Please do not destroy or throw away this publication. If you have no further use 

for it, write to the Geological Survey at Washington and ask for a frank to return it

UNITED STATES DEPARTMENT OF THE INTERIOR 

Harold L. Ickes, Secretary

GEOLOGICAL, SURVEY 

W. C. Mendenhall, Director

Bulletin 857-D

NOTES ON THE GEOLOGY OF THE

ALASKA PENINSULA AND

ALEUTIAN ISLANDS

BY

STEPHEN R. CAPPS

Mineral resources of Alaska, 1932 

(Pages 141-153)

UNITED STATES

GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1934

For sale by the Superintendent of Documents, Washington,' D.C. .----. Price 5 cents
CONTENTS

	Page 

Abstract ____________________________ _____________________________ 141

Introduction__ .__.-.__.___________L_____________..V -__..-__..--_ 141

Geography _._-_..__--______-_-__.__-__.__--__--__-___-_._--_-.___ 142

The rocks.________________________-_.__,______... 143

Glaciation_._.._-_._..--____._____-_________--____-__________--_ 146

Local observations-________________________________________________ 147

Unalaska Island__________---___^______-____--__---___--_____- 147

Atka Island..___...___..._..._......._._..__._______..______. ' 149

Adak Island...___________________________________ 149

Kiska Island_______---___--___--____---_-__-_---_--______-_-. 150

Attu Island.____________________________________________ 151

Cold Bay and Lenard Harbor___-_..____________-__.__..______ 152

Dolgoi Island and Pavlof Bay.___________________ ______________ 152

ILLUSTRATION

Page 

PLATE 5. Map of the Alaska Peninsula and Aleutian Islands.______-__- 146

n
NOTES ON THE GEOLOGY OF THE ALASKA PENINSULA 

AND ALEUTIAN ISLANDS

By STEPHEN R. CAPPS

ABSTRACT

During the spring of 1932 an opportunity was offered by the United States 

Navy for a geologist to accompany an expedition organized to make a reconnaissance of the western part of Alaska Peninsula and the Aleutian Islands. 

This expedition visited several localities the geology of which was little known. 

It was found, as had already been expected, that the islands west of Unimak 

Pass are composed mainly of basic volcanic lavas and fragmental materials, 

into which have later been injected dikes, sills, and considerable masses of 

intrusive rocks, some of which are of acidic types and of granitic texture. 

These westward islands are bordered both to the north and south by depressions 

2,000 fathoms or more in depth, and the islands have apparently been built up 

from that depth by the ejection and extrusion of volcanic materials since early 

Tertiary time. No rocks of proved pre-Tertiary age were seen, and the only 

sedimentary materials present may well have been derived from the erosion of 

the volcanic islands after they were built up above sea level. On the Alaska 

Peninsula pre-Tertiary sediments through which the volcanic materials broke 

to the surface are abundantly present.

There,is evidence that all the larger islands and the higher portions of the 

peninsula were severely glaciated during Pleistocene time. Each of the larger 

islands was the center of ice accumulation and dispersal, and the present 

topography, except upon recently active volcanic cones, shows strongly the 

effects of glacial sculpture.

In the spring of 1932 an expedition was organized by the United 

States Navy for reconnaissance hydrographic work in the Alaska 

; Peninsula and Aleutian Islands. This expedition, in command of 

Lt. K. H. Harrell, sailed from Bremerton, Wash., April 20 on the 

U.S..S. G(mnet, a mine sweeper, and proceeded northward by way of 

southeastern Alaska. In addition to the regular complement of the 

ship the expedition included two amphibian planes with the necessary 

officers, pilots, and mechanics. Permission was extended by the Navy 

to the Geological Survey for a geologist to accompany the expedition, 

and it was the writer's good fortune to be assigned to that work. 

Inasmuch as the planes were flown as far northward as Seward, the

59018 34 141
142 MINERAL RESOURCES OF ALASKA, 1932

Gannet accompanied them to that point, with few stops. From 

Seward westward the planes were carried aboard ship to Unalaska 

and from that time forward whenever long moves were made. The 

main purpose of the expedition was to make a reconnaissance air 

survey of the shores of the Alaska Peninsula and the Aleutian Islands. To accomplish this the Gannet was moved to harbors at 

intervals of a few hundred miles, and the shores between those harbors were examined and photographed from the air.

It was recognized in advance that the program of work outlined 

by the Navy would give the 'geologist opportunity to visit only a 

few scattered localities and that even at those places the stops would 

be short and the chances for geologic. work limited. Nevertheless, 

it was believed that whatever information could be gathered would 

be valuable, particularly as most of the area to be visited was difficult 

of access, and little was known of its geology. The following notes 

on the geology of the region have little ^areal significance, for through-, 

out the region protected harbors are few and the coast line in general 

is rugged and cut into sea cliffs by the waves, so that landing by 

small boat is dangerous or impossible in ordinary weather. The 

writer therefore had opportunity to visit only the immediate shores 

of the harbors in which the ship was anchored. A large number 

of flights by plane were made, however, and these, though failing to 

yield detailed geologic data, served to broaden the understanding 

of the region as a whole. The writer wishes here to acknowledge 

his indebtedness to the officers of the expedition, w:ho on .all occasions 

assisted him in every possible way and whose Unfailing courtesy 

and friendliness made the summer one to be remembered with pleasure. The microscopic study of the thin section of rocks collected 

on this expedition was made by J. B. Mertie, Jr., and he determined 

the rock types that will be referred to here.

GEOGRAPHY

The Alaska Peninsula and the Aleutian Islands form one of the 

great structural festoons with which the Pacific Ocean basin is bordered (pi. 5)-. This festoon is punctuated at short intervals .by 

volcanoes either still active or so young that erosion has not yet de^ 

stroyed their characteristic shape. From Mount Spurr, on the north, 

the northernmost vent that has been recognized, to Attu Island, on 

the west, a distance of 1,575 miles along the curve of the arc, there 

are active or moderately young volcanoes at intervals not greater 

than 110 miles, and for most of the distance they are separated by 

only TO miles or less. Throughout the Alaska Peninsula the volcanoes 

have broken out through older sedimentary or igneous rocks, by which 

they are now flanked. In the Aleutian Islands there are few if any
GEOLOGY OP ALASKA PENINSULA AND ALEUTIAN ISLANDS 143

exposures of the basement rocks, and the islands are largely constructional, having been built up to and above sea level by the accumulation of lavas and volcanic fragmental material ejected from 

below.

Little oceanographic work has been done in the region off the 

Aleutian Islands, but enough soundings have been made on the Pacific si.de to show that there the water deepens to 1,000 fathoms within 

25 miles, more or less, and still farther south the bottom drops off 

to a deep that shows soundings between 3,000 and 4,000 fathoms. 

Bering Sea, northeast of a line passing from Unimak Pass just south 

of the Pribilof Islands, -i$ shallow, being all less than 100 fathoms 

deep, and much of it less than 50 fathoms. A line of soundings 

taken by the fathometer on the Ga/nnet in 1932, extending along an 

irregular course from a point north of Aniukta Pass to Attu Island, 

at varying distances from the intervening islands, shows that north 

of the islands the 1,000-fathom line lies close to the island festoon 

and that at a distance of 50 miles or more from them there is a remarkably smooth-floored depression at a depth of 2,000 to 2,200 

fathoms. The shape of this depression between the islands and the 

cpntinental mass, which includes much of Bering Sea, is not known, 

but it is significant that the island arc rises as a sharp ridge separating deeps of 2,000 fathoms or more both to the north and south.

THE BOCKS

In the various localities visited by the writer in the Aleutian 

Islands in 1932 no sediments or other basement rocks were seen that 

are definitely older than the volcanic rocks of which the islands are 

chiefly composed. So far as is known, the beginning of volcanism 

along the great arc occurred in Eocene time. Knappen l reports that 

the Eocene sediments of the Aniakchak district grade upward into 

stratified bentonitic clays and volcanic sediments, and Capps 2 found 

volcanic materials in Eocene beds near Mount Spurr. Thus it is 

shown that volcanism in the belt including the Alaska Peninsula 

and Cook Inlet regions began sometime in the Eocene, and it seems 

fair to presume that volcanic activity in the Aleutian Islands began 

at about the same time. It is reported that sandstones and shales 

occur in places in the Aleutian Islands, and indeed sedimentation 

must have been in progress around the borders of each island ever 

since it first built itself up high enough to be attacked by waves or 

by subaerial erosion. It is possible also that considerable blocks of 

the basement rocks were carried upward by the up-welling magmas

1 Knappen, R. S., Geology and mineral resources of the Aniakchak district: U.S. Geol, 

Survey Bull. 797, p. 195, 1929. 

3 Capps, S. R., The Mount Spurr region': U.S. Geol. Survey Bull. 810, p.- 161, 1930.
144 MINERAL RESOURCES OF ALASKA, 1932

or were ejected as fragments during periods of explosive violence, 

but the impression gained by the writer is that the islands west of 

Unimak Pass are composed very largely of materials of volcanic 

derivation and lavas, tuffs, and intrusive rocks injected into these 

volcanic materials and secondarily of sedimentary materials derived 

from these volcanic rocks by erosion.

Sedimentary rocks of Tertiary age are known to occuf at intervals 

along the Alaska Peninsula and at a few places in the Aleutian 

Islands, and they have yielded collections of invertebrate fossils and 

of plant remains. In the present investigation such rocks were seen 

at Unga Island and in Pavlof Bay, where ordinary sandstones and 

shales occur. In the Aleutian Islands most if not all of the Tertiary 

fossils have been collected from volcanic tuffs that are certainly 

younger than the beginning of volcanic activity at those places. 

Among the localities where such fossiliferous Tertiary materials 

occur may be listed Umnak,3 Unalaska, Akutan, and Atka Islands.* 

Atwood 5 lists a number of fossiliferous localities on the Alaska 

Peninsula that have yielded both invertebrate and plant remains, 

which were assigned to the Eocene. More recent studies of the invertebrate fossils, however, point to an age within the Tertiary 

younger than Eocene. At any rate, inasmuch as these fossils are'all 

of Tertiary age and as volcanism in this area is known to have begun 

at least by early Tertiary time, these fossiliferous beds do not necessarily represent basement rocks through which the volcanic materials 

found their way to the surface but more likely are sediments deposited on the flanks of preexisting land masses. On the Alaska Peninsula there is abundant proof that a pre-Tertiary land mass existed, 

and that the volcanoes there broke through Jurassic and Cretaceous 

sediments. In the Aleutian Islands, so far as the writer knows, 

there are no unquestioned occurrences of pre-Tertiary rocks, and it 

appears likely that those islands did not come into existence until 

they were built up to the surface of the ocean by volcanic activity, 

probably in early Tertiary time.

Many writers who have discussed oceanic volcanic islands have 

assumed that the presence on them of rocks of granitic composition 

and texture indicates that these granitic rocks are a part of the basement through which the vents broke and are older than the volcanoes 

themselves. The facts observed in the Aleutian Islands seem to cast 

some doubt upon this assumption, or at least to raise the question 

whether it is not possible for lavas of granitic or dioritic composition

8 Hulten, E., letter to author.

4 Grewlngk, Constantin, Beitrag zur Kentniss der orographischen und geognostischen 

Beschaffenheit der Nordwest Kiiste Amerikas, mit den anliegenden Inseln : Russ.-k. mineral. Gesell. St. Petersburg Verb., Jahrg. 1848-49, pp. 123-278, 1850.

5 Atwood, W. W., Geology and mineral resources of parts of the Alaska Peninsula: 

U.S. Geol. Survey Bull. 467, pp. 49-58, 1911.
GEOLOGY OF ALASKA PENINSULA AND ALEUTIAN ISLANDS 145

composing a part of the series of magmas from a volcano to be injected into earlier flows and tuffs, and there to cool under sufficient 

cover to yield typical granites or diorites. Such granitic rocks were 

observed on Unalaska Island, at the head of Captains Bay, where 

fairly coarse quartz monzonite and medium-coarse quartz-pyroxene 

diorite were collected. These coarse-grained intrusives have been 

interpreted as indicating the presence there of an old granitic terrane 

which existed before the volcanoes came into being and through which 

they broke their way to the surface. No opportunity was available to 

study the contact relations of these rocks with the associated volcanic materials, but inasmuch as most of Unalaska Island is unquestionally volcanic, it seems possible that the diorites are intrusive into 

the volcanic rocks, rather than that they are older than the volcanic 

activity. This inference is strengthened by the Occurrence on other 

islands of smaller intrusive masses of acidic composition and fairly 

coarse grain in places where there seems no reason to doubt that they 

represent intrusives into lavas and tuffs. Thus on Adak Island an 

andesite or latite porphyry was found intruded into volcanic tuff. 

On Kiska Island small masses of quartz latite cut basalts and basalt 

tuffs. On Attu Island sodic diorite is intrusive into basalts. The case 

for believing that the granitic rocks of the Aleutian Islands are of 

Tertiary age and therefore later than the beginnings of volcanic 

activity there is strengthened by the fact that granitic intrusive 

masses of monzonitic habit are numerous in southwestern Alaska, 

where they have been definitely determined to be of Tertiary age.

With the exception of the acidic rocks just mentioned, all the specimens collected in the Aleutian Islands are of basic composition and 

include a considerable variety of basalts, basalt porphyries, basalt 

tuffs, and obsidian.

 Although all the larger islands of the Aleutian chain are composed 

mainly of volcanic rocks, it is a notable fact that most of the active 

or comparatively young volcanic peaks rise near the north ends of 

the larger islands, or on the northern islands in the chain, whereas 

in general erosion has attacked the southern islands and the southern 

portions of the larger islands so vigorously as to destroy the conical 

shape of the vents, and indeed to make difficult the identification of 

the sites of the volcanoes that served as vents for the materials of 

which the islands are built. It thus appears that the earlier volcanoes tended to lie some distance south of the line upon which the 

present vents occur, and that there has been a slight northern migration of the line of activity.

The visitor cannot help being impressed by the striking linear 

arrangement of those vents that are still active or that still possess 

their characteristic geomorphic form. No volcanic cone was recog-
146 MINESAL EESOUBCES OP ALASKA, 1932

nized on the Near Islands, at the westward end of the arc, though 

the Semichi Islands and Buldir Island may be volcanic. But from 

Kiska Island eastward to Unimak Island and thence northeastward 

to Mount Spurr, west of Cook Inlet, the recognizable volcanic cones 

lie along a smoothly arcuate line with remarkable closeness. The 

only conspicuous departure from this irregularity is Bogoslof Island, 

which lies about.30 miles north of the line. It thus appears that 

this great chain of volcanoes lies along a single great rift or tear 

in the crust, with perhaps a short spur running, through Bogoslof. 

In the westward islands the rift lies toward the northern edge of 

the island festoon. At Unimak Island and in the lower part of 

Alaska Peninsula the volcanoes lie about centrally between the 

Pacific and Bering Sea, but still farther north the rift swings to 

the east of the peninsula and lies close to the Pacific littoral. This 

shift from the north edge of the islands to the northeastern part 

of the peninsula in no way affects the perfection of the great arc 

or tlie remarkable alinement of vents along it.

GLACIATION

All the larger and more mountainous of the Aleutian Islands 

that were visited, as well as the mountainous portions of the western 

part of Alaska Peninsula, show pronounced evidence of severe 

glaciation during Pleistocene time, and in a few places of high elevation on the peninsula valley glaciers still persist. Thus an ice 

stream flows southwestward along the base of the Aghileen Pinnacles, just west of Pavlof volcano, and its sapping of a jointed lava 

flow has produced the extraordinary ruggedness of the ridge on 

which the Pinnacles stand. At the time that area was seen from the 

air, in late June, the high mountains were so extensively covered 

with snow that it was impossible to make out the actual extent 

of glacial ice, though the glacier appeared to be several miles long. 

Another rather short glacier has its source in the crater of the 

enormous extinct Veniaminof volcano, west of Chignik, and flows 

southward to the base of that volcanic mountain to drain into the 

Pacific.

The small development of the existing glaciers, however, is no 

measure of the extent of the ice at the climax of the Pleistocene 

glaciation, for all those areas that reached an elevation of as much 

as 2,000 feet were able to hold perpetual snows and to generate 

glaciers. On the Pacific side of the peninsula these glaciers pushed 

down into the sea and developed a rugged, fiorded coast. On the 

Bering Sea side they pushed out onto the lowlands, and although 

the limits they reached have not been accurately determined, it 

seems certain that a. combination of glacial deepening of basins
U.S. GEOLOGICAL SUiiVEY

170° 175

BULLETIN 807 PLATE 5 

155° 150°

175° EAST OF GREENWICH 180 165° WLST OK GREENWICH 160

MAP OF THK ALASKA PENINSULA AND ALEUTIAN ISLANDS.
GEOLOGY OF ALASKA PENINSULA AND ALEUTIAN ISLANDS 147

and glacial deposition of morainal material has provided the sites 

for many of the large lakes that form so conspicuous a feature of 

these lowlands. Throughout this area, however, including both the 

peninsula and the Aleutian Islands, the glacial ice was of local origin, 

and as the distance to the sea was nowhere great, the ice streams 

never reached great length.

In such of the larger and more rugged of the Aleutian Islands as 

were visited it is evident that each island developed a separate, 

individual ice cap, in which only the higher peaks and ridges stood 

above the glacial ice, and the glacial movement was radially away 

from the divides toward the sea. Thus Unalaska, Atka, Adak, 

Kiska, and Attu Islands were each almost completely covered by 

glacial ice that not only filled the higher valleys but pushed out 

over the lowlands to the sea, and it seems certain that the other large 

islands not visited, including Unimak, Akun, Akutan, Umnak, 

Kanaga, Tanaga, Amchitka, and Agattu, each supported a separate 

ice cap that covered most of the land surface. The former presence 

of great glaciers on these islands is witnessed by such features as 

broadly U-shaped valleys heading in cirques, sharp, rugged interstream ridges, tarn lakes, fiorded coast lines, erratic boulders, and 

a multitude of other features characteristic of regions that have 

been covered with ice. One can picture in mind what a forbidding 

region this must have been at the time of maximum glaciation, 

and how effective a barrier to the migration of land animals along 

the coast this almost continuous mass of ice must have interposed.

The accomplishments of normal erosion agencies since the retreat 

of the last great glaciers are relatively trivial. Some sharp postglacial gulches mark the mountain slopes, the stream valleys in 

many places present an alternation of shallow rock canyons with 

basins of deposition, and beaches, bars, and spits have been built by 

waves and currents in protected places, but on the whole the landscape is much as it was when the ice first melted away from it.

LOCAL OBSERVATIONS

UNALASKA ISLAND

In traverses carried along the shores of Iliuliuk Bay and on 

Amaknak Island (the island on which Dutch Harbor is situated) 

the only rocks observed were basic lavas and tuffs that have a 

prevailing dip of about 15°-30° N. These volcanic rocks are cut 

by numerous dikes and sills of similar composition. The southeast 

shore of Iliuliuk Bay was not examined in detailbut appears also 

to be composed entirely of volcanic rocks, though their structure 

is more diverse than that of the rocks on Amaknak Island. In thin 

section three specimens of rock taken from the shores of Amaknak
148 MINERAL RESOURCES OF ALASKA, 1932

Island proved to be basaltic tuff, basalt porphyry, and olivine 

basalt porphyry. No rocks other than those of volcanic origin 

were seen.

A short trip to the head of Captains Bay, southwest of Unalaska, 

showed the presence there of abundant granitic gravel discharged 

by the Shaishnikof Eiver, which flows from the high interior of 

Unalaska Island northeastward into the head of Captains Bay. 

The pebbles of this gravel, examined in thin section, were found to 

be fairly coarse grained quartz monzonite, of a type which the writer 

saw nowhere else in the Aleutian Islands. Associated with the quartz 

monzonite gravel on the bars of the river are minor amounts of 

chloritized tuff and a small proportion of volcanic rocks ranging 

from light-colored lavas and porphyries to basalt. A short trip 

southward from the head of Captains Bay showed quartz-pyroxene 

dibrite in.place. The abunda'rice of 'quartz monzonite and of diorite 

in the gravel of the Shaishnikof River indicates that a large part of 

its basin lies in granular intrusive rocks, although the portion of the 

island bordering all of Unalaska Bay except the upper end of Captains Bay is evidently made up of volcanic materials. The cliffs in 

the neighborhood of Cape Cheerful at the west end of the bay, show 

a great thickness of lava flows; those toward the base of the cliffs 

are relatively thin, but three flows from 200 to 500 feet thick appear 

toward the top of the cliffs.

A brief examination of the shores of the head of Chernufski 

Harbor, on the northwest coast of Unalaska Island near the site 

of an abandoned village, showed that this portion of the island is 

also composed entirely of volcanic rocks. Two specimens of bedrock 

both proved to be basalts. On the Pacific slope of the bar on which 

the village is located the beach is composed almost exclusively of 

highly polished pebbles of a dark to light green fine-grained rock 

that in thin section proved to be an obsidian tuff composed of both 

rounded and angular fragments of unaltered volcanic glass, now 

silicified and cemented together by very fine grained quartz. Some 

fragments of plagioclase and other original minerals are still visible, 

but the bulk of the rock is made up of the volcanic glass. Native 

stone implements of a similar rock have been found in the region 

by archeologists, and it is likely that the material for them was 

obtained from this locality. The rock, though hard to work, takes 

on an exceptionally smooth polish and can be shaped into superior 

pointed weapons.

Unalaska Island, like all the other high mountainous islands of 

the Aleutian chain, was severely glaciated in Pleistocene time by 

glaciers that originated on the mountain slopes and pushed radially 

out to sea in all directions. This island apparently acted as the
GEOLOGY OF ALASKA PENINSULA AND ALEUTIAN ISLANDS 149

center for the development of a local ice cap, so that not only the 

mountain valleys but the lowlands and the lower portions of the 

intervalley ridges were overridden and sculptured by glacial ice. 

In places this ice scour has given strong surface expression to the 

structure of the underlying lavas and tuff. Thus on Amaknak 

Island, in the area between Dutch Harbor and the head of Iliuliuk 

Bay, glacial plucking has developed a series of steplike ridges that 

as viewed from the air show steep scarps on their south face and 

smooth dip slopes on the north slope. The whole island bears conspicuous evidence of strong glacial sculpturing, and except for the 

steep wave-cut cliffs on exposed shores and the building of deltas, 

bars, and spits in protected places, postglacial erosion has been slight, 

and the land forms are much as they were when the glaciers receded.

ATKA,-ISLAND

A portion of the shore of Nazan Bay, Atka Island, near the village 

of Atka, was examined, and the rocks there all proved to be volcanic. Two thin sections cut from rocks collected near the village 

were both found to be porphyritic basalts, considerably altered. In 

one the phenocrysts of plagioclase were almost entirely altered to 

chloritic minerals and epidote. Some of the basalt flows show prominent columnar jointing, and associated with the flows are hard, wellcemented tuffaceous rocks in which large angular fragments are 

distinguishable.

It is reported that the natives formerly mined native copper on 

Salt Island, a small island that lies just northwest of the center of 

Atka Island. This copper probably occurs in an amygdaloidal lava 

flow. At the time of visit the higher peaks of the island were concealed in clouds, but it is said that the volcanic peak on the north 

end of the island shows signs of having been active at no very remote 

date, though the cone is considerably dissected. It is also reported 

that a hot spring a few miles above the head of Korovin Bay, on 

the northwest side of Atka Island, boils violently at intervals.

ADAK ISLAND

Opportunities for close observation of the geology of Adak Island 

were confined to the Bay of Islands and the head of Three Arm Bay, 

on the west shore of the island, though a comprehensive view of all 

the north half of the island, except where concealed by clouds, was 

obtained from the air. The portion of Adak Island studied is composed entirely of volcanic rocks,, none of the basement rocks through 

which the volcanic materials were erupted being seen. The south and 

west shores of the Bay of Islands are made up of dense, hard flow 

rocks and massive tuffs. The flows include andesite or latite por-
150 MINEEAL EESOUECES OF ALASKA, 1932

phyry and basaltic greenstone. .The islands at the entrance of the 

bay appear to consist dominantly of tuffaceous material, with only 

subordinate amounts of flow rocks.

Between the Bay of Islands and Andrew Bay, on the north shore of 

Adak Island, there is a volcanic peak whose top was continually in 

clouds during the time of visit but whose lower slopes contain large 

quantities of loosely consolidated ashes, tuff, and fragmental material, indicating comparatively recent activity. The materials, being 

little indurated, have suffered much more from postglacial erosion 

than the harder rocks around the Bay of Islands, and they have 

slumped extensively on being undercut by the waves.

Adak Island as a whole shows severe glacial erosion and appears 

to have been almost entirely covered by a local ice cap at the time 

of its maximum glaciation. The Bay of Islands, Three Arm Bay, 

and the Bay of Waterfalls (on the south shore) are all glacial fiords, 

and high on the slopes of the bordering mountains glacial lakes lie 

in basins excavated by the ice in the volcanic rocks. Erratic boulders 

occur, even on the high slopes, although over most of the surface of 

the island the bedrock has been ice-scoured and is covered only by 

a thin layer of rock-disintegration products, soil, and vegetation.

KISKA ISLAND

Opportunity was offered to study that part of Kiska Island immediately adjacent to Kiska Harbor, on the east shore. On entering 

this harbor, Little Kiska Head, at the northwest extremity of Little 

Kiska Island, stands out as a fine example of columnar basalt, exposed in steep, wave-cut cliffs. The north and west shores of the 

harbor are composed mainly of loosely indurated volcanic tuff, with 

fragments 1 inch in diameter or less. This tuff is bedded, dipping 

5°-10° N., and, though standing in nearly vertical cliffs where wave 

erosion- is active, is nevertheless so poorly indurated that it crumbles 

readily under the hammer. Near the north entrance to the harbor 

the tuff is overlain by basaltic lava. The shores and the mountain on 

the south side of the harbor are composed of lava flows that include 

dark basaltic rocks, some of which are partly glassy, and lightercolored lavas, among which a quartz latite was identified.

Only the shores of Kiska Harbor were studied at close range, but 

as seen from the air Little Kiska Island and all of the eastern and 

northern portion of Kiska Island appear to contain only volcanic 

rocks.

At the northern end of Kiska Island is a volcanic cone some 4,000 

feet high that was entirely snow-covered almost to sea level in late 

May 1932. Its slopes show only moderate dissection, and the aviators of the expedition thought they detected fumaroles on its upper 

slopes.
GEOLOGY OF ALASKA PENINSULA AND ALEUTIAN ISLANDS 151

Kiska Island shows the effects of glacial erosion, but as most portions of the island are less than 1,500 feet above sea level the glacial 

scouring was in most places not severe.

ATTU ISLAND

As seen from the air Attu Island is a rugged mountainous island, 

of which the main peaks and ridges rise to elevations of 3,000 to 

3,500 feet. So far as is known, there are no active volcanic vents on 

this island, and indeed no mountains were seen whose contours were 

constructional. The whole island has been heavily glaciated during 

Pleistocene time, and the present topography has been produced by 

the glacial modification of the preexisting stream-eroded landscape. 

As in most of'the other islands of the Aleutian chain, the shores of 

Attu Island are exposed to the full force of the surf and are generally clifflike,. with little or no beach exposed. Attu Island has 

almost no low-lying areas, the mountains nearly everywhere descending abruptly to the shore and being edged by steep cliffs against 

which a constant surf breaks, so that landing by a small boat is dangerous or impossible, and access on foot is difficult. Close-range 

study of the geology was confined to the shores of Chichagof Harbor, 

on the north side of the island, in the vicinity of the village of Attu. 

At the time of visit, in late May, the island was almost entirely snowcovered down to the shore line, and tidal lagoons were still covered 

with ice..

The north shore of Chichagof Harbor is composed mainly of 

basaltic lavas, some of which are full of irregular crusts. Associated 

with the basic flow rocks are some coarser-grained acidic rocks that 

in thin section are seen to consist of sodic diorite, in which chlorite 

and epidote are present as alteration products. Some unaltered pyroxene is present, and magnetite and epidote were noted as accessory 

minerals. This sodic diorite is believed to represent an intrusive into 

the more basic lavas. At one place it is cut by a stockwork of quartz 

stringers that have a fairly definite linear arrangement, striking 

northeast and dipping nearly 90°. No metallic mineralization was 

noted in this quartz.

The south shore of Chichagof' Harbor is composed of a series of 

massive basalt flows, some of which are amygdular, and of coarse 

massive basic tuffs consisting of lava fragments in a lava matrix. 

These beds dip about 45° NE. Some of the basalt beds contain small 

cubes of iron pyrite, both as scattered grains in the matrix and as 

bunches along tiny cracks. There are also a good many quartz 

stringers cutting the basalts, but no metallic minerals were seen in 

them.
152 MINERAL RESOURCES OF ALASKA, 1932 

COLD BAY AND LENARD HARBOR

A brief examination of the shores of Lenard Harbor, an arm of 

Cold Bay near the southwest end of Alaska Peninsula, showed that 

the rocks bordering the harbor are composed, exclusively of volcanic 

materials. One specimen, as studied in thin section, proved to be 

glassy basalt porphyry, with phenocrysts of plagioclase and smaller 

ones of augite in a matrix composed of plagioclase, magnetite, and 

a reddish-brown glass. Another very similar rock had phenocrysts 

of plagioclase and augite in a fine-grained groundmass of labradorite, 

augite, and magnetite. The rocks in place, as well as the boulders 

on the beach, include a considerable variety of lavas and tuffs, ranging in color from brick-red through gray, brown, and green. Some 

are highly vesicular, and others are fairly coarse grained and massive. In places the cliffs show well-developed columnar jointing.

The mountains bordering Lenard Harbor show strong evidence 

of glacial erosion. A large valley tributary to the harbor from the 

north shows glacial sculpturing to a height of 1,500 feet above its 

floor, and its tributary valleys are cirques. At the niouth of Lenard 

Harbor the glacier surface formerly stood at least 500 feet above sea 

level. Similarly the mountains west of Cold Bay show glacial 

erosion, and the valleys are well-sculptured glacial troughs.

The west and northwest shores of Cold Bay are bordered by a 

fairly flat foreland faced at the water's edge by steep wave-cut bluffs 

that rise almost vertically for 50 to 100 feet above sea level. At the 

one place where these bluffs were examined the material was found 

to consist mainly of fine bedded sandy silt studded with pebbles and 

boulders as large as 4 feet in diameter. The silt was certainly 

deposited in standing water, yet the boulders and pebbles are as 

certainly of glacial origin, for many are beautifully striated and 

faceted. The upper few feet of the bluff is composed of sandy silt 

containing no large boulders and only a few small pebbles. This 

upper layer is probably wind-blown material.

The material exposed at the locality examined appears to represent 

sediments deposited in a lake or a protected bay at a time when the 

bordering mountains were glaciated and the glaciers were discharging into the water. At that time the land surface stood at least 100 

feet lower than at present, and much of the area now bordering Cold 

Bay on the west was submerged. Glacial ice pushed down into this 

area from the mountains both east and west of Cold Bay.

DOLGOI ISLAND AND PAVLOF BAY 0

The shores of upper Dolgoi Harbor, Dolgoi Island, are composed 

of dense, hard volcanic tuff that consists of subangular to wellrounded fragments of various basaltic rocks and individual crystals
GEOLOGY OF ALASKA PENINSULA AND ALEUTIAN ISLANDS 153

of plagioclase and augite. This tuff exhibits extensive alteration. 

It dips 10°-15° S. Locally the tuff is overlain by flows of columnar 

basalt, and the islands of the lower harbor are also basalt flows. 

These flow rocks in thin section are seen to consist of a glassy basalt 

porphyry in which phenocrysts of plagioclase and augite lie in a 

matrix of plagioclase, magnetite, and a reddish-brown glass. Similar flows of columnar basalt were observed from the air on the shores 

of Volcano Bay, northwest of Dolgoi Island. A brief examination 

of the east shore of Pavlof Bay, near Settlement Point, showed that 

the point itself is composed of basaltic lava, lying with apparent conformity upon a series of well-indurated impure gray sandstone that 

weathers buff. Associated with the sandstone are a few thin shale 

beds, but they are so thin that they form a very small percentage of 

the total exposure. Both sediments and the overlying lava dip a 

few degrees to the northwest.

A large creek that flows into Pavlof Bay just northeast of Settlement Point discharges gravel that includes, in addition to basic 

lavas, a considerable proportion of pebbles of hard black argillite. 

Nothing definite is known of the age of the sandstone, described 

above, or of the argillite. The light color and moderate induration 

of the sandstone suggests that it is of Cretaceous age or younger.

o