lichen ground 
HIS CONTRIBUTIONS TO GENERAL GEOLOGY

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age) at the s

present fores

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H20 CONTRIBUTIONS TO GENERAL GEOLOGY

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active slumping on 

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the lake.

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27. 100 100 15 North........ Not examined..-......... Not examined.

300 300 15 ....do. 

1,200 600 50 -_-_do.

30. 1,200 500 .do.

Small lake in central 

depression about 150 ft 

in diam.

Complex of pingos consisting mainly of a 

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pingo drains across 

northwest pingo 

through deepest trench.

Complex of pingos of several ages; not examined 

in detail.

Not examine

Southeast ping

generation sh

small spruce, 

much as 30 ft 

northwest pin

mature spruc

Lowest pingo h

growth of Sph

and Labrador

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H22 CONTRIBUTIONS TO GENERAL GEOLOGY

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PINGOS IN CENTRAL ALASKA H23

The Fairbanks Creek pingo, now largely destroyed by mining operations, was an irregular but grossly lenticular ice mass enclosed by layers 

of peat and organic silt (figs. 10, 11). A bed of peat in the silt was 

arched over the western part of the ice body. The upper surface of 

the ice was arched and lay about 2.5 to 3 feet below ground surface 

along about two-thirds of the exposure. This upper ice surface, where 

less than about 3 feet from the ground surface, probably coincided 

with the permafrost table. The base of the ice lens was irregular.

Structures in the ice core were probably the result of internal melting after its exposure during mining operations. The ice was clear 

and unfractured; it appeared blue from a distance but white at closer 

view as a result of internal reflections from crystal faces. Most of the 

ice consisted of equidimensional and randomly oriented crystals 3 to 

10 inches in diameter and contained many air bubbles. Near the base 

was a series of horizontal bands composed of vertically oriented, nearly 

fibrous ice crystals. A dark band of dirty ice, inclined gently upstream, 

extended through the approximate center of the mass (fig. 10).

The Fairbanks Creek pingo was transected by at least three vertical 

ice wedges about 3 feet wide, which extended downward through the 

pingo ice and interrupted the dirt bands; ice in the wedges differed 

from the clean pingo ice in having a much smaller grain size and a 

vertical foliation. The pingo's smooth surface, low relief, absence of 

microrelief, and vegetation similar to that on the surrounding slopes 

suggest that the Fairbanks Creek pingo was relatively young. However, placer mining had been conducted in the immediate vicinity for 

at least 50 years (T. L. Pewe, written commun., 1963), and ice wedges 

as large as the two that transected the western part of the ice mass 

must have required many centuries to form.

Considering the stratigraphic relationships of ice wedges and 

Quaternary deposits in many mining exposures, T. L. Pewe (oral 

commun., 1963) was convinced that few if any ice wedges have formed 

in central Alaska within the last 5,000 years. Although the ice wedges 

in the Fairbanks Creek pingo may indeed have been several thousand 

years old, ice wedges in other pingos, discussed later, appear to be 

growing at present. Perhaps pingos constitute especially favorable 

sites for the formation of frost cracks that eventually become ice 

wedges. The following factors may be significant: (1) The coefficient 

of thermal contraction is higher for pure ice than for mineral soil or 

peat, (2) many pingos form sharply defined topographic convexities, 

and their interiors may cool more rapidly and deeply during winter 

than do adjoining flat areas, and (3) the insulating winter snow cover 

occasionally may blow from some pingos and cause them to cool more 

rapidly than adjacent snow-covered flat areas.
H24 CONTRIBUTIONS TO GENERAL GEOLOGY

FIGURE 10. The Fairbanks Creek pingo (19) as exposed in 1957. Projecting 

pillar near man's head is one of the ice wedges. Dark bands near middle of 

ice mass are bands of dirty ice. Photograph by D. M. Hopkins, 1957.

McKINXJBY CREEK PINGO (21)

The McKinley Creek pingo, a low, broad mound that obstructs a 

swampy drainageway west of lower McKinley Creek (lat 65°09' N., 

long 150°20' W.) in the Manley Hot Springs area (fig. 5) seems to 

represent a slightly more advanced stage of development than the 

Fairbanks Creek pingo. It is one of the broadest and lowest observed, 

but the apparent relief is much greater than true relief because low 

herbaceous vegetation on the margins grades inward to trees at the 

summit (fig. 12). The concentrically zoned vegetation reflects a nice 

adjustment to increasingly better drainage toward the center of the 

pingo. The wettest part of the marginal swale contains Carex sedges; 

at the base of the pingo slopes these sedges grade into a ring composed 

mostly of Eriophorum (cottongrass) tussocks which higher up mix 

with black spruce saplings 2 to 3 feet tall. The moderately well drained 

summit area is covered with young birch and willow trees mixed with 

black spruce trees, 15 feet high and 3 inches in diameter at breast 

height; the ground cover is a mosaic of Labrador-tea, cranberry, blue-
PINGOS IN CENTRAL ALASKA H25

FIGURE 11. The Fairbajoks Creek pingo (19). (By T. L. Pew6, unpub.

data.)

berry heath plants, and moss and caribou lichen. The summit has 

been burned over, and traces of an old scrub forest are present.

The height, lack of microrelief, and absence of a crater or springs 

suggest that this pingo is moderately young but old enough for vegetation to have become delicately adjusted to variations in drainage conditions. Because at least two generations of trees have grown there, 

the McKinley Creek pingo cannot be less than several decades old.

PIONEER CREEK PINGO (23)

The broad, oval Pioneer Creek pingo is considerably higher than 

the McKinley Creek pingo and has a more complex topography. We 

believe that it represents a later stage in pingo development. It is 

on the steep southwestern side of Pioneer Creek valley (lat 65° 12' N., 

long 150°06' W.) in the Manley Hot Springs area (fig. 5). Its most 

conspicuous feature is the large oval summit crater which is associated with springs, collapse trenches, minor scarps, and benches (fig. 

13). The pingo is covered with a mature forest; individual trees are 

as much as 130 years old. The soil shows incipient profile development 

adjusted to the present drainage and slope conditions. A large crater 

and fractures in the summit area suggest that the pingo has ceased to
H26

S

CONTRIBUTIONS TO GENERAL GEOLOGY

100 FEET

VERTICAL AND HORIZONTAL 

SCALE

FIGURE 12. The broad, low McKinley Creek pingo (21). Dashed line indicates 

approximate slope of hill east and west of pingo.

X- -X'

1200'-l

1100'-

1000'

Silty talus

FIGURE 13. Diagrammatic plan and hand-level profile by K. L. Pierce of the 

Pioneer Creek pingo (23), Manley Hot Springs area. Dashed lines in plan 

view are form lines.
PINGOS IN CENTRAL ALASKA H27

grow and that ablation of the ice core has begun. Because the pingo 

blocks a deep ravine developed along a fault in the bedrock and because 

there are flowing springs at the summit, this pingo appears to be a 

clear example of those fed by ground water flowing through bedrock 

and is among the few pingos in which ice was found a few feet beneath 

the surface of the crater.

DISCOVERY PINGO (18)

The Discovery pingo, one of the first observed in the upper Tanana 

valley, is north of the Tanana River in the Mount Hayes C-l quadrangle about 4.5 miles east of Sand Lake at lat 63°45' N, long 144°13' 

W. Discovery pingo appears to be more advanced in its development 

than the Pioneer Creek pingo. The hillock is nearly circular in plan 

(fig. 14), is moderately high and broad, has side slopes ranging from 

20° to 30°, and is marked by a round summit pond whose banks are 

actively collapsing (fig. 15). The crater rim is breached by an outlet, 

but the pond was not high enough to overflow through the outlet in 

July 1960. Large blocks of silt with trees growing on them have slid 

into the pond; the outer slopes of the pingo are slightly benched, apparently owing to collapse. A mature spruce-birch forest grows on the 

flanks and crater rim and contrasts with the scattered and deformed 

small spruce trees in the surrounding muskeg. The pingo is composed 

of stratified sandy silt that contains lenses of arkose and peat and a 

few small granitic boulders; this material was deposited by runoff 

from the adjacent hillside prior to the formation of the mound.

An excavation in the flank of the pingo shows that a slightly developed subarctic Brown forest soil profile, consisting of 3 to 5 inches 

of brown-stained silt, has formed since the pingo was arched up. A 

fragment of spruce wood collected at a depth of 2.5 feet in the silt has 

a radiocarbon age of about 7,000 years (dated 5,050 B.C. ±150 years, 

specimen T-303; Reidar Nydal, written commun., 1961). The wood was 

deposited in silty colluvium before the pingo was formed, and therefore the pingo is less than 7,000 years old. The formation of a slightly 

developed soil, however, indicates that the pingo must be several centuries old; it is more likely to be several thousands of years old.

PINGO 12

Pingo 12, located 6 miles west of the Taylor Highway in the Tanacross C-3 quadrangle (lat 63°41' N., long 142°27/ W.; fig. 4), is typical of pingos in a very advanced stage of decay. It is on a gently inclined south-facing slope and about 2,000 feet from a small stream. 

All that remains is a segment of the crater rim, about 30 feet high on 

the downslope side, that confines a large pond about 500 feet long and 

400 feet wide (fig. 16). The crater rim rises about 25 feet above the
H28 CONTRIBUTIONS TO GENERAL GEOLOGY

""* *\

FIGURE 14. The Discovery pingo (18). Dense stand of large birch trees on the 

pingo contrasts sharply with the open stand of small black spruce in the 

surrounding muskeg. Photograph by H. L. Foster, August 1960.

pond surface on the southwest side, but elsewhere it is low and marshy. 

Aquatic vegetation is encroaching on the pond. The crater rim showing no evidence of recent slumping, appears to be stabilized. The rim 

of the crater is gently rolling; it may be composed of former hummocks or slump blocks. Spruce trees as much as 2 feet in diameter on 

the crater rim attest to a prolonged period of stability.

PINGO 7

Pingo 7, a compound pingo, is an example of a regenerated mound 

that rose near a collapsed crater and of a stage of development beyond 

that of pingo 12. Second-generation pingos are not uncommon, and 

some show three or more generations of mounds. Pingo 7 is in the
PINGOS IN CENTRAL ALASKA H29

c

2500'-

2000' - 

1500'- 

1000'-

^

o M

^_^ c

^^^^^ ^

Granite

0 1000 FEET

FIGURE 15. The Discovery pingo (18), in typical location, on valley side. No

vertical exaggeration.

500 FEET

FIGURE 16. Pond and crater rim of very old pingo (12). No vertical exaggeration.

Tanacross C-3 quadrangle at approximately lat 63°34' N., long 142° 

19' W., in forested terrain, and in an area where pingos are common. 

It is about 400 feet across and 40 feet high and has a small pond 30 

by 35 feet at the summit. An older pingo to the southeast has a low 

rim, no more than 20 feet high, enclosing a marsh draining to the 

northeast; the northwest rim of the older pingo forms the southeast 

rim of pingo Y. The form of pingo Y suggests that it also may be quite 

old, because its stage of development is comparable to the mature-appearing Discovery pingo. Pingo Y is composed of rounded gravel, pink 

and green volcanic debris, clay, silt, and sand. Ice is exposed in the 

crater wall in several places and on the south-southwest side on the 

flank; the inner wall is very steep and is actively slumping, so that 

trees on the rim are being destroyed. The occurrence here and elsewhere of two or more pingos of different ages suggests complex cyclical
H30 CONTRIBUTIONS TO GENERAL GEOLOGY

episodes of pingo growth, decay, blockage of ground-water flow, and 

renewed formation of pingo ice in or near the older pingos.

SIZE AND SHAPE

The central Alaskan pingos that have been inspected by geologists 

have an average height of approximately 40 feet, average lengths of 

500 to 600 feet, and usually do not have equidimensional ground plans. 

The range in height is from 10 feet to about 100 feet, and the range 

in length is from 50 to 1,450 feet. If these figures are representative, 

the largest open-system pingos in Alaska are probably smaller than 

the largest closed-system pingos of the Mackenzie delta area (Mackay, 

1962, p. 26); pingos there range in height from 10 to about 150 feet 

and in diameter from about 100 to nearly 2,000 feet. The average size, 

however, is about the same (Stager, 1956). The Alaska pingos are 

also much smaller than some open-system pingos that are described by 

Miiller (1959, p. 116-117) in eastern Greenland and that have a maximum height of 160 feet and a maximum diameter of 1 mile. Pingos in 

central Alaska are circular, elliptical, or irregular in plan. Their 

outlines are commonly obscured by the growth of groups of mutually 

interfering mounds. Slopes range from 1° or 2° to about 35°.

HYDROLOGY

All the pingos observed are in areas where artesian ground water 

confined beneath perennially frozen and impermeable sediments may 

be expected; none are in the basins of preexisting lakes or drained 

lakes. Many pingos have active and apparently perennial springs 

emerging from craterlike areas, "crater lakes," or collapse trenches 

on the flanks. For example, active springs were flowing from pingos 

in the Circle area late in the exceptionally cold winter of 1961-62 

(Lloyd Spetzman, oral commun., 1962). Many of the pingos in the 

Manley Hot Springs area give rise to minor streams, and many lie at 

the upper ends of patches of willow vegetation that appear to be growing on the sites of annual aufeis accumulations. Some spring-fed crater 

ponds discharge nonflammable gas, probably air, and thus indicate 

further the presence of artesian pressure.

Evidently many of the pingos in central Alaska are the sources, 

or are closely associated with the sources, of perennial flows of water 

having temperatures above freezing. If this conclusion is correct, 

pingos should be a useful guide to areas where modest perennial supplies of ground water can be obtained.

Ground-water requirements for the growth of open-system pingos 

probably include the following:
PINGOS IN CENTRAL ALASKA H3jl

1. A restricted amount of artesian flow beneath, or in rare cases within, 

the permafrost, as suggested by Muller (1959, p. 116); a large 

volume of ground water would bring too much heat into the system and prevent the freezing of the pingo.

2. Water temperatures and permafrost temperatures very near freezing ; very cold water will freeze more readily, and frozen ground 

near the melting point has the minimum tensile strength. This delicate temperature balance must be maintained during the growth 

of the pingo; if temperatures drop below freezing, the source of 

ground water will be closed off, and if they rise too high, the pingo 

will melt and become a spring.

3. Blockage of ground-water flow near the base of the slope to localize 

the site for pingo formation; this is probably caused by an abrupt 

increase in depth of permafrost at the base of the slope and in the 

valley-fill sediments and prevents subpermaf rost water from continuing into the valley and eventually reaching a surface stream. 

Permafrost apparently does thicken downslope in many valleys 

(Hopkins and others, 1955, p. 129), especially in minor drainages. 

The fact that flood plains of large streams, on the other hand, are 

mostly free of permafrost might partly explain why few pingos 

occur in the valley of large rivers.

The precise role that ground water plays in the doming of pingos 

is very uncertain, and theoretical consideration of the forces required 

reveals large gaps in our understanding of the quantitative aspects of 

the process. Considering the highest pingo measured in this study 

(about 100 feet) and assuming the maximum probable thickness of 

overburden (about 50 feet) based on observations of the Fairbanks 

Creek pingo and by Mackay (1962, p. 28), the static load at the base 

of the center of such a pingo would be 3 to 4 atmospheres. However, 

additional force is required to dome the perennially frozen layer by 

subpermafrost or intrapermafrost water, and this force apparently 

may be greater than the static load. On the basis of studies by Berezantsev (1947), Muller (1959, p. 63) estimated that the force required 

to overcome the tensile strength of perennially frozen ground is 2.5 to 

18 kilograms per cubic centimeter or 2.4 to 17.4 atmospheres. Hence, 

the total force required may be roughly 6 to 22 atmospheres.

According to Eoger Waller, U.S. Geological Survey (oral commun., 

1965), su'bpermafrost water in central Alaska where found in drill 

holes is ordinarily under artesian pressure, and flowing wells are common. The highest measured pressure, in a well near Anchorage, was 

about 2 atmospheres. High artesian pressures measured elsewhere in 

the United States were generally not higher than about 10 atmospheres 

(Kobert Bennett, U.S. Geol. Survey, oral commun., 1965). Theoretical
H32 CONTRIBUTIONS TO GENERAL GEOLOGY

figures for the calculated maximum hydrostatic head at the sites 

of pingos are indeed high: as much as or more than 20 atmospheres. 

In view of the limited available ground-water data and the abundance 

of pingos in central Alaska, it seems unlikely, however, that extreme 

artesian pressures would actually be present. The writers conclude 

that pingos may rise as a result of a hydraulic lift effect, whereby relatively low artesian pressure is amplified by confinement under a pingo, 

as suggested by Miiller (1959, p. 62-66), or as a result of a combination 

of freezing pressure and artesian pressure. Nevertheless, the mechanism of open-system pingo formation is very poorly understood.

MICRORELIEF

Some pingos of interior Alaska are smooth and devoid of microrelief, but more commonly they have craters, ponds, hummocks, benches, 

escarpments, and trenches. Trenches resulting from the collapse of 

frozen ground are common near persistent seepages on the flanks of 

pingos that lie below steep north-facing slopes; they have not been 

seen on pingos that lie on the lower parts of gentle slip-off slopes. 

Pingos of both types commonly have stepped scarps and benches on 

some parts of their flanks; the origin of these is uncertain, but they may 

be frost cracks that have been modified by creep.

The most common type of microrelief consists of crisscrossing networks of trenches comparable in pattern and scale to the trenches that 

often result from the partial thawing of ice wedges. On some pingos 

these trenches are steep walled, flat bottomed, and narrow, and they 

separate flat-topped miniature plateaus; on other pingos, networks of 

low, wide troughs separate convex mounds. The trenches range in 

width from a few inches to 5 feet and in depth from 1.5 feet to 20 feet 

below the intervening mounds and plateaus. In general, the narrowest 

and shallowest trenches are associated with incompletely developed 

networks on young-looking pingos; the widest, deepest trenches are 

associated with mature pingos that show signs of losing part of their 

ice core.

Narrow, steep-walled trenches are commonly associated with recent 

disturbance of the vegetation. The Hutlinana Bluff pingo (22), for 

example, has at its summit a narrow, sharp-walled crack that has very 

recently cut the Sphagnum mat; black spruce trees are tilted toward 

the crack as though the ground immediately adjoining the crack were 

collapsing into it. One of the trenches on the Bean Ridge pingo (20) 

is vertical walled, 1.5 feet wide, 1 to 3 feet deep, and crossed by the 

bared root of a 15-year old tree; this trench must have formed after

the root had grown within the last 15 years. In contrast, wide trenches

having gently sloping walls and separated by convex mounds are gen-
PINGOS IN CENTRAL ALASKA H33

erally found in pingos that support an undisturbed mature forest; the 

floors of these trenches show no evidence of recent cracking.

DISCUSSION

SPECULATIONS ON THE ORIGIN OF TRENCH NETWORKS

The origin of trench networks on pingos is uncertain, but at least 

two possibilities exist: (1) They are a result of distension of the surface during the original uplift of the mound, or (2) they are related 

to ice wedges that formed after the mound was uplifted.

Closed-system pingos in northern and western Alaska commonly are 

nearly bisected along their long axis by a large, central, deep, V-shaped 

trench several tens of feet in width and depth. This is probably a tension crack that formed at the time the pingo was arched. A secondary 

trench pattern on the flanks of closed-system pingos resembles those 

patterns described in central Alaska. However, no pingos in central 

Alaska were observed to be bisected by a large, deep, axial trench, and 

therefore we believe that the trench networks on these pingos are not 

related to tension fractures for the following reasons:

1. Trench formation appears to progress over a period of many years. 

The youngest pingos have few or no trenches, and the most complete trench patterns are found on the older pingos.

2. Although the first crack to form may be centrally located, it nevertheless develops many years after the original pingo formed.

3. The trenches have the pattern and dimensions of ice-wedge 

polygons.

4. Ice wedges were observed in the pingo ice of the Fairbanks Creek

pingo (19).

A hypothetical sequence for the formation of trenches related to 

ice wedges is shown in figure 17. Although T. L. Pewe (oral commun., 

1962) believes, on the basis of his extensive studies of ground ice, that 

ice wedges are not forming today in most environments in the Fairbanks district, pingos may provide those unusual and ideal conditions 

for their formation. Apparently frost cracks are forming in pingos at 

present in the Manley Hot Springs area, as mentioned previously, and 

such cracks may be the first stage in the formation of ice wedges and, 

later, trenches.

AGE

Individual pingos in interior Alaska differ widely in age, but all are 

more than 10 years old and none are probably more than about 7,000 

years old.

Soil profiles reveal some information about the age of pingos, and 

radiocarbon dates from prepingo soils found at the top of the pingos 

reveal an upper limit of their age. Most pingos have formed in sites
H34 CONTRIBUTIONS TO GENERAL GEOLOGY
PINGOS IN CENTRAL ALASKA H35

where the soil is saturated throughout the thawing season and where 

Half Bog or Tundra soils form. These consist of mixtures of organic 

debris and reduced dark-gray silt or mottled mixtures of reduced 

gray silt and oxidized yellow, orange, or red silt; neither type shows 

pronounced profile zonation. Some of the more gently sloping and 

poorly drained pingos still have such a soil, and these must be fairly 

young. Silt in well-drained environments in time acquires a welldefined subarctic Brown profile or a Podzol profile showing pronounced vertical zonation. In the Manley Hot Springs area such soils, 

believed to be of late Wisconsin age or younger, generally show profiles 

having 2 to 3.5 feet of oxidation; if the drainage is especially good, a 

leached ashy gray podzolic layer an inch thick is present just beneath 

the organic mat

In east-central Alaska, loess on well-drained terrace gravel at the 

junction of the Robertson and Tanana Rivers (Tanacross C-6 quadrangle) has developed a similar and distinct Brown forest soil profile. 

A thin gravel bed underlying the loess at a depth of 3 to 5 feet has a 

radiocarbon age between 4250±250 years (W-Y56) and 5650±200 

years (W-753). Hence a fully developed subarctic Brown forest soil 

may form in less than about 4,000 years. In contrast, silt on the high, 

well-drained pingos shows only a poorly developed soil profile, consisting of a 3- to 6-inch zone of oxidation grading down into gray or 

olive unoxidized silt. The soil profile on these pingos must represent 

a shorter span of time than the much thicker soil profile developed 

on loess of Wisconsin or Recent age on stable well-drained surfaces. 

None of the pingos showing this poorly developed soil profile can be 

more than a few thousand years old.

FIGURE 17. Postulated origin and development of trench networks on a pingo.

A, Winter year 1. Ground water under artesian pressure seepsi to surface; 

outlet is frozen; and water intrudes along bedding plane in silt or in temporary unfrozen zone between top of permafrost and base of winter frozen 

layer, then freezes.

B, Winter, several years later. Large ice laccolith has formed. Channel to surface is now obstructed, and water find its outlet elsewhere. Growth to this 

stage may take a few or several tens of years.

C, Winter, several tens of years later. Frost cracks develop (width greatly 

exaggerated).

D, Summer, shortly after stage C. Ice wedges have developed in the pingo 

and in the overburden where pingo ice is deeply buried.

E, Summer, several tens to several hundreds of years later. Deep thaw follows 

summer fire. Central part of pingo melts down uniformly where pingo was 

just below active layer. Near margins of pingo, where ice wedges had 

extended into the overburden, trenches and mounds form.
H36 CONTRIBUTIONS TO GENERAL GEOLOGY

The few pingos dated by the radiocarbon method are of Recent age. 

As stated previously, Discovery pingo (18) in the upper Tanana area 

is not older than about 7,000 years and is probably somewhat younger. 

A youthful-appearing pingo near Circle, Alaska, has a maximum 

radiocarbon age of 5,720±65 years (Krinsley, 1965). The age was 

obtained from organic material directly overlying convoluted lacustrine or alluvial sand beds, and hence the pingo was probably uplifted 

after an episode of intensive frost action. Craig (1959) obtained a 

maximum age for a pingo in the tundra-covered Thelon River valley, 

Northwest Territories, (lat 64°19' N., long 102°41' W.). Organic material in laminated silt 1.5 to 4.5 feet from the upper surface has a 

radiocarbon age of 5,500±250 years. The organic material included 

pollen indicating a forest vegetation; Craig infers that the pingo 

formed as a result of the same climatic change that caused the forest 

to revert to the present tundra vegetation.

Radiocarbon datings of organic material in the upper strata of two 

Mackenzie delta pingos near Tuktoyaktuk, Northwest Territories, 

places a maximum age of 12,000ih300 years for the Ibyuk pingo and 

6,800±200 years for the Sitiyok pingo (Miiller, 1962). Miiller believes 

that the time of doming of the Ibyuk pingo was no earlier than 7,000 

to 10,000 years ago and that the Sitiyok pingo probably formed about 

4,000 years ago. Hence there is rough agreement in maximum age for 

at least five pingos in northwestern North America. Mackay (1962, 

p. 60) also concludes on the basis of vegetation types, humus thickness, 

widths of ice wedges, peat accumulation, and cliff recession that the 

Mackenzie delta closed-system pingos are a few thousand years old.

The vegetation living on individual pingos ranges in age from a 

few decades to more than a century. Drainage generally has been improved on the pingos as a result of the arching above surrounding 

terrain, and on most pingos the vegetation has responded to the improved drainage. Several pingos support trees well over a century old 

that could not have begun to grow until the pingo had formed; still 

older generations of trees are represented by rotting stumps and fallen 

logs. These pingos cannot be less than two centuries old. The McKinley 

Creek pingo (21) gives the impression of being a young, possibly 

incipient pingo, yet the vegetation there is nicely adjusted to soil 

drainage conditions. Trees were not cored on this pingo, but it must 

be at least several decades old. As several pingos, for example, pingo 

20 and the southeastern part of pingo 29 (fig. 5), support vegetation 

that is immature but appropriate in composition to the present drainage conditions, they probably are not more than a few decades old. 

The north end of complex pingo 30 is the only mound in this area on 

which the vegetation seems slightly maladjusted to the topography; 

this part of the pingo group may not be much more than 10 years old.
PINGOS IN CENTRAL ALASKA H37

Many of the pingos in the Manley Hot Springs quadrangles occur 

in areas of which aerial photographs taken in 1951 and 1959 are available. According to the sets for both years, no pingos have apparently 

changed during the 8-year period. As some of the seemingly youngest 

pingos in the region appear in both sets of photographs, it is likely that 

all the recognized pingos in the Manley Hot Springs area formed 

before 1951.

The growth of open-system pingos in interior Alaska is apparently 

a result of the Recent climate, but there is no clear evidence of any 

response to fluctuations of the Recent climate in pingo growth or 

decay. Pingos apparently have been forming continuously for several 

thousand years, probably for the last 4,000 years, and just possibly for 

the last 7,000 years. Moreover, there is no strong independent evidence, 

particularly from the botanical record, that there have been major 

climatic fluctuations in interior Alaska (W. S. Benninghoff, written 

commun., 1963; Heusser, 1957, p. 65). There have been variations of 

the Recent climate in coastal Alaska to the south (Heusser, 1960), on 

the Arctic slope to the north (Livingstone, 1955; Tedrow and Walton, 

1964), and on the plains of the Northwest Territories directly to the 

east (Craig, 1959); however, these changes at present cannot be linked 

with the pollen record or the history of pingo development.

Although no pingos are known to be of Pleistocene age, they might 

have formed in the periglacial interior of Alaska as they did in Wurm 

time in Wales and in Belgium (Pissart, 1963). It is known that opensystem pingos, and possibly closed-system pingos, pass through a 

sequence of stages and eventually become lakes having uneven, raised 

shores. Possibly some of the hundreds of small lakes in the valleys of 

the Yukon-Tanana Upland, sometimes regarded as thaw ponds, may 

be Pleistocene pingo craters.

CONCLUSIONS

Nearly 300 pingos or pingolike mounds have been located in central 

Alaska, between the Alaska and Brooks Ranges in the forested zone 

of discontinuous permafrost. Although the distribution is uneven, it 

is likely that other pingos may be identified in central Alaska and in 

the Yukon Territory to the east. Inasmuch as the pingos may occur in 

concentrations as high as 10 per 100 square miles, it is remarkable that 

they have not been recognized in any numbers until recently.

Pingos show a preference for sites on south- and southeast-facing 

slopes, near the base of the slopes and the transition between valleyfill deposits and the slope mantle, and are commonly alined parallel 

to contour lines. They are composed of a variety of surficial materials,
H38 CONTRIBUTIONS TO GENERAL GEOLOGY

primarily silty colluvium and valley-fill material; none have been 

found on glacial drift.

Pingos may pass through several stages which in sequence may include (1) low domes supporting vegetation and covered with soil 

similar to that of the surrounding area, (2) domes of low to medium 

height (10 to about 40 feet) having little microrelief but covered with 

vegetation reflecting improved drainage conditions, (3) domes of moderate height having mature forests, trenches, scarps, steps, craters, 

ponds, springs, and slightly developed forest soil profiles, (4) collapsed domes having large central depressions or ponds, disrupted 

vegetation, and low crater rims of tilted and disturbed surficial material, and (5) collapsed domes having second- or third-generation 

pingos forming on their flanks.

Little is known about the mechanics of pingo formation, but theoretical calculations of a hydrostatic head suggest sufficient pressure is 

normally present, and if combined with freezing pressures, both the 

tensile strength of the frozen layer and the weight of the static load 

can be theoretically exceeded.

The pingos observed are mostly circular or elliptical; a few are very 

irregular in plan and probably represent several coalescing mounds. 

They range in size from about 50 to 1,450 feet in diameter and 10 to 

100 feet in height. Their average size is roughly the same as the average 

Mackenzie delta pingo, but the largest are much smaller than the 

biggest pingos of eastern Greenland. One or more of the following 

features are commonly present: (1) Summit craters, some asymmetrically placed, commonly occupied by springs or a lake, (2) trench 

networks, (3) slumped flanks and crater sides, (4) disturbed vegetation, and (5) hummocks.

Very limited observations of the internal structure show that pingo 

ice forms in, but near the top of, the permafrost and that pingo ice may 

be crosscut by large ice wedges extending through the ice lens. The 

common occurrence of trench networks might be explained by the 

melting of ice wedges that at one time extended above the surface of 

the main ice lens. Trench networks are especially conspicuous on 

mature cratered pingos that show the effects of collapse.

Aside from their scientific interest, which includes investigation of 

such problems as the mechanics of formation, relationships to thermal 

regime and permafrost, ground-water hydrology, and internal structure or crystal fabrics, pingos have definite, although limited, practical value. Crater ponds and springs, which flow even in very cold 

winters, are direct sources of water and should be good indicators of 

ground water at moderate depths. 'Some steep-sided pingos having 

ground ice at shallow depths could be tunneled and used as
PINGOS IN CENTRAL ALASKA H39

semipermanent cold storage facilities, a technique employed in northwestern Alaska (Porsild, 1938, p. 58). To the engineer, presence of 

pingos would indicate very poor foundation conditions for buildings, 

roads, towers, or other permanent structures. To the geologist, they 

are reliable indicators of permafrost, and in rare cases, they might be 

useful in predicting the trace of fracture zones and faults.

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