The Pleistocene Geology of 

Amchitka Island, 

Aleutian Islands, Alaska 

GEOLOGICAL SURVEY BULLETIN 1478 

The Pleistocene Geology of 

Amchitka Island, 

Aleutian Islands, Alaska 

By LEONARD \1. CARD, JR. 

GEOLOGICAL SURVEY BULLETIN 1478 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1980 
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary 

GEOLOGICAL SURVEY 

H. William Menard, Director 

Library of Congress Cataloging in Publication Data 

Gard, Leonard Meade, 1923-

The Pleistocene geology of Amchitka Island, 

Aleutian Islands, Alaska. 

(Geological Survey bulletin ; 1478) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:1478 

1. Geology, Stratigraphic--Pleistocene. 2. Glacial 

epoch--Alaska-~chitka Island. 3. Geology--Alaska 

--Amchitka Island. I. Title. II. Series: United 

States. Geological Survey. Bulletin ; 1478. 

QE75.B9 no. 1478 [QE84.A38] 557.3s 80-607888 

[551.7'92'097984] 

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

Washington, D. C. 20402 
CONTENTS 

Abstract 

Introduction --------------------------------------------------------------

Deposits at South Bight----------------------------------------------------

Lacustrine and fluvial deposits of early Pleistocene age _________________ _ 

Marine deposit of the South Bight I transgression _______________________ _ 

Beach deposit of the South Bight I transgression _______________________ _ 

Beach deposit of the South Bight II transgression _______________________ _ 

Terraces on Amchitka _____________________________________________________ _ 

lligh plateaus -------------------------------------------------------- The 125--- to 131-m surface ___________________________________________ _ 

The 67- to 76-m terrace _______________________________________________ _ 

The 37- to 49-m terrace _______________________________________________ _ 

Other marine terraces _________________________________________________ _ 

Subaerial terraces _________________________________________________ _ 

Intertidal bench ________________________________________ ------------

Submarine terraces ____________________________________ - -----------

Correlation of terraces with marine deposits at South Bight _____________ _ 

Deposits at Constantine graben ___________________________________________ _ 

Older glacial deposits __________________________________________________ _ 

South Bight I(?) marine deposit _______________________________________ _ 

Younger glacial deposits _______________________________________________ _ 

Glacial outwash at Rifle Range fault ________________________________________ _ 

Glaciation of central and western Amchitka _______________________________ _ 

Extent of glaciation _______________________________________ --- _-------------

Pleistocene tectonism _____________________________________ --- _-------------

Correlation of marine transgressions ___________________________ - ------------

References cited ____________________________________________ -- _-------------

ILLUSTRA TIOl'\S 

FIGURE 1. Index map of the Aleutian Island chain _____________________ _ 

2. Map showing surfaces of planation, Amchitka Island _________ _ 

3. Photograph showing sea cliff at South Bight _________________ _ 

4. Diagrammatic geologic cross section of South Bight Graben, Amchitka Island ___________________________________________ _ 

Page 

1 

1 

5 

5 

11 

11 

14 

17 

18 

19 

19 

20 

20 

20 

21 

22 

23 

25 

26 

26 

28 

31 

31 

32 

32 

34 

37 

Page 

3 

4 

5 

6 

5. Photograph showing stratum of logs projecting from tilted 

lakebeds of early Pleistocene age at South Bight____________ 10 

6. Map showing distribution of marine terraces, east end of Amchitka Island ____________________________________________ 12 

7.-11. Photographs showing: 

7. Faulted South Bight I marine sand in cliff at South Bight 13 

8. The South Bight I beach deposit 1.5 km northwest of 

South Bight _______________________________________ 14 

III 
IV CO;\ITENTS 

FIGURES 7-11. -Continued Page 

TABLE 

9. Intertidal bench, cut on breccia of Banjo Point Formation, exposed at low tide __________________________ 21 

10. Sea cliff retreat owing to freeze and thaw ____________ 22 

11. Rhythmically bedded glacial pond deposit of alternating 

sand and silt overlain by 2-m-thick till of unsorted 

stony clayey sand -------------------------------- 29 

TABLES 

Page 

1. Diatoms collected from lower part of lacustrine deposits of early 

Pleistocene age at South Bight, Amchitka Island, Alaska____ 7 

2. Number of pollen and spores collected from lacustrine deposits 

of early Pleistocene age at South Bight, Amchitka Island, 

Alaska--------------------------------------------------- 9 

3. Correlation of Pleistocene deposits, Amchitka Island __________ 25 

4. Correlation of Quaternary marine transgressions on Amchitka 

Island____________________________________________________ 35 
THE PLEISTOCENE GEOLOGY OF 

AMCHITKA ISLAND, ALEUTIAN ISLANDS, 

ALASKA 

By LEONARD l\1. GARD, JR. 

ABSTRACT 

Tilted and faulted lakebeds of early Pleistocene age, overlain by two interglacial 

marine deposits, have been downfaulted and thereby preserved in a graben at South 

Bight, Amchitka Island, Alaska. During the early Pleistocene, trees grew on this now 

treeless Aleutian Island, and the pollen assemblage reflects an average July temperature several degrees warmer than at present. 

The two marine deposits were laid down during high-level stands of the sea, here 

named the South Bight I and South Bight II marine transgressions. These 

transgressions carved two broad terraces on Amchitka Island. The older of these, South 

Bight I, which is undated, is tentatively correlated with Termination III of Broecker 

and VanDonk 11970\. The fossiliferous South Bight II deposit, dated by the uraniumseries method at 127,000±8.000 years, is correlated with Broecker and VanDonk's 

Termination II. Two small terraces on Amchitka at 6-9 m and 15-18 mare thought to 

represent high sea stands that formed the Barbados I and II reef tracts 82,000 and 

105,000 years ago. High terrace remnants at 125-131 m are tentatively correlated 

with Termination IV of Broecker and VanDonk. 

Evidence of two glaciations is found on Amchitka Island. The earlier glaciation, 

represented by well-indurated diamictite of unknown age, appears to have occurred 

prior to the South Bight I marine transgression and may be as old as Kansan. The later 

glaciation, which is younger than the South Bight II transgression, probably covered 

the island in late Wisconsin time. Several thin icecaps probably were present simultaneously during the Wisconsin, and distribution of distinctive erratics indicates that 

one ice sheet, centered at the low east end of the island, moved from east to west at 

least 8 km and had a source area that could have been no higher than 73 m above 

present sea level. 

INTRODUCTION 

Multidisciplinary earth science studies by the USGS (U.S. Geological Survey), conducted on and near Amchitka Island from 1964 to 

1972, were made in support of the development of the island as an 

underground nuclear test site by what was then the USAEC (U.S. 

Atomic Energy Commission, now the U.S. Department of Energy). 

These investigations were the first comprehensive geologic studies 

made in the Aleutians since the reconnaissance mapping by the 

1 
2 PLEISTOCENE GEOLOGY OF AMCHITKA ISLAND, ALASKA 

USGS during the 1940's and 1950's. The original reconnaissance 

geologic reports (Coats, 1956; Powers and others, 1960) included descriptions of glacial and interglacial deposits. 

During the course of the more recent investigations, the Pleistocene geology was studied in detail, mainly to determine the 

Quaternary tectonic stability of Amchitka. This paper summarizes 

the results of that study. 

The Aleutian Islands are probably one of the least accessible areas 

in the world. Weather and logistics make geologic operations difficult, 

often unpleasant, and very expensive. Many of the islands display 

flights of marine terraces; yet, because of inaccessibility, little attempt has been made to study those terraces and their relationship to 

worldwide Pleistocene marine transgressions. To date, Pleistocene 

marine deposits are virtually unrecognized by any studies on these 

islands. 

Amchitka Island, one of the western Aleutian Islands, lies at about 

lat 5P30' N. and long 179° E. (fig. 1). It is the second most southerly 

island in the chain and lies only slightly north of the latitude of the 

northern tip of Vancouver Island, British Columbia. The island is 

about 65 km long and 5-8 km wide and trends northwest. The southeastern half is low lying, generally less than 150m above sea level, 

while the western half of the island rises to a rugged mountainous 

segment 356m high that gives way northwestward to a plateau about 

250 m above sea level. 

Although the Pleistocene history is far from complete, geologic 

strata on Amchitka Island reveal more than has been reported so far 

from any other Aleutian island. Graben faulting has preserved an 

unusual series of Pleistocene deposits near the eastern end of the island. Sediments are preserved at and near South Bight, at Constantine 

Harbor, and along the Rifle Range fault (fig. 6). Evidence is found for at 

least two glaciations separated by a fossiliferous marine deposit. 

Marine transgressions have carved at least four well-developed 

marine terraces on Amchitka Island (fig. 2), two of which are warped, 

tilted, and faulted and are correlated with fossiliferous marine and 

beach deposits. Marine regressions have cut one broad extensive surface and at least two smaller discontinuous terraces, now all below 

sea level. 

Subaerial marine terraces are much better developed on the eastern half of Amchitka Island, perhaps because erosion and beveling 

during late Oligocene or early Miocene time had already reduced the 

rocks of earlier Tertiary age to a low gently sloping surface. On the 

steep flanks of the mountainous western end, formed by Miocene 

volcanic rocks, the Pleistocene terraces are restricted to narrow discontinuous ledges and notches. 
s'Slo 

....., ~ 

160° 58 ° 164° 168° 

KAMCHATKA 

Bering 

lslandn 

\\ KOMANDORSKIYE 

~ ISLANDS 

~ttu 

Agattu ~ 

NEAR 

ISLANDS 

172° 176° 180° 

BERING SEA 

t.s< IY-<tr Kiska -11Jt0 c2 •-c .S Kanaga 

176° 

Rat"' ~ o o." s T anaga ~ (_;Adak Atka 

Amchitka v,O.'\. ·;~ )) '!;1uc-,· ~~ 0 

)f-.\\ •1 ANDREA Aml ia ~ I NOF t'-i\' DELAROF ISLANos 

ISLANDS 

0 200 

172° 168° 164° 

~t Matthew 

t?o 

400 

PRIBILOF 

<;~ISLANDS 

s ~~a 

\

s '--~ " (__r--f; "(o-f. PuD"o· Unimak 

o~ 0 ~~ Unalaska 

.,Yumnak 

Sam alga 

PACIFIC 

600 KILOMETERS 

160° 

ALASKA 

OCEAN 

FIGURE 1.-Index map of the Aleutian Island chain. Amchitka Island lies at about lat 51 °30'N and long 179°E. 

58° 

54° 

50° 

...... 

:....: 

o--.] 

:;o 

0 

u 

c 

n 

o--.] 

0 

z 

C,..j 
51° 

EXPLANATION 

~ 214-275 METERS 

-152-214 

[2] 125-131 

ts:S1 67-76 

~ 37-49 

~ 15-18 

06-9 

[ill -46 

~-91 

./J 

--toA 

7o 

--L Fault- bar and ball on downthrown side 

Qls Landslide 

179° 

BERING SEA 

FIGURE 2.-The planation surfaces (patterns) of Amchitka Island and offshore. Altitudes determined at 

bases of abandoned sea cliffs, except for surfaces between 152 and 214m and between 214 and 275m. 

Altitudes from Holmes and Narver unpublished topographic map, 1:6,000. 

~ 

'i:l 

t""" 

M 

c;; 

...., 

0 

n 

M 

z 

M 

(') 

M 

0 

t""" 

0 

(') 

-< 

0 

"''"j 

;J> 

~ 

n 

:r: 

=1 

:;::: 

;J> 

c;; 

t""" 

;J> 

z 

.o 

;J> 

t""" 

;J> 

(/) 

:;::: 

;J> 
DEPOSITS AT SOUTH BIGHT 5 

DEPOSITS AT SOUTH BIGHT 

LACUSTRINE AND FLUVIAL DEPOSITS OF EARLY PLEISTOCENE AGE 

South Bight graben, about 800 m wide, was once a closed structural 

depression which, in early Pleistocene time, formed a quiet treeflanked lake along what is now the south shore of the island. Submarine contours suggest that the depression may have been as much 

as 7 km long. Since then, coastal erosion has destroyed the south- and 

west-bounding bedrock shores of the lake and has left a cross section 

of sediments that were deposited in the subsiding graben exposed in 

the sea cliff at the head of the bight (fig. 3). 

The earliest Pleistocene record to be found is more than 80 m of 

tilted and faulted lacustrine and fluvial sediments exposed in the 

lower part of the sea cliff(fig. 4). These sediments constitute the lower 

part of strata referred to by Powers, Coats, and Nelson (1960, p. 541) 

as ~~tilted sedimentary rock at South Bight." I have divided this lower 

part into three units: The lowest unit consists of more than 63 m of 

fine-grained carbonaceous lake sediments; the middle unit is composed of 28 m of fluvial sand and pebble gravel; and the highest unit, 

only 3.5 m thick, resembles the carbonaceous lacustrine sediments of 

the lowest unit. 

FIGURE 3.-Sea cliff at South Bight. Lakebeds (1) of early Pleistocene age form lower 

part of cliff at left (north) and are conformably overlain by South Bight I interglacial 

marine deposits (2). Both are tilted and faulted (3) and beveled (4). Unconformity (4) 

has been tilted down to south (right) and is overlain by South Bight II interglacial 

beach deposits (5). Sea cow localities at (6). White sand bed above unconformity at 

right of picture is composed of cross-stratified shell fragments. Photograph by Robert 

H. Morris, U.S. Geological Survey. 
NNW 

METERS 

60 

40 

20 

SEA 

LEVEL 

EXPLANATION 

Quaternary deposits: 

{~~ ·} :J Dune sand 

~ South Bight II interglacial 

~ beach deposit 

r--1 South Bight I interglacial 

~ marine deposit 

Lacustrine and fluvial(?) deposit ..... Tertiary deposits: 

f

6

0

o 

0

o "J Amchitka Formation 

0 100 200 METERS 

SSE 

FIGURE 4.- Diagrammatic geologic cross section of South Bight graben, Amchitka Island. Faults (heavy lines) generalized in 

part; arrows show direction of movement. Black triangle, Hydrodamalis location. Vertical exaggeration x 2. 

0':1 

r-"" 

t""l 

IJl 

...., 

0 

(") 

tTl 

z 

tTl 

C'l 

tTl 

0 

r0 

C'l 

.....::: 

0 

"Tl 

> 

s:: 

n 

:r: 

...., 

;;::: 

> 

IJl 

r-

> z 

t:! 

> 

r-

> 

IJl 

;;::: 

> 
DEPOSITS AT SOCTH BIGHT 7 

The lowest unit , mainly carbonaceous clayey silt and sand, records 

graben subsidence and quiet water conditions. The base of this unit is 

not exposed. These strata contain freshwater diatoms, pollen, carbonized logs, and wood fragments. Thin layers of altered ash indicate 

contemporaneous intermittent eruptions of nearby volcanoes. 

A sample containing diatoms (table 1) was collected from near the 

TABLE 1.- Diatoms collected from lower part of lacustrine deposits of early Pleistocene 

age at South Bight, Amchitka Island, Alaska 

[f, frequent ; R, rare. No taxa are common or abundant. F, freshwa ter forms; M, marine forms; S, sal ine or 

brackish-water fo rms; X, no ecologic information. Identified by G. W. Andrews, written commun. , 1974] 

Achnanthes coarctata Brebisson ___ _______________ ___ _ 

Achnanthes flexella (Kuetzing) Brun _______ _________ _ Achnanthes oestrupii (A. Cleve) Hustedt __ __________ ____ ____ _ _ Achnanthes sp. A _____ _______ ___ _______ __________ _ 

Brebissonia aff. B. boeckii (Ehrenberg) Grunow ___________ ___ _ 

Cocconeis sp. A --------------------- - - ---------- - - ___ ___ _ _ 

Cocconeis sp. B ----------------- - ----- - - -------------------

Coscinodiscus sp. A - - - ------------- - - - - -------------- ------- Cymbella cistula (Hemprich) Grunow ______________ ____ ____ _ _ Cymbella laevis Naegeli _________ _________ ______ _______ _____ _ 

Cymbella naviculiformis Auerswald ___ _____ _____________ ____ _ _ 

Cymbella tumida (Brebisson) Van Heurck ___________ ___ __ ___ _ 

Didymosphenia aff. D. geminata (Lyngbge) M. Schmidt __ __ ___ _ Diploneis finnica (Ehrenberg) Cleve ________ ______ _____ ____ _ _ Diploneis ovalis (Hilse) Cleve ____________ __________ _ _____ ___ _ Diploneis sp. A ___ ___ __ _________ _________ ___ ______ _____ ___ __ _ 

Epithemia turgida (Ehrenberg) Keutzing ________ __ ______ ___ _ Eunotia robusta Ralfs ___________ _______________ ___ ____ ___ _ 

Eunotia sp. A ------ ----------------------- ---- -- - - __ _____ _ Fragilaria construens var. Binodis (Ehrenberg) Grunow ____ __ _ _ Frustulia rhomboides var. Amphipleuroides Grunow _ ____ ___ _ Gomphonema intricatum Kuetzi ng ___ ____ __ _____________ ___ _ Grammatophora angulosa Ehrenberg ______________ ___ _______ _ Gyrosigma strigile (W. Smith) Cleve ___ ___ _________ ________ ___ _ 

H ydrosera sp. ___ ____ ________ ___ ___ ___ ___________ ____ __ __ - _--

Melosira distans var. Lirata f. seriata Mueller ________ _______ _ Melosira granulata (Ehrenberg) Ralfs _________________ _______ _ 

Navicula bacillum Ehrenberg __ _____ _______________ _ Navicula inflexa (Gregory) Ralfs ___ _____ ____ ____ ____ _ 

Navicula peregrina (Ehrenberg) Keutzing _________________ ___ _ Navicula pseudoscutiformis Hustedt ________ ___ ___ _____ _____ _ Nitzschia aff. N. amphibia Grunow ________ _______________ __ _ _ Nitzschia plana W. Smith ____ _____ _______ ______ ____ _______ ___ _ 

Pinnularia gibba Ehrenberg _____________ _________ _ ____ ___ _ 

Pinnularia mesolepta (Ehrenberg) W. Smith _____ ________ __ ___ _ Pinnularia nodosa Ehrenberg _______ _____ ______ ________ __ ___ _ Pinnularia sp. A ______________ ___________ _________ ___ ____ __ _ _ 

Raphoneis angustata Pantocsek ___ _______ __________ __ __ __ ___ _ Rhopalodia gibba (Ehrenberg) 0 . Mueller ________ _________ ___ _ Strauroneis salina W. Smith _____ _______ __ ____ __ ___ ____ _____ _ Surirella oval is Brebisson ____ __ ___________ ____________ _______ _ 

Tetracyclus emarginatus (Ehrenberg) W. Smith __ ___ ____ ___ _ Tetracyclus lacustris Ralfs ____ ____________ ______ _____ __ ___ _ 

Tetracyclus rupestris (A. Braun) Grunow ________ _______ ____ ___ _ 

Frequency 

R 

R 

f 

R 

R 

R 

R 

R 

R 

R 

R 

R 

R 

R 

f 

R 

f 

f 

R 

R 

R 

R 

R 

f 

R 

f 

f 

R 

f 

R 

R 

R 

R 

R 

R 

R 

R 

R 

R 

R 

f 

R 

f 

f 

Ecology 

F 

F 

F 

X 

s 

X 

X 

X 

F 

F 

F 

F 

F 

F/S 

F/S 

X 

F/S 

F 

X 

F 

F 

F 

M 

s 

M 

F 

F 

F 

s 

s 

F/S 

F 

s 

F 

F 

F 

X 

M 

F 

s 

s 

F 

F 

F 
8 PLEISTOCE NE GEOLOGY OF AMCHITKA ISLAND, ALASKA 

lowest part of the exposed lakebeds. These fossils were identified by 

G. W. Andrews, USGS, who reported on the collection as follows: 

This is an unusual assemblage in that it contains a re~atively large number of taxa, 

none of which could be considered more than "frequent" in the sample examined. Many 

of the specimens are broken, which suggests that they may have been transported to 

the site of deposition, rather than having been deposited at the place of growth. 

Geologic age: Only one extinct species was identified, Rhaphoneis angustata, previously known only from rocks of Miocene age. On the basis of the occurrence of 

Epithemia and Rhopalodia, the assemblage cannot be older than middle Pliocene. All 

of the taxa, with the one exception, are known to be living today, which is in accord 

with your early Pleistocene age determination. Late Pliocene and Pleistocene nonmarine diatom assemblages are so similar to living assemblages that I cannot make 

detailed age determinations in this interval. 

Paleoecology: This assemblage contains a preponderance of freshwater diatoms, and 

many of the taxa are cool-water forms. It also contains a sizable percentage of salttolerant freshwater species as well as some that prefer saline or brackish water. The 

three rare species of marine diatoms are probably of little significance. Deposition 

possibly occurred in the slightly saline upper reaches of an estuary or perhaps in a 

slightly saline or brackish pond. The largely broken nature of the diatom valves, as 

well as the trace number of marine species, may indicate that some diatoms were 

blown into the deposit. This may also account for the low frequency of many species, as 

well as the mixing of freshwater, saline, and marine species. 

Spru<;e, pine, and perhaps alder and birch grew on Amchitka at 

least part of the time that the lake was in existence; ferns appear to 

have dominated the local vegetation. Two samples collected from different horizons and submitted for analysis (table 2) contained tree 

pollen. According to Estella B. Leopold (USGS, written commun., 

1971): 

The absence of dicotyledon Tertiary relicts indicates that these samples are post 

Miocene and probably post Pliocene. The presence of spruce and pine pollen (total tree 

countof 12 percent in Sample No. D4615) indicates that these trees, and perhaps alder, 

grew at Amchitka at the time of deposition. The great abundance of Polypodiaceae 

spores suggests that ferns dominated the local vegetation. The remainder of the assemblage includes plant groups typical of your Quaternary samples from the Amchitka 

area. The spruce count in Sample No. D4615 is like that from modern surface samples 

taken at latitudinal tree line in the Brooks Range. The presence of pine pollen, 

however, makes this assemblage of particular interest, because the nearest pine is 

either in the interior ofthe upper Yukon of Canada (Pinus banksiana) or in Chukotka 

in Siberia (Pinus pumilla). 

The assemblage suggests that average July temperatures during the Tertiary were 

warmer by zoe than now. These samples could be either Pliocene or early Quaternary 

in age, and of these an early Quaternary age seems more likely. 

Subsequently, I collected a series of six samples from throughout 

the section. Remarkably, none of the six samples contained tree pollen. Certainly the presence of four separate layers containing logs as 

much as 20 em in diameter indicates that trees were at least intermittently present during the life of the lake. Of these six samples, 

Leopold said: 

All six samples contain abundant monolete spores of the Polypodiaceae type and 
DEPOSITS AT SOUTH BIGHT 9 

TABLE 2.- Number of pollen and spores collected from lacustrine deposits of early Pleistocene age at South Bight, Amchitka Island, Alaska 

[Identified by Estella B. Leopold, written commun., 1971. - ,not found or absent; +,very rare or present but not as a whole pollen grain] 

Pollen or spores 

Picea _____________________ __ _______ __ __ __ _______________ _ 

Pinus -------------------------------------------------- Alnus -------------------------------------------------- Betula -------------------------------------------------- Ericales - -------------------------------------- --------- Gramineae ---------------------------------------------- Cyperaceae __ ____ ____________________ __________ __ _______ _ 

Rubiaceae, Galium type ---------------------------------- Compositae ______________ ___ ____________________ ________ _ 

Labiatae ------------------------------------------------ Lycopodium sp. ___ _________ ____________________ __ _______ _ 

R tJ~b~f&e~;~ icc ia ______________________ -= == ============= ======= _______ __===__========= ________=== ___====== _____ _ 

Unident. dicotyledons ___ ___ ___ ______ ___ __ _______________ _ Polypodiaceae ___ __ ______ ______ ___ __ __ __ __________ ____ ___ _ 

Moss spores ________________ __ ___________________ _________ _ 

U.S. Geological Survey 

Paleobot. Loc. Nos. 

D4615 (WC- 54-70 ) 

4 

8 

1 

2 

3 

2 

+ 

+ 

+ 

1 

7 

92 

3 

D4614 (WC- 53--70) 

+ 

8 

3 

3 

+ 

+ 

+ 

5 

183 

constitute about 95 percent of the count; sparse herbaceous pollen grains are present; 

that is, Galium, Cyperaceae. No tree pollen of any kind was seen. 

It is of interest that tree pollen is noted in two samples lower down in this section 

(USGS Paleobot. Sample Nos. D4614 and D4615), but the pollen seems to be completely 

absent in these six samples. The assemblage includes few plant types and, therefore, is 

a bit dangerous to interpret ecologically. But it does seem probable that the assemblage 

represents tundra and that at the time of deposition the trees were absent from or very 

rare on this island. If this conclusion is valid, we can presume that the climate has 

worsened following the deposition of Sample Nos. D4614 and D4615. 

A specimen of the lakebeds that contains a thin altered ash layer 

was carefully oriented and removed, and its remanent magnetism 

was determined using a spinner magnetometer. Two samples were 

cut from the specimen and both were determined to be reversely 

magnetized. According to G. D. Bath (USGS, written commun., 1974), 

~~The directions suggest that this particular section of the lakebeds 

was deposited at a time when the Earth's field was generally opposite 

to its present direction." Results from a single sample are generally 

considered to be inconclusive by most paleomagnetic investigators; 

however, these magnetic data tend to confirm an early Pleistocene 

age for these deposits. 

Thin beds of angular boulders, wood fragments, and logs extend 

into the fine-grained lake sediments from the north ~~shore," probably 

recording. times of graben subsidence and sloughing of debris from the 

north-bounding fault scarp. The layers consist of wood fragments and 

sharply angular boulders as much as 30 em in diameter. These layers 
10 PLEISTOCEN E GEOLOGY OF AMCHITKA ISLAND, ALASKA 

extend laterally into the lakebeds for about 30 m from the fault, and 

this sloughing is marked farther out from Hshore" by a stratum of logs 

(fig. 5). Four such horizons were noted in the exposed section of 

lake beds. Wood from one of these horizons, collected by H. T. 

Shacklette for radiocarbon dating (Sample No. W2250), produced a 

date in excess of 38,000 years (Spiker and others, 1977, p. 349, Sample No. W2250). Either the graben block must have been subsiding 

evenly during this time, as no angular unconformities have been 

observed in the lakebeds, or the graben already had subsided before 

the beds were deposited. In the latter case, the debris layers may have 

been produced by earthquakes or by normal bank sloughing. Evenness of the bedding suggests that the lake was deep enough that the 

bottom sediments were not disturbed by wave action. 

Overlying the lacustrine deposits with slight erosional unconformity are 28 m of cross-stratified sand and strongly iron-stained and 

weathered pebble gravel; these deposits document changed conditions 

of deposition. Whether this deposition records uplift nearby or 

whether it is outwash from an early glaciation noted at Constantine 

Harbor is uncertain. Subsequently, the graben again reverted to 

quiet lacustrine conditions for a short time and 3.5 m more of carbonaceous sand and silt were deposited conformably on the sand and 

gravel. 

FIGURE 5.-Stratum of logs projecting from tilted lakebeds of early Pleistocene age at 

South Bight. Arrows point to prominent logs. "Shore" was to left. 
DEPOSITS AT SOLTH BICHT 11 

These beds of early Pleistocene age may correlate with carbonaceous conglomerate and tuff layers on Tanaga Island, which contain a 

similar pollen assemblage (Fraser and Barnett, 1959, table 1, locality 

14). 

MARINE DEPOSIT OF THE SOUTH BIGHT I TRANSGRESSION 

Overlying the lacustrine deposits with apparent conformity, as 

much as 18m of even-bedded well-compacted marine sand and gravel 

(fig. 4) provide the first depositional record of a Pleistocene interglacial rise of the sea level at Amchitka. This rise in sea level is here 

named the South Bight I marine transgression. These marine sediments constitute the upper part of the ((tilted sedimentary rock at 

South Bight" of Powers, Coats, and Nelson (1960, p. 541). 

A basal deposit of about 3.5 m of well-rounded, iron-stained, sandy 

gravel, composed of local rock types averaging 10-14 em in diameter, 

grades upward without a definite break to even-bedded sand with 

scarce, discontinuous stringers of well-rounded pebbles. The sand 

contains unworn spicules of the siliceous tetraxial marine sponge 

Dichotriaenes (R. M. Finks, Queens College, written commun., 1972). 

Although these sponge spicules are not diagnostic of age, water 

depth, or temperature, they do indicate that the sediment is of marine 

origin, according to Finks. No other fossils were found in these strata. 

The sand is fine to medium grained, well compacted, but not lithified. 

The sand is rarely cross-stratified, and cut-and-fill structures and 

ripplemarks are lacking. These conditions suggest that sea-level rise 

was swift and that the lake in the graben was rapidly overwhelmed 

and transformed into a closed depression on the sea bottom lying 

offshore. The lake was relatively unaffected by currents or wave action. Initially, gravel and then sand were trapped in the depression as 

sea level rose. No evidence of contemporaneous volcanism is preserved. I have correlated this apparently deepwater marine deposit 

with a prominent marine terrace whose sea-cliff base now lies at 

about 67-76 m above present sea level (figs. 2 and 6), for reasons that 

will be given later. 

Renewed, but now asymmetrical, subsidence of the South Bight 

graben tilted the entrapped marine deposit and underlying lacustrine 

and fluvial deposits southward. During tilting, the beds were broken 

by a myriad of small normal faults having less than 3 m of displacement, generally downthrown on the north side (fig. 7). Density of 

these faults increases toward the south boundary of the graben. 

BEACH DEPOSIT OF THE SOUTH BIGHT I TRANSGRESSION 

A deposit of very coarse gravel, which includes subangular boulders as large as 1.5 min diameter and which contains only a minor 
51° 

15' 

30" 

St M_akarius 

Point 

~ 

EXPLANATION 

I ~;~~r METERS 

15-18 

6-9 

Grampus Point 

~ Fault-bar and ball on 

downthrown side 

CS) Closed depression 

0 2 3 KILOMETERS 

FIGURE G.-Distribution of marine terra~es (patterns), eastern end of Amchitka Island. Note closed sea-bottom depressions in Constantine Harbor and St. Makarius Bay. Solid lines indicate bases of abandoned sea cliffs, dashed where 

indistinct or absent. Numbers on solid lines indicate altitude of base of sea cliff, in meters. Altitudes converted from 

feet to nearest whole meter. Altitudes from Holmes and Narver unpublished topographic map, 1:6,000. 

~ 

~ 

""0 

r 

M 

[JJ 

_, 

0 

() 

M 

z 

M 

CJ 

M 

0 

r 

0 

CJ 

>-< 

0 

.., 

;l> 

~ 

() 

:t 

_, 

;;:: 

;l> 

[JJ 

r 

;l> 

z 

_t:J 

;l> 

r 

;l> 

[JJ 

;;:: 

;l> 
DEPOS ITS AT SO L'T H BIGH T 13 

FIGURE 7.-Faulted South Bight I marine sand in cliff at South Bight. Note offset in 

uncommon pebble bed. 

amount of sand, lies on the 67- to 76-m terrace at 65-m altitude in a 

gravel pit 1.5 km northwest of South Bight. This deposit was thought 

to be of glacial origin by Powers, Coats, and Nelson (1960, p. 544) 

because of the presence of ((exotic rocks." 1 They speculated that it 

might have been reworked by marine action subsequent to deposition. The presence of battered sponge spicules, which I found in a rare 

pocket of iron-stained sand, coupled with the coarseness of the gravel, 

leads me to conclude that this is a beach deposit and that it is related 

to the 67- to 76-m terrace on which it lies. Unfortunately for future 

students of this area, marine shell material now lying on the floor of a 

gravel pit in the deposit was in gravel dug from the South Bight II 

deposit 1.5 km to the southeast and stockpiled here during the AEC 

occupation. 

This coarse beach gravel lies on a smoothly eroded, undulating 

bedrock surface that is composed of apophyses of East Cape pluton 

and metamorphosed host rock. This surface is now exposed in many 

places by excavation. Nowhere could I find striations on the bedrock; 

it appears to be a wave-worn surface. Schafer (in Gates and others, 

1

It should be noted here that, with but one exception to be discussed later, I have never found any rocks exotic to 

Amchitka in the Pleistocene deposits. Those referred to by Powers, Coats, and Nelson (1960, p. 542 and 544) can all 

be accounted for as various facies of a quartz diorite pluton at East Cape (Carr and others, 1970) and its metamorphosed roof pendants and aureoles. In fact, the existence of ice-rafted off-island exotic rocks would present the 

paradox of their somehow having been deposited 30m above present sea level during glacial times when sea level 

may have been as much as 90 m lower or would require that extensive ice or tree rafting took place during 

interglacial times. 
14 PLEISTOCENE GEOLOGY OF AMCHITKA ISLAND, ALASKA 

1971, p. 783) described a similar situation on Shemya Island, for 

which he hypothesized that "the bedrock was cut by marine erosion, 

covered by marine deposits, and then glaciated." Indeed, the Amchitka gravel deposit also appears to have been glaciated, as it not 

only shows evidence of frost stirring in the upper part of the deposit 

(fig. 8) but also is overlain by scattered striated boulders (none was 

found within the deposit). The gravel owes its preservation to its 

unusual coarseness, which is due to the relatively wide spacing of 

joints in the hard, homogeneous underlying rock from which it was 

derived. 

BEACH DEPOSIT OF THE SOUTH BIGHT II TRANSGRESSION 

At South Bight, the South Bight I marine deposit and underlying 

lakebeds of early Pleistocene age were tilted, faulted, and then truncated by erosion during a new eustatic high-level stand of the sea. 

The resultant angular unconformity (fig. 4) must represent a considerable hiatus for which no depositional record has been found. During 

that time, at least one glacial period must have occurred. This highlevel stand of the sea, here named the South Bight II marine 

transgression, must have persisted for a considerable length of time, 

FIGURE 8.-South Bight I beach deposit 1.5 km northwest of South Bight. Gravel above 

base of entrenching tool is iron stained, and many elongate cobbles have been turned 

vertically by frost action (center of picture). 
DEPOSITS AT SOCTH BIGHT 15 

for it cut a broad terrace whose sea-cliff base now generally lies between 37 and 49 m above sea level (figs. 2 and 6). The basis for 

correlating this terrace with the South Bight II transgression will be 

discussed in the section entitled ((Correlation of Terraces with Marine 

Deposits at South Bight." 

This transgressive sea deposited a 6- to 30-m-thick wedge of highly 

fossiliferous coarse sand- to boulder-sized be.ach gravel conformably 

on the erosion surface (fig. 3). Continued uneven subsidence of the 

graben lowered the erosion surface and the overlying deposit and 

tilted them southward. The erosion surface beneath the South Bight 

II beach deposit, in the graben, ranges from 34 m above sea level at 

the north side of the graben to 15 m above sea level at the south side. 

The thin northern part of this beach gravel crosses the northbounding graben fault with no apparent offset, implying that movement on that fault ceased before the end of South Bight II time. Here 

Powers, Coats, and Nelson (1960, p. 542) observed that the beds in the 

gravel pit at the north side of the graben were covered by several feet 

of rounded to angular cobbles and boulders, ((many of them mica 

schist and biotite granite, entirely foreign to the bedrock exposed in 

the Aleutian Islands." (See footnote, p. 13) Some of these boulders are 

faceted, and those of a finer grained nature commonly display striations. This exposure has since been destroyed by excavation, but faceted and striated boulders can be found scattered on the surface of 

the beach deposit in this area. 

Some beds, especially in the lower part of the beach deposit near 

the south end of the exposure, are composed mainly of cross-stratified 

shell fragments and broken urchin tests and spines. Discontinuous 

pods as much as 1 m thick, composed almost entirely of unbroken 

pecten shells, are also present. At the base of the deposit, subrounded 

boulders partly coated with kelp-secreted carbonate have blocks of 

algal carbonate and thick oyster shells wedged in between them. 

Near the south end of the exposure, the middle part of the deposit 

contains beds of subangular boulders as much as 60 em in diameter 

that probably were derived from the south-bounding fault, perhaps 

during fault movement, and deposited in deeper water before they 

could become rounded. The deposit generally decreases in grain size 

upward and northward, although scattered subrounded boulders are 

present throughout. In the central part of the exposure, several 30-

cm-thick beds of pumice pebbles crop out. The pebbles are wellrounded, hornblende-bearing, white frothy pumice that average 2.5 

em in diameter but may be as large as 7 em in diameter. The pumice 

originated in eruptions of one of the nearby active volcanoes. The 

buoyant, low-density fragments apparently floated to this area, were 

rounded by wave action, and were stranded on the interglacial beach. 
16 PLEISTOCENE GEOLOGY OF AMCHITKA ISLAND, ALASKA 

Other than the pumice gravel, the sand and gravel of the beach 

deposit appear to be all of local origin, with one exception. This exception is an angular cobble of fossiliferous limy argillite that was found 

about 6 m above sea level near the south end of the sea cliff at South 

Bight. The cobble was not found in place but was lying on the slope 

composed of South Bight I sands. It apparently had fallen from the 

bouldery South Bight II beds above although the possibility that it 

came from South Bight I sands cannot be ruled out. In thin section, 

the argillite is seen to be composed of about 50 percent angular to 

subangular chert, slate, and schist fragments Hfloating" in a calcareous matrix, according to F. M. Byers, Jr. (USGS, written commun., 

1973). Fossils are Clinocardium cf. C. decoratum (Grewink), 

Macoma? fragments, and ~~teredo borings" in silicified wood fragments. According to W. 0. Addicott (USGS, written commun., January 24, 1972), the ~~Clinocardium in this sample is similar to a species 

recorded from strata of Miocene and Pliocene age from the Aleutian 

Islands and the Bering Sea area." Rocks of this description are foreign 

to Amchitka, but the above description of the rock composition and 

fossil content is similar to the description of the Miocene(?) Chuniksak Fo~mation of Attu Island (Gates and others, 1971, p. 731). Because of these facts and the angularity of the specimen, I conclude 

that this exotic rock was tree rafted to the South Bight area during 

the South Bight II transgression. If this conclusion is correct, presence of this exotic rock at Amchitka implies that trees may have been 

present in the Aleutians during this interglacial period. 

The beach deposit contains both vertebrate and invertebrate fossils 

(Cushman and Todd, 1947; Powers and others, 1960; Allison, 1973), 

as well as the fossil coralline algae Lithothamnion. Uranium-series 

dating of bone and shell material indicated an age of 127,000±8,000 

years for this deposit (Szabo and Gard, 1975). A recent study by Allison (1973) of the rich invertebrate fauna provides evidence for 

paleotemperatures and paleoecology. He reported that the mean February water temperature probably was about 3.9°C, and that the 

mean August sea temperature was between 10.0°C and 11.7°C; these 

compare with present-day temperatures of 3.9°C for February and 

10.0°C for August. 

Remains of four marine mammal taxa have been found in this 

deposit (Gard and others, 1972). These are Steller's Sea Lion 

(Eumetopias cf. E.jubata Schreber), walrus (Odobenus rosmarus Linnaeus), whale, and Steller's Sea Cow (Hydrodamalis gigas Zimmermann). 

The sea cow bones found here are the first recorded discovery of 

Steller's Sea Cow remains in situ in Pleistocene deposits. Stejneger 

(1883) described the excavation of a partial sea cow skeleton from 
TERRACES ON AMCHITKA 17 

what probably is a Pleistocene terrace deposit about 12 feet (3. 7 m) 

above sea level and 500 feet (152 m) inland on Bering Island. The 

giant beast, now extinct, was discovered in 1741 by the German naturalist G. W. Steller along the coast of Bering Island, one of the 

Komandorskiye Islands, the most westerly islands in the Aleutian 

chain (fig. 1). The sea cow, whose closest living relatives are the 

manatee and dugong, p_robably had survived the onslaughts of hunting man only because the Komandorskiye Islands were unknown to 

man until the Bering expedition was shipwrecked there in 1741. 

After that, it was only a matter of time-27 years-until these huge 

sluggish beasts were slaughtered to extinction. They were easy to 

catch and good to eat, and Russian ships regularly stopped there for 

victualing. 

Parts of the skeleton of a young, but apparently nearly full-grown 

individual, as well as parts of two others, were found in the deposits of 

the South Bight II marine transgression (Whitmore and Gard, 1977). 

The Pleistocene animal was toothless, as was his 18th century counterpart, and undoubtedly also subsisted on kelp. The South Bight 

specimen is indistinguishable from the modern species although it 

was probably somewhat larger. A female specimen measured by Steller was 25 feet (7.5 m) long and weighed as much as 22,000 pounds (10 

metric tons) (Scheffer, 1972). 

TERRACES ON AMCHITKA 

At least seven planar surfaces lying well above sea level exist on 

Amchitka (fig. 2). The two highest of these are the plateaus that top 

the mountainous western third of the island; they probably were not 

formed by marine erosion. A third surface, lying between 131 m and 

7 4 m above sea level, lower than the high plateaus but found only on 

the eastern two-thirds of the island, is of indeterminate origin but has 

an indistinct suggestion of a sea cliff at 125-131 m above sea level 

near the middle of the island. 

The four lowest surfaces have unquestionably been carved by 

marine transgressions and are backed by distinct sea cliffs. These 

terraces are increasingly more dissected from lowest to highest, suggesting that they decrease in age downward. The upper two are broad 

and well developed, indicating that, at least twice during the Pleistocene, sea level stood higher than at present for long periods of time. 

The lower two are small and discontinuous, generally best preserved 

on promontories, and are virtually undissected. The terraces have 

been identified by the altitudes of the knickpoints at the base of their 

sea cliffs. ·The main terrace surfaces, of course, lie at lower altitudes. 

These four terraces are best developed on the eastern two-thirds of 

the island, because that part of the island has had low subdued topog-
18 PLEISTOCENE GEOLOGY OF AMCHITKA ISLAND, ALASKA 

raphy since Miocene time and is composed of readily eroded Tertiary 

mudflows and turbidites; hence, broad terraces were more readily 

carved there by high sea stands. The steep, mountainous western 

third of the island is composed mainly of hard lava flows and breccias, 

and these high sea stands are represented there only by small 

benches and notches, probably because the steep slopes on hard rocks 

prevented the formation of broad terraces. Many of the small benches 

probably have been destroyed by subsequent coastal erosion. 

HIGH PLATEAUS 

The western third of Amchitka consists of a mountain segment and 

high plateaus (Powers and others, 1960). The mountain segment is a 

sinuous ridge that rises as high as 366 m above sea level and roughly 

parallels the northwest-trending island axis. The ridge is a remnant 

of a Miocene volcanic complex that has been sapped by valley glaciers 

on the north and south. 

Northwest of the mountain segment, a high dissected plateau, cut 

on the Miocene volcanic rocks and ranging in altitude from 214 to 275 

m, extends to the northwest (fig. 2). Several isolated, high, relatively 

planar areas, ranging from 152 to 214m in altitude, are found west, 

south, and east of the mountain segment. It is uncertain whether the 

lower surface should be considered separately from the higher one. 

Although Powers, Coats, and Nelson (1960, p. 525) considered them 

one surface, I have separated them, mainly because the segments to 

the south and east are isolated from the higher plateau. 

Powers, Coats, and Nelson (1960, p. 532) suggested that the high 

plateau is modified from a marine platform that has been elevated 

about 800 feet (244m). To me, this seems unlikely. Relative sea level 

during the Miocene was at least 23 m lower and probably was as 

much as 90 m lower than at present. A subaerial basalt flow of probable Pliocene age near present sea level further indicates that 

Pliocene sea level also was at or below its present position. These 

relations would require that during late Miocene and early Pliocene 

time the island sank 244 m relative to sea level, was eroded, and was 

then elevated to about its present altitude. Except for some rounded 

cobbles scattered on a high mesa about 220m above sea level at the 

northwestern end of the island (Powers and others, 1960, p. 525), 

evidence is lacking that the island has ever been 244 m lower since 

the Miocene. The cobbles could easily be lag from Miocene bedrock 

conglomerate (Powers and others, 1960, p. 525) or, more likely, remnants of outwash from glaciers covering the plateau. Altiplanation 

seems to offer a simpler explanation for the origin of these high surfaces than large-scale lowering and raising of the island relative to 

sea level. 
TERRACES ON AMCHITKA 19 

Sand and gravel composed of hornblende andesite, lying south of 

Chitka Point between about 152 and 183m, was interpreted by Powers, Coats, and Nelson (1960, p. 541) as a possible littoral-zone deposit, although they stated that the deposit might be correlative with the 

Miocene Chitka Point Formation. I examined this deposit in detail in 

several fresh excavations made for road construction during the AEC 

occupation of the island and concluded that this unfossiliferous, 

poorly sorted, strongly weathered, and iron-stained sand and gravel 

is part of the Chitka Point Formation in which conglomerates and 

lahars are common. 

Similar material makes up at least part of the large landslide lying 

2 km north of this exposure. Where it is well exposed on the shoreline, 

however, the interstitial particles are tuffaceous, unlike those seen at 

152m. 

THE 125- TO 131-M SURFACE 

Remnants of this warped and dissected surface are found at only 

four places on the eastern two-thirds of Amchitka (fig. 2). No remnants at the 125- to 131-m altitude were noted on the western third of 

the island, although a few notches are seen at about 152 m that might 

be upwarped remnants of this surface. At the northwestern end of the 

surface, south of Chitka Point, only two indistinct scarps are found 

that are suggestive of ancient sea cliffs. Bases of these scarps lie 

between 125 and 131 m above sea level, and the surface is identified 

relative to these bases. 

The surface generally slopes to the southeast, probably owing to 

post-terrace deformation; it descends from 131m near Chitka Point to 

84 m near mid-island. Only three small remnants are found on the 

eastern end of the island, where they have been notched by the sea 

cliff bounding the next lower surface. Two of these eastern remnants 

reach altitudes of 7 4 and nearly 107 m above sea level on the island 

block south of Constantine graben, and the third one lies 77 m above 

sea level near East Cape (fig. 6). Whether this surface was the product 

of Pleistocene wave erosion is uncertain because marine deposits 

were not found on it. 

THE 67- TO 76-M TERRACE 

This wave-cut terrace is found on the eastern two-thirds of Amchitka from Chitka Cove eastward (fig. 2). In the central part of the 

island, it is backed by a 5-km-long sea cliff whose base lies 70-76 m 

above sea level. Curiously, I have been unable to identify this terrace 

anywhere on the mountainous western third of the island; it has 

probably been obliterated by coastal erosion. 
20 PLEISTOCENE GEOLOGY OF AMCHITKA ISLAND, ALASKA 

The 67- to 76-m terrace is best developed near the eastern end of 

the island on the island block that lies south of Constantine graben 

(fig. 6). Here it forms most of the surface of that segment and displays 

well-developed sea cliffs which are cut on the remnants of the 125- to 

131-m surface. One of these sea cliffs was described by Powers, Coats, 

and Nelson (1960, p. 527). A 7-m-high conical knob, lying about 1.5 

km west of this cliff near the airport terminal building, is interpreted 

as an ancient sea stack whose base lies 67 m above sea level. Near 

East Cape, another small rise is surrounded by a less well-developed 

cliff whose base lies about 70 m above sea level. On this terrace, at an 

altitude of 65 m, lies the coarse, bouldery beach deposit of the South 

Bight I marine transgression. 

THE 37- TO 49-M TERRACE 

This prominent terrace is best developed on the low central part of 

the island, where it displays a 13-km-long sea cliff facing the Pacific 

coast. It is dotted with lakes and ponds, many of which are entirely 

confined by turf. Unlike the higher terraces, the 37- to 49-m terrace is 

present as narrow benches on both the north and south sides of the 

mountainous western third of the island and as a 700-m-wide bench 

around the northwestern tip. This terrace was cut during the South 

Bight II marine transgression. It was erroneously reported as lying at 

52 m above sea level in an earlier paper (Gard and others, 1972). 

Although present on both sides of Chitka Cove at an altitude of 40 

m, the terrace is absent in the cove itself. Across the island, on the 

Pacific Coast, it lies at only 27 m above sea level. This disjunction 

suggests that faulting or tilting may have occurred in this area. The 

part that lies at 27m occurs in conjunction with a segment of the 15-

m terrace (too small to show in fig. 2) or else it might have been 

misidentified. 

OTHER MARINE TERRACES 

SLTBAERIAL TERRACES 

Two brief high-level stands of the sea cut terraces at 15-18 m and 

6-9 m above sea level on Amchitka. The terraces are discontinuous 

but at most places have well-developed sea cliffs; they are best 

manifested at Rifle Range and Omega Points (fig. 6). At a few places, 

especially near the western end of the island, the 15- to 18-m terrace 

seems to be as high as 25m. No known deposits or fossil records are 

associated with either terrace; their undissected surfaces and sea 

cliffs suggest that they may be quite young, and they may represent 

brief interstadial high-sea stands during the Wisconsin Glaciation. 
TERRACES ON AMCHITKA 21 

Carbon-14 dates of9,810±160 yr B.P. on a soil lying at 5 m above sea 

level near Silver Salmon Lake (R. F. Black, 1974, Sample No. I3902) 

and 8,500±250 yr B.P. (Meyer Rubin, written commun., January 26, 

1972, Sample No. W2660) for a peat sample collected as part of this 

study from 2.6 m above sea level at Rifle Range Point provide 

minimum ages for the lower terrace. 

It has been suggested (Powers, 1961) that a small marine terrace is 

present at 3-5 m above sea level on islands in the Aleutians and 

elsewhere around the Pacific Ocean. Although scattered remnants of 

fiat surfaces at about this altitude are found at the mouths of some 

streams on Amchitka, it is debatable whether these were surfaces 

formed by a high-level stand of the sea or whether they are merely 

related to storm berms. If the terrace to which Powers referred 

elsewhere in the Pacific is the 6- to 9-m terrace of this paper, then 

the island has undergone minor tectonic uplift since that terrace 

was cut. 

1:'-JTERTIDAL BE:'-JCH 

The intertidal bench actively being developed around Amchitka 

(fig. 9) and other Aleutian Islands was inferred by Powers, Coats, and 

Nelson (1960, p. 527) to be the result of frost action on the sea cliffs 

and abrasion of loose stones on the ramp that lies between the actively retreating cliff and the bench, rather than by direct action of 

waves on the cliff face. The frost-riven rubble is then removed by 

FIGURE 9.-Intertidal bench, cut on breccia of Banjo Point Formation, exposed at low 

tide. Firmly cemented erosion-resistant boulders project above bench surface. Berm 

at outer edge of bench probably preserved from freeze-thaw mechanical weathering 

by constant wave splash. Note water-filled eroded joints. 
22 PLEISTOCENE GEOLOGY OF AMCHITKA ISLAND, ALASKA 

storm waves. Direct evidence of cliff retreat by frost action is displayed on the Pacific coast of Amchitka. The actively retreating cliff 

shown in figure 10 is interrupted by a cone of bedrock that is being 

preserved from frost action by a stream cascading over its surface. 

The Holocene bench is better developed along the Pacific coast than 

it is along the Bering coast of the island. This same phenomenon 

holds true for the fossil sea cliffs and terraces, which are also better 

developed on the south side of Amchitka than they are on the north 

side. I believe this is true mainly because the more southerly exposure to the low winter sun allows more repetitive thawing and freezing to occur on the south-facing cliffs during the winter months when 

the sea cliffs are commonly ice coated. Pacific storms are probably 

stronger and generate larger, more powerful waves, which would aid 

by more rapid removal of frost-riven rubble. Fraser and Barnett 

(1959, p. 240) also noted the better development of the tide-level 

bench on the south side of Kanaga Island, but they attributed it 

mainly to southward tilting of that island, which they believed would 

favor water-level and subaerial weathering. 

SuBMARI NE TERRACES 

During glacial advances, sea level was considerably lower than at 

present, because large volumes of water were stored in massive continental glaciers. Around Amchitka Island, detailed bathymetric 

FIGURE 10.- Sea cliff retreat owing to freeze and thaw. Bedrock cone is protected from 

freezing by stream cascading over its surface. Bedrock is breccia of Banjo Point 

Formation. Note low-angle joint that extends from cliff at right across bedrock cone. 

Ocean at near high tide covers intertidal bench. 
TERRACES ON Al\IC:HITKA 23 

charts (compiled by Continental Shelf Data Systems Division of 

Doeringsfeld, Amuedo, and Ivey, Denver, Colo.) indicate the presence 

of several submerged marine terraces; the two that are best developed 

are shown in figure 2. The largest of these is very prominent and is 

found on both the Bering and Pacific sides of the island. It consists of a 

smooth platform displaying many shallow closed depressions lying 

between -91 m and -100m and is nearly 3 km wide on the Pacific 

side of the island. 

The width and continuity of the -91 m submarine terrace suggest 

that it is far too large to have been formed by only one proglacial 

marine regression. In fact, the present submarine distribution of the 

subaerially deposited lavas of the Chitka Point Formation suggests 

that sea level may have been relatively as much as 91 m lower during 

Miocene time and that the terrace may actually have been formed at 

that time and may subsequently have been enhanced by one or more 

proglacial regressions. A wave-cut origin in mid-Tertiary time for 

this surface was first suggested by Gates and Gibson (1956, p. 134). 

Areas of smooth, hummocky sea-bottom topography, with numerous closed depressions lying above the -91 m terrace, are seen in 

places on the bathymetric map; these areas suggest the presence of 

submerged glacial deposits on the slopes surrounding the island. Possibly the reason that terraces shallower than -46 mare poorly developed is that when sea level stood lower during the height of glaciation, the presence of glacial ice along the shoreline inhibited terrace 

development. 

A distinct change in the submarine topography occurs at about -45 

to -50 m around the island. Above this depth, the topography is very 

rugged and irregular; below, it is smooth. A scarp occurs at this break 

and on the Pacific side of the island; a narrow, discontinuous bench 

lies at the base of the scarp. This change in topography was noted by 

Powers, Coats, and Nelson (1960, p. 531) who observed that the 

rugged topography suggested a stream-dissected slope. 

Less well-developed scarps and benches are found at -25 and -77 

m. Other small notches doubtless are present; but lack of density of 

soundings, especially near shore, prevent adequate delineation of 

such features. 

No terraces appear to lie below the -91 m terrace; however, at the 

outer edge of that terrace, at -100 m, the contour interval on the 

bathymetric charts changes to 5 fathoms (9 m) and any small terrace 

might not easily be detected. 

CORRELATION OF TERRACES WITH MARINE DEPOSITS AT SOUTH BIGHT 

The thick, well-exposed marine deposits at South Bight owe their 

preservation to subsidence of the graben, and thus they now lie 
24 PLEISTOCENE GEOLOGY OF AMCHITKA ISLAND, ALASKA 

somewhat below the terraces to which they must be related. The only 

marine deposit found on a terrace is the bouldery beach gravel lying 

on the 67- to 76-m terrace 1.5 km northwest of South Bight, which 

apparently was too coarse to be removed by erosion. So, with this one 

exception, direct correlation between deposits and terraces cannot be 

made. I have correlated deposits with terraces indirectly using the 

erosional unconformity between the South Bight I and South Bight II 

deposits as a point of reference. 

Because of the soft nature of the underlying deposits, cutting of this 

surface must have been rapid and likely occurred soon after sea level 

first rose to that position. As the base of the South Bight II beach 

deposit is conformable to this surface, it seems probable that the high 

stand of the sea that cut this surface also promptly deposited the 

sediments on it and thus preserved it from destruction. 

That uneven subsidence of the South Bight graben was taking 

place during the South Bight II transgression is evidenced by the 

southward tilt of the erosion surface and corresponding southward 

thickening of the overlying beach sediments. The highest point on the 

erosion surface, which now lies 34 m above sea level at the north side 

of the graben, must therefore be related to a terrace that now lies at 

least that high above sea level. Three such terraces are preserved on 

the eastern end of the island: the 37- to 49-m terrace, the 67- to 76-m 

terrace, and small remnants of the 125- to 131-m surface (fig. 6). 

If the unconformity and overlying South Bight II beach deposit are 

related to the 125- to 131-m surface, they would have to have been 

downfaulted more than 90 m since deposition. If they are related to 

the 67- to 76-m terrace, minimum downfaulting would have been 31 

m. In either case, this would mean that not only were the lakebeds of 

early Pleistocene age and the South Bight I marine beds deposited at 

least that much higher, but also a tremendous amount of subsequent 

erosion must have occurred during the relatively short 127,000 years 

since deposition of the South Bight II beds. It seems most logical to 

correlate the unconformity and overlying South Bight II beach deposit with the 37- to 49-m terrace on the basis of altitude similarity and 

relative freshness and size of the terrace. In this case, only a 

minimum downward displacement of about 3 m is required for the 

unconformity at the fault on the north side of the graben. Maximum 

displacement of the unconformity along the south-bounding graben 

fault has been about 22 m. As mentioned earlier, the 37-to 49-m 

terrace is displaced downward 10m along the south-bounding fault at 

North Bight (fig. 6). 

If correlation of the erosion surface and South Bight II deposit with 

the 37- to 49-m terrace is correct, then the South Bight I deposit must 

be related to a higher terrace. It has already been pointed out that the 
DEPOSITS AT CONSTANTINE GRABEN 25 

sponge spicule-bearing sand at South Bight is not a beach deposit but 

probably was deposited in an offshore submarine depression. Presence of sponge spicules in the coarse boulder beach gravel, which lies 

on the 67- to 76-m terrace, suggests that this deposit is the beach 

equivalent of the spicule-bearing marine deposit in the graben (table 

3). Thus, I have correlated the 67- to 76-m terrace with the South 

Bight I transgression. 

DEPOSITS AT CONSTANTINE GRABEN 

Glacial deposits are uncommon on Amchitka and, like the interglacial deposits, are generally restricted to topographically low and protected areas. Hummocky sea-bottom topography and closed depressions suggest that glacial deposits lie in greater quantity offshore 

than on the island. The most extensive glacial deposits above sea 

level are found in Constantine graben. 

Till-like sheets of distinctly different age, separated by fossiliferous 

marine sand and gravel, crop out for about 400 m along the cliff at the 

northwest corner of Constantine Harbor. These were first described 

by Coats (1956, p. 92) as ~~three successive layers of till and interbedded fluvioglacial sediments." Later, Powers, Coats, and Nelson (1960, 

p. 544) also noted that ~~three thin sheets ofbouldery till are separated 

by irregular lenses of sorted cobbles." I have been able to distinguish 

only two tills here. 

TABLE 3.-Correlation of Pleistocene deposits, Amchitka Island 

[Leaders( ______ ) indicate no data] 

Age South Bight 

Late Pleistocene __ Thin till and striated 

boulders. 

South Bight II beach 

deposit; fossiliferous; 125,000 years. Unconformity _____ _ 

Middle Pleistocene South Bight I 

marine deposit 

(sponge spicules). Lakebeds _________ _ 

Fluvial gravel 

(iron-stained) 

outwash?. 

Early Pleistocene Lakebeds (logs, pollen, reversed polarity). 

Gravel pit 1.5 km northwest of South Bight 

Striated boulders 

and frost-stirred 

cobbles. 

Coarse boulder 

beach deposit 

(sponge spicules). 

Constantine Harbor and Constantine graben 

Till, striated boulders, and 

lake beds. 

Unconformity. 

Shallow lagoonal 

deposit (sponge 

spicules, fossil 

shells). 

Indurated till 

(diamictite). 
26 PLEISTOCENE GEOLOGY OF AMCHITKA ISLA:'\!D. ALASKA 

OLDER GLACIAL DEPOSITS 

The lowest Pleistocene unit exposed here consists of 2-10 m of 

well-indurated bouldery diamictite that lies on a smooth surface of 

obsidian breccia bedrock of the Amchitka Formation. As noted by 

Powers, Coats, and Nelson (1960, p. 544), the diamictite is primarily 

composed of material derived from the glassy underlying bedrock and 

is ''well cemented, probably by secondary minerals weathered from 

the finely ground andesitic glass that is so abundant in the matrix." 

The diamictite displays rude stratification, and contains discontinuous, chaotically distorted, thin sandstone beds and subangular to 

round cobbles and boulders set in a well-indurated muddy matrix. 

About 50 percent of the stones are weathered obsidian breccia derived 

locally, a few are fresh diorite, and the remainder are unweathered 

basalt and andesite, which are smoothly worn and commonly faceted 

and striated. One medium-grained, granitic-type rock is deeply 

weathered. Facies changes and the rude stratification apparently led 

Powers, Coats, and Nelson (1960) to conclude that this unit consisted 

of more than one till. I believe that this is all one unit and, because of 

the rude stratification it displays, I refer to it as a diamictite. That the 

deposit was closely associated with ice is shown by its heterogeneous 

texture and content of striated and faceted rocks; perhaps it was let 

down from thin, melting sediment-rich ice. This is the lower of my 

two tills. At one place, it is overlain by about 1 m of extremely well 

indurated varved clay, which is, in turn, overlain by 0.3 m of unfossiliferous sand, well cemented by iron oxide. The diamictite sheet has 

been downdropped about 10-15 m to the south by an east-trending 

fault. 

SOUTH BIGHT I(?) MARINE DEPOSIT 

The diamictite at Constantine Harbor is unconformably overlain 

by as much as 12 m of fossiliferous, shallow-water marine silt, sand, 

and gravel on the south side of the fault. The marine deposit is absent 

on the north side of the fault; but, because the fault is not exposed, it 

could not be determined whether the marine sediments are displaced 

by or deposited against the fault. A section measured in this deposit is 

as follows: 

0.9-3. 7 m Cross-stratified, fine-grained sand. Laminae dip 

0.3-0.9 m 

1.8 m 

0.9 m 

northeast. Contains sponge spicules. Uppermost 

bed. 

Pebble to cobble gravel, well-rounded; many pebbles and cobbles have weathering rinds. 

Sand and gravel, even-bedded. No fossils. 

Sandy silt. No fossils. 
0.6 m 

3.5-4.0 m 

8-12 m 

DEPOSITS AT CO:\STANTINE GRABE:\ 27 

Fine-grained sand, even-bedded. Contains pelecypod molds from which the carbonate is leached. 

Silt, yellowish-brown, well-compacted. No fossils. 

Total thickness 

All beds in this marine deposit are firmly compacted and 

moderately iron stained but not cemented. Poorly preserved 

pelecypods, from which the carbonate has been leached, collected 

from this deposit were identified by W. 0. Addicott (USGS, Cenozoic 

Sample No. M5005) as follows: 

Macoma cf. M. inquinata (Deshayes) 

Macoma cf. M. nasuta (Conrad) 

?Macoma 

?Spisula 

This poorly preserved assemblage is suggestive of a Pleistocene or 

Holocene age according to W. 0. Addicott (written commun., 1972), 

who further stated that ~~Macoma nasuta has never been recorded 

from Hopkins' (1967) Pleistocene transgressions and is restricted 

today to areas east and southeast of the Aleutian Chain. The other 

doubtfully identified species, M. inquinata, is a circum-North Pacific 

bivalve and ranges northward into the Bering Sea." Neither these 

pelecypods nor sponge spicules were reported in the South Bight II 

deposit (Allison, 1973). 

Addicott stated further that "this association of Macoma spp. is 

suggestive of very shallow water conditions-intertidal or upper 

reaches of the inner sublittoral zone * * * . Assemblages such as this 

in which Macomas are the dominant element are often indicative of 

shallow water bays or marine estuaries." The fact that these fossils 

are not recorded from other transgressions suggested to Addicott that 

they might be very young-possibly very late Pleistocene or 

Holocene. A Holocene age is not likely, however, because this deposit 

underlies a till which appears to be of Wisconsin age. Further, weathering rinds on cobbles, compaction of beds, and iron staining suggest 

considerable antiquity for this deposit. Amchitka lies 8°-14° south of 

the latitude of most of the type localities of other recorded 

transgressions (Hopkins, 1967), which suggests that water temperatures and ocean currents at Amchitka may have restricted these 

bivalves to this more southerly latitude. 

This marine deposit, presumably interglacial, cannot be correlated 

with certainty with either the South Bight I or South Bight II interglacial deposits. It is certainly not Holocene; and its apparent antiquity, discussed above, suggests that it is older than the South 

Bight II transgression. 

It is tempting to correlate these beds with the South Bight I 

transgression, owing to the presence of sponge spicules. Although this 
28 PLEISTOCENE GEOLOGY OF AMCHITKA ISLAND, ALASKA 

is a tenuous correlation because of the lack of time significance of 

sponges, nevertheless, South Bight I deposits are the only deposits on 

Amchitka in which I have found sponge spicules; they were looked for 

but not found in the South Bight II deposits. Until new evidence is 

found, therefore, I am tentatively correlating these beds with the 

South Bight I transgression (table 3). 

The altitude of the base of this shallow water deposit is lower than 

one would expect for such an old deposit; it is only 9 m above sea level. 

If it is of South Bight I age, it has either been downfaulted more than 

50 m from the 67- to 76-m terrace level, or (more reasonably) it was 

deposited during a brief pause in the transgressive or regressive 

phases of South Bight I transgression. 

YOUNGER GLACIAL DEPOSITS 

A thin till overlies the marine deposit and older faulted diamictite 

at the northwest corner of Constantine Harbor, but it has not been 

offset by the fault there. This till, traceable intermittently throughout much of Constantine graben, generally is less than 2 m thick. It 

consists of a heterogeneous mixture of brown clayey sand containing 

less than 10 percent rounded to subrounded, unweathered, polished 

pebbles and cobbles randomly distributed throughout. It is compact 

but not indurated. On fresh exposures, bedding is not apparent, but 

on weathered surfaces a faint suggestion of stratification can be seen 

(fig. 11). 

A sandy and silty, rhythmically bedded, unconsolidated lake deposit (fig. 11), containing random pebbles and cobbles, underlies the till 

throughout much of Constantine graben. This deposit was described 

in detail by Powers, Coats, and Nelson (1960, p. 544-546). Individual 

beds average about 30 mm in thickness. The deposit is as much as 6 m 

thick, but the base is not exposed. The generally fine-grained aspect 

and even bedding suggest that it was deposited in a pond or lake at 

the margin of a retreating ice sheet. About 350 m inland from Constantine Harbor, where varvelike layers of sand and silt are well 

exposed, the clayey silt layers throughout several meters of section 

contain vertical tubes filled with sand from the overlying layers. 

These tubes are about 5 mm wide by 25 mm long, are round in cross 

section, and terminate in rounded bottoms. No bifurcations of the 

tubes were observed. They are lined with about 1 mm of iron-oxidecemented silt, which helps to preserve them. Their origin is uncertain, as no organic material was found in any of them. 

Random subangular pebbles and cobbles, distributed throughout 

the section, disrupt some of the thin beds; their presence suggests 

that these rocks had been dropped from floating ice. Powers, Coats, 

and Nelson (1960, p. 546) concluded, correctly, I believe, that the 
DEPOSITS AT CONSTANTINE GRABEN 29 

FIGURE H .-Rhythmically bedded glacial pond deposit of alternating sand and silt 

overlain by 2-m-thick till of unsorted, stony, clayey sand. Weathered surface oftill at 

right shows faint trace oflayering, which suggests that it may not have been directly 

deposited from melting ice. Dark band at top of face is peat containing thin ash 

layers. Shovel at lower right is about 1.3 m long. 

deposit probably ~~was accumulated in a local pond, largely of glacial 

melt water, and derived almost wholly from ice-transported material." These lakebeds, which commonly dip south or southeast toward 

the Constantine fault, are offset in places by small faults. 

Extensive excavation for construction material has exposed glacial 

striations on bedrock surfaces in Constantine graben (Powers and 

others, 1960, p. 545). Although not conclusive evidence, the location 

and attitude of the striations suggest that ice moved from east to west 

here. No till was observed here, but striated boulders are scattered on 

the higher ground surface to both the north and south. 

Till about 3 m thick underlies the lake deposit. This till, exposed in 

only a few places low on the north side of the graben 450 m inland 

from the harbor, is a brownish-gray stiff clay containing scattered 

rounded and polished stones ranging from 2 to 25 em in diameter. 

Across the graben to the south, sandy outwash and gray to brown 

clayey till, containing polished pebbles, are in fault contact with bedrock along the Constantine Fault. 

In most places, the glacier that deposited this drift left only randomly scattered erratic boulders and cobbles, whereas till(?), less 

than 0.5 m thick, and scattered faceted and striated boulders overlie 

the South Bight II beach sediments at South Bight. In and near Constantine graben, a concentration of erratics is lying on the ground 

surface. Many of them are distinctive quartz diorite from a pluton, 
30 PLEISTOCENE GEOLOGY OF AMCHITKA ISLAND, ALASKA 

which crops out near East Cape, and associated contactmetamorphosed breccia of the Amchitka Formation. These rock types 

could only have come from near South Bight where these rocks crop 

out. 

The presence of these erratics at Constantine graben (one faceted 

and striated boulder of metabreccia measured about 3 x 2 x 1 m) indicates that the transporting glacial ice must have moved a distance of 

at least 8 km from east to west. Although this ice sheet apparently 

was extensive on the eastern end of the island, there is no indication 

that it extended much farther west than the Constantine graben 

area. It must have been thin and incapable of much erosion as it 

overrode, but failed to completely remove, the boulder beach gravel of 

the South Bight I transgression at the gravel pit near South Bight. In 

fact, this beach deposit probably was the source of many of the 

erratics found at Constantine graben. Such evidence indicates that 

the snow line, at least on Amchitka, must have been substantially 

lower than the 183 m estimated for the Near Islands (Schafer, in 

Gates and others, 1971) or the 122m estimated for Unalaska (Drewes 

and others, 1961, p. 647). I know of no evidence to support the suggestion (Coats, 1956, p. 96; D. M. Hopkins, written commun., 1968, cited 

in Pewe, 1975, p. 21) that Illinoian(?) ice or ice of any other age came 

from present ocean areas off Amchitka Island and overrode the east 

end of the island. Indeed, the ocean depths alone preclude such an 

occurrence. 

This younger glaciation has not been directly dated, yet it must 

have occurred between 127,000 and 10,000 years ago, as it overlies 

dated deposits of the South Bight II marine transgression and is overlain by carbon-14 dated soils. It must be of Wisconsin age. Soils older 

than about 10,000 years have not been found on Amchitka. The oldest 

carbon-14 date determined was reported by Everett (1971) as 

10,550± 160 yr B.P. from sapric organic material directly overlying 

bedrock; however, the exact location was not reported. A date of 

9,800± 160 yr B.P. (Sample No. 13902) was reported by Black (197 4) 

for a soil lying on weathered bedrock about 5 m above sea level near 

Silver Salmon Lake, and a date of 9,200±130 years (Sample No. 

13905) was obtained from a peat sample, collected by Black, that 

directly overlies the marine deposit at the northwest corner of 

Constantine Harbor. At this place, the younger till had been removed 

by erosion prior to accumulation of this peat. 

A carbon-14 date of 8,090±300 years (Meyer Rubin, written commun., 1974, Sample No. W2849) was determined for a sample from a 

15-cm-thick, strongly undulating soil that lies on about 0.6-1.2 m of 

eroded crossbedded dune(?) sand. The dune sand overlies the younger 

till 14 m above sea level at a point 550 m inland from Constantine 
GLACIATIO:'\ OF CENTRAL A:'\D WESTERN AMCHITKA 31 

Harbor. The soil horizon was, in turn, overlain by 2-3m of apparent 

dune sand, which contains several thin soil horizons. The dune sand 

that overlies the sampled soil conforms to that undulating surface, 

but the layers gradually flatten upward until they become horizontal. 

The very unevenness of the sampled soil and the underlying sand 

suggest that the undulations might have been caused by downslope 

slumping of the dune sand. 

GLACIAL OUTWASH AT RIFLE RANGE FAULT 

On the north side of the Rifle Range Fault, a deposit of sand and 

gravel (not shown in fig. 6) covers an area of as much as 0.5 km2

• This 

deposit was mainly destroyed by excavation during World War II, but 

fresh exposures in the remaining portion were created by subsequent 

excavation during the AEC occupation. The deposit is as much as 6 m 

thick and consists of firmly compacted, even-bedded sand with interstratified gravel beds. No fossils were found. Cut-and-fill structures are common and, in one exposure, sand and cobble gravel display steep initial dips similar to those seen in kame deposits. This 

deposit is presumed to be glacial outwash; its age is not known but it 

is probably related to the last glaciation. 

The deposit is broken by several normal faults seen in fresh exposures. These faults roughly parallel the Rifle Range Fault and display 

the same sense of movement, down to the north. A few faceted and 

striated boulders were found scattered on top of this deposit. As no 

erratics from the distinctive outcrops near South Bight were found, I 

infer that the eastern ice sheet never reached this far west. 

GLACIATION OF CENTRAL AND WESTERN AMCHITKA 

The Quaternary geology of the central and western parts of Amchitka Island has been studied in less detail, but it is reasonable to 

assume that ice also covered those parts of the island, as they are 

topographically higher than the eastern end. Except for scattered 

striated cobbles of local rock, depositional evidence of glaciation 

northwest of the Rifle Range Fault is sparse; erosional evidence in the 

form of rock -defended lakes lying west of the middle of the island 

suggests that ice also covered that part of the island. In the 

mountainous western third of the island, glacially eroded U -shaped 

valleys, heading in cirques, indicate that glaciers filled the valleys, 

but glacial deposits are uncommon on land. Near the western end of 

the island, scattered erratic boulders lie on the weathered and altered 

dissected plateau surface. These boulders of fresh andesite, as large 

as 1.5 min diameter, appear to have come from an outcrop of Miocene 
32 PLEISTOCENE GEOLOGY OF AMCHITKA ISLAND, ALASKA 

lava about 5 km to the east. Scattered well-rounded cobbles on Square 

Bluff, at the northwestern tip of the island (Powers and others, 1960, 

p. 525), are probably either remnants of conglomerate of the Chitka 

Point Formation of Miocene age or, more likely, lag concentrate of 

outwash from ice that covered the plateau. 

EXTENT OF GLACIATION 

The sea-bottom topography suggests that glaciers extended 

perhaps as much as 8 km beyond the present shoreline on the south 

side of the island. More than 200 closed depressions, mostly much less 

than 0.5 km across, lie on the south side of the island between -9 m 

and -91 m, as shown on bathymetric charts where the contour interval is 1 fathom (1.83 m). As many as 20 large depressions, averaging 

about 2 km in maximum dimension, are seen on the -91 m terrace 

where the contour interval is 5 fathoms (9.15 m). These depressions 

have 4-8 m of closure. Perhaps they represent some pre-Wisconsin 

glaciation. 

Closed submarine depressions, almost nonexistent on the north 

side of the island, suggest that ice extended much farther to the south 

than it did to the north. This asymmetry is consistent with the 

asymmetry of glaciation on the Alaska Peninsula (Coulter and 

others, 1965). 

Most of the closed depressions above -91 m probably represent ice 

plucking of bedrock, but in several coves and bays the depressions are 

very suggestive of closure by moraines, in that the topography there 

is very smooth. Constantine Harbor and St. Makarius Bay each have 

two sets of closed depressions (fig. 6). At Constantine Harbor, the 

inner depression has 2. 7 m of closure, with a lip at -33 m; the outer 

depression also has 2.7 m of closure, with a lip at -39 m. At St. 

Makarius Bay, the inner depression has a closure of 10m, with the lip 

at -32 m; the outer depression has 8.25 m of closure, with the lip at 

- 34 m. I infer these closed depressions lie behind terminal moraines, 

probably of the east-end icecap. Similar occurrences of closed depressions were also noted at Kiska Harbor and at Tanaga Bay at those 

respective islands. 

PLEISTOCENE TECTONISM 

Local movement on faults at South Bight and Constantine grabens 

and at the Rifle Range Fault is recorded in the Pleistocene deposits. 

Till and outwash of the Wisconsin age are in fault contact with bedrock along the Constantine Fault, and a deposit of outwash(?) sand 

and gravel near the Rifle Range Fault is broken by minor faulting. 
PLEISTOCENE TECTO;-..;ISM 33 

At South Bight, the north-bounding fault, which had been moving 

since at least early Pleistocene time, apparently ceased to move before the end of South Bight II time, as the upper part of these deposits 

are unaffected by that fault. Movement continued for some time on 

the south-bounding fault. The 37- to 49-m terrace at North Bight has 

been downdropped about 10 m by this south-bounding fault, but 

movement ceased before cutting of the 15- to 18-m terrace, which is 

not offset at North Bight where it crosses the south-bounding fault 

(fig. 6). 

Southward tilting of the island segment lying between Constantine 

and South Bight grabens is demonstrated by variation in sea-cliff 

altitudes (fig. 6). The 67- to 76-m (South Bight I) terrace is tilted more 

than 6° southward, whereas the 37- to 49-m (South Bight II) terrace is 

tilted less than 1 o to the southeast. Tilting started after South Bight I 

time and continued until shortly after South Bight II time, as demonstrated both by these terraces and by the structure in the South Bight 

graben. 

Lack of widely divergent altitudes of most abandoned sea cliffs 

suggests that differential tectonic movement on the island during the 

Pleistocene has been minor. The fact that the terraces appear to be 

older in ascending altitude suggests that the entire island may have 

been rising throughout the Pleistocene; yet the proximity to sea level 

of the lower Pleistocene lakebeds, although obviously downfaulted, 

tends to contradict, this conclusion. Several islands in the Delarof 

Islands, east of Amchitka, have conspicuously tilted prominent terraces (Morris, 1971) not seen on Amchitka. Correlation of terraces 

between different islands in the Aleutians cannot be made on the 

basis of common altitudes, as differential movement of the islands 

has obviously occurred. 

The -91 m submarine terrace, so prominent around Amchitka, 

also extends around Rat and Kiska Islands to the west, yet it is either 

poorly developed or absent around the Delarof Islands east of Amchitka Pass. This suggests that the Rat Island block has been more 

stable tectonically than the Delarof Island block during the Pleistocene and possibly longer. 

Powers, Coats, and Nelson (1960, p. 531) recognized variations in 

depth of the submerged terraces around Amchitka, but lack of density 

of soundings hampers attempts to refine these observations. They 

noted that most of the segments of divergent depths lie around the 

eastern half of the island; this fact suggests that fault activity may 

have been more recent there. 

Sea level is believed to have been relatively stable for the past 

2,000-4,000 years. The presence of a broad intertidal bench found at 

many places around Amchitka and other Aleutian Islands indicates 
34 PLEISTOCENE GEOLOGY OF AMCHITKA ISLAND, ALASKA 

that, despite the high seismicity of the Aleutian Arc, most of the 

islands have remained isostatically stable during this time. Certain 

abandoned sea cliffs on some of the Delarof Islands, however, suggest 

that several meters of uplift may have occurred there during the past 

several thousand years (Morris and Bucknam, 1972). 

CORRELATION OF MARINE TRANSGRESSIONS 

Any evaluation of the Pleistocene geology on Amchitka Island 

would be incomplete without an attempt to correlate events there 

with well-documented Pleistocene events elsewhere in the world. 

Certainly the termination concept of Broecker and VanDonk (1970) 

provides a reasonable sequence of high sea levels of middle and late 

Pleistocene age with which to correlate. 

The flight of marine terraces on Amchitka indicates that at least 

four and possibly five high sea stands occurred there during the Pleistocene. Although only one of these terraces is dated, their order of 

succession and relative sizes are such that we can at least speculate 

on the correlation of the other terraces with high sea stands 

elsewhere (table 4). 

The uranium-series date 2 of 127,000 years (Szabo and Gard, 1975) 

for the South Bight II transgression leads me·to correlate that event 

with Broecker and VanDonk's (1970) Termination II and Mesolella 

and others' (1969) Barbados III, which presumably are related to the 

Sangamon Interglaciation of the central United States. Even disregarding the uranium-series date, the extent and size of the South 

Bight II terrace (37-49 m), the lowest major terrace on the island, 

indicate that it represents a lengthy high sea stand that apparently 

was never followed by a marine transgression of nearly the same 

magnitude. 

I tentatively correlate the 6- to 9-m and the 15- to 18-m terraces on 

Amchitka with the Barbados I and II reef tracts (Mesolella and 

others, 1969) on the basis of their relative position, size, and lack of 

dissection. These lowest Amchitka terraces, both younger than the 

127,000-year-old South Bight II transgression, represent only brief 

marine transgressions on Amchitka. The Barbados I and II reef tracts 

also are less extensive than the 125,000-year-old Barbados III tract 

(Mesolella and others, 1969, p. 272) and presumably represent less 

time of development. 

The South Bight I transgression (67- to 76-m terrace) on Amchitka 

is difficult to correlate with a well-established Pleistocene event, as it 

2

Despite skepticism voiced by certain workers (Kaufinan and others, 1971) of the validity of uranium-series dating of marine mollusks in noncarbonate sediments, this date is in general agreement with the present concept of the age of the last major interglaciation throughout widely separated areas (Broecker and VanDonk, 1970, table 6). 
CORRELATION OF :\IARINE TRANSGRESSIONS 35 

TABLE 4.--Correlation of Quaternary marine transgressions on Amchitka Island 

This paper 

6- to 9-m terrace 

15- to 18-m terrace 

South Bight II 

37- to 49-m terrace 

127,000±8,000 yrs. 

South Bight I 

67- to 76-m terrace 

125- to 131-m terrace 

[Leaders ( _ _ ___ I indicate no data J 

Broecker and VanDonk, 1970 (fig. 31 

Termination I 

11,000±600 yrs. 

Termination II 

127,000±6,000 yrs. 

Termination III 

225,000± 15,000 yrs. 

Termination IV 

Mesolella and others, 1969 

ltable 11 

Barbados I 

82,000 yrs. 

Barbados II 

105,000 yrs. 

Barbados III 

125,000 yrs. 

Hopkins. 1973 !fig. 101 

Pelukian II. 

Pelukian I. 

Kotzebuan. 1 

Einahnuhtan. 

'Recent dating by B. J. Szabo 1 written commun .. 19801 of mollusks collected from Kotzebuan deposits by D. M. 

Hopkins suggests that the Kotzebuan transgression may be considerably younger than indicated previously by 

Hopkins 119731. 

lacks diagnostic fossils and is undated. Yet, because of its size, it 

obviously represents a major marine transgression preceding the 

South Bight II transgression. It must have been followed by two 

glaciations: the older occurred during the hiatus represented by the 

unconformity at South Bight, which was caused by lowering of sea 

level and erosion (but for which no deposits are recognized), and the 

younger overlies the South Bight II deposits. Assuming that the terrace sequence represents a complete record of transgressions on Amchitka, I tentatively correlate the South Bight I transgression with 

Termination III of Broecker and VanDonk (1970, p. 194), which occurred about 225,000 years ago. If my tentative correlation of the 

marine deposits at Constantine Harbor with the South Bight I 

transgression is valid, then the underlying diamictite must be preIllinoian-possibly Kansan-in age. 

Correlation of the Amchitka marine transgressions with those of 

mainland Alaska is made separately here because there is considerable uncertainty in this regard. 

Allison (1973) stated that the fossil mollusks in the South Bight II 

deposit ttoffer evidence in favor of a Kotzebuan or possibly even a 

Pelukian age" for the South Bight II transgression. Because the presence of till overlying the deposit was believed by D. M. Hopkins to 

preclude a Pelukian (Sangamon) age for the deposit (Allison, 1973, p. 

45), Allison concluded that the South Bight II fauna ttprobably" were 

Kotzebuan (pre-Illinoian). I feel this is not a valid conclusion. Presumably, Hopkins assumed that the overlying till would have to be 

Illinoian because, on mainland Alaska, Illinoian ice was far more 
36 PLEISTOCENE GEOLOGY OF AMCHITKA ISLAND, ALASKA 

extensive than was Wisconsin ice. However, there is good evidence for 

extensive Wisconsin ice sheets on islands in the Aleutians. 

On mainland Alaska, there seems little doubt that glaciation of 

pre-Wisconsin (Illinoian?) age was far more extensive than that of 

Wisconsin age (see, for example, Coulter and others, 1965). It appears, however, that Wisconsin glaciers were extensive enough in the 

Aleutians to have covered most of what is now land surface and to 

have removed most evidence of the pre-Wisconsin glaciation. On 

Umnak Island, in the eastern Aleutians, earlier studies by Byers 

(1959, p. 349) suggested that extensive glaciation of the Nikolski 

plain ~~was late Pleistocene, possibly Wisconsinan in age." Extensive 

Wisconsin Glaciation there is further substantiated by Black (1974, 

p. 126), who stated that on Umnak ~~no glacial deposits have been 

recognized earlier than late Wisconsinan, when an extensive ice 

sheet covered all but the highest peaks up to 10,000 or 11,000 years 

ago." Soils and peat older than 10,000 years have not been found on 

Umnak. In the western Aleutians, Schafer (in Gates and others, 

1971) recognized that extensive ice sheets covered the Near Islands 

(Attu, Agattu, and the Semichi Islands); he suggested that they were 

~~no older than Wisconsinan." 

Lack of organic material yielding carbon-14 dates older than about 

10,000 years, not only on Amchitka Island but also on 13 other islands from Kiska to Tanaga in the central Aleutians (Gard, unpub. 

data, 197 4), strongly suggests that icecaps or permanent snowfields 

must have covered most of these islands during late Wisconsin time. 

Thus, evidence from other islands at the extremities of the Aleutian chain supports findings on Amchitka that an ice sheet of Wisconsin and probably late Wisconsin age covered Amchitka Island. 

In a recent paper, Hopkins (1973) has compared the later part of his 

Beringian marine transgressions with Broecker and VanDonk's 

climatic curves. He recognized two stages of the Pelukian transgression. He correlated Pelukian I with the 127 ,000-year-old Termination 

II of Broecker and VanDonk (1970), and he believed (1973, p. 536) 

Pelukian II ~~may correspond to either Barbados I or II and thus may 

be either 80,000 or 105,000 years old." If the 127 ,000-year-old South 

Bight II transgression is correctly correlated with Termination II, 

then it could also be correlative with the Pelukian I transgression. 

Correlation of the higher and older Amchitka transgressions with 

Beringian transgressions is less certain. The South Bight I transgression (67- to 76-m terrace) was obviously one of major proportions and 

immediately preceded the South Bight II transgression. Table 4 

shows that Hopkins (1973) correlated the Einahnuhtan transgression 

with Termination III of Broecker and VanDonk. I am strongly tempted to correlate South Bight I with Hopkins' Kotzebuan transgression 
REFERENCES CITED 37 

although he did not correlate Kotzebuan with any of the 

transgressions at Barbados. This correlation remains unresolved. 

It must be emphasized again that all the above correlations of Amchitka transgressions are based on the relative positions and sizes of 

the terraces with the one dated major transgression-the South 

Bight II transgression. 

Correlating terraces between islands on the basis of corresponding 

altitudes is unreliable because of differential Quaternary tectonic 

movement between islands and island blocks; sequences of terraces 

on each island will need to be studied on their own. Only then can 

reliable correlations between the effects of marine transgressions on 

different islands be attempted. 

It is hoped that this paper will inspire and aid future students of the 

Aleutian Pleistocene not only to expand this work on Amchitka, especially the early Pleistocene history, but also to study the terraces and 

search for marine deposits on other islands. Such work will greatly 

extend our know ledge of the Pleistocene in the Aleutians and will 

help to further determine the Pleistocene tectonic history of the Aleutian chain. 

REFERENCES CITED 

Allison, R. C., 1973, Marine paleoclimatology and paleoecology of a Pleistocene invertebrate fauna from Amchitka Island, Aleutian Islands, Alaska: Palaeogeography, 

Palaeoclimatology, Palaeoecology, v. 13, no. 1, p. 15-48. 

Black, R. F., 1974, Geology and ancient Aleuts, Amchitka, and Umnak Islands, Aleutians: Arctic Anthropology, v. 11, no. 2, p. 126-140. 

Broecker, W. S., and VanDonk, Jan, 1970, Insolation changes, ice volumes, and the ~ 

record in deep-sea cores: Geophysics and Space Physics Reviews, v. 8, no. 1, p. 

169-198. 

Byers, F. M., Jr .. 1959, Geology of Umnak and Bogoslof Islands, Aleutian Islands, 

Alaska: U.S. Geological Survey Bulletin 1028-L, p. L267-L365. 

Carr, W. J., Quinlivan, W. D., and Gard, L. M., Jr., 1970, Age and stratigraphic 

relations of Amchitka, Banjo Point, and Chitka Point Formations, Amchitka Island, Aleutian Islands, Alaska, in Changes in stratigraphic nomenclature by the 

U.S. Geological Survey, 1969, by G. V. Cohee, R. G. Bates, and W. B. Wright: U.S. 

Geological Survey Bulletin 1324-A, p. A16-A22. 

Coats, R. R., 1956, Reconnaissance geology of some western Aleutian Islands, Alaska: 

U.S. Geological Survey Bulletin 1028-E, p. E83-E100. 

Coulter, H. W., Hopkins, D. M., Karlstrom, T. N. V., Pewe, T. L., Wahrhaftig, Clyde, 

and Williams, J. R., compilers, 1965, Map showing extent of glaciations in Alaska: 

U.S. Geological Survey Miscellaneous Geologic Investigations Map I-415. 

Cushman, J. A., and Todd, M. R., 1947, A foraminiferal fauna from Amchitka Island, 

Alaska: Cushman Laboratory for Foraminiferal Research Contributions, v. 23, pt. 

3, p. 60-72. 

Drewes, Harald, Fraser, G. D., Snyder, G. L., and Barnett, H. F., Jr., 1961, Geology of 

Unalaska Island and adjacent insular shelf, Aleutian Islands, Alaska: U.S. 

Geological Survey Bulletin 1028-S, p. S583-S676. 
38 PLEISTOCENE GEOLOGY OF AMCHITKA ISLAND. ALASKA 

Everett, K. R., 1971, Composition and genesis of the organic soils of Amchitka Island, 

Aleutian Islands, Alaska: Arctic and Alpine Research, v. 3, no. 1, p. 1-16. 

Fraser, G. D., and Barnett, H. F., Jr., 1959, Geology of the Delarof and westernmost 

Andreanof Islands, Aleutian Islands, Alaska: U.S. Geological Survey Bulletin 

1028-I, p. I211-I248. 

Gard, L. M., Jr., Lewis, G. E., and Whitmore, F. C., Jr., 1972, Steller's Sea Cow in 

Pleistocene interglacial beach deposits on Amchitka, Aleutian Islands: Geological 

Society of America Bulletin, v. 83, no. 3, p. 867-869. 

Gates, Olcott, and Gibson, W. M., 1956, Interpretation of the configuration of the 

Aleutian ridge [Alaska]: Geological Society of American Bulletin, v. 67, no. 2, p. 

127-146. 

Gates, Olcott, Powers, H. A., Wilcox, R E., and others, 1971, Geology of the Near 

Islands, Alaska, with a section on Surficial geology, by J.P. Schafer: U.S. Geological Survey Bulletin 1028-U, p. U709-U822. 

Hopkins, D. M., 1967, Quaternary marine transgressions in Alaska, in Hopkins, D. M., 

ed., The Bering Land Bridge: Stanford University Press, p. 47-90. 

--- 1973, Sea level history in Beringia during the past 250,000 years: Quaternary 

Research, v. 3, no. 4, p. 520-540. 

Kaufman, Aaron, Brocker, W. S., Ku, T. L., Thurber, D. L., and others, 1971, The 

status ofU-series methods of mollusk dating: Geochemica et Cosmochimica Acta, 

v. 35, no. 11, p. 1155-1183. 

Mesolella, K. J., Matthews, R K., Brocker, W. S., and Thurber, D. L., 1969, The 

astronomical theory of climatic change-Barbados data: Journal ofGeology, v. 77, 

no. 3, p. 250-274. 

Morris, R H., 1971, Marine terraces of the western Aleutian Islands, Alaska: U.S. 

Geological Survey Report USGS-474-139, 23 p.; available from U.S. Department 

of Commerce National Technical Information Service, Springfield, VA 22161. 

Morris, R H., and Bucknam, R. C., 1972, Geomorphic evidence of late Holocene vertical stability in the Aleutian Islands, Alaska: Seismological Society of America 

Bulletin, v. 62, no. 6, p. 1365-1375. 

Pewe, T. L., 1975, Quaternary geology of Alaska: U.S. Geological Survey Professional 

Paper 835, 145 p. 

Powers, H. A., 1961, The emerged shoreline at 2-3 meters in the Aleutian Islands, in 

Russell, R J., ed., Symposium, Baton Rouge, Pacific island terraces-eustatic?: 

Zeitschrift fiir Geomorphologie Supplementband 3, p. 36-38. 

Powers, H. A., Coats, R. R, and Nelson, W. H., 1960, Geology and submarine physiography of Amchitka Island, Alaska: U.S. Geological Survey Bulletin 1028-P, p. 

P521-P554. 

Scheffer, V. B., 1972, The weight of the Steller sea cow: Mammalogy Journal, v. 53, no. 

4, p. 912-913. 

Spiker, Elliott, Kelley, Lea, Oman, Charles, and Rubin, Meyer, 1977, U.S. Geological 

Survey radiocarbon age dates XII: Radiocarbon, v. 19, no. 2, p. 332-353. 

Stejneger, Leonhard, 1883, Contributions to the history of the Commander Islands, No. 

1-Notes on the natural history, including descriptions of new cetaceans: U.S. 

National Museum Proceedings, v. 6, p. 58-89. 

Szabo, B. J., and Gard, L. M., Jr., 1975, Age of the South Bight II marine transgression 

at Amchitka Island, Aleutians: Geology, v. 3, no. 8, p. 457-459. 

Whitmore, F. C., Jr., and Gard, L. M., Jr., 1977, Steller's sea cow (Hydrodamalis gigas) 

of late Pleistocene age from Amchitka, Aleutian Islands, Alaska: U.S. Geological 

Survey Professional Paper 1036, 19 p.