GEOLOGICAL SURVEY CIRCULAR 42

February 1949

PRELIMINARY REPORT OF

PERMAFROST INVESTIGATIONS

IN

THE DUNBAR AREA, ALASKA
UNITED STATES DEPARTMENT OF THE INTERIOR

J. A. Krug, Secretary

GEOLOGICAL SURVEY

W. E. Wrather, Director

WASHINGTON, D. C.

Free on application to the Director, Geological Survey, Washington 26, D. C.
PRELIMINARY REPORT OF PERMAFROST INVESTIGATIONS 

IN THE DUNBAR AREA, ALASKA

ByTroyLPewe

INTRODUCTION

The purpose of this report Is to present 

a preliminary description of the Permafrost 

conditions In the Dunbar region. The report 

Is the result of reconnaissance work on 

August 6, 1948.

The Dunbar area Includes a strip along 

the Alaska Railroad from Standard to Berg, a 

distance of 20 miles (fig. I)f an area that 

has been withdrawn by the government for 

possible unit agricultural settlement. The 

area lies in the interior of Alaska on the 

southern border of the Yukon-Tanana Plateau.

GENERAL GEOLOGIC SETTING

In the Dunbar area the Yukon-Tanana 

Plateau consists of gently rolling, wellrounded hills about 1,200 to 2,000 feet high. 

The lowlands are at an altitude of about 400 

feet.

Prom Standard to Dunbar, a distance of 

about 7 miles, the railroad is in the relatively narrow valley of lower Qoldstream River. At 

Dunbar the river empties out into the broad 

lake-covered alluvial plain termed the Hinto 

Plats. Prom Dunbar to Berg, a distance of 13 

miles, the Minto Plats adjoin the hills, and 

the railroad traverses a narrow strip at the 

base of the hills. The Flats are northwest of 

the track and the hills southeast.

The area from the Tanana River to Standard can be divided into three main terrain 

divisions: (1) the floodplain, (2) the depositional slope the low-lying area between 

the floodplain and the hills and extending 

into the valleys and (3) the hills.

FLOODPLAIN

As the floodplain of the Tanana River is 

outside the government withdrawal area, It will 

not be discussed.

DEPOSITIONS SLOPE

The lower gentle slopes of the hills and 

locally the valley bottoms have been tentatively termed the deposltional slope. The contact 

between the depositional slope and the hills 

is an.arbitrary one. The sediment both of the 

lower hills and the depositional slope is silt,

but the silt of the depositional slope has a 

higher organic content. Where the steeper 

slope of the hills gives way to the more 

gentle slope of the .depositional area, the 

drainage becomes sluggish and the watersaturated ground is frozen. The contact between the hills and the depositional slope 

area is therefore tentatively placed at the 

contact between the frozen gentle slope and 

the unfrozen steeper slope. This contact is 

generally at a higher altitude on the northfacing slopes than on the south-facing slopes.

Sediment of the depositional slope is 

mostly muck. Muck is a dark, fetid, highly 

organic silt that generally contains abundant 

remains of animals of the glacial period. In 

the creek valleys muck overlies creek gravels 

which, in turn, rest on bedrock.

Permafrost;

The sediment of the depositional slope is 

permanently frozen to a depth of at least 150 

feet. This? permafrost probably consists of a 

continuous body extending from hill to hill 

(fig. 2). The thickness of the frozen ground 

probably decreases toward the hills where it 

pinches out at the base of the hill. The 

creek gravels under the muck are frozen. It 

Is not known if the ground is thawed under the 

river. Depth to permafrost is about 12 to 24 

inches in the bottom land, but where permafrost 

thins toward the hills the depth to permafrost 

increases.

Perhaps the most important characteristic 

of the frozen ground in the depositional slope , 

is the presence of many large clear-ice masses 

of irregular shapes and sizes. The presence 

of these ice masses is indicated by the following:

(1) Exposure of ice masses In'railroad 

cuts through the lower end of 

tan-colored silt ridges.

(2) Exposure of ice masses in well at 

Standard. An ice mass at least 

15 feet thick was encountered. I/

(3) Thermokarst pits in the upper part

of the depositional slope probably 

near the contact of the frozen and 

nonfrozen ground. Pits 20 feet 

long, 10 feet wide, and 8 feet 

deep were noticed.

August 6,

Oral cooHunication, railroad section chief
IOO 0 IOO 200 30O 400 MILK*

Figure 1. Index map of Alaska, showing location of Duribar area.

Alaska Railroad

6oldstr*am

North South

Vertical exaggeration X 8 T.L. PeW 

1948

6.OOO I6.0OO_____24.0 OO Fttt

Approximate horizontal scalt

Figure 2. Sketch showing probable permafrost distribution in lower Ooldstream River Valley near. 

Dunbar, Alaska, based on prediction from known permafrost distribution in upper 

Qoldstream Valley.
(4) Presence of cave-In-lakes. As Icethaws and pita are formed they 

sometimes hold water which thaws 

the bank causing the lake to 

enlarge by the caving In of the 

bank, with trees and other plants 

therefore slumping Into the water.

(5) Presence of polygonal ground.

(6) Sinking of railroad tracks.

(7) Similarity to the depositional 

slope of the Fairbanks area 

about 41 miles away.

Economic application

Construction. The depositional slope 

is less favorable for construction than the 

floodplain or hill area due to permafrost 

conditions. The permanently frozen, finegrained muck is subject to much heaving and 

slumping. The melting of the ice masses is 

a major factor in the slumping of the ground.

Agriculture.' Polygonal mounds and pits 

may form In areas cleared for farms on the 

depositional slope. This is what has occurred 

on the depositional slope in the Fairbanks 

area. When the overlying vegetative coyer is 

removed, the underlying ice masses begin to 

melt. If the Ice masses are in a polygonal 

network, the area in the middle of the Ice 

polygon net Is left standing In relief as the 

surrounding ground sinks. In the Fairbanks 

area, mounds begin to form in the second or 

third year after the field is cleared.

In addition to mounds, many pits may 

form in the fields. These pits are the result of" the melting of a mass of ice. They 

have been termed theraokarst pits and form 

readily when the land is cleared. A few such 

pits have already formed In'uncleared areas 

in the Dunbar region under present climatic 

conditions. When such land is cleared, 

pitting will probably be Intensified.

The formation of such mounds and pits 

is detrimental to farming. Sometimes fields 

must be abandoned and frequently cultivation 

must be stopped and the field used for pasture. The important factor to note Is not 

that the homesteader can still use the 

field if he constantly combats the melting, 

but that it takes valuable time, money, and 

soil to fight these conditions resulting 

from permafrost. As farming in this region 

is near-marginal at best, such added difficulties may prove insuperable.

Qround water. As permafrost is near - 

the surface in the depositional slope, 

ground water in most places is not available 

to shallow wells. Beneath the confining 

permafrost, water is generally available but 

at depths sometimes exceeding 150 feet. The 

water In many'instances would be unpalatable, 

This is true at Berg.

HILLS

The hills in the area are of Birch 

Creek schist mantled with 1 to 40 feet of 

tan-colored silt. Lower hills and lower 

slopes have the greatest thickness of silt. . 

Like loess, silt has the property of standing in vertical cliffs. On the steep slopes 

the silt Is cut into parallel vertical 

ridges, but on the lower slopes the silt is 

cut into rather small flat-topped remnants 

and small eliptical knobs. The remnants 

farthest away from the valley bottom are 

probably not frozen and those in the bottom 

have a depth to permafrost of over 4 feet. 

Some of the lower ends of the silt ridges 

contain ice masses as revealed in railroad 

cuts.

Because of the relatively steep slopes, 

drainage is generally good and no permafrost 

occurs. Some frozen ground is found on the 

north-facing side of the hills, but this 

has tentatively been placed In the depositional slope area.

Economic application

Construction.-^Because of difficulties 

resulting from permafrost, the hills offer 

the best location for the construction of 

buildings.

Agriculture. The combination of greater 

solar head and no slumping of the ground in 

the field makes, the gentle,south-facing hill 

slopes the best location for farms. However, 

the amount of gently sloping land is small and 

most slopes are too steep for cultivation. The 

type of soil on the slopes yields very readily 

to gullying and soil erosion.

Qround water. To obtain water on the 

hills, wells probably would have to be continued down into the underlying bedrock. This 

may prove too expensive for the homesteader. 

Some water may be obtained from springs at the 

base of the hills. The location of such 

springs and the possibility of their winter 

use has not been definitely determined.