Geology and 

Submarine Physiography 

of Amchitka Island 

Alaska

By HOWARD A. POWERS, ROBERT R. COATS, and WILLIS H. NELSON

INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGICAL SURVEY BULLETIN 1028-P

Prepared in cooperation

with the Departments of

the Army, Navy, and Air Force

 \

*

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1960
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D.C.
PREFACE

In October 1945 the War Department (now Department of the 

Army) requested the Geological Survey to undertake a program of 

volcano investigations in the Aleutian Islands-Alaska Peninsula 

area. The first field studies, under the general direction of G. D. 

Robinson, were made during the years 1946-48. The results of the/ 

first year's field, laboratory, and library work were assembled as 

two administrative reports, and most of these data have been revised 

for publication in Geological Survey Bulletin 1028. Part of the 

early work was published in 1950 in Bulletin- 974-B, "Volcanic Activity in the Aleutian arc," and in 1951 in Bulletin 989-A, "Geology 

of Buldir Island, Aleutian Islands, Alaska," both by Robert R. Coats. 

During the years 1949-54 additional fieldwork was carried out under 

the direction of H. A. Powers. Unpublished results of the early 

work and all of the later studies are being incorporated as parts of 

Bulletin 1028. The geological investigations covered by this report 

were reconnaissance. The factual information presented is believed 

to be accurate, but many of the tentative interpretations and conclusions will be modified as the investigations continue and knowledge grows.

The investigations of 1946 were supported almost entirely by the 

Military Intelligence Division of the Office, Chief of Engineers, 

U.S. Army. From 1947 to 1955 the Departments of the Army, Navy, 

and Air Force joined to furnish financial and logistic assistance. 

The Geological Survey is indebted to the Office, Chief of Engineers, 

for its early recognition of the value of geologic studies in the 

Aleutian region, and to the several military departments for their 

support.

CONTENTS

	Page 

Preface_ _ _-___--_--____---______-_______-__________----__--_---- in

Abstract__.__ ________._________--_______________ 521

Introduction. _____________________________________________________ 522

Geography__ ____________________________________________________ 522

Physiography ___ _ _________________________________________-^_-_. 524

Mountain segment__---_____--_-_______--___-___----_-_------ 524

High plateau_-______-__________________^__________-______-__ 525

Chitka Point ridge______________ _ ___ _ ___________ 525

Lower plateaus______________________________________________ 526

Ponds_____________________.__._._.___________ 526

Stream valleys.____-__--_-______-_-___________-----_----_- 526

Inland sea cliffs_______________________________________________ 527

Shoreline features___________________________________________ 527

Submerged topography_______________________________________ 531

Interpretation of topography__________________________________ 532

Stratigraphy____________________________________________-___----__ 533

Amchitka formation.__________________________________________ 533

Banjo Point formation_______________________________-___-----_ 536

Quartz diorite______________________________.-_-_ 539

Chitka Point formation._______________________________________ 539

Pyrite-rich rock_______________________________________________ 541

Gravel of hornblende andesite____________________--_-____---_- 541

Tilted sedimentary rock at South Bight-__________________------_ 541

Interglacial beach deposit at South Bight___________---__------_ 542

Glacial deposits.____________________________________________ 543

Mantle.______________________________________ 547

Structure__ _---_---_-___--______-__________________--_-___------ 547

Regional- _----_-__-__-__________________________-__-_ ______ 547

Local. ______-------.__________________________ 547

Geologic history..__________________________________________________ 549

References cited.__________________________________________________ 551

Index ____________________________________________________________ 553

ILLUSTRATIONS

PLATE 69. Topography of part of the Aleutian Ridge and geologic map of

Amchitka Island________________________________ In pocket

Page 

FIGURE 77. Map of the Alaska Peninsula and Aleutian Islands________ 523

78. Profile of Chitka Point and offshore area, with inferred origin

of features.____________________________________ 529

TABLE

Page 

TABLE 1, Chemical and spectrographic analyses and norms of volcanic

rocks from Amchitka Island____-_______-___-----_.----_- 534

INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGY AND SUBMARINE PHYSIOGRAPHY OF 

AMCHITKA ISLAND, ALASKA

H By HOWARD A. POWERS, ROBERT R. COATS, and Wn-us H. NELSON

ABSTRACT

Amchitka Island isi the exposed part of a segment of the Aleutian Ridge 

just west of the southernmost part of the Aleutian arc. It has been disturbed 

by faulting, differentially uplifted, and dissected by marine, stream, and glacial erosion. The oldest rocks, the Amchitka formation, are volcanic ash, tuff, 

breccia, and lava flows generally of andesitic composition, apparently erupted 

and emplaced under the ocean. These rocks were deformed, uplifted, and 

subjected to erosion before and during deposition of the interbedded conglomerate, basaltic breccia, and tuff of the Banjo Point formation. A meager 

Banjo Point fauna is Oligocene or possibly early Miocene in age. Rocks of 

both these formations were intruded by small bodies of quartz diorite.

Flows of feldspathic basalt and andesite, interbedded with marine conglomerate in the lower part and subaerial in the upper part, make up the 

next younger formation, the Chitka Point formation. In places, rocks of the 

lower part of the formation were deformed before eruption of the flows of 

the upper part. The subaerial lava flows were submerged at least 600 feet, 

and possibly 1,100 feet. Erosion reduced the mass to a surface of low relief, 

possibly a s ubmarine shoal, now represented by an erosion surface at an altitude of about 1,100 feet. Marine sand and cobble conglomerate, perhaps 

formed during this erosion, are now exposed at altitudes of up to 600 feet.

Uplift of at least 500 feet and possibly more than 1,100 feet took place differentially and spasmodically during late Tertiary and Quaternary time. A 

wave-cut platform (perhaps cut during a time of high interglacial sea level) 

forms a partly dissected plateau between 700 and 900 feet in altitude. A 

younger abandoned sea cliff, whose base is at an altitude of 240 feet, and a 

beach deposit, whose top is at 135 feet, are inferred to be structurally dislocated remnants of shoreline features of a late Pleistocene interglacial high 

sea level, as the fauna of the beach deposit is characteristic of warmer water 

than now surrounds the island. Movement along faults bounding the Constantine Harbor graben occurred during deposition of late glacial deposits.

Stream-cut ridge and valley topography, modified by glacial action, extends 

below sea level. The ridges terminate in a line of cliffs at the edge of a 

plane surface at a depth of about 165 feet. This surface slopes seaward and 

joins a wide, much flatter bench that extends to the edge of the continental 

shelf. Both surfaces are interpreted as wave-cut benches associated with shore521
522 INVESTIGATIONS OF ALASKAN VOLCANOES

lines of lower late-glacial sea levels. The deeper bench is block faulted in the 

eastern part, but the minus 165 foot bench is not disturbed by these faults. 

Sea cliffs cut by postglacial marine erosion have been abandoned by a relative lowering of sea level of 6 or 8 feet that took place before Aleut settlement more than 4,000 years ago.

INTRODUCTION

Amchitka is the southernmost island of the Rat Island group, which 

is located west of the center of the Aleutian Island arc. (See fig. 

77.) Its rocks contain the record of geologic events during part of 

Tertiary time, and from its physiography some of the events of 

Pleistocene time can be inferred.

This report is the result of about 50 days of field reconnaissance 

by several men during 4 summers. R. R. Coats, geologist, and 

Will F. Thompson, Jr., recorder, spent 4 days in 1946 and 10 days 

in 1947 on the east end of the island. In 1949, as a guest aboard the 

U. S. C. G. S. ship Explorer, H. A. Powers was put ashore at several 

localities around the west end of the island. A larger party, working in two-man teams from the U. S. G. S. motorship Eider, made 

observations in 1951 in some of the larger areas not previously visited. Corps of Engineer topographic maps, scale 1:24,000, were used 

in the early work, and Coast and Geodetic Survey topographic maps, 

scale 1:20,000, were available in manuscript stage to the 1951 field 

party. Dennis P. Cox, Joseph P. Dobell, Richard Q. Lewis, Willis 

H. Nelson, Richard A. Robie, George L. Snyder, Edward C. Stover, 

Jr., and Howard A. Powers, in charge, made up this party.

The fossils collected were identified in the laboratory by K. E. 

Lohman, F. S. MacNeil, R. B. Stewart, F. M. Swain, and Ruth Todd, 

of the U.S. Geological Survey.

Parts of this work would have been impossible without the cooperation of a great many members of the United States Army, 

Air Force, Navy, Coast and Geodetic Survey, and Coast Guard. 

Especial thanks is extended to Lt. Col. R. E. Ware, post engineer, 

Adak, and Lt. Col. C. E. Johnson, port commander, Adak, in 1946; 

and to Comdr. H. A. Karo, captain, Explorer, in 1949. Credit for 

the success of difficult small-boat landings and logistic support of 

the party in 1951 is gratefully placed with Carl Vevelstad, captain, 

and Charles E. Best, engineer, of the Eider. 0

GEOGRAPHY

Amchitka Island lies between long 178°37' E. and long 179°29' E., 

and between lat 51°21' N. and lat 51°39' N.; it is about 35 miles 

long, in a northwesterly direction, and from 3 to 5 miles wide. Before the Russian occupation in the 16th century, the island apparently supported a relatively large Aleut population. It has long 

been uninhabited and is not visited regularly by any means of trans-
w

r

170° 180° 170°

FIOCBB 77. Map of the Alaska Peninsula and Aleu
524 INVESTIGATIONS OF ALASKAN VOLCANOES

portation. In the summer of 1951, a wharf in Constantine Harbor 

was in a fair state of repair, and an airstrip was still in existence, 

both being relics of an extensive military occupation between 1943 

and 1950.

The climate is maritime (Arctic Weather Central, 1950); mean 

annual temperature between 1943 and 1948 at the airbase (225-ft 

alt) was +40°F, and both daily and seasonal variations were small. 

Extremes recorded were +15 °F and +65 °F. During the summer, 

fog occurred more than 50 percent of the time and complete overcast 

more than 65 percent. Summer winds averaged about 20 miles per 

hour, and seldom exceeded 70 miles per hour. In winter months, 

there was less fog and overcast; winds averaged 25 miles per hour, 

and frequently exceeded 100 miles per hour. Precipitation averaged 

35 inches annually, including snowfall of 70 inches.

Vegetation is an almost complete cover of alpine-zone-type moss, 

grass, and associated flora, except on the steepest cliff faces and on 

the wave-swept rock bench above mean tide level. There are no 

trees.

The island is within the Aleutian Island Wildlife Reservation, 

and especial protection is afforded the sea otter, which is now present 

along the coasts in large numbers. Fox and rats, both introduced, 

were the only land mammals noticed on the island.

PHYSIOGRAPHY

The island contains landforms of varied aspect, ranging from 

rugged mountains northeast of Windy Island to swampy tableland south and west of Cyril Cove (pi. 69). All the varied topography seems, however, to have been developed from plateaus of low 

relief, disturbed more or less by block faulting, and modified to 

greatly different degrees by dissection.

MOUNTAIN SEGMENT

The segment of the island between Chitka Point and Windy Island 

is mountainous. The divide between drainage to the Pacific Ocean 

and the Bering Sea is a sinuous ridge with several summits exceeding 1,100 feet in altitude, separated by 4 passes at about 750 feet; 

an outlying peak (alt 990 ft) south of Chitka Cove is separated 

from the main mountain ridge by a broad pass with an altitude 

of about 400 feet. Two prominent long spurs extend southward from 

the main ridge, and three extend northward; they form divides 

between major compound amphitheatre valleys that head in the 

main ridge.

The summits of the ridge, and of parts of the spurs, are gentle 

slopes eroded across the local rock structure. Each summit flat is 

small and is bounded by the truncating steep slopes of the amphi-
GEOLOGY AND PHYSIOGRAPHY OF AMCHITKA ISLAND 525

theatre valleys. The surfaces are underlain by several feet of 

colluvium made up largely of angular fragments of local rock that 

is virtually undecomposed chemically. Clearly these flats are remnants of an erosional surface of very low relief. No features were 

observed that suggest whether or not the surface has been glaciated, 

or even whether the cut surface is subaerial or submarine in origin. 

The steep headwalls of the amphitheatre valleys are in part rock 

faces and in part inactive talus covered with vegetation. At the 

base of the steep slopes is a bench or remnants of a dissected valley 

floor at an altitude of between 700 and 800 feet in most places; a 

few slopes extend as low as 400 feet. Most of the bedrock surface 

exposed in the benches or valley floors is grooved and striated and 

many outcrops have the form of roches moutonnees. Tarns are 

present on some of the benches and valley floors. To some extent, 

the altitude of the benches on which the tarns occur appears to depend upon that of the exhumed top of relatively impermeable rock 

that underlies more permeable volcanic breccia and flow rock.

HIGH PLATEAU

In the segment of the island northwest of Windy Island, the 

drainage divide meanders across a partly dissected tableland, most 

of whose surface is between 700 and 850 feet in altitude; a small 

knob near the center of the segment extends to slightly above 925 feet. 

East of the knob, the part of the ridge that is above an altitude of 

825 feet is generally undissected. West of the knob, a strip of 

similarly undissected surface slopes from an altitude of 925 feet to 

725 feet where it branches into a north-south strip that includes all 

the surface above the 700-foot contour. Two separate areas of 

undissected surface cap the two mesas at the west end of the island.

The high plateau surface is underlain by colluvium of angular 

rock fragments. On the north mesa of Bird Cape, very well rounded 

pebbles and cobbles make up about 10 percent of the colluvium, but 

it is not evident whether they are lag from bedrock conglomerate 

in the mesa or cobbles related to marine planation of the surface. 

At lower altitudes, most bedrock surfaces are polished and striated, 

as they are east of Windy Island.

CHITKA POINT RIDGE

The crest of the island and of the spur extending north to Chitka 

Point are undissected, rounded surfaces underlain by colluvium of 

broken country rock of the Chitka Point formation that here is 

especially closely jointed and locally is abnormally decomposed by 

the breakdown of abundant pyrite.

A short segment of the island crest that slopes gently southeast 

from the junction with the Chitka Point spur to the 500-foot contour
526 INVESTIGATIONS OF ALASKAN VOLCANOES

line is underlain by permeable interbedded sand and cobble gravel 

of hornblende andesite.

LOWER PLATEAUS

The east half of the island is tableland made up of several segments 

at different altitudes ranging from about 135 to about 500 feet above 

sea level. Much of the plateau surface is underlain by a residual 

mantle of small rock fragments derived from underlying closely 

jointed sedimentary rock; parts of the surface are on unshattered 

rock that is grooved and striated and commonly in roche-moutonnee 

form. Ponds are numerous, both on the plateau tops and the dissected margins. A few ice-scoured knobs appear here and there in 

all the plateau segments but are most abundant east of Makarius 

Bay. From the distribution of these patches of scoured rock, it is 

surmised that the surface of all the plateau has been overridden by 

glacial ice, but that evidence of the sculpture has been preserved 

only on surfaces of rather massive rock. Postglacial stream dissection is almost negligible, but stream dissection older than the 

last glaciation modified each of the plateau segments to a different 

degree.

FONDS

Many ponds occupy depressions in the surface of bedrock or in 

alluvium near sea level. A great many others, however, are confined 

entirely by turf sealed with fine mineral and organic material. Water 

levels in 2 adjacent ponds separated by a divide about 5 feet wide 

differed by V/2 feet. In a ditch through the divide, only turf was 

found to a depth of at least 3 feet below the bottom of the lower 

pond. The two groups of summit ponds due west of Banjo Point and 

southwest of Cyril Cove are all of this type. The abundance of 

ponds thus does not necessarily indicate a large number of closedcontour bedrock depressions. Further, the shape and arrangement 

of these ponds do not necessarily have a geological significance. The 

arrangement of ponds in bedrock surface depressions, however, is 

related to geological features; some are located along the outcrop of 

less resistant rock, and the alinement of some ponds reflects the 

outcrop of zones of abundant fracture.

STREAM VALLEYS

Streams are all short and small, and some flow throughout the 

year. All the streams occupy valleys which are apparently glacially 

scoured as their broad, shallow, concave profiles appear much too 

large for the present streams. Most streams enter the ocean in rapids 

or falls over bedrock; a few are entrenched in short, narrow floors 

of alluvium near their mouths. The pattern of stream valleys shows
GEOLOGY AND PHYSIOGRAPHY OF AMCHITKA ISLAND 527

many straight-line elements controlled by zones of fracture or, less 

commonly, by soft beds.

In the western segment of the low plateau, between its west junction with the mountain segment and the 200-foot contour near Banjo 

Point, the plateau top is moderately dissected by ice-scoured stream 

valleys, and the margins toward both coasts are deeply indented by 

valleys. The altitude of these partly dissected plateau tops ranges 

from 225 feet to 500 feet. In contrast, the plateau top east of 

Makarius Bay, mostly between 200 and 240 feet in altitude, is not 

dissected, though the margins are deeply eroded by valleys. Streams 

across the summit occupy no perceptible valleys, but, at the edge 

of the plateau, they enter broad, shallow valleys that become broad 

and deep in a short distance toward the coasts. The same is true 

of the segment west and south of Cyril Cove whose flat summit is at 

an altitude of between 125 and 150 feet. Possibly the planed tops 

were formed after the valleys that indent the margins had been 

eroded.

INLAND SEA CLIFFS

South of Constantine Harbor, a bedrock knob rises above the 

240-foot level of the summit plateau to about 325 feet. The margin 

of this knob is an almost unbroken cliff 10 to 50 feet high. This 

cliff is the best example on the island of an inferred elevated sea 

cliff. Eock outcrops in this knob, the cliff, and the plain have been 

scoured by ice; the cliff has not been breached by stream erosion.

Several short reaches of cliff fronting the Pacific, south and west 

of Banjo Point, are indicated as raised sea cliffs on plate 69, although they might be parts of a faultline scarp or they may be 

neither. These are cited as the least convincing examples of inferred raised sea cliffs. Other probable raised sea cliffs rise from 

dissected planed surfaces at different places and altitudes.

SHORELINE FEATURES

Most of the littoral zone is a rock bench, perhaps 50 feet wide on 

the average but in places as much as a quarter of a mile wide, much 

of which is covered with living seaweed. Seaward, the bench is 

truncated by a steep cliff that apparently extends as much as 30 feet 

below water. Landward, the bench terminates against the base of a 

cliff 50 to several hundred feet high.

The weed-covered part of the bench (wet bench) is a nearly plane 

surface cut across bedrock structures approximately at mean sea 

level. Continuity of the wet bench is broken by boulder-floored 

channels into which waves slosh even at low tide. The shape, 

dimensions, orientation, and abundance of these channels seem to 

be determined by zones of fractured rock or by soft beds. Above
528 INVESTIGATIONS OF ALASKAN VOLCANOES

the general plane of the wet bench (weed-covered surface) knobs, 

ridges, and platforms rise usually to a height of no greater than, 

about 6 feet, but, rarely, to several tens of feet. Where these 

features are isolated they are composed of rock more resistant to 

erosion than that in the bench, but along the landward margin at 

the base of the cliff where they abound and form a nearly continuous 

dry bench they are not exclusively of resistant rock.

The landward margin of the rock bench is commonly surmounted 

by a cobble or boulder beach deposit whose top is 5 to 15 feet above 

the level of living seaweed. The lower boulder beaches support 

vegetation only of a sort that becomes established between storms; 

debris from the military occupation of the island is incorporated in 

them. The higher beaches and the face of the cliff behind them 

are covered with a thick turf-and-root mat of long established 

vegetation. Some of these features are illustrated by the profile off 

Chitka Point (fig. 78).

The rock bench is absent at the mouths of many stream valleys, 

and a sand beach terminates an alluvial fill in a few of the wider 

valley mouths. Storm beaches, with light vegetation, are as much as 

15 feet above the level of living seaweed and have a higher, heavily 

vegetated beach behind them in some places. In a few places the 

rock bench is missing at the base of a bare rock cliff that exposes 

massive rock broken by widely spaced joints.

The rock bench approximately at sea level is much wider and more 

extensively developed along Aleutian shores than along apparently 

comparable shores at lower latitudes; its development approaches 

that of the strandflat of high latitudes. It is inferred that rapid 

disintegration of surface rock by daily freezing and thawing is 

an important factor here, though not as effective as Nansen's shore 

erosion by frost in producing the strandflat (Nansen, 1922). In 

the Aleutian Islands, the effective base of frost erosion may be determined by the position of the ground-water table (Bartrum, 1926).

Long records of daily changes in temperature in the Aleutian Islands 

are not available, but at sea level perhaps nearly a fifth of the days 

have light freezing nighttime temperatures and thawing daytime temperatures. Rock of the sea cliffs always contains abundant moisture, 

even above the water table. Consequently a shallow surface layer 

of the rock is subjected to disruption by frost action in minute open 

spaces. Waves at nearly every high tide, and during every storm, 

wash over the rock surface up to several feet above mean sea level 

and remove the surface layer as fast as it is loosened. This accelerates the disintegration caused by frost action by keeping a nearly 

fresh surface constantly exposed to attack in the zone reached by 

the waves. These two processes form a cut rock bench, most rapidly 

on permeable rock in which bedding planes and fractures are closely
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spaced, and least rapidly on impermeable rock with widely spaced 

fractures. The daily freezing cannot take place below the water 

table, as the ground-water temperature at the shore is influenced 

strongly by the ocean temperature, which does not reach 32°F. 

Theoretically, this should fix the position of the bench surface at 

water-table level, but, practically, the bench is cut below this level 

by some abrasion as the waves move the loosened material back and 

forth across the bench. However, the abrasive action is effectively 

reduced and locally is stopped completely for an unknown length 

of time after the bench surface is cut below the level of mean tide 

because a protective mat of seaweed rapidly is established as soon as 

the water cover is favorable.

Concurrently a different process of erosion is developing a cut 

bench at a deeper level. Below the water table, where the impact 

of each wave crest puts the water in fractures under a differential 

pressure, blocks are loosened and removed by the waves down to a 

level that is determined in part by local rock structure and in part 

by wave depth, which is influenced by local offshore configuration. 

The level appears to be between 12 and 30 feet below sea level. 

This process seems most effective on massive, impermeable rock with 

widely spaced joints, and least effective on permeable rock with 

closely spaced joints and bedding planes. Thus, on coasts cut in 

jointed gabbro or massive flow lava, a steep cliff extends 15 to 30 

feet below sea level, with no tide-level bench, but on coasts cut in 

closely fractured pillow-lava flows or bedded sedimentary rock a 

weed-covered bench at low-tide level is truncated at its seaward 

margin by a submerged cliff that extends down to the deeper wavequarried bench (fig. 78).

The features of the shoreline that have been described apparently 

include 2 sets of beach deposits: 1 set is associated with activity at 

the present sea level, and 1 set is 6 to 8 feet higher and has been 

undisturbed by wave activity long enough to be covered by a peatlike mat of roots and permanent vegetation. The cliff behind these 

beaches likewise is protected from present marine erosion. Dwelling 

sites of Aleut inhabitants commonly found on the crests of the 

higher, abandoned beach ridges may be as much as 4,000 years old. 

(Byers, 1959). Similar features of an inactive shoreline are characteristic of the Near Islands (Powers, personal communication, 

1950). If the beaches are products of activity at 2 levels of the sea, 

it follows that the rock bench must include features of 2 sea levels. 

It is inferred that the knobs and platforms that rise about 6 feet 

above the seaweed-covered rock flats are remnants of an older tidelevel rock bench that is being reduced to a bench in equilibrium with 

present sea level (fig. 78).
GEOLOGY AND PHYSIOGRAPHY OF AMCHITKA ISLAND 531

SUBMERGED TOPOGRAPHY

Submerged topography surrounding the island is shown on plate 69 

by contour lines at 100-foot intervals to a depth of 400 feet, and by 

contours at 300-foot intervals starting at 600 feet. More detailed 

topography to a depth of 400 feet around the east half of the island 

is shown by supplemental contours at 25-foot intervals. The 25-foot 

contour is omitted because soundings were too scarce to locate it 

adequately (heavy growth of kelp prevented operation of the surveying launches). The 50-foot contour is below the bench being 

cut by present shoreline erosion. However, a profile off Chitka Point 

(fig. Y8), where inshore soundings were made, shows this bench in 

relation to the local examples of more deeply submerged topographic 

features. The topography between 50- and 150-foot depths all along 

the coast suggests ridges and valleys of a stream-dissected slope. 

This pattern changes abruptly between the 150- and 175-foot contours; the change must be real as there is no coincident change in the 

number of soundings per unit area. Within this zone of change, the 

ridges terminate in a steep cliff whose base is at depths ranging 

from 160 to 180 feet. Examples of these cliffs that have been 

identified on the charts of detailed soundings are marked on plate 69 

with short lengths of dotted lines with a number giving the 

depth of the base of the cliff. The shape of the 100-foot contour 

shows that the ridge and valley topography continues all around 

the island; that of the 200-foot contour shows a slope with very 

little relief. ,-..- 

The shelf-break (Dietz and Menard, 1951, p. 1996) at the outer 

edge of the insular shelf (Gates and Gibson, 1956, p. 132), is at a 

depth of about 325 feet throughout most of its length, though 

locally short segments are at depths ranging from 300 to 390 feet. 

Most of the segments of divergent depth are around the east half 

of the island. The margin of the shelf is not greatly indented, and 

the shape of depth contours below 325 feet in general suggests faultblock and grabeii, rather than ridge and valley, topography. The 

seaward part of the insular shelf between depths of about 290 and 

325 feet is a wide bench with very low gradient on the Pacific Ocean 

side, and locally on the Bering Sea side, of the island. It will be 

referred to as the shelf-break bench. Details of local topography, 

on this shelf-break bench are shown on plate 69 by symbols showing 

the altitude of the knobs and the depths of the depressions. Along 

the inland margin of this bench are several .abrupt slopes .whose 

bases are at depths ranging from 265. to 310 feet around, the western 

part of .the island^ and from .200. to .310 around, the .eastern part. 

The tops of these short stretches of abrupt slope are at depths of 180 to 220 feet. .................. ^ ...
532 INVESTIGATIONS OF ALASKAN VOLCANOES 

INTERPRETATION OF TOPOGRAPHY

The landmass exposed to erosion, broadly speaking, has the appearance of a fault block elevated and tilted to the southwest. The 

Bering slope might be a faultline scarp, and the gentle southern 

insular slope between depths of 400 to 3,000 feet might be the tilted 

surface of the postulated fault block. The greater encroachment 

of the Pacific shore onto the block may be due largely to the 

original asymmetry, but may also be in part to a greater rate of 

erosion; although Bering Sea storms are frequently as severe as 

those of the Pacific Ocean, the usual daily waves of the Pacific 

are stronger.

It is suggested that the high plateau is modified from a marine 

platform whose inshore edge has been elevated about 800 feet, and 

that the higher summits are relics of islands shaped by the sea that 

formed the platform. Plateaus and benches (and cliffs rising from 

them) at heights of 400 to 500 feet might be relics of features of a 

second elevated shoreline. If so, they have been displaced relative 

to each other by faulting.

On this emerged landmass the pattern of stream valleys was 

established, and a good deal of subaerial erosion was accomplished, 

before the cutting of a shoreline whose best preserved (but faultdisturbed) remnant now stands at an altitude of 240 feet. It is inferred that the same stream-cut valley and ridge topography extends 

below present sea level (nicked by the benches and cliffs of the 

present shoreline) and is almost obliterated by the bench and cliffs 

of a shoreline now submerged at about 165 feet (fig. 78). Almost 

no suggestion of ridge and valley topography exists on the deeper 

shelf-break bench, which is believed to be part of a shoreline dislocated by subsequent faulting and now submerged at depths ranging from 220 to 310 feet. The closed-contour depressions in the 

shelf-break bench are probably the result of several processes. Those 

west of Aleut Point, in the pass between Amchitka and Rat Islands, 

perhaps are due largely to scour by strong currents. Those south 

of the mountainous part of the island perhaps represent scouring 

by glacial ice, as each of the larger depressions is offshore from 

a large amphitheatre that has been glaciated. Those east of the 

lT9th meridian perhaps are in part structural and in part glacial 

depressions, judging from their shapes and alinement. Evidence of 

glaciation on the Near Islands platform, which is equivalent in part 

to the shelf-break bench, is presented by Scruton (1953) and discussed by Gates and Gibson (1956, p. 135).
GEOLOGY AND PHYSIOGRAPHY OF AMCHITKA. ISLAND 533

STRATIGRAPHY

AMCHITKA FORMATION

Water-laid beds of volcanic breccia and some thin-bedded finegrained tuff, all interbedded with pillow flows of lava, are the oldest 

recognized rocks of the island and are here named "the Amchitka 

formation." The material is andesitic and usually porphyritic with 

phenocrysts of plagioclase, pyroxene, and amphibole. Pyroclastic 

fragments are angular to subangular; none is well rounded. Beds 

of both coarse and fine fragments are indurated. All the rock is 

broken by joints.

These rocks make up the northern part of the western third, and 

most of the eastern fourth, of the island; small, isolated exposures 

are found at and west of Cyril Cove. No correlation of individual 

beds was possible between the two large areas of outcrop. The 

total thickness of these rocks is perhaps several thousand feet. The 

formation is overlain, with angular unconformity, by tuff and 

conglomerate of the Banjo Point formation and lava and conglomerate of the Chitka Point formation. It is invaded by a stock 

and dikes of quartz diorite, as well as dikes of basalt and andesite.

The geologic age of these rocks is unknown, but they must be at 

least as old as early Tertiary inasmuch as the overlying Banjo 

Point rocks are of early middle Tertiary age.

In eastern Amchitka, the most abundant rock of this formation 

is lapilli tuff and tuff-breccia in beds 10 to 100 feet thick. The 

fragmental texture is commonly conspicuous, and the fragments 

range in size from less than a tenth to 3 or 4 inches in greatest 

dimension. Whole and broken crystals of plagioclase, amphibole, and 

pyroxene are common, but glassy fragments are most abundant and 

range in vesicularity from dense obsidian to pumice. The rock is 

mottled dull yellowish brown and olive green to blue green with 

flecks and streaks of white. Much of the glass is altered to nontronite, and some of the interstitial space is occupied by zeolites; the 

rock as a whole is well indurated, though porous in patches. Adjacent to some of the intrusive bodies, and along a few zones rich 

in pyrite, the rocks are altered to fine anhedral quartz and alkalic 

plagioclase, calcite, sericite, epidote, chlorite, leucoxene, and iron 

oxides.

The type section of these rocks is exposed along the south coast 

from South Bight westward to long 179° 18' E. Beds of coarse 

tuff-breccia, each more than 100 feet thick, alternate with flows of 

pillow lava 5 to 20 feet thick and with layers of thin, evenly bedded 

fine indurated yellow and green tuff 5 to 50 feet thick.
534 INVESTIGATIONS OF ALASKAN VOLCANOES

The upper bed of latite-tuff-breccia, exposed over a large area 

south of Constantine Harbor and all of Kirilof Point, contains 

abundant angular fragments of dense black obsidian up to 3 or 4 

inches across in a mottled brown-green matrix of vesicular pumice 

fragments and shards. In a few places, bipolar bombs of aphanitic 

lava (in part hollow) as much as 30 inches long are embedded in 

the tuff. A core-drill hole south of Constantine Harbor penetrated 

this bed for 300 feet without passing through it. The glass of the 

vesicular matrix material is altered to nontronite and has zeolites 

throughout; heulandite, mordenite, and apophyllite were identified 

by X-ray examination by Fred Hildebrand of the U.S. Geological 

Survey. The dense black obsidian is shattered by fractures lined 

with nontronite and in places with zeolites. Small bodies of the 

glass, between fractures, contain scattered microphenocrysts of calcic 

andesine and clinopyroxene. The glass is clear and light brown in 

thin flakes and sections. The index of refraction of the clearest 

pieces of glass ranges from 1.520 to 1.523 (±0.001). A sample of 

the glass, reasonably free of zeolites but not of nontronite, has the 

chemical composition given in table 1.

TABLE 1. Chemical and spectrographic analyses and norms of volcanic rocks from

Amchitka Island . .

[Analyses by U.S. Geological Survey] ' .

1 2 3 4

Chemical analyses

[Analysts: col. 1, E. J. Tomasl; cols. 2,3, and 4, L. Kehl]

SiO2 _ ------

A12 03

Ti02 ... ... --------

Fe2 03 - ---------

FpO

MnO -----------

MgO__ ---__ _

CaO--.. ---------

Na2 0 -----------

K20_ -----------

H20- __________

H2 0+ _-.__--_-

P2 05 .--. -----

C02 -----------

Cl. . ___-_-- -

F__-----_--

S___ ------

BaO__-----_ 

TotaL -----------

Less 0 _-_-__ -__-

Total. __________

59. 13

15. 10

.57

2. 14

2. 71

. 12

1. 32

3.76

3.76

3.54

1.48

5. 75

. 27

.01

.23

. 07

.02

nd

99. 98

.09

99.89

58. 84

17. 94

.67

2.36

3.41

. 12

3. 33

7.09

3.55

1.67

.07

.73

.20

.01

ind

.04

nd

.04

100. 07

. 02

100. 05

54.59

18. 49 -.76

2.86

440

. 15

4.24

9.05

3.35

.92

.30

.82

. 17

. 15

nd

.03

nd

.03

100. 31

.01

100. 30

58. 04

18. 14

.67

2.92

3. 14

. 13

3.69

7. 59

3.36

1.41

.23

.52

.20

.01

nd

.03

nd

.03

100. 11

. 01

100. 10

See footnotes at end of table.
535

TABLE 1. Chemical and spectrographic analyses and norms of volcanic rocks from 

Amchitka Island Continued

i 2' 34

Spectrographic analyses 2 

[Analyst: P. R. Barnett]

B---_----_-------

Ba____-__-___-____

Co.. .._-...:...-..

Cr__ 

Cu... _._....._._..

Ga______-_________

Mo. .__.. . ._....

Li... _-____--_____

Ni._ 

Pb_ ____

Rb

So.... .........

,Sr. ...............

V.. ......... ......

Y

Yb_. ._.-....__.__.

Zr. _______________

0.006 

.20 

.001 

. 0005 

.003 

.002 

.0005 

nd 

<. 0005 

. . 004 

. nd 

<. 0005 

.08 

.01 

<. 001 

<.0001 

.030

<0. 001 

.06 

.002 

.0040 

.009 

.001 

<. 0001 

.0007 

.002 

<.0001 

.002 

.002

 io|

.02^ 

.004 

. 0003 

.010

<0. 001 

.04 

.003 

.0040 

.009 

.001 

<. 0001 

.0030 

.002 

<. 0001 

.001 

.003 

.06 

.03 

.004 

.0003 

.008

<0. 001 

.04 

.002 

.0030 

.008 

.001 

<. 0001 

.0010 

.002 

<. 0001 

.001 

.002 

.09

: .02

.003 

. 0002 

.010

Norms

Q_.__. ............

or ___ ____________

ab. ..___._____..__

ari________________

wo___ ______ _______

fs.. ...............

en_____________.__

cc____________-___

il_________ _ _____

mt ______ __ ---._

op________________

hl_________________

Water. _______

CIPWsymbol__:___

15. 21

20.80

29.87

14.85

.75

2.45

3.30

.80

3. 20

. 67

.40

7.23

(1)11.4.2.3"

12. 12

9.70

29.87

28. 08

2.55

3.30

8.30

1. 30

3.35

.34

.80

11.4(5) .3".4

6.69

5.24

28.30

32.53

4.46

4. 62

10. 60

.32

1.41

4.30

.34

1. 12

II."5.3.4"

12.09

8.34

28.30

30. 15

2.72

2.51

9.20

1.30

4. 18

.34

.75

11.4(5) .3.4

1 nd=not determined.

» Looked for but not found: Ag, As, Au, Be, Bl, Cd, Cs, Oe, In, La, Nb, Pt, Sb, Sn, Ta, Th, Tl, U, W, 

and Zn.

1. Obsidian tuff-breccia, Amchitka formation: sparse phenocrysts of andesine and clinopyroxene. Specimen No. 51-P-27. Lat 51°22'15" N.; long 179°16'00"E.

2. Columnar lava, Amchitka formation; rich in phenocrysts of labradorite-andesine, ortho- and clinopyroxene. Specimen No. 51-P-31. Lat 51°27'40" N.; long 179°09'25" E.

 3. Flow banded lava, Chitka Point formation; rich in phenocrysts of calcic plagioclase, olivine, ortho- and 

clinopyroxene. Specimen No. 49-P-36. Lat 51°34'50" N.; long 178°48'45'' E.

4. Scoriacoous lava, Chitka Point formation; rich in phenocrysts of calcic plagioclase, brown hornblende, 

ortho- and clinopyroxene. . Specimen No. 49-P-46. Lat 51°35'56" N.; long 178°42'40" E.

In one quarry exposure the bottom of this tuff-breccia is mixed 

into a mass of microcrystalline lava; laterally this mixed contact 

changes to a sharp contact of tuff-breccia on pillow lava. In one 

exposure the pillow lava seems to have burrowed into the tuffbreccia. Two or three other flows of pillow lava, 10 to 20 feet thick, 

are interbedded with lapilli tuff at other localities, notably at long 

179° 19'E. on the south coast.
536 INVESTIGATIONS OF ALASKAN VOLCANOES

Banded, richly porphyritic, andesitic rock is exposed in several 

disconnected outcrops just west of Cyril Cove. The chemical composition of this rock is given in table 1. It could not be determined 

whether the rock body is a shallow sill or a relatively thick lava 

flow. The banding, emphasized on weathered surfaces by abrupt 

changes in color from light to dark gray, apparently is due to slight 

differences in crystallinity of the groundmass and not to segregation 

of the phenocryst minerals that make up about half of the rock. 

Calcic plagioclase is most abundant, then clinopyroxene and hypersthene, and a few large crystals of apatite and opaque oxide. 

A very few "ghosts" of amphibole crystals are suggested by the 

shape of patches of opaque dustlike iron oxide.

Along the west shore of Chitka Cove the most abundant rock is 

light gray mottled with green, porphyritic, and fractured by closely 

spaced vertical and diagonal joints, and looks like an outcrop of a 

shallow intrusive body of porphyry. However, the body grades 

upward into thin-bedded fine-grained thoroughly indurated siltstone 

about 6 feet thick. Overlying the siltstone, with apparently a depositional contact, is a greenish-gray rock, mottled with light gray, 

similar in appearance to the lower porphyry. Neither the top of the 

upper bed nor the base of the lower was seen; each has an exposed 

thickness exceeding 200 feet.

The lower rock contains rounded crystals of quartz (up to 1 mm 

in diameter) and aggregates of secondary minerals having the shape 

of large euhedral crystals of feldspar, amphibole, and clinopyroxene, 

all in a fine-grained matrix. All constituents except the quartz 

phenocrysts are altered to calcite, sericite, epidote, alkalic plagioclase, 

chlorite, fine anhedral quartz, leucoxene, pyrite, and iron oxide. 

The upper rock contains the same secondary-mineral assemblage 

but no phenocrysts of quartz and ghosts of only feldspar phenocrysts. 

It is inferred that these two bodies originally were thick beds of 

unsorted-crystal vitric tuff whose clastic texture has been destroyed 

by hydrothermal alteration and recrystallization.

Westward in the larger outcrop the original mineral constituents 

of the rocks are also altered, but the fragmental texture of lapilli 

tuff can commonly be seen in spite of the alteration, and the beds 

are at most a few tens of feet thick. The formation is broken 

into small fault blocks, tilted at random, with dips of up to 45°.

BANJO POINT FORMATION

Bedded marine sandstone, conglomerate, tuffaceous shale, and some 

lapilli tuff of basaltic composition make up the stratigraphic unit 

here named the Banjo Point formation. These rocks occupy the 

entire width of the island westward from Makarius Bay to about 

5 miles west of Banjo Point, and also several square miles of Saint
GEOLOGY AND PHYSIOGRAPHY OF AMCHITKA ISLAND 537

Makarius Point, and lie on thin-bedded tuff and pillow lava of the 

Amchitka formation with an angular unconformity of 5° to 10°. 

On the north coast west of Kirilof Point, sandstone beds of the 

Banjo Point formation transgress a westward-sloping surface eroded 

on tuff-breccia. West of Banjo Point, the basaltic tuff and sandstone 

are separated by a fault contact from hornblende-andesite breccia of 

the Chitka Point formation; elsewhere they are cut by dikes of 

similar porphyritic andesite. Everywhere the top of the formation 

is the present erosion surface. The formation could be as little as 

400 feet thick, though it probably was originally much thicker; no 

marker beds could be traced through the many fault blocks to permit 

reconstruction of a complete section.

The geologic age of the Banjo Point formation is placed, on the 

basis of a meager fauna, in the range Oligocene to Miocene. A 

fauna of 36 species of Foraminifera is contained in two collections, 

one from Banjo Point and the other from the south coast almost 

due west of Banjo Point. Eighteen of these species are identified 

with or closely related to known species (Todd, 1953). The assemblage resembles most nearly the fauna of two formations, the 

Oligocene Tumey formation of Atwill (1935) in California and the 

Bastendorff shale of Eocene and Oligocene age in Oregon. Fragments of diatoms, genus Coscinodiscus and Arachnoidiscus, according 

to K. E. Lohman, were found in the foraminiferal material but are 

too fragmentary and corroded to be identified as to species. Some 

very poor molluscan material from Banjo Point, possibly representing Lucina, Pododesrrws, and a buccinoid gastropod (according 

to R. B. Stewart) adds nothing to the age determination. A poorly 

preserved specimen from the south coast at lat 51°24'32" N".; long 

179°9'44" E., is identified by F. S. MacNeil as Chlamys aff. G. 

washburnei Arnold and considered by MaoNeil to be of probable 

middle Tertiary age (late Oligocene or early Miocene). MacNeil 

states that 

Chlamys washburnei was described from beds on the Yachates River, Oregon, 

which have been assigned variously to the Pliocene (Arnold [1906, p. 119- 

120]), middle Miocene(?) (Weaver [1942, p. 86]), and to the early Oligocene 

(Yokes, personal communication, 1954). It is associated, according to Yokes, 

with two crinoids, Isocrinus oregonensis and /. nehalemensis, which Moore 

and Yokes [1953, p. 116] described from beds correlated with the Keasey formation.

The stratigraphic position of the identified specimen of Chlamys 

with respect to the two collections of Foraminifera is not positively 

known. Many small fragments of pecten shells occur in beds both, 

below and above the foraminiferal layer at both localities, and there 

is no geologic evidence to suggest that the Foraminifera and the 

pecten are not part of the same fauna. No rational choice between
538

Oligocene and Miocene for the age of the fauna appears possible 

with the meager data at hand.

Compilation of a complete section of the rocks of the Banjo Point 

formation was not feasible, but a partial section exposed in the north 

coast at Banjo Point is considered the type section of the formation.

Eroded present surface. ft in

Conglomerate, tuffaceous, unsorted _________________ 100+ 0

Lapilli tuff, bedded __________________________ 25 0

Conglomerate _ _ __-__~______________ 10 0

Sandstone _______________________________ 1 0

Siltstone, tuffaceous, fossiliferous ______ ____ 2

Sandstone ___________________________:____ 8

Conglomerate _ ___ __________________ 2+ 0

Total _______________________________ 138+ 10 

Base of formation not exposed.

The thick upper unit contains a few large (4-ft diameter) rounded 

boulders scattered at random in a mass of unsorted lapilli to siltsize fragments of basaltic composition, apparently the product of 

a nearby volcanic eruption. A few thin, lenticular streaks of sorted 

material give a very crude bedding, conformable to the surface and 

to the bedding of the underlying tuff. The contact of the tuffaceous 

conglomerate on the tuff is abrupt.

The lapilli tuff is poorly sorted but is bedded in units 4 to 12 

inches thick, and contains angular lapilli of basaltic scoria in a matrix 

of vesicular^ vitric, similar-appearing material. .,

The two units of conglomerate contain well-rounded to subrounded 

cobbles of various sorts of basaltic rock in a matrix of small subrounded to angular fragments of basalt, vitric tuff, and fragments 

of barnacle and pecten shells. The finer material in these beds is 

the product of the erosion and transport of volcanic rock rather than 

of pyroclastic eruption and initial submarine deposition.

The two beds of sandstone are similar and contain subrounded to 

subangular coarse to medium-grained sand size particles of volcanic 

material, both crystalline and vitric, They are indurated by limonitic 

cement.

The tuffaceous siltstone is a compact nonlaminated bed of vitric 

shards altered to nontronite, rich in carbonized plant fragments. 

It contains shells of immature mollusks and Foramiriifera. . The 

angularity of the shards makes it appear to be a bed of vitric ash 

initially deposited in water, rather than a product of erosion.

Elsewhere, partial sections of this formation contain beds showing 

similar litholpgic materials present in locally different proportions 

and arrangement. In 1 exposure on the Pacific shore southwest of 

Banjo Point is a unit of thin-bedded vitric tuff 80 feet thick; a
GEOLOGY AND PHYSIOGRAPHY OF AMCHITKA ISLAND 539

few hundred yards to the west is a bed, over 30 feet thick, composed 

entirely of beach-rounded cobbles and boulders up to 2 feet across.

QUARTZ DIORITE

Quartz diorite and related rock form a stock about 2 miles in 

diameter at the east end of Amchitka that intrudes the Amchitka 

formation. Dikes of the same sort of rock, too small to map, intrude 

the Amchitka and the Banjo Point formations. In the east cliff of 

South Bight, the quartz diorite is cut by a 4-foot dike of hornblende 

andesite, similar to some lava of the Chitka Point formation. Cobbles of the quartz diorite were not found in any of the preglacial 

conglomerate.

The age of the stock and dikes of quartz diorite is considered to 

be middle to late Tertiary, inasmuch as the rock is younger than 

the Banjo Point formation and older than part, if not all, of the 

Chitka Point formation.

Much of the rock is medium grained and light to medium gray, 

and is composed of plagioclase, augite (commonly uralitized), quartz, 

and orthoclase. The mass ranges from local granodiorite facies to 

gabbro, but the bulk is quartz diorite.

CHITKA POINT FORMATION

Lava flows and flow breccia of porphyritic andesite, interbedded 

with marine conglomerate, are here named the Chitka Point formation. The conglomerate is confined to the lower part of the formation; lava flows and breccia emplaced above water form the upper 

part. These rocks underlie most of the mountainous west half of 

the island. They lie on the Amchitka formation with angular unconformity, and dikes of porphyritic rock similar to lava of the 

Chitka Point formation cut the Banjo Point and Amchitka formations. South of Chitka Point they are overlain by a gravel deposit 

of hornblende andesite. Thickness of the formation probably exceeds 

1,000 feet.

The age of the Chitka Point formation is estimated as middle to 

late Tertiary because it overlies the early Tertiary Amchitka formation and its dikes cut rock of early to middle Teritary age; there 

is no evidence to rule out the possibility that it might be early 

Quaternary.

No complete section of the formation was compiled; several incomplete sections (described below) give a picture of its general 

characteristics. At Chitka Point, the type locality, a thick sheet of 

poorly sorted, indurated conglomerate contains many carbonized 

broken fragments of woody material. This wood was unidentifiable 

as to species, but its presence, together with well-rounded cobbles in 

the conglomerate, indicates the nearness of land. The conglomerate
540 INVESTIGATIONS OF ALASKAN VOLCANOES

is interbedded with porphyritic pyroxene andesite lava. This is 

overlain at a 200-foot altitude on the ridge to the south by a thick 

lava flow of porphyritic hornblende andesite. Similar-appearing 

hornblende andesite, with and without plagioclase phenocrysts, is 

the only rock exposed along the crest of the island for 10 miles to 

the west. In the north-facing cliff at long 178°48' E. a 400-foot 

thickness of hornblende porphyry lava and breccia lying on an 

eroded surface of the Amchitka formation is capped by almost 100 

feet of darker basaltic andesite with plagioclase and pyroxene 

phenocrysts. Both of these rocks are loose textured, contain open 

vesicles and interstitial spaces, and appear to have been emplaced 

on land. Both dip about 15° SW. At long 148°44' E. on the south 

coast, a flow of hornblende andesite that dips about 15° SW. lies 

with angular unconformity on a section of 3 flows of pyroxene 

andesite interbedded with 2 thin layers of conglomerate that dip 

30° 1ST.

The most abundant constituents of the conglomerates are very 

well-rounded to subrounded cobbles of unaltered porphyritic andesite 

similar to rocks of the interbedded flows; less abundant, but common, are cobbles and pebbles of the altered rocks from the Amchitka 

formation containing pyrite crystals. In some restricted localities, 

particularly on the north coast, all constituents of the conglomerates, 

as well as of the interbedded lava, are hydrothermally altered.

The most abundant rock type in the Chitka Point formation is 

porphyritic hornblende-pyroxene andesite. The analysis of one sample is given in table 1. More than half the rock is phenocrysts. About 

2 percent is large (up to 5 mm) crystals of brown hornblende with 

thick reaction rims of calcic plagioclase and opaque oxide. Hypersthene and clinopyroxene crystals up to 1 mm across make up 

about 15 percent of the rock; zoned phenocrysts of calcic plagioclase 

make up about 40 percent of the rock. The groundmass is plagioclase crystals in light-brown glass.

The next most abundant rock type, porphyritic andesite, is represented by the analysis in table 1. The sample is from a thick 

columnar-jointed dark-colored flow. The rock is richly porphyritic. 

About 1 percent of the rock is now masses of bowlingite, pleochroic 

in green and yellow, surrounded by thin rims of small pyroxene 

crystals; the bowlingite was formerly olivine in euhedral 1-mm 

crystals. Euhedral crystals of pyroxene, both hypersthene and 

zoned clinopyroxene, make up 10 percent of the rock. Some hypersthene occurs as cores in clinopyroxene. About 25 percent of the 

rock is phenocrysts of calcic plagioclase, conspicuously zoned with 

oscillatory zoning and fritted cores. The groundmass is light-brown 

glass with plagioclase laths and crystallites of opaque oxide.
GEOLOGY AND PHYSIOGRAPHY OF AMCHITKA ISLAND 541

Less common are two other rock types, one that contains only 

clinopyroxene as phenocrysts, and one with phenocrysts of green 

hornblende as the only ferromagnesian constituent; both are rich 

in plagioclase.

PYRITE-RICH ROCK

Rocks of the Amchitka and Banjo Point formations contain pyrite, 

up to a third of the volume of the rock, in several localities. 

Weathered outcrops are conspicuous because of their gossan-red 

color. In detail, the pyrite-rich rock follows fractures of diverse 

strike direction, but zones of pyrite enrichment trend within a few 

degrees of east. These zones are most abundant in the west third 

of the island, common east of Makarius Bay, and present but not 

conspicuous in the rest of the island.

In rocks of the Chitka Point formation, pyrite is not common but 

is present in halos or narrow zones of alteration that suggest 

fumarolic activity.

One sample of pyrite-rich rock from the south shore of Chitka 

Cove contains only trace amounts of copper, lead, and zinc; no 

sulfides other than pyrite were seen in hand-lens inspection of many 

outcrops.

GRAVEL OF HORNBLENDE ANDESITE

Bedded sand and gravel composed entirely of fragments of hornblende andesite lie on the crest of the island for 6,000 feet southeast 

of the 600-foot contour south of Chitka Point. The deposit is mostly 

coarse moderately well sorted sand with several interbeds, 5 to 

15 feet thick, of cobble to boulder conglomerate. The material is 

compact but not indurated. The dip of about 12° SE. is possibly 

wholly initial. At least 1 of the conglomerate beds is made up of 

angular to subangular slabs, up to 18 inches in greatest dimension, 

of porphyritic hornblende andesite. The other conglomerate beds 

contain well-rounded cobbles and boulders up to 2 feet in diameter, 

with interstitial coarse sand. Lag deposits of these cobbles and 

boulders superficially resemble raised boulder-beach deposits where 

outcrops strike across the crest of the ridge. The formation seems 

to be a littoral-zone deposit whose thickness probably does not 

exceed 100 feet.

The relative age of these rocks is not apparent from the available 

data. They may be entirely younger than the Chitka Point formation, or perhaps correlative with part of it.

TILTED SEDIMENTARY ROCK AT SOUTH BIGHT

Beds of silt, sand, and gravel crop out on the shore at the head 

of South Bight and at the head of the small unnamed bight across 

the island to the northeast. Presumably they are continuous between
542 INVESTIGATIONS OF ALASKAN VOLCANOES

these outcrops, and they may occupy a total area of less than a 

quarter square mile.

The lowest 200 feet of the beds dip about 12° SE. These are 

layers, in random sequence, a few inches to 2 feet thick, of carbonaceous sandy silt, fine to medium-size sand, and pebbly sand to 

sandy fine gravel. A few layers of light-colored siliceous volcanic 

ash, each less than 2 inches thick, are interbedded. The general 

color is pale reddish yellow to greenish yellow, and the constituents 

are compacted but not indurated. Many fragments of partly carbonized 

wood are scattered through the silt beds, but no other megafossils 

are exposed, and no microfossils were found in one composite sample 

containing material from a dozen different beds throughout the section. This series of relatively uniform beds of fine material grades 

conformably upward into about 150 feet of less well bedded mediumsize to coarse sand with irregular layers of well-rounded gravel. 

The dip of these beds diminishes to 7° or 8° SE. at the top of the 

section.

The base of these beds is not exposed. Their lateral terminations, 

both to the northwest and southeast, are poorly exposed but appear 

to be steep fault contacts against the Amchitka formation. The top 

is truncated by a nearly horizontal erosion surface about 40 feet 

above sea level, overlain by fossiliferous beach sediment of Pleistocene 

age. The deposit apparently is part of a small fault block, tilted 

and dropped between blocks of resistant rock of the Amchitka 

formation. The upper layers of coarser material may have been 

deposited after some of the tilting and faulting had taken place.

INTERGLACIAL BEACH DEPOSIT AT SOUTH BIGHT

At the head of South Bight, about 75 feet of poorly bedded loose 

f ossiliferous coarse sand and gravel lies conformably on the erosion 

surface that truncates the lower tilted beds of sand and gravel. 

Inland, this fossiliferous gravel is covered with clifftop dune sand 

except in a gravel pit back from the north end of the cliff. At the 

north edge of this excavation, the crudely bedded sand and gravel 

grade into a gravel beach deposit with seaward-dipping streaks of 

sand and concentrated shell fragments. The highest exposed beach 

material, at an altitude of 135 feet, is overlain by spoil from the 

gravel pit. In the west wall of the excavation, shell-bearing coarse 

sand and fine gravel are covered by several feet of rounded to 

angular cobbles and boulders, many of them of mica schist and 

biotite granite entirely foreign to the bedrock exposed in the Aleutian 

Islands. Several of these erratic blocks and boulders are scattered 

on the highest exposed beach material.

Foraminifera, ostracodes, and several mollusks collected from the 

raised-beach material indicate that it is of Pleistocene age. It is
543

probably interglacial, and perhaps the youngest interglacial age, 

inasmuch as the deposit has not been removed by erosion.

Foraminifera. This fauna was reported by J. A. Cushman and 

Ruth Todd (1947), who compared the collection with that of Timms 

Point, Calif., and concluded that ". . . the fauna would seem to be 

either Pleistocene or Pliocene in age, probably the latter." In a 

letter on this subject (Jan. 7, 1955), Miss Todd said:

I think that most foram workers are now generally agreed on the Pleistocene, 

not Pliocene, age of the Timms Point material as indicated by the mollusks, 

and I am sure that at present Pliocene would not be preferred over Pleistocene 

for this Amchitka deposit. The 1947 report states that While the fauna is 

decidedly one of cold waters, it is not by any means arctic, and most of the 

species found today are in waters to the south of this area. Certain of the 

species, those of Quinqueloculina and Cibicides, are known from nearshore deposits and comparatively shallow water. The great abundance of the two species of Globigerina, pelagic forms, would indicate that ocean currents from 

warmer areas, such as the present Japan current, also influenced this area. 

It would seem probable, therefore, that the fauna was deposited in shallow 

water, nearshore, and on an exposed coast.

Ostracodes. Dr. Fred M. Swain examined and reported as follows 

on ostracodes from the same material that contained the Foraminifera :

Bythocypris sp.

Bairdia cf. inflata (Norman)

Cythereis sp. aff. C. angulata (G. O. Sars)

Cytheropteron, sp. nov.?

Eucythere"! sp. or new genus

New genus aff. Paracytheridea

It is difficult to determine the age accurately, but at present I see no reason 

to believe the deposit is earlier than Pleistocene.

Mollusks. The mollusks were identified by R. B. Stewart and 

F. S. MacNeil as follows:

Balanus sp.

Astarte actis (Dall)

Panomya ampla (Dall)

Pecten (Chlamys) n. sp.? aff. albidus Dall

Pecten (Chlamys) beringianus (Middendorf)

Pecten (Chlamys) islandicus (Mtiller)

Venericardia (Cyclocardia) aff. crassidens (Broderip and Sowerby)

Epitonium greenlandicum (Perry) n. var. incomplete

This small marine fauna, probably from fairly shallow water, could be of 

Pleistocene age or possibly uppermost Tertiary. A. leffingwelli (Dall) has been 

reported from no locality younger than the Nome Inter-beach.

GLACIAL DEPOSITS

Till is found at three places and gravel of glacial origin at several 

others. As a group these deposits are believed to be of glacial
544 INVESTIGATIONS OF ALASKAN VOLCANOES

origin because in a few places they lie on ice-polished and icestriated surfaces, they contain fragments of metamorphic rock and 

mica granite rock types not found in place in any of the Aleutian 

Islands, and some of the cobbles and boulders in the till are striated 

and many are faceted.

A small patch of till was exposed just above a 300-foot altitude on 

the crest of Amchitka about 1% miles west of Banjo Point, but it 

was later destroyed during road construction. Along the northwest 

shore of Constantine Harbor, in the upper part of the sea cliff, three 

thin sheets of bouldery till are separated by irregular lenses of 

sorted cobbles. The material of the till is mostly local country rock, 

much of it broken from adjacent outcrops of the thick obsidian tuff 

breccia, but some is lava from more distant localities and some is 

quartz diorite of the type now exposed in largest quantity at East 

Cape. Several of the boulders are of exotic rock types. The till is 

well cemented probably by secondary minerals weathered from the 

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

larger body of till inland from the head of Constantine Harbor is 

composed mostly of fine material with cobbles and boulders, some 

exotic and a few striated, scattered throughout the mass. The 

abundant fine matrix material apparently was derived from beds 

of fine sand and silt that underly the till.

Two deposits in a graben due west of Kirilof Point and two on 

the island crest southwest of Ivakin Point contain sand and gravel 

(with some exotic rock types), some poorly sorted and some well 

sorted. Most of the material in these deposits has been excavated 

for construction use, leaving few outcrops undisturbed. In 1946, 

the deposit south of Ivakin Point was 3 to 6 feet thick and consisted 

of unfossiliferous unbedded gravel containing less than 10 percent 

sand. Fifty percent of the gravel particles exceeded 1 inch in 

diameter. A few boulders up to 5 feet in diameter were subangular, 

and those up to 1 foot were subrounded to rounded, particularly 

those of quartz diorite, which was very common. These deposits 

are considered to be of glacial origin because of the presence of the 

exotic rocks, but it is not known how much of the washing 

and sorting of the material accompanied the deposition, or how 

much, if any, may have been caused by marine reworking subsequent 

to deposition.

Unconsolidated fine sand and silt, in a varvelike arrangement of 

thin beds, is exposed almost continuously across the island in the 

graben depression connecting the heads of Makarius Bay and Constantine Harbor. The total thickness of these beds is not known. 

The highest outcrop against the south wall of the graben is about 

75 feet above sea level, and the bottom is not exposed in the deepest
GEOLOGY AND PHYSIOGRAPHY OF AMCHITKA ISLAND 545 >

> cuts. In most exposures these beds are overlain unconformably by 

gravel or till at a height of 50 to 75 feet.

 ^ In a borrow pit 3,000 feet east of the head of Makarius Bay, a 

50-foot section of the bedded sand and silt is capped at an altitude 

of TO feet by glacial till and a few patches of water-laid sorted sand. 

At one exposure in this area, a thin lens of pebble conglomerate 

breaks the orderly, varvelike alternation of beds about 20 feet below 

the top of the section; this break cannot be traced into other areas.

Another borrow pit, 1,000 feet to the north across the graben, 

exposes bedded silt and sand overlapping rock of the Banjo Point 

formation. The loose bedded silt and sand dips about 4° E., and the 

^ older bedded rock dips 25° in a S. 20° W. direction. The surface of 

> the older rock, recently exposed by quarrying, shows glacial polishing and striae that plunge about 25° E. on the polished face that 

dips 30° SE.

In another large borrow pit, 1,000 feet inland from Constantine 

Harbor along the north wall of the graben, the bedded sand and silt

 dip as much as 20° E. in a section broken by many minor faults.

i The most conspicuous feature of these deposits is the regular 

interlayering of thin beds of silt and sand. In detail, a coarse layer

^ grades into the overlying fine layer, but the change from fine to 

overlying coarse is abrupt; the deposit is a series of graded beds 

rather than alternating beds of coarse and fine material. The 

graded beds range from 14 to 1 inch in thickness. The material in

A. the coarser part is fine to very fine sand; that of the finer part is 

about a half very fine sand, a third silt, and a sixth clay size. The 

clay-size material, in a laboratory test, remained in suspension in

£ cold water only a few hours. The mineralogy of grains larger than 

clay size, determined by K. Q. Lewis, is:

50 percent plagioclase grains, rounded edges

i 30 percent rock aggregate, largely feldspar, magnetite, and chlorite 

10 percent chalcedony and other cryptocrystalline material 

5 percent quartz, angular-edged fragments 

5 percent heavy minerals, in order of abundance, green hornblende,

 J brown hornblende, magnetite, biotite, muscovite, augite, and green 

chlorite.

No rocks exposed in any of the western Aleutian Islands contain 

muscovite, and biotite is very rare; all the other constituents could 

be of local origin. Both types of mica present in the sand probably 

came from the same source that furnished the cobbles and boulders of 

schist and mica granite found as erratics in the till and gravel.

Diatoms are the only fossils found in these beds. The following 

were identified by K. E. Lohman:
546 INVESTIGATIONS OF ALASKAN VOLCANOES

[R, rare; F, frequent; C, common]

Arachnoidiscus cf. A. ehrenbergii Bailey; R.

cf. A. longi Brown, found in beds near Jeremie, Haiti, possibly as old as 

Eocene; R.

oamaruensis (Schmidt) Brown found in Oligocene near Oamaru, New 

Zealand, and lower middle Miocene near Nanggoeland, East Java; F.

sp.; F.

Aulacodiscus cf. A. nigricans Tempere and Brun, found in Pliocene near Sendai, Japan; R.

sp.; R.

Campylodiscus sp.; C. 

Endictya cf. E. oceanica Ehrenberg; F.

robusta (Greville) Hanna and Grant; R.

sp.; F.

Orammatophora sp.; R. 

Hyalodiscus sp.; R. 

Isthmia nervosa Kutzing; C. 

Melosira cf. M. sol (Ehrenberg) Kutzing; C.

sp.; F.

Pseudopyxilla baltica (Grunow) Forti; R. 

Rhabdonema arcuatum var. robustum (Grunow) Hustedt; F.

biquadratum Brun, found in Pliocene near Sendai, Japan, and lower Pliocene Sisquoc formation in Purisima Hills, Calif.; F.

valdelatum Tempere and Brun, found in Pliocene near Sendai, Japan; F.

sp.; F. 

Triceratium sp.; F.

This is an amazing assemblage of short-ranging diatoms representing Oligocene, Miocene, Pliocene, and Recent species. The shattered and eroded condition of the individual valves strongly suggests reworking of older beds. The 

assemblage is almost certainly no older than middle or early Pliocene since it 

contains two species not known from older rocks. It could be early Pleistocene, with exposures of older diatom-bearing sediments contributing debris and 

diatoms to the deposits.

The absence of nonmarine diatoms indicates that the original diatom-bearing sediments were deposited under marine conditions. However, it is possible 

that the present deposit is entirely reworked, and could have been deposited 

under nonmarine conditions with no contemporaneous diatoms (nonmarine) 

present. .~ rv

The age of this deposit probably is Pleistocene since it lies'on-a 

glaciated surface and is covered by glacial gravel and till, confirming Lohman's conclusion that a large part of the diatom assemblage 

is reworked from older sediments. The physical characteristics of 

the sediments and the geography of the deposit limit the choice of 

possible conditions under which deposition could have taken place. 

The thin, even beds must have been deposited in quiet water, undisturbed by wave action or strong currents. The grading within 

each bed suggests sorting by settling of a sedimentary load dumped 

periodically into a basin. Probably the deposit was accumulated in 

a local pond, largely of glacial melt water, and derived almost 

wholly from ice-transported material.
'.«.*!

GEOLOGY AND PHYSIOGRAPHY OF AMCHITKA ISLAND 547

MANTLE

A mantle, from a few inches to a few feet thick, of broken but 

undecomposed fragments of the local bedrock is present over most 

of the island. In places, silt and clay particles fill the interstices 

in the upper part of the layer of loose fragments. This layer is 

thickest on bedrock that is transected by closely spaced joints and 

bedding planes, and thin or absent on rock with widely spaced 

joints and no bedding planes. It is inferred that frost splitting has 

been the most active agent in forming this residual mantle. 

" Iri a few places, favorable both to undisturbed deposition and 

preservation, thin beds of very fine volcanic ash give evidence that 

the island has been showered by material from postglacial eruptions 

of nearby active volcanoes. On most of the island surfaces such 

material has been incorporated in the mantle of turf and is not 

identifiable as bedded ash.

STRUCTURE 

REGIONAL

Amchitka, Rat, and Kiska Islands are located along the northeast 

edge of a segment of the Aleutian Ridge (Gibson and Nichols, 1953, 

pi. 1) that might be considered, in a general way, as a great fault 

block tilted southwestward toward the Aleutian Trench. In profile, 

across the west end of Amchitka, the north face of the block drops 

1,500 feet in 7,500 feet (about 1,075 feet per statute mile), and the 

top surface of the presumed block descends southwestward 3,000 

feet in a distance of 80,000 feet (about 200 feet per statute mile), 

then drops 7,800 feet in 50,000 feet (about 830 feet per statute mile) 

to the intercept with the Aleutian Bench (Gates and Gibson, 1956, 

p. 143).

This large tilted block is composed of rocks that are greatly deformed by block faulting (summarized for Amchitka in paragraphs 

following), so the inferred top surface of the block must be a plane 

ofe'"e'p9sion, perhaps related to the ridge shelf, defined and described 

by Gates and Gibson (1956, p. 133). This tilted surface that forms 

the upper part of the south insular slope is crossed by several scarps 

and canyons that may be the margins of blocks dislocated by cross 

faults in the supposed large block. Adjacent to Amchitka Island, 

these features can be traced across the surface of the eastern part 

of the insular shelf and into the block and graben shapes of the 

island in the vicinity of Constantine Harbor; similar features to the 

west cannot be traced across the insular shelf.

LOCAL

On Amchitka Island many lines of topographic features such as 

elongated pond depressions, straight valleys, and straight canals
548 INVESTIGATIONS OF ALASKAN VOLCANOES

through the shore bench are prominent on aerial photographs and 

have been indicated on plate 69. Those lines known or presumed 

from the geology to represent the outcrop trace of softer layers of 

interbedded rock are omitted; those shown probably represent traces 

of fracture zones, joints, and faults. Some of the faults located in 

outcrop lie on such a line of physiographic features, but many 

observed faults are not marked by such features.

All the mapped faults and attitudes of disturbed beds seem 

adequately explained as resulting from normal faulting on planes of 

moderate to steep dip. In the western part, fault strikes are about 

equally distributed between northwest, north, and northeast; in the 

east half of the island, north and northeast directions predominate. 

In contrast, most of the dikes of the youngest volcanic rock and 

the zones of pyrite enrichment strike within a few degrees of east, 

a direction that is not well represented by the mapped lineations or 

fault strikes.

Faults, both observed and inferred, are most numerous and structural disturbance is the greatest, in the rocks of the Amchitka 

formation. Glacial deposits are involved in fault movements along 

the Constantine Harbor graben. At the head of the harbor, in the 

cliff of both the north and the south shore, are 2 faults that strike 

N. 80° E. and dip about 75° toward each other. On each fault 

the graben block has dropped, bringing till into fault contact with 

tuff-breccia of the Amchitka formation. About 3,000 feet westward 

from the harbor shore, along the south wall of the graben, a vertical 

fault cuts fluvioglacial sand and gravel and the north block has 

dropped at least 30 feet. Overlying the fault, without displacement, 

is about 2 feet of the uppermost silty till. This movement apparently took place during the last glacial activity. Movement after 

late interglacial time and before the last glaciation took place along 

a fault that strikes east of north from South Bight, if the inference 

is correct that both the raised beach at an altitude of 135 feet at 

South Bight and the raised marine platform and cliff at 240 feet 

west of South Bight are features of the shoreline of a late interglacial stand of the sea.

The region is subject to earthquakes, but no physiographic traces 

of very recent fault movement are to be seen on the ground or on 

aerial photographs of the island.

At least 2 uplifts separated by 1 submergence, with respect to 

sea level, are inferred from the stratigraphy. The oldest rocks were 

all deposited below sea level and consist entirely of unreworked 

volcanic material. The next younger formation contains rocks deposited in both submarine and subaerial environments, including 

beds of detrital material derived from erosion of rock of the oldest 

formation. Subaerial lava flows of the third younger formation
GEOLOGY AND PHYSIOGRAPHY OF AMCHITKA ISLAND 549

that appear to extend below present sea level are covered in part 

by marine conglomerate that is now as much as 600 feet above 

sea level.

GEOLOGIC HISTORY

The oldest deposits (the Amchitka formation) are andesitic rocks, 

products of explosive and effusive submarine volcanic activity, with 

no land sufficiently near to contribute products of erosion. It is 

believed that this sea was located on a continental rather than an 

oceanic segment of the earth's crust, because all the igneous rocks 

of Amchitka resemble chemically the suites of andesitic rocks distributed around the margin of the Pacific Ocean.

Uplift, with some deformation, brought part of this submarine 

volcanic deposit above sea level. Products of erosion of this land 

are interbedded with water-laid tuff and, rarely, a lava flow (the 

Banjo Point formation of Oligocene or Miocene age). These 

volcanic rocks are basaltic, but relatively rich in plagioclase. Both 

the older andesite and the basalt are represented by cobbles in the 

beds of conglomerate. The assemblage of fossil Foraminifera 

suggests an environment of disconnected marine lagoons partly 

sheltered from the open ocean; the abundance of conglomerate suggests shoals and numerous small islands. Tension fractures formed 

during this time are represented by dikes of basalt that strike 

generally between northwest and northeast. After an unknown 

lapse of time, small stocks, sills, and dikes of quartz diorite were 

emplaced in the rocks of both of the preceding formations.

Eruption of andesitic lava was resumed (the Chitka Point formation) after an unknown lapse of time, and in the region of western 

Amchitka at least one volcanic pile was built subaerially to a 

height of more than a thousand feet. Dikes of similar andesitic 

material trend within a few degrees of east, as do lines of hydrothermal alteration and zones of introduced pyrite. However, most 

of the alteration and pyritization is found in rock older than the 

Chitka Point formation, so it is inferred that the east-trending lines 

of weakness and pyritization are associated with the preceding 

plutonic activity but persisted through the time of younger Chitka 

Point volcanism. The age of this youngest volcanic activity on 

Amchitka is unknown.

The region was next submerged, and marine conglomerate was 

derived from and deposited on the subaerial Chitka Point formation. 

(Some of this conglomerate is now found as much as 600 feet above 

the lowest exposures of subaerial Chitka Point formation at present 

sea level.)

Erosion reduced the surface of Chitka Point formation to low 

relief, perhaps a completely submerged wave-cut shoal; it is not 

known whether the oldest erosion surface (now above 1,100 feet in
550 INVESTIGATIONS OP ALASKAN VOLCANOES

altitude) on the present island was cut by submarine or by subaerial 

erosion. The total submergence of these subaerial volcanic rocks, 

over 800 and perhaps over 1,100 feet, seems to be too great to attribute to a rise of sea level; perhaps all of the submergence should 

be considered as resulting from structural depression.

Uplift followed this episode of marine planation, and the merging 

island was benched by shoreline erosion at 2 levels (at the present 

altitudes of 500 and 800 feet). These shorelines may represent 

pauses during structural uplift, or possibly fluctuation of sea level 

in early Pleistocene time. As the island emerged, vigorous stream 

erosion established the pattern and major dimensions of the present 

valleys and ridges. The pattern extends below the present sea level, 

attesting that some stream erosion took place during a time of lower 

sea level, possibly a time of glacial maximum.

It is inferred that the structural uplift halted before late Pleistocene time and that the shelf-break bench was eroded at a low sea 

level of a middle Pleistocene glacial maximum, coincident with the 

erosion of the now-submerged stream pattern. Benches and shoreline cliffs also were eroded at a high sea level of at least the last 

interglacial stage.

Block faulting occurred on a small scale between the last interglacial and the latest major glacial maximum. In the eastern part 

of the island, segments of the interglacial shoreline were displaced 

to altitudes ranging from 135 to 240 feet, and the shelf-break bench 

was offset from a depth of 325 feet to as much as 390 feet. During 

the lowered sea level of the last major glacial maximum, shoreline 

features were cut around the island at a depth of about 165 feet; 

these features apparently have not been offset by faulting.

Postglacial stream erosion has modified the glaciated topography 

very little, but marine erosion has made great changes in the shoreline. A good deal of the present sea cliff marks the position of a 

shoreline cut at a sea level about 6 to 8 feet higher than the present 

shoreline. At least 4,000 years ago, before early Aleut inhabitants 

built their dwellings on many of its storm beaches, this shoreline 

was abandoned by a slight lowering of sea level.

The most recent marine erosion has modified much of the wave-cut 

bench and beach deposits of the abandoned shoreline and, in 

favorable localities, has cut a new sea cliff.

The region is still structurally unstable as attested by frequent 

earthquakes, but no recent earth movement has been sufficient to 

form visible scarps or offsets in present topography.
GEOLOGY AND PHYSIOGRAPHY OF AMCHITKA ISLAND 551

REFERENCES CITED

Arctic Weather Central, 1950, Climate, weather, and flying conditions of Alaska 

and Eastern Siberia: llth Weather Squadron, Elmendorf Air Force Base, 

Alaska.

Arnold, Ralph, 1906, The Tertiary and Quaternary pectens of California: U.S. 

Geol. Survey Prof. Paper 47, 264 p.

Atwill, E. R., 1935, Oligocene Tumey formation of California: Am. Assoc. 

Petroleum Geologists Bull., v. 19, no. 8, p. 1192-1204.

Bartrum, J. A., 1926, "Abnormal" shore platforms: Jour. Geology, v. 34, p. 

793-806.

Byers, F. M., Jr., 1959, Geology of Umnak and Bogoslof Islands, Aleutian Islands, Alaska: U.S. Geol. Survey Bull. 1028-L, p. 267-365.

Coats, R. R., 1956, Reconnaissance geology of some western Aleutian Islands: 

U.S. Geol. Survey Bull. 1028-E, p. 83-100.

Cushman, J. A., and Todd, Ruth, 1947, A foraminiferal fauna from Amchitka 

Island, Alaska : Cushman Lab., Contr., v. 23, no. 297, p. 60-72.

Dietz, R. S., and Menard, H. W., 1951, Origin of abrupt change in slope at 

continental shelf margin: Am. Assoc. Petroleum Geologists Bull., v. 35, 

p. 1994-2016.

Gates, Olcott, and Gibson, William, 1956, Interpretation of the configuration 

of the Aleutian Ridge: Geol. Soc. America Bull., v. 67, p. 127-146.

Gibson, William, and Nichols, Haven, 1953, Configuration of the Aleutian Ridge, 

Rat Islands Semisopochnoi I. to west of Buldir I.: Geol. Soc. America 

Bull., v. 64, no. 10, p. 1173-1188.

Moore, R. C., and Yokes, H. E., 1953, Lower Tertiary crinoids from northwestern Oregon: U.S. Geol. Survey Prof. Paper 233-E, p. 113-147.

Nansen, Fridtjof, 1922, The strandflat and isostacy: Christiana, p. 1-313.

Scruton, P. C., 1953, Marine geology of the Near Islands Shelf, Alaska: U.S. 

Geol. Survey open-file report, p. 1-200.

Todd, Ruth, 1953, Foraminifera from the lower Tertiary of Amchitka Island, 

Aleutian Islands: Cushman Found. Foram. Research, Contr., v. 4, pt. 1, 

no. 70, p. 1-7.

Weaver, C. E., 1942, Paleontology of the marine Tertiary formations of Oregon and Washington: Washington Univ. Pub. in Geology, v. 5, pt. 1-3, 789 p.

INDEX

Page

Acknowledgments_ _... .... ..... 522

Aleut Point....________________ 532

Aleutian Bench. ... 547

Aleutian Ridge... 547

Aleutian Trench_._______________ 547

Amchitka formation, age... . 533,549

dikes and stock___ .. . . 539

faults..._______._._______ 542,548

origin.....________________ 533,549

pyrite.... 541

spectrographic analyses . . 534-535

Amchitka Island..._____.___ 532,533,544,547

Amphitheatre valleys....________ 524-525,532

Analyses, table_____ . 534-535

Andesite, hornblende.._ ..... 526,539,541

porphyritic.____._________-_ 536,540

subaerial and submarine______...... 549

Banjo Point______________ 527, 536,537,544 

Banjo Point formation, age and description_ 533,

536-539, 549

dikes________ ... 539

pyrite.__ . 541

Beach deposits, above present sea level .... 528,

530,542,549,550 

age..... .__.___.- __ . 542-543

at present sea level___-____ 528,530,550

Beach ridges.._________________ 530

Benches, above sea level________ 525,532.550

at sea level...... 527,528,530,550

below sea level___________ 531,532,550 

Bering slope.. . 532 

Bibliography________._......_ 551

Bird Cape._._...... 525

Chitka Cove.._____________ 524,536,541

Ohitka Point.____________ 524,528,539,541

Chitka Point formation, age________ 539,549

analyses 534-535,540-541

description,.____....._.. .. 539

origin.....__________________ 549-550

pyrite.____._ ... ...._ _ 541

Climate..___________________ 524,528 

Cobble beach deposits____________ 528 

Colluvium.-________..._ 525 

Constantine Harbor__.___ 527,534,544,545,547 

Constantine Harbor graben.._ __ 548 

Cyril Cove_______.______ 524,527,533,536

Depressions, above sea level__ __ 526,547-548 

below sea level._____________ 531,532

Dikes, composition__________ 533,537,539,549 

strike...___________________ 548,549

Drainage divide_______________ 524,525

Dry bench. __________________ 528

Page

East Cape. ________________ 544

Erosion, frost__________________ 528,547

glacial_________________ 526,527,532

marine- _____ 528,530,532,550

stream....____________ 526,527,531,550

Fault blocks...._________ 624,531,542,547,550

Faultline scarp________________ 532 

Faults, Amchitka formation.__............ 536

strike_____________________ 648 

Fossils....__________________ 537,543,546

Fractures ....._ ___.___ . 527,548, 549

Qrabens-_____________ 531,544, 545.547,548

Gravel, beach...._________. 526,539,541,542

glacial__________________ 543-546

Hornblende andesite.____________ 526,541

Insular shelf.._______.....______ 531,547

Insular slope... 532,547 

Ivakin Point ._________________ 544

Joints________.____________ 548

Kirilof Point...______________ 534,537,544 

Kiska Island..________________. 547 

Knobs . . 525,526, 527,528, 530,531

Location and extent of area______.__ 522 

Lohman, K. E., quoted..., . ... 546

MacNeil, F. S., quoted... . . ... 537 

Makarius Bay....________ 526,527,536,541,545

Near Islands._________________ 630 

Near Islands platform______________ 532

Platforms. _________________ 628,530,548 

Ponds_________________.__ 626, 547-548 

Pyrite..______._____________ 541,548

Quartz diorite, age and description........ 539

dikes and stock_____________ 539,549

Rat Islands.__________________ 632,547 

Ridge shelf._______________ 647

Ridges-....________________ 528,531,532 

Roches moutonnfees- _________-_-. 525,526 

Rock bench________________ 527,528,530

Saint Makarius Point____________ 536-637 

Sand, age_____________________ 542,546 

glacial___________________ 544,545,548 

Sea cliffs _______________ 527,528,550 

Sedimentary rock.. ___.......... 526,541-542

Shelf-break bench..___________. 531,532,550 

Shoal, Chitka Point formation________ 549

553
554 INDEX

Sill.- .

Silt. .... ... .

ftfrnMrQ

Streams .

Talus .

Tarns __ _. _ - _ .

Page 

___________ 549

_________ 546,547

__ 533,539,541,542,548

_______ 533,539,549

__________ 528

.. _____ 526-527, 532

__ __ . ______ 526

. __ . __ ........ 525

Till.... - ____ ..

Unconformities .......__

Valleys- __ .. ._.. .

TV^ct bencn

Windy Island _____

Page 

... __ .. 544-545,548

. ____________ 524

. ______ 537, 539, 540, 545

. ______ 525-526, 531, 532

.- _ 524,525,527,528,530

 _________ 524.525

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