Geology of the Delarof and 

\ Westernmost Andreanof Islands te

Aleutian Islands, Alaska

By GEORGE D. FRASER and H. FRANK BARNETT

., INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGICAL SURVEY BULLETIN 1028-1

Prepared in cooperation with the 

Departments of the Army, Navy, 

and Air Force

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1959
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

The Library of Congress has cataloged this publication as follows:

Fraser, George DeWitt, 1920-

Geology of the Delarof and Westernmost Andreanof Islands, Aleutian Islands, Alaska, by George D. Fraser and 

H. Frank Barnett. Washington, U.S. Govt. Print. Off., 

1959.

v, 211-248 p. illus., maps (4 fold. col. in pocket) profile, tables, 

25 cm. (U.S. Geological Survey. Bulletin 1028-1. Investigations 

of Alaskan volcanoes)

"Prepared in cooperation with the Departments of the Army, Navy, 

and Air Force."

Bibliography: p. 244-245.

1. Geology Aleutian Islands. 2. Rocks, Igneous. i. Barnett, 

Henry Frank, 1921- joint author, n. Title: Delarof and westernmost Andreanof Islands, Aleutian Islands, Alaska. (Series: U.S. 

Geological Survey. Bulletin 1028-1. Series: U.S. Geological Survey. Investigations of Alaskan volcanoes)

557.984

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

Washington 25, D. C.
PREFACE T "*' ;

The U. S. Geological Survey, in response to the October 1945 

request of the War Department (now Department of the Army), 

made a reconnaissance during 1946-54 of volcanic activity in the 

Aleutian Islands-Alaska Peninsula area. Results of the first 

year's research, field, and laboratory work were hastily assembled 

as two administrative reports to the War Department. Some of 

the early findings, as recorded by Robert R. Coats, were published 

in Bulletin 974-B (1950), "Volcanic Activity in the Aleutian Arc," 

and in Bulletin 989-A (1951), "Geology of Buldir Island, Aleutian 

Islands, Alaska."

Unpublished results of the early work and all data gathered 

in later studies are being published as separate chapters of 

Bulletin 1028.

The investigations of 1946 were financed principally by the 

Military Intelligence Division of the Office, Chief of Engineers, 

U. S. Army. From 1947 until 1955 the Departments of the Army, 

Navy, and Air Force jointly furnished financial and logistic 

assistance.

m

CONTENTS ' ^ __

	Page

Abstract__________.__________-...___________ 211

Introduction-_______---_--_------_-__--____------__-______________ 212

Geography____--__--__--_---_------__---__---_-______-__-____-__ 213

Location...._____. ___ ___________ 213

Climate and vegetation_______________________________________ 213

; Landforms___-____-_---_--________-______--____________ . 214

Geology__ ....................................... _ __ 215

Layered rocks-___-___---------_---_--___---_-_---_---_-----__- 215

Amatignak Island_________________________________________ 215

UlakIsland--__--------------_---__----.................. 216

Central Delarof Islands and southern parts of Tanaga and Kanaga

Islands______________________________ 217

Early shield volcanoes___________________________________ 225

Recent volcanic cones___--___________-______.______________ 225

Surficial deposits.-.--------------.--------- -_._-...-.- 226

Fine-grained intrusive rocks..___________________________________ 227

Coarse-grained intrusive rocks_________________________________ 230

Structure...-.___.....___._.._______________._. 232

Major structures.__---_-_--___-_____-__--_.__-.___________ 232

Minor structures.------------_----__--_----...____-__.___- 233

Submarine topography....---......____-_.-_-___...._-.._.. 235

Physiography._ ................................................... 237

Geologic history.._________________________________________________ 243

References cited......____--___---__--___-___-_---__-----_-________ 244

Index..-...-.-------_-----.------------------.-.------.----..-__- 247

ILLUSTRATIONS

[Plates 27-30 In pocket]

PLATE 27. Generalized geologic map of Delarof and westernmost 

Andreanof Islands.

28. Geologic map of western peninsula of Kanaga Island.

29. Geologic map of Kavalga Island.

30. Geologic map of Ogliuga and Skagul Islands.

31. Gareloi Volcano from Skagul Island.-_.-_-_-_._____ Facing 212

32. Kavalga Island, looking west.-__-_--_.-_-_-----.__ Facing 213 

FIGURE 44. Index map of Aleutian Islands------_----_-______-_._____ 213

45. Detailed submarine topography of central Delarof Islands area. 236

46. Surfaces of planation and drainage map, central Kanaga

Island.___--__---._-_-______.____________ 239

47. Selected topographic profiles showing orientations of planar

surfaces of five islands of Delarof group_________________ 242

TABLES

TABLE 1. Fossils from Kanaga and Tanaga Islands.--___..__.....___ 218 

2. Chemical composition of rocks from Delarof and westernmost

Andreanof Islands.___________________________________ 222

INVESTIGATIONS OF ALASKAN VOLCANOES

GEOLOGY OF THE DELAROF AND WESTERNMOST 

ANDREANOF ISLANDS, ALEUTIAN ISLANDS, ALASKA

By GEORGE D. FRASER and H. FRANK BARNETT

ABSTRACT

The layered rocks of the Delarof and westernmost Andreanof Islands 

are divisible into five units, generally younger from south to north. Unit 

1, the rocks of Amatignak Island, is composed of altered, well-bedded 

tuffaceous rocks including thick, finely banded, penecontemporaneously deformed argillite. These rocks apparently grade northward into unit 2, the 

altered pillow lavas and pyroclastic deposits of Ulak Island which are interpreted as a near-source facies of unit 1. The age of these two units is 

unknown but presumably is Tertiary. Unit 3 is a composite sequence of 

almost unaltered shallow marine and subaerial volcanic rocks composed 

of tuff-breccia, lava, pillow lava, and subordinate sedimentary rocks. This 

units extends in a belt across the central Delarof Islands and the southern 

parts of Tanaga and Kanaga Islands. Upper Tertiary fossils are found in 

the rocks on Tanaga and Kanaga; the rocks of the central Delarofs are 

correlated with these because of lithologic similarities. The rocks in unit 

3 may be partly equivalent, extreme facies of the Amatignak rocks, or 

they may be younger. The contact is not exposed. Unit 4 is composed of 

Pleistocene precaldera lava that forms fragments of old shield volcanoes 

near the presently active cones. This lava, younger than most of unit 3, 

apparently intertongues in places with the upper part of the older sequence. 

Unit 6 comprises the composite cones of presently active Gareloi, Tanaga, 

and Kanaga Volcanoes. All five bedrock units are andesitic to basaltic in 

composition.

Units 1, 2, and 3 are cut by many dikes, dike swarms, and sills of andesitic or basaltic composition. One limburgite intrusive mass and one light 

rhyodacite dike represent divergent compositions.

The older units are also cut by coarse-grained sills, dikes, and plutons, 

mostly of granodioritic composition. Ilak Island is part of a granodiorite 

pluton cut by aplite and andesite dikes; none of the invaded rock is exposed 

and the age is unknown.

None of the rocks have been regionally metamorphosed; even low-grade 

foliated rocks, such as slate and phyllite, are absent.

Submarine contours reveal a major trench and ridge structure for the 

Aleutian arc. Islands are actually peaks of a great arcuate mountain range. 

In this area the range rises 25,000 feet from a foredeep on the Pacific side 

and 13,000 feet from a deep on the north. The main ridge is apparently

211
212 INVESTIGATIONS OF ALASKAN VOLCANOES

an arch bounded on the south by a north-dipping thrust zone and on the 

north by a high-angle fault zone, downthrown to the north. Volcanoes are 

located along the inferred fault on the north. A group of transverse submarine and subaerial linear elements suggests further that the area has 

been segmented by high-angle faulting. An Aleutian island arc was probably 

formed in this general area as early as Miocene time, but the present 

volcanoes and the island locations and shapes were formed much later, 

probably in Pleistocene time.

Several stages of late Tertiary or Quaternary wave planation and subsequent differential tilting characterize the late history of the area; sporadic 

central-type volcanism at its north edge began in early Pleistocene and has 

continued to the present time. Widespread deposition of Recent ash, and 

historic eruptions from Gareloi, Tanaga, and Kanaga Volcanoes, are the 

most recent volcanic events. Glaciers were active on Tanaga and Amatignak 

during the Pleistocene, and ice patches are now present near dormant east 

Tanaga volcanoes and on the high slopes of Gareloi Volcano.

INTRODUCTION

Most of the western Aleutian Islands here described were 

mapped in reconnaissance (fig. 44) during the field season of 

1952 as part of a general study of the geology of the Aleutian 

Islands. The Geological Survey's motorship Eider transported 

the field parties during the frequent changes of base camps. Those 

who participated in the field study were: R. E. Wilcox (geolpgistin-charge), H. F. Barnett, B. H. Bieler, G. D. Fraser, E. H. 

Meitzner, W, H. Nelson, R. A. Rpbie, and G. L. Snyder.

Two-man traverses were made on foot from base camps established along the coasts. Coverage is summarized as follows: 

Kanaga (south of latitude 51°54') 7 camps, 34 traverses; Tanaga 

(south and southeast coasts) 4 camps, 8 traverses; Amatignak 

(east coast) and Ulak, 3 traverses each; Kavalga, 2 traverses; 

Unalga, Ilak, Skagul, Ogliuga, 1 traverse each. Traverses ranged 

from 4 to 12 miles, and averaged about 7.

Contour maps prepared in 1943 by the Corps of Engineers, 

U. S. Army, at a scale of 1:25,000 were available for Kanaga 

Island. The Delarof Islands were mapped on Coast and Geodetic 

Survey topographic manuscript sheets, scale 1:20,000. For 

southern Tanaga the only maps available were form-line Coast 

and Geodetic Survey topographic hydrographic charts (Nos. 

9145, 9146), scale, 1:40,000. High-altitude aerial photographs 

cover most of the area.

Data included here for northern Kanaga are based upon the 

reconnaissances of Robert R. Coats, who also touched briefly 

at some of the Delarof Islands and part of Tanaga (Coats, 1950, 

1952, 1956b, c; written communication, 1954).
GEOLOGICAL SURVEY 

AERIAL PHOTOGRArH OF GARELOI VOLCANO FROM SKA

Wave-planed surface of Skagul Island contrasts with volcanic cone 15 miles northwest. No

Ogliuga (not shown) are unique among the Delarof Islands in that the usual sea cliff Photographic and Charting Service (MATS), United Slates Air Force.
GEOLOGICAL SURVEY 

AERIAL PHOTOGRAPH OF KAVALGA ISLAND, LOOKIN

Note wave-planed upper surface, sea cliff, partly covered storm beach, and present day rock ben

may have originated as boulders and irregularities on wave-cut surface; compare with fea

Photo Air Photographic and Charting Service (MATS), United States Air Force.
DELAROF AND WESTERNMOST ANDREANOF ISLANDS 213 

GEOGRAPHY 

LOCATION

The 10 islands discussed lie along the Aleutian Islands arc 

between longitude 177°03' and 179°09' W., and between latitude 

51°13' and 51°56'30" N. (fig. 44). The two large islands of 

Kanaga and Tanaga have areas of about 100 and 160 square 

miles, respectively, and are the westernmost of the Andreanof 

group. Four of the eight Delarof Islands, Skagul, Ogliuga, 

Kavalga, and Unalga, constitute a narrow projection which extends 35 miles west of the larger islands and is separated from 

them by Tanaga Pass. Ulak and Amatignak form a spur trending 

southwestward from this westerly projection and from the general 

curvilinear trend of the Aleutian chain. Ilak and Gareloi are, 

respectively, 5 and 15 miles south and north of the line between 

Skagul and Unalga. The total area of the scattered Delarof 

group is about 65 square miles.

FIGURE 44. Index map of the Aleutian Islands, Alaska, showing location of the Delarof and 

westernmost Andreanof Islands.

CLIMATE AND VEGETATION

Weather in the Aleutian Islands varies locally and is related 

to the interaction of ocean currents, moving air masses, and 

island topography. Winds, clouds, rain, and fog generally prevail 

the year around. Low heavy fogs often limit visibility to 100 

feet for days at a time, particularly in the summer, and traverses 

must often be completed solely by compass and aneroid barometer 

observations. Annual precipitation is about 40-50 inches.

468596 59 2
214 INVESTIGATIONS OF ALASKAN VOLCANOES

In the western Andreanof s summer winds originate mostly from 

the southeast, south, and southwest. Average velocities are about 

25 miles per hour; during sudden storms velocities increase to 

70 miles per hour. From October to March prevailing winds 

are from the north and west. Excessively strong, gusty downslope winds, sometimes exceeding 100 miles per hour, result from 

the rapid spilling over of air masses dammed against the windward sides of mountains or ridges.

The North Pacific drift from the southwest warms the cold 

waters brought south from the Bering Sea. Winter temperatures 

seldom fall as low as 0°F. The mean temperature of the warmest 

month (usually August) is about 55°; the daily temperature 

ranges in summer from 46° to 75°F. Total annual snowfall, 

falling mostly from December through March, averages 70 inches, 

but little accumulates except at higher mountain altitudes.

Vegetation is abundant as a consequence of the temperate humid 

marine environment. Mosses, grasses, fungi, and a wide variety 

of flowering plants (many with edible parts) blanket the slopes 

to a thickness of 2 or 3 feet. The only trees are a few stunted 

recumbent willows.

Except at gullies and cliffs the plant cover is unbroken on the 

flat or gently rolling uplands below 1,500 feet altitude. The irregular, hummocky, springy surface makes walking slow less 

than 2 miles per hour is an average rate. Rock outcrops in the 

interiors of all the flat islands are rare and widely separated. 

Geologic investigations in such islands were largely confined to 

the sea-cliff and rock-bench zone around the island perimeters.

LANDFORMS

The landforms among the western Andreanof Islands vary 

greatly. Kanaga and Tanaga Islands exhibit compound landforms. 

Lofty volcanic heights at the north give way southward and westward to poorly drained lowlands surrounded by sea cliffs. The 

highest, least eroded, and most recently active volcanic areas 

are at the extreme north of both islands. Like several of the 

larger Aleutian Islands, Kanaga has a narrow low-lying westward 

extension.

Gareloi Island is a conical volcano which rises abruptly from 

the ocean floor a few thousand feet below sea level to a summit 

5,160 feet in altitude (pi. 31). Two small glaciers hang on the 

steep, slightly gullied slopes. The shoreline is precipitous along 

most of its length. An eruption in 1929 opened a series of vents; 

it spilled dark lava from a rift zone southeast of the summit vent
DELAROF AND WESTERNMOST ANDREANOF ISLANDS 215

and spread pyroclastic materials over the southern face of the 

volcano. This recently active, elevated mass is north of lower 

lands, paralleling the arrangement on Kanaga and Tanaga.

Amatignak Island is a dissected glaciated upland marked by 

cirques, sharp aretes, troughlike valleys, and small intermontane 

depressions. The island has 1,875 feet of relief. No constructional 

volcanic landforms were seen.

The other islands of the Delarof group are low, flat-topped 

featureless platforms bounded mostly by sheer cliffs and sea-level 

rock benches; they resemble the southern parts of Kanaga and 

Tanaga Islands. Small ponds commonly dot the poorly drained 

surfaces. Ulak has a somewhat higher relief than the low 

Delarofs.

The flat Delarof Islands are dotted with small conical mounds 

(pis. 31, 32) generally only a few feet high and too small to show 

on a topographic map. A few larger mounds are visible as closed 

contours (pi. 30); dense vegetation, possibly the result of fertilization by birds, gives some credence to the idea that the mounds 

are "bird perches." Some of the structures may have begun as 

irregularities on the wave-planed surface (pi. 32).

GEOLOGY

LAYERED BOOKS 

AMATIGNAK ISLAND

Character. Sedimentary rocks composed of volcanic detritus 

make up most of the bedrock on Amatignak Island (pi. 27). The 

rocks are of dark-gray, greenish-gray, olive-green, bluish-gray, 

and light-brown tuffaceous argillite and siltstone, tuffaceous sandstone, and tuffaceous breccia. Greenish colors are most common. 

Grain sizes less than one-half inch predominate, but volcanic 

bombs are visible in a few beds, and altered volcanic glass is 

seen in thin section; accordingly many of these rocks have j>een 

classified as volcanic wacke (Williams, Turner, and Gilbert, 1954). 

Generally the rocks are well indurated, chloritized, pyritized, and 

silicified. Cryptocrystalline quartz veinlets are common.

A common rock of the island is a finely laminated argillite, 

banded green and gray. The rock is brittle, breaks with a conchoidal fracture, and often shows structures recording penecontemporaneous deformation. Similar argillite has been found on 

Ulak Island and elsewhere in the Aleutian Islands in association 

with altered pillow lavas and marine pyroclastic deposits.

Microscopic examination of specimens in the sand- and siltsize range confirmed their origin as tuffaceous sediments. One
216 INVESTIGATIONS OF ALASKAN VOLCANOES

specimen had fragments (< 0.04 mm) of green chloritic material, 

oligoclase-andesine, and augite cemented by carbonate. Another, 

with zeolitic(?) cement had particles (< 0.2 mm) of chloritized 

volcanic glass containing feldspar laths and other crystals, and 

crystal fragments of unaltered augite and albitized plagioclase 

(< 0.09 mm). Most of these rocks are of andesitic or basaltic 

composition.

Many andesitic or basaltic dikes and small sills intrude the 

sedimentary rocks along the eastern and southern coast. Small 

dioritic or diabasic sills are also found; a large quartz diorite 

sill forms Knob Point on the east coast. Shipboard observations 

and study of aerial photographs suggest large areas of massive 

rock, perhaps plutonic, in the northwest quadrant of the island.

Age. No precise age can be assigned to the rocks of Amatignak 

Island. Parts of some formations on Attu, Agattu, Adak, and 

Unalaska dating from late Paleozoic to early Miocene have similar 

lithology. The two oldest stratigraphic units in the Rat Islands, 

generally coarser grained than the rocks of Amatignak, have been 

altered and intruded by quartz diorite. Fossils from the Rat 

Islands indicate an Eocene (?) to Miocene age for the younger 

sequence intruded by quartz diorite, but the older unit is not 

fossiliferous. Mesozoic fossils have been found only on Atka 

Island; Paleozoic fossils have been found in one small area on 

Adak (Coats, 1956a). Tertiary fossils have been found on Attu, 

Agattu, and Unalaska in rocks resembling those of Amatignak. 

On Amatignak, structures resembling worm tubes were found in 

two outcrops; one unidentifiable foraminifer, probably post-Cretaceous, was found in thin section. The rocks of Amatignak are 

provisionally assigned to the Tertiary and are probably Miocene 

or older.

The rocks on Amatignak have been tentatively correlated with 

those on nearby Ulak Island, and both are probably at least as 

old as those on the central Delarof Islands which have not been 

regionally altered or invaded by plutonic rocks (p. 217).

ITL.AK ISLAND

The rocks of Ulak Island resemble those of Amatignak. They 

are greener and more altered than those of the central Delarofs. 

The layered sequence is composed of altered palagonite breccia 

and tuff, basaltic pillow lavas, and tuffaceous sedimentary rock. 

Those rocks are intruded by andesite and basalt dikes and sills. 

In contrast with the well-bedded sedimentary section on Amatignak, the rocks of Ulak are dominantly igneous and pyroclastic.
DELAROF AND WESTERNMOST ANDREANOF ISLANDS 217

J>

Pillow lavas, abundant on Ulak, were not observed on Amatignak. 

The rocks of Ulak are chloritized, but the pillow lavas and tuff 

are apparently not spilitic. ;

j A 2-foot thick layer like the distinctively banded argillite 

found on Amatignak lies between altered tuff-breccia layers on 

the west coast of Ulak. This suggests a correlation of the rocks 

of the two islands, and possible facies change from fine-grained, 

well-bedded sedimentary rocks on Amatignak to more dominantly

' volcanic rocks on Ulak.

j Bedding on the south half of Ulak Island dips toward Amatig-

' nak, and beds on northeast Amatignak dip steeply east, suggesting 

either a syncline beneath the straits between Amatignak and

', Ulak or drag on an interisland fault (pi. 27). If there is a 

fault between Amatignak and Ulak, and if the relative altitudes 

indicate relative movement, then the apparent facies change may

* be better explained as a change upward in the section toward 

Ulak. The rocks on Ulak are believed to be about the same age 

as the rocks on Amatignak.

CENTRAL DELAROF ISLANDS AND SOUTHERN PARTS OF TANAGA AND 

^ KANAGA ISLANDS

Character. Unaltered pyroclastic rocks and lava make up the 

bulk of the other islands of the area (pis. 27-30). Tuff-breccia 

and tuff are interlayered with black, gray, or brown andesitic 

and basaltic flows, pillow lava, and flow breccia. Tuff-breccia 

is most common, lava flows are abundant, and tuff is somewhat 

less common. Sedimentary rocks are least abundant, except on 

southeastern Tanaga. Plates 28, 29, and 30 show proportions of 

_, the rock types in selected areas.

Volcanic centers have not been found. The relative scarcity 

of reworked volcanic material and the abundance of coarse debris 

suggest that the volcanic centers were nearby. Fossils (table 1) 

indicate that many of the beds are of shallow marine origin and 

some of them are subaerial.

The composition of these rocks, as revealed by field mapping, 

brief petrographic study, and four chemical analyses seems to be 

consistently basaltic and andesitic, with no representatives of 

more siliceous types of lava. Table 2 presents chemical analyses 

of nine samples from Kanaga, Tanaga, and the Delarof Islands. 

The first four analyses are of rocks considered in this section. The 

main differences between rocks of the sequence apparently are 

textural and structural rather than chemical. All sections studied 

under the microscope fall in the basaltic and andesitic groups
218 INVESTIGATIONS OF ALASKAN VOLCANOES

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TABLE 2. Chemical composition of rocks from the Delarof and wester

[Analyst, E. J. Tomasi]

Constituents

BlOt.. ....... ...............................

AliOt. ....... .............................. .

FeO. _ .. 

MgO-.. 

CaO.-- 

NajO-.. 

KtO. ........ ... ...................... ... ...

^Q-. .....................................

~H.tO+. ....... . ....... ....... ... .... .....

TlOt................... .....................

cot...... . .. ...-. ............ ......

PtOt... ... ... ... ... ... ..... ... ... ... .....

G\.... ......................................

F-. ........................................

B..... .......................................

MnO...... _. 

Less O ____ ..-- _ .. ___ .. __ .. ...

Total--.-...... 

Locality, name, and occurre

Aphanitic rocks

Unalga

Augite 

andesite, 

flow

52-N-64

53.29 

18.49 

3.76 

3.79 

. 4.48 

9.08 

3.55 

.68 

.78 

.60 

.93 

.12 

.22 

.00 

.03 

.00 

.13

99.93 

.01

99.92

Skagul

Augite 

andesite, 

flow

52-W-64

55.56 

18.96 

4.52 

3.16 

2.70 

8.09 

3.95 

.78 

.62 

.37 

.60 

.01 

.40 

.05 

.03 

.00 

.18

99.98 

.02

99.96

Ogliuga

Labradoriteaugite 

andesite, 

flow

52-W-70

48.94 

18.88 

4.49 

5.27 

6.09 

10.37 

2.98 

.47 

.53 

.64 

.95 

.03 

.22 

.01 

.03 

.00 

.18

100.08 

.01

100.07

S. Tanaga

Andeslne 

basalt, flow 

or sill

52-Fs-94

51.75 

16.01 

4.39 

6.84 

4.00 

8.73 

3.71 

.70 

.79 

.82 

1. 39 

.01 

.53 

.08 

.05 

.01 

.21

100.02 

.04

99.98

2.82

NW. Kanag

Dark 

rhyodaclte, 

dike

' 52-Sn-37

58.03 

17.16 

2.85 

3.87 

3.16 

6.95 

3.77 

2.07 

.08 

.74 

.76 

.03 

.30 

.13 

.04 

" .00 

.16

100.10 

.05

100. 05

»Includes 0.14 percent BaO, not shown.
DELAROF AND WESTERNMOST ANDREANOF ISLANDS 223

whether the rock is pyroclastic or lava, and the sedimentary rocks 

appear to have been derived from similar rocks. Significant 

variations within the basaltic and andesitic groups are indicated 

by a wide variation in type and frequency of mafic phenocrysts, 

and it seems probable that a few of the lighter colored rocks, 

in particular the pyroclastic deposits, would fall in the dacite or 

rhyodacite groups of the chemical classification of Rittmann 

(1952). No rocks as light colored as the rhyodacite dike (specimen 52-Sn-l 00) were found in the layered sequence, and at least 

90 percent of the sequence is probably more basic than dacite.

The root names (andesite, basalt) used in this report for 

analysed aphanitic rocks are those appearing in the chemical 

classification of Rittmann. In the text the full Rittmann name 

is given parenthetically for each analysed rock. Pigeonite appears 

in the calculated Rittmann name for specimens 52-N-64, 52- 

W-64, and 52-W-70, but no pigeonite was found in the modes. 

Moreover, pigeonites, as used by Rittmann (1952, p. 78) are 

"mixed crystals of diopside and clinohypersthene" not further 

defined by optic angle and may, therefore, include augite and 

other clinopyroxenes as defined by others.

Specimen 52-N-64 is a dark, greenish-gray augite andesite 

(Rittmann: pigeonite andesite). Phenocrysts of augite, magnetite, and zoned (oscillatory) calcic plagioclase are set in a groundmass of plagioclase microlites («An50 ) and opaque oxides. The 

larger plagioclase phenocrysts are often poikilitic and usually 

contain several zones of dusty inclusions.

Specimen 52-W-64 (Rittmann: pigeonite andesite) is a lightgray porphyritic augite andesite. The phenocrysts (<2 mm) 

are progressively zoned calcic plagioclase, augite, magnetite, and 

one ghost crystal of hornblende ("opacite"). The groundmass 

contains microphenocrysts of plagioclase, clinopyroxene, and 

opaque oxide in a cryptocrystalline matrix.

Specimen 52-W-70 is dark-gray labradorite-augite andesite 

(Rittmann: pigeonite labradorite andesite) with numerous black, 

subangular to rounded cryptocrystalline inclusions (0.1-1.3 cm) 

of unknown composition, but possibly altered basaltic glass or 

tuff. The inclusions are opaque on the borders and are crowded 

with opaque dust throughout; small crystals of plagioclase and 

clinopyroxene have begun to form within the inclusions. The 

igneous matrix for these inclusions is markedly porphyritic with 

zoned, poikilitic crystals of calcic plagioclase (wide dusty zones 

and areas within the crystals), augite, and corroded olivine in 

a groundmass of labradorite microlites and opaque oxides. There 

is a marked flow structure in the groundmass.
224 rNTVESTIGATIONS, OF ALASKAN VOLCANOES. '

Specimen 52-Fs-94 (Rittmann: andesine basalt) is a black, 

very fine grained rock. About 1 percent consists of scattered 

phenocrysts of plagioclase, clinopyroxene, and pseudomorphs after 

olivine. The groundmass has a strong flow structure and consists 

of microlites of calcic andesine, clinopyroxene, and opaque oxides.

Other rocks of this group are generally similar. Augitic 

pyroxene, hypersthene, oxyhornblende, and olivine are common 

mafic phenocrysts. Phenocrystic plagioclase is usually labradorite 

or more calcic and strongly zoned. Andesites with both clinopyroxene and orthopyroxene are perhaps the most common rocks, 

but many fall close to the andesite-basalt boundary and are difficult to classify without chemical analyses.

Throughout the islands areas of bleached and iron-stained 

rocks, believed to result from fumarolic activity, are common; 

one large, poorly defined area contains abundant epidote stringers 

which probably indicate a different type of alteration. This area 

extends from Kanaga Bay to Cape Chlanak and northward toward 

Naga Point on the east coast of Kanaga Island. An andesitic 

dike swarm and at least one granodioritic dike invade the volcanic 

complex in this area and may be related to the epidotization.

Plates 28, 29, and 30 show typical areal distributions of the 

rocks. Outcrops are confined largely to coastal areas; many 

major lithologic changes occur across and along strikes. Bedding 

attitudes vary greatly and in thick, coarse pyroclastic deposits 

or massive tuff layers are often impossible to obtain; local unconformities exist in many outcrops. Key beds have not been 

found; consequently, mapping of structures and separating even 

large areas into broad map units is not possible in reconnaissance. 

Much more field and laboratory detail is necessary before an 

adequate generalized map of the area can be made.

Age. Fossils were found on Kanaga and Tanaga Islands. Rocks 

of the central Delarof Islands are grouped with the fossiliferous 

rocks because of lithologic similarities and geographic proximity. 

Table 1 is a summary of the paleontologic evidence for dating 

rocks on Kanaga and Tanaga. The rocks are believed to be largely 

Miocene and Pliocene but may include some Pleistocene.

Similar paleontologic evidence for rocks older than Miocene is 

lacking. The epidotized rocks between Kanaga Bay and Naga 

Point resemble Finger Bay volcanics known to be, in part, 

Paleozoic (Coats, 1956a); but upper Tertiary fossils were found 

in the altered area on Kanaga (table 1, loc. 1). A basal contact 

with older rocks has not been found. The upper contact with lava 

of the precaldera shield volcanoes is apparently unconformable 

in some areas and gradational elsewhere (p. 225).
DELAROF AND WESTERNMOST ANDREANOF ISLANDS 225 

EARLY SHIELD VOLCANOES

The radially dipping lava flows of partially destroyed early 

shield volcanoes on northern Kanaga and Tanaga Islands have 

been mapped separately on plate 27, although the contact with 

rocks farther south is probably transitional in part. Coats (1956b) 

has described those rocks for northern Kanaga and mapped them 

in more detail than is shown here. In one area on the northwest 

coast of Kanaga Island, lava flows of ancient Mount Kanaton 

(Coats, 1956b, p. 72), which predate caldera subsidence, overlie 

the fossiliferous beds (Iocs. 4, 5, and 10 in table 1) with apparent 

angular discordance. The time interval between deposition of 

these contrasting rock types may be very short. Because the 

fossiliferous sedimentary rocks are characterized by variable 

initial dips and local unconformities, an apparent unconformity 

here may lack significance. Lava indistinguishable from that of 

the shield volcano is intercalated farther south with pyroclastic 

debris and sediment typical of southern Kanaga. A break in 

topographic slope separates ancient Mount Kanaton from planed 

southern Kanaga; this critical area is covered by a thick ash, 

soil and tundra mantle. On the northeast coast of Kanaga Island, 

6.5 miles south of Kanaga Volcano, lava flows of ancient Mount 

Kanaton (?) are interlayered with pyroclastic deposits, and a 

gradational contact is suggested.

The corresponding contact on Tanaga Island where the topographic break is even less well developed than on Kanaga, was 

not visited.

The upper contact on both Kanaga and Tanaga is based largely 

on topography and location of undissected volcanoes within caldera 

remnants.

Age. Coats (1956b, p. 78) has assigned a late Tertiary and 

Pleistocene age to the lava of ancient Mount Kanaton and associated precaldera volcanic centers on Kanaga Island. He has 

postulated a late Pleistocene date for caldera formation. Corresponding rocks on Tanaga Island are assumed to be about 

the same age.

RECENT VOLCANIC CONES

Gareloi, Tanaga, and Kanaga Volcanoes, which have relatively 

undissected composite cones with records of historic activity 

(Coats, 1950), and dormant remnants of slightly older volcanoes 

on Tanaga Island are of Recent age. Relations of Recent volcanoes 

with the early shield volcanoes on Tanaga have not been determined. The rocks composing the three active cones have initial 

dips as steep as 35° and rest unconformably on older rocks in
226 INVESTIGATIONS OF ALASKAN VOLCANOES

Kanaga and Tanaga. The younger cones are built within calderas 

formed in older volcanoes.

Gareloi and Kanaga Volcanoes and parts of northern Tanaga 

have been studied by Coats (1950, 1952, 1956b, c; 1954, written 

communication). The rocks of Kanaga Volcano and ancestral 

Mount Kanaton are basaltic or andesitic and their lime and alkali 

parameters show the province to be calc-alkaline (Coats, 1952, 

p. 485). Basaltic and (or) andesitic rocks occur on Gareloi 

Volcano, and presumably exist on Tanaga Volcano. The strikingly 

similar chemical composition of these rocks and those farther 

south indicates that this sector of the Aleutian arc has been 

dominated by basaltic and andesitic rocks since Miocene time 

and perhaps longer.

The material which forms the present surfaces of these conical 

volcanic mountains is largely postglacial. That glaciation was 

followed by volcanic eruptions is evident on northern Tanaga 

Island where an older glaciated cone is capped by lava extruded 

from a reactivated volcanic vent.

SURFICIAL DEPOSITS

Deposits of volcanic ash and soil covered by heavy tundra 

vegetation (without permafrost) mantle the islands inland from 

the sea cliffs. Locally the ash on northern Kanaga, which contains thin layers of pumiceous lapilli and reddish-brown soil zones, 

reaches a maximum thickness of 20 feet and sometimes forms 

dunes. The ash blanket is continuous on the low, flat islands 

of Unalga, Kavalga, Ogliuga, and Skagul, but quite thin and discontinuous on Ulak and Amatignak. Quaternary volcanoes nearby 

were probably the source. Because of abundant loose, fresh 

pumiceous lapilli in the deposits of the central Delarofs, the 

ash layer there may be younger than the stratified sequences 

with intercalated soil layers on Kanaga and Tanaga Islands. A 

thin dark-gray soil is usually formed on the surface of the ash. 

Peaty deposits have formed in some boggy areas.

On several islands, notably Ogliuga, Skagul, and Kanaga, extensive sand dunes are stabilized by vegetation (pis. 28, 30). 

Stream-transported alluvium, mostly fine pyroclastic material, 

locally fills small basins and valleys. Deltas form at protected 

bay heads.

Discontinuous gravel, cobble, and boulder beaches fringe all 

the coves and bays of the islands, commonly with a wave-cut 

bench offshore. Abandoned beach deposits at higher level are 

sometimes preserved. Coats (1956c, p. 92) reports that southern 

Tanaga is mantled by marine gravels resting on a wave-cut plat-
DELAROF AND WESTERNMOST ANDREANOF ISLANDS 227

form, and he also reports that similar boulder gravels on Ogliuga 

are overlain by coarse volcanic cinders.

Glacial debris was recognized in cirques and cols at higher altitudes on Amatignak and on northern Tanaga Island. Ulak Island 

presents a scoured appearance, but no morainal material was 

found.

Coats (1956b, pi. 15) has mapped an andesitic ash layer of 

caldera age on northern Kanaga Island. This layer, conspicuous 

as coarse and fine pyroclastic debris on the surface of Kanaton 

Ridge, is buried beneath more extensive layers of younger ash 

south of ancient Mount Kanaton. On the south bank of a large 

lake 6.4 miles south of Kanaga Volcano the older ash is beneath 

about 20 feet of the finer Recent ash as a decomposed zone of 

coarse pumice one foot thick. Neither the younger ash nor the 

buried ash are mapped in this report.

FINE-GRAINED INTRUSIVE ROCKS

Many shallow intrusive rocks, usually andesitic or basaltic, 

either porphyritic or aphyric, cut the bedrock units south of 

Recent volcanic cones and older shield volcanoes. Nearly vertical 

dikes are usual, either as single units or as dike swarms.

The most conspicuous dike swarm is in the area on southeastern 

Kanaga Island extending 2^ miles north of Cape Chlanak. The 

dikes are andesitic and contain needle hornblende. Most trend 

northwest, but several at Cape Chlanak trend west.

Basalt. Many basalt dikes, most trending northwest, are on 

the north coast of Kanaga Island about 5 miles northwest of 

Kanaga Bay. Several domelike masses of basalt, as much as 400 

yards in greatest dimension, are visible along the coast in the 

same area. One basalt mass, with a radius of 150 to 200 yards, 

in cross section is steep sided, bulbous, and concentrically divided 

into layers about 10 feet thick. Each concentric layer has a 

separate set of radial columnar joints.

These rocks contain conspicuous, sporadically large inclusions 

of dark gabbroic rocks rich in anorthite, olivine, clinopyroxene, 

and chromite. Many of the inclusions have been stretched into 

vague schlieren. One of the inclusions covers a surface area of 400 

square feet or more. Less abundant angular inclusions of tuffaceous(?) siltstone, apparently derived from the sedimentary and 

pyroclastic rocks, are also found.

The basalt is characterized by a strong fluidal structure of calcic 

plagioclase microlites. A great variety of dark minerals olivine, 

orthopyroxene, clinopyroxene, and oxyhornblende may appear
228 INVESTIGATIONS OF ALASKAN VOLCANOES

in the same slide. Some of the mafic minerals are probably xenocrysts from gabbroic inclusions. Elongate vesicles filled with secondary minerals are present in several outcrops.

These intrusions may be one of the many types of volcanic 

domes usually found in and near volcanic vents (Williams, 1932), 

but more probably they are bulbous developments in shallow sills. 

Evidence of doming was found at one contact with overlying tuffbreccia ; basalt dikes as much as 60 feet wide also invade the tuffbreccia. Although the sill-like character of the Kanaga structures 

is not certain, the joint patterns do suggest tabular masses (either 

sills or flows) and this origin seems more likely than plugs or 

domes within vents.

Limburgite. A half mile inland from the coastal area which 

shows domed sills (?) and dikes, a fragmental rock with abundant 

olivine-rich inclusions as much as a foot in diameter forms an isolated outcrop about 10 by 50 feet. The olivine inclusions, both 

rounded and tabular, are mixed with less abundant, very angular 

inclusions of tuffaceous limestone in a limburgite matrix. Ross 

and others (1954, p. 701) examined some of these inclusions and 

reported that chromium diopside, enstatite, and chromium spinel 

are associated with dominant olivine in the ultrabasic inclusions. 

Possibly this outcrop is a petrographically extreme development 

of the inclusion-rich, domelike structures just described. The 

abundance, size, and angularity of the inclusions and their association with sedimentary inclusions suggest a shallow nearby source 

(cf. Ross and others, 1954, p. 693).

Andesite. On southwestern Kanaga (pi. 28) thick, coarsely 

porphyritic sills of andesite comprise the sea cliff for long distances. One of these sills has vertical columns 10 feet across and 

at least 100 feet high. Topography on the headland 1 mile southeast of Western Point suggests a thickness greater than 300 feet. 

Top and bottom contacts were not observed, but nearly vertical 

end contacts at two locations and one exposure of contact breccia 

suggests either postemplacement faulting or forceful transgressive intrusion.

These rocks are characterized by ragged phenocrysts of biotite 

and (or) oxyhornblende with "opacite" rims. The rocks also contain abundant phenocrysts of zoned plagioclase (labradorite-oligoclase) with or without dusty zones. Progressive, oscillatory, and 

reversed zoning occur. Clinopyroxene and orthopyroxene, magnetite, and a few rounded quartz grains are also present. The rocks 

are probably siliceous andesite, but chemical analyses would be required to classify them exactly.
DELAROF AND WESTERNMOST ANDREANOF ISLANDS 229

"Specimen 52-W-38 (Rittmann: labradorite daciter)" is from a 

light-gray columnar sill 2.5 miles northwest of Cape Tusik on 

southern Kanaga. This sill is finer grained and megascopically less 

porphyritic than the sills on southwest Kanaga. Phenocrysts, 

mostly less than 1 millimeter in diameter, make up 25 percent of 

the rock as follows: abundant zoned labradorite-bytownite with 

thin rims of oligoclase-andesine and, commonly, oscillatory zoning; less abundant oligoclase-andesine crystals surrounded by a 

zone crowded with dark inclusions, then a rim of labradoriteoligoclase with strong oscillatory zoning (included here are many 

phenocrysts of labradorite with center parts showing patchy extinction); clinopyroxene and orthopyroxene, both rare (<0.5 

mm); abundant subhedral opaque oxides (<0.2 mm); rare 

sphene; ghost hornblende (<2 mm) now composed of fine pyroxene, opaque oxide, and feldspar; rounded quartz relics (<0.6 

mm) surrounded by rims of granular clinopyroxene in a brown, 

weakly anistropic matrix. The strongly fluidal groundmass is composed of: abundant lath- and blocky-shaped crystals of zoned 

oligoclase-andesine (<0.2 mm); many stubby crystals of clinopyroxene (<0.15 mm); many blocky crystals and grains of 

opaque oxide (<0.07 mm); irregular grains and masses of carbonate, some within pyroxene crystals and some within plagioclase 

crystals; abundant interstitial isotropic, material having a low index of refraction and a fracture pattern suggestive of cristobalite 

or opal.

Both the western and southern Kanaga sills are complex rocks. 

Feldspars of several types showing internal zones crowded with 

inclusions and oscillatory or reversed compositional zones, 

rounded quartz relics, and hornblende ghosts all suggest hybridization or at least a very complex cooling history.

Rhyodacite. Specimen 52-Sn-100 (Rittmann: rhyodacite) is 

from a very light-colored podlike body about 200 yards wide, exposed 4.8 miles west of the mouth of Kanaga Bay on the south 

coast of Kanaga Island. This was the only light rhyodacite found 

in the Delarof-Kanaga area. Quartz, oligoclase, and biotite are 

present in the white to light-green glassy matrix (n^ 1.476) 

which makes up about 85 percent of the rock.

Specimen 52-Sn-37 (Rittmann: dark rhyodacite) is of quite different composition (table 2) and would be classified microscopically as vitrophyric labradorite dacite. This specimen is a darkgray rock from the chill border of a small dike on northwest Kanaga. Its texture is porphyritic (« 35 percent phenocrysts) with 

a hyalo-felty ground mass and strong flow structure. Phenocrystic
230 INVESTIGATIONS OF ALASKAN VOLCANOES

plagioclase (<4 mm) is occasionally uniform, but usually shows 

either progressive or distinct oscillatory zoning. Composition 

ranges from An72-46 , with most of the determinations about An54 . 

A few of the crystals contain dusty zones of glassy inclusions. Augite (often as an overgrowth on hypersthene), hypersthene, 

opaque oxides, and rare ghost hornblende with a rim of granular 

pyroxene are also present as phenocrysts; small grains of cristobalite(?) are less conspicuous, and these are all set in a brown 

glass (n between 1.506 and 1.510) containing numerous plagioclase (An32-43 ) microlites. Potash, present in the analysis, is not 

expressed mineralogically and must be contained, along with occult quartz, in the low-index glass.

Summary and age. Most of the aphanitic intrusions studied 

microscopically have a porphyritic texture with phenocrysts of 

zoned calcic plagioclase (rarely, more sodic types) and one or 

more of the following dark minerals: augitic pyroxene, orthopyroxene, oxyhornblende, olivine, and (rarely) biotite. The compositions and textures present in the dikes and sills are more variable than those found in the flows of the intruded sequence, but 

most of the small intrusive masses are andesite or basalt.

No single period of intrusion can be assumed. Dikes found in 

the chloritized rocks on Ulak Island are themselves altered; however, fresh-appearing andesite dikes cut the granodiorite pluton 

that constitutes Ilak Island, and most of the dikes in the Naga 

Point-Kanaga Bay altered area are fresh. If alteration is associated with subjacent plutonic invasion, diking probably both preceded and followed that event. Diking almost certainly accompanied extrusive volcanism throughout the Tertiary.

One dike which cuts the fossiliferous sequence on northwest 

Kanaga (specimen 52-Sn-37) stops at the contact with overlying 

Kanaton lava. The dike raises the lava flows slightly but does not 

cut them, suggesting that it failed to penetrate the massive flows; 

thus some of the dikes may be Quaternary.

COARSE-GRAINED INTRUSIVE ROCKS

Coarse-grained intrusive rocks are found in several areas. A 

thick sill of quartz diorite makes up the outer end of Knob Point 

(pi. 27) on Amatignak Island. Other sills of diabase or diorite and 

perhaps larger plutons exist on that island.

Ilak Island is composed of medium- to coarse-grained granodiorite (specimen 52-Sn-167) and quartz diorite cut by numerous 

small aplite dikes (specimen 52-Sn-181) and a few andesite dikes 

10 to 200 feet wide. The largest andesite dike has a contact with
DELAROF AND WESTERNMOST ANDREANOF ISLANDS 231

the granodiorite indicating that both rocks may have been fluid at 

the same time. There are many rounded inclusions of coarse- and 

fine-grained material in the granodiorite, but contacts with the 

host rock are not exposed.

A granodiorite dike occurs on Kanaga Island between Cape 

Chlanak and Naga Point. Possibly the alteration (p. 224) and 

coarse dikes, together with the andesite dike swarm (p. 227), indicate a plutonic mass relatively close to the surface in that area.

Inclusions and boulders. Inclusions ranging in composition 

from dunite to granite are found in the shallow intrusions and 

flows on Kanaga. Large rounded boulders of gabbroic to granitic 

composition and also boulders of schist and gneiss were found in 

association with inclusions of similar rock in the lava cropping 

out along the shore of an inland lake 400 feet above sea level. Near 

tfce southeast tip of Skagul boulders of diorite, coarse-grained 

granite gneiss, and fine-grained granite occur. Boulders of coarsegrained rocks, possibly of local derivation from sills known to 

exist on the island, were also found on Amatignak.

Some of these boulders undoubtedly are weathered-out inclusions; others may be ice-rafted from known plutonic areas. One 

boulder on Amatignak was locked in the roots of a driftwood tree. 

As there are no trees in the Aleutians, this boulder may have come 

from a great distance, probably from the west. Large areas of 

metamorphic rock are not necessary to account for these boulders 

of gneiss and schist. On Unalaska Island the foliated rocks are 

confined to sheared roof pendants and narrow border zones on 

large plutons. No large areas of regionally metamorphosed rocks 

of even slate grade are known in the Aleutian Islands, and there 

is no evidence for an oldland of metamorphic and igneous rocks. 

However, inclusions in late Tertiary aphanites indicate that phir 

tonic rocks of diverse composition are present beneath the islands 

in some areas.

Age. All dated plutons in the Aleutian Islands are Tertiary> 

The Ilak pluton and other coarse-grained rocks in the DelarofKanaga area cannot be dated, nor is it certain that they are all 

the same age. At least two episodes of coarsergrained intrusion 

are possible. The inclusions in lava of late Tertiary age on 

Kanaga indicate older rocks, and the dike there permits inference 

of intrusion after late Tertiary. However, it cannot be stated defi^ 

nitely that two episodes are required; apophyses from one intrusion could cut some upper Tertiary rocks, and the same intrusion 

could supply inclusions for slightly younger upper Tertiary lava 

flows.
232 INVESTIGATIONS OF ALASKAN VOLCANOES

A middle Tertiary episode of intrusion, in the southern parts 

of several islands or groups of islands, followed by general uplift, 

erosion, and late Tertiary, central-type volcanism along the north 

edge of the chain has been suggested for other areas (Gates, Fraser, and Snyder, 1954). Possibly the Ilak pluton is one of several 

roughly synchronous intrusions belonging to this episode. The 

coarse-grained sills on Amatignak may be connected with magma 

chambers that supplied andesitic lava at different times during the 

Tertiary, and may or may not be genetically related to the granodiorite on Ilak.

STRUCTURE

The Aleutian Islands may be considered as peaks of a submerged mountain range rising 25,000 feet from the floor of the 

Aleutian Trench and 13,000 feet from the deep on the concave 

side. Major configurations are apparent from regional studies of 

submarine contours (Murray, 1945). The Aleutian arc is single 

in its western part and therefore is probably simpler and younger 

than double arcs in the circum-Pacific belt (Umbgrove, 1947, p. 

185).

Trend lines. Most of the area is covered by the sea and many 

structures must be presented as interpretations (pi. 27) from 

fragmentary subaerial and submarine data (pis. 27-31). Structures interpreted by trend lines on plate 27 are of three types: (1) 

linear subaerial drainage basins, (2) straight shorelines, and (3) 

submarine scarps, troughs, and slopes. Most of these alinements 

probably reflect faults whose precise locations are not known. 

Some of the linear features may reflect folds. Individual trend 

lines offer no positive proof of structural control. Together the 

trend lines form a geometric pattern which would be difficult to 

obtain by nonstructural methods. We believe that the submarine 

topography is controlled by longitudinal, perpendicular transverse, and oblique transverse fractures, which apparently are 

geometrically and genetically related to the arc as a whole.

MAJOR STRUCTURES

In general the main ridge north of the Aleutian Trench probably is a complex arch bounded on both sides by faults. One thrust 

zone dipping north from its trace on the north side of the Aleutian 

Trench is indicated by seismic evidence and by comparison with 

better known arcs where the seismic data furnish compelling evidence for such a fault (Benioff, 1949; 1955).

A second major fault bounds the arch on the north. Alinement 

of volcanoes along the north edge of the islands has been noted by 

all workers in the area. Gates and Gibson (1956) and Snyder
DELAROF AND WESTERNMOST ANDREANOF ISLANDS 233

(1957) have demonstrated b.y detailed submarine contouring that 

this alinement, in some areas, is coincident with, a high-angle, 

normal (?) fault downthrown on the north side. A 1,200-foot submarine scarp is evident west of the Delarofs between Little Sitkin 

and Semisopochnoi Volcanoes. A similar fault or fault zone is 

probably buried beneath coalescing volcanic piles in the DelarofAndreanof area.

This latter fault appears to have been offset by transverse 

faults. The Delarof sector shows southerly displacement relative 

to the Rat and Andreanof sectors, so that Gareloi Volcano lies 

south of lines connecting volcanoes on either side. Amchitka Pass, 

west of the Delarofs, and Tanaga Pass probably were formed by 

large transverse faults.

The Amchitka Pass area is unique for the Aleutian chain. Amatignak Island, east of the pass, is farther south than any other 

island of the chain and is bounded on the northwest by a 6,000-foot 

depression (pi. 27). Amchitka Island, west of the pass, and the 

Southern Delarof Islands appear to be dragged south with respect 

to adjacent parts of the chain. West of Amchitka Pass the trend 

of the arc is northwest rather than west. Most significant is the 

345-mile arcuate ridge, Bowers Bank (Murray, 1945), that curves 

north and west of Amchitka Pass. The Amchitka Pass area, therefore, is the site of an intersection of two large mountain ranges, 

but an analysis of the structures is not possible with available 

data.

The trend of the main arch is shown on plate 27. Its crest passes 

north of the central Delarofs and south of Tanaga and Kanaga. 

Unfaulted segments of the south flank of the arch show a smooth 

slope more gentle than that to the north, where longitudinal faulting modified by volcanic deposition gives a different profile. How 

much of the movement on this arch is bending and how much is 

discontinuous movement on high-angle longitudinal faults similar 

to the north boundary fault is unknown. Abundant high-angle 

dikes on most of the islands may indicate a tensional environment 

(Anderson, 1951, p. 3). Dynamic metamorphism and tight folding 

are absent, possibly confirming the hypothesis of tension near the 

crest of the main arch. The entire arch may be a drag feature on 

the main underlying thrust zone, or it may be the anticlinal lip of 

a tectogene formed by plastic deformation (cf. Benioff, 1955; 

Vening Meinesz, 1955).

MINOR STRUCTURES

Folds. In addition to the main arch, dip reversals indicate several smaller folds (pi. 27). Data are fragmentary, and local
234 INVESTIGATIONS OF ALASKAN VOLCANOES

changes in dip resulting from conditions of deposition and faulting make all of these folds questionable.

Probable northeast-trending folds on Kavalga and Tanaga (and 

also on Adak Island 10 miles east of Kanaga) contrast with possible northwest-trending folds on Amatignak Island. Unusually 

steep beds on northeast Kavalga are probably dragged on a strong 

north-trending fault zone (pis. 27 and 29).

The large and poorly exposed area of central Kanaga shows 

many beds trending north and northeast; many have dips too 

steep or persistent to be attributed to chaotic deposition. A dome 

at Cape Tusik and northerly folds or faults elsewhere may explain 

some of these anomalous attitudes.

Faults. The major longitudinal and transverse faults believed 

to exist beneath the sea have been mentioned. Probably some of 

the faults that cut the islands have not been found because of the 

lack of lithologic marker beds, erasure of scarps by wave planation, and cover by ash and tundra. Faults that have been recognized in the field are mostly high-angle shear zones, silicified and 

stained with iron oxides. North- and northwest-trending faults of 

this type are evident on Kavalga (pi. 29).

A small reverse fault on the south shore of Kanaga Island (pi. 

27) shows apparent vertical displacement of 20 feet. A steaming 

fissure passing through Kanaga Volcano strikes N. 65°-70° W. 

On western Kanaga large andesite porphyry sills commonly end 

in steep side contacts that are probably faults (pi. 28). Small 

faults are shown on Ulak and Tanaga (pi. 27).

A possible north-trending fault between Ulak and Amatignak 

is suggested by the different attitudes of the beds on the two islands, and by the steepening of the beds on Amatignak. Relative 

altitudes of the two islands and the dragged beds suggest that the 

west side is upthrown (pi. 27).

Calderas. Coats (1950, p. 43) found incomplete calderas on 

both Kanaga and Tanaga Islands, the northern sections presumably destroyed by faulting or marine erosion. These structures 

are approximately coincident with the east and south margins of 

coastal areas of volcanic rocks of Quaternary age shown on 

plate 27 for Kanaga and Tanaga Islands. Possibly the north rims 

of these structures were never completely formed. Arcuate normal 

faults, downthrown on the north and roughly alined with a larger 

east-west structural trend, may explain the calderalike rims better 

than the theory of partly eroded conventional calderas. (Williams, 

1941, p. 242; van Bemmelen, 1954, p. 88-92.)
DELAROF AND WESTERNMOST ANDREANOF ISLANDS 235 

SUBMARINE: TOPOGRAPHY

Submarine contours in most of the area from longitude 180° 

east to Adak Strait, and including northern Tanaga Island and 

Amatignak, are shown on plate 27. The map was prepared from 

U. S. Coast and Geodetic Survey boat sheets at several larger 

scales. The contour interval is 300 feet. The map scale and the 

density and precision of location of soundings give the contour 

lines the status of form lines. Figure 45 is a detailed map of the 

bottom topography around the central Delarof Islands.

Many sea valleys, ridges, scarps and passes show strong alinements miles in length. These alinements commonly trend in one 

of four directions: west, almost parallel with the terrestrial Aleutian Ridge line; obliquely northwest; obliquely northeast; or 

roughly north at right angles to the ridge. The linear character of 

ridges, valleys, and scarps, and the presence of blocklike seahighs 

suggest that much of the bottom topography reflects geologic 

structure.

The islands of Unalga, Kavalga, Ogliuga, and Skagul emerge 

from a shallow submarine platform orientated generally east-west 

along the Aleutian arc (pi. 27 and pi. 31). The platform extends 

west of the Delarof s across Amchitka Pass; east of this group of 

islands it appears to be offset to the north and presumably is coincident with the flat southern parts of Tanaga and Kanaga Islands.

The crest of the Aleutian Ridge between the 180th meridian 

and Skagul Island is bounded on the south by a steep slope at 

least 1,500 feet high. At the foot of the slopes just south of Kavalga and west of longitude 179°50' W. the bottom flattens to 

form a shallow trough with seaknolls alined along its outer edge. 

The knolls are flat topped with a steep northern margin and a more 

gentle southern slope. Probably the trough was once continuous 

along the Delarof Islands and across Amchitka Pass, but it is now 

disrupted north of Amatignak by a 2,700-foot slope which drops 

into a large depression whose maximum depth is 6,000 feet. The 

depression is a triangular area between the Amatignak-Ulak Islands block, a seahigh 16 miles northwest of Amatignak, and the 

crest of the Aleutian Ridge. The greatest depths are at the southeast corner of the triangle.

The east-west, or longitudinal, trend is dominant along the central Delarof s and west to longitude 180°, but it is only generally 

evident east of Tanaga Pass as the boundary of the emergent 

Aleutian Ridge. The submarine topography north of the islands is 

only partly contoured, but additional evidence of U. S. Coast and 

Geodetic Survey hydrographic charts and analogy with the Kiska-
236 INVESTIGATIONS OF ALASKAN VOLCANOES
DELAROF AND WESTERNMOST ANDREANOF ISLANDS 237

Amchitka area (Gibson and Nichols, 1953, pi. 1) indicates the 

slopes are steeper than those south of the islands.

Tanaga Pass and the pass east of Kavalga Island (fig. 45) have 

features with strong northwesterly alinements (N. 35° W. to N. 

50° W.). The east side of Ulak Island is the west edge of a parallel-sided, flat-bottomed trench trending northwest; the pass between Kavalga and Unalga apparently is an extension of this 

trend offset to the east. Other features with evident northwesterly 

trends intersect the seahigh 16 miles northwest of Amatignak and 

bound it on the northeast; they can be seen on central Ulak and 

southeast Tanaga as linear subaerial features. The north margin 

of Tanaga Bay trends northwest, in line with the linear zone on 

the island.

The most obvious northwesterly alinements of bottom topography are in the Delarof Islands-Amchitka Pass area; such alinements are largely absent east of Tanaga Pass.

A zone of closely spaced northeasterly trends (N. 25° E. to N. 

50° E.) is apparent south of Ilak and Tanaga Islands, and may be 

the main structural control on Kanaga Pass. This trend also outlines the south margin of the Amatignak-Ulak Islands block. Submarine valleys and seaknolls oriented northeast are evident in 

Amchitka Pass to a lesser extent.

Northerly trends are also evident and two north-south scarps 

are particularly noticeable: one is south of Kanaga Island, in line 

with Adak Strait; the second is in Amchitka Pass, near the 180th 

meridian and intersects the Aleutian Ridge crest at the base of a 

steep west-facing slope. Amchitka Seavalley (Gibson and Nichols, 

1953, pi. 1) also trends north. Skagul Pass (pis. 27, 31) is an example of this trend, and many smaller alinements have been contoured.

The southwest peninsula of Ulak Island is triangular. This 

shape is probably the result of the intersection of northwest-, 

northeast-, and north-trending structural breaks. In general, the 

orientation of long straight coastlines and interisland passes 

throughout the area shown on plate 27 is closely coincident with 

the four main directions of submarine alinements (p. 235), with 

the directions of subaerial trends, and with the strike of dikes.

PHYSIOGRAPHY

Kanaga Island. Landforms on Kanaga Island illustrate the 

concept of constructive volcanism coupled with continual erosion. 

The symmetrical, complete cone of Kanaga Volcano at the north 

end of the island is the most recent constructional landform.
238 INVESTIGATIONS OF ALASKAN VOLCANOES

Steam issues from a vent near the summit, and flows of blocky 

lava in 1906 (Coats, 1947, p. 93) indicate present day volcanic 

activity. Continuing construction of the cone has inhibited dissection by streams. The shoreline is concentric around the volcano 

and is scarcely eroded by the sea.

The arcuate calderalike scarp of Kanaton Ridge south and east 

of Kanaga Volcano confines the younger deposits within a halfbowl depression (Coats, 1956b, p. 76). Many branching streams 

flow southward from the scarp lip. They are underfit in broad 

valleys which radiate from Kanaton Ridge, and are separated by 

almost continuous sharp interfluves of high relief. The valleys 

were developed upon the slopes of the ancient Kanaton Volcano. 

Cliff recession on the west shore has carved bluffs at least 400 feet 

high into the rocks of the old volcano and the underlying sedimentary rocks.

Planed surfaces on Kanaga Island. About 3 miles south of the 

lip of Kanaton Ridge, the streams join a transverse drainage direction in the flatlands below 500 feet in altitude. This northwestsoutheast drainage is the first expression of the older, wave-planed 

surfaces of the southern and western parts of Kanaga.

Five major levels of planation are inferred from the topographic 

profiles (fig. 46). Each level is continuous and bounded by distinct breaks in slope many feet high. Abundant ponds and lakes 

on the several levels emphasize their flatness. The irregularly 

dashed lines on the map, which indicate the levels, were drawn 

from drainage patterns and slope changes shown on topographic 

maps and aerial photographs.

The broad central area of the island is topped by the highest 

planar surface, level 1, which has an altitude of more than 500 feet 

on the northwest and 400 feet on the southeast. (On the map, level

1 includes a minor surface about 50 feet below the uppermost flatlands.) A steep scarp 100 to 150 feet high separates level 1 from 

level 2, at an altitude of 260 to 280 feet on the northwest and 

slightly more than 200 feet on the southeast side of the island. 

Level 2 is easily seen at the narrowed neck separating the northern volcanic highlands from the rest of the island. The highest 

flat ponded area west of the central area, along the narrow prolongation of the island, is correlated with level 2; presumably level

2 also can be correlated with the higher altitudes at the extreme 

west end of Kanaga, north and west of Cape Chunu. Level 3, 

mostly about 220 feet in altitude, is well defined near the west edge 

of the central area; it makes up much of the slender western extension of the island and may be present locally along the steep
DELAROF AND WESTERNMOST ANDREANOP ISLANDS 239
240 INVESTIGATIONS OF ALASKAN VOLCANOES

coast of the central part of the island on the northwest. Level 4 

is at altitudes from 100 to 120 feet and is best preserved on the 

western elongation of the island; it is definable, however, near the 

west coast of the narrow neck area and as small remnants elsewhere. Level 5 (not shown in fig. 46) includes the narrow benches 

along the present shoreline. Sheer cliffs commonly separate level 

5 from higher levels. Minor terraces which are suggested by the 

topographic map configurations and are locally apparent in the 

field, particularly in the levels below level 2, are not discussed here.

The planed surfaces are almost continuous in the broad central 

region and western elongation of Kanaga Island (fig. 46), but 

along the east coast north of Naga Point only levels 1 and 2 are 

present. Level 2 merges with the south slopes of Kanaton Ridge. 

The continuity of the surfaces inland from Cape Chlanak, southeastern Kanaga, is less apparent because of extensive stream erosion parallel with a broad northwest-trending dike swarm.

The drainage divide for most of Kanaga Island, excluding the 

volcanic highlands, is close to the north shore (fig. 46). Streams 

flowing northward into the Bering Sea from the central region 

drop as much as 600 feet per mile down a series of cliffs and 

scarps; those flowing southward into the Pacific Ocean are not so 

steep. The asymmetry is as marked farther west, but altitudes 

are lower. The tendency of the drainage to take southerly courses 

across most of the central region is commonly thwarted by structural controls, mostly bedding and diking.

Presumably, the greater age and higher altitudes of the central 

area are responsible for the close coincidence of drainage direction with four or five known structural trends. On level 1 the 

streams flow southwest or northeast, roughly parallel with the 

length of the island. Streams below level 1 commonly follow the 

other structural trends. Southeast drainage is dominant along 

the east coast. The two deeply incised valleys leading into Kanaga 

Bay from the north reflect a north-trending structural control visible in Kanaga Bay and extending north along the west margin of 

northern Kanaga Island (pi. 27). The parallel southward drainage along the island's western elongation may be simply downslope or may follow northerly strikes in the area.

Sea-level benches now forming on the south side of Kanaga are 

notably broader and more continuous than those on the north. This 

difference in width is consistent with the concept of a planation 

platform of marine origin tilted by up-faulting at the north. The 

fault scarp, subject to direct wave action, would recede with much 

of its steepness preserved; the newly inundated platform areas
DELAROF AND WESTERNMOST ANDREANOP ISLANDS 241

across the island, on the other hand, would favor water-level and 

subaerial weathering and the consequent development of a wider 

bench (Hills, 1949). Predominant southerly winds probably also 

contribute to the formation of broader benches along the south 

coast.

Physiographic evidence and geometric construction suggest 

southeastward tilting of the entire Kanaga Island block, excepting the volcanoes. A slope of at least 150 feet in 5.4 miles is indicated across central Kanaga. This slope is opposite to the prevalent dips.

Tanaga Island. The physiography of Tanaga Island is similar 

to that of Kanaga. The northernmost part of the island is, like 

Kanaga, a mountainous area that has been active volcanically during and since the Pleistocene. The volcanoes are alined east-west, 

and include relatively undissected, ash-covered cones and dissected 

shield volcanoes, some heavily glaciated. The glaciated peak nearest Gusty Bay has crevassed ice around the summit depression, and 

at least four recent cones to the east and south have poured lava 

and ash upon radial valleys and ridges. Tanaga Volcano and its 

twin to the west are located within an arcuate structure, possibly 

a caldera. The volcano at the northwest tip of the island appears 

also to have been built up within a similar structure.

Ponded areas and abandoned beach'ridges resemble those on 

Kanaga, and imply a related emergence. Coats (1956c, p. 92) 

states that most of south Tanaga is a wave-cut platform mantled 

by marine gravel. Correlation with the planed surfaces of Kanaga 

Island has not been attempted.

The Delarof Islands. The low, nearly flat Delarof Islands (pis. 

31, 32) appear to be blocks planed smooth by waves and later 

faulted. Profiles drawn transverse to the trend of main topographic breaks on five islands (fig. 47) show the apparently unrelated orientations of subaerial planar surfaces.

Bedding traces are well exposed on wave-planed surfaces 

around most of these islands. On Ulak, where the ash cover is 

thin, a west-northwest bedding trend is reflected inland by alinement of ponds. This higher surface probably resulted from wave 

planation, possibly followed by glacial scouring.

Glacial erosion has etched a north-south backbone through the 

south half of Amatignak Island. .The ridge crests trend northnortheast across the north half. A high area near the northwestern coast is underlain by more massive rocks. No continuous 

surfaces of planation are evident around the island. Either the 

emergence of Amatignak was later than the benching observed on 

nearby islands, or glaciation removed all trace of wave-cut planes.
242 INVESTIGATIONS OF ALASKAN VOLCANOES

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DELAROP AND WESTERNMOST ANDREANOF ISLANDS 243 

GEOLOGIC HISTORY

The pre-Miocene history of this area is unknown, but two possible interpretations are suggested by available data: (1) Rocks 

in the Amatignak-Ulak area were deposited in early Tertiary in a 

fairly stable geosynclinal trough or on its flanks and were invaded 

and altered by granodiorite plutons. The area was uplifted and 

eroded during or after the time of intrusion. Later, in Miocene 

time, marine and subaerial volcanism began in the central Delarof-southern Kanaga area. These young Tertiary chaotic deposits 

contrast markedly with the well-bedded older rocks farther south. 

(2) Another possibility is that rocks in the Amatignak-Ulak area 

are actually the well-bedded, deeper water facies of Miocene deposits farther north (which are nearer their source) and the alteration of the southern facies is a result, not of greater age, but 

either of proximity to late Tertiary plutons or of different diagenetic conditions within the geosyncline.

The known geologic history, then, begins in Miocene time with 

the deposition of poorly bedded volcanic rocks. East-west normal 

faults probably localized a chain of volcanoes near the latitude of 

the central Delarofs, and this zone apparently moved northward 

to its present location. Depth contours of the present volcano belt 

on the north edge of the islands reveal many volcanic cones. 

These, and the volcanoes projecting above the sea, form a series 

of coalescing volcanic piles. Conditions in Tertiary time may have 

been similar to those of today; but shifting volcanic centers, concurrent and subsequent erosion of volcanic landforms, and deposition of Recent volcanic ash have all obscured the record. No centers have been found for the Tertiary deposits; and large areas 

of coarse pyroclastic rocks, not obviously related to volcanic centers, suggest episodes of generalized eruption (much of it submarine) during which mudflows and flood eruptions may have 

been important.

Many local unconformities record unstable conditions during 

late Tertiary time and no persistent lithologic units were deposited. Lava flows, pillow lavas, and poorly sorted tuff-breccia are 

the dominant rocks; many dikes and, locally, large shallow sills 

invaded this sequence. Some of the sills were uncovered by erosion 

and large columnar talus blocks from them were incorporated in 

later sedimentary and pyroclastic beds. Relief features produced 

by faulting, erosion, and deposition were masked by subsequent 

floods of lava and coarse pyroclastic rocks. Pockets of well-sorted 

tuff, some of which contain fossils, filled depressions on southwestern Kanaga.
244 INVESTIGATIONS OF ALASKAN VOLCANOES

During the entire period of volcanism recorded here, andesitic 

and basaltic rocks were dominant, with only sporadic and minor 

contributions of more acid rocks. Shallow-water marine life left 

a fragmentary record, and forested land areas existed where today there are no trees.

Near the beginning of Pleistocene time broad shield volcanoes 

formed near the sites of present volcanoes. These volcanoes were 

partly destroyed by caldera-forming processes, and Recent volcanoes of sharply conical form grew in the old, incomplete calderas. At almost the same time another volcano, Gareloi, formed 

oh the ocean floor. This late volcanism was all confined to the 

northern ends of the islands.

Wave planation carved a series of platforms on the late Tertiary volcanic rocks during the Pleistocene; Amatignak Island, 

because it has not been planed, may have been uplifted in Pleistocene time. Glaciers modified Amatignak and the higher volcanic 

areas, and may have overridden the flat areas on other islands, 

particularly Ulak.

REFERENCES CITED

Anderson, E. M., 1951, The dynamics of faulting, 2d ed. revised, Edinburgh,,

Oliver and Boyd, 206 p. 

Bemmelen, R. W. van, 1954, Mountain building, The Hague, Martinus

Nijhoff, 177 p. 

Benioff, H., 1949, Seismic evidence for the fault origin of oceanic deeps:

Geol. Soc. America Bull., v. 60, p. 1837-1856.

 1955, Seismic evidence for crustal structure and tectonic activity:

Geql. Soc. America Special Paper 62, p. 61-73. 

Coats, R- R., 1950, Volcanic activity in the Aleutian arc: U. S. Geol. Survey

Bull. 974-B, p. 35-49.

 1951, Geology of Buldir Island, Aleutian Islands, Alaska: U. S. Geol. 

Survey Bull. 989-A, p. 1-26.

 1952, Magmatic differentiation in Tertiary and Quaternary, volcanic 

rocks from Adak and Kanaga Islands, Aleutian Islands, Alaska: Geol. 

Soc. America Bull., v. 63, p. 485-514.

 1956a, Geology of northern Adak Island, Alaska: U. S. Geol. Survey 

Bull. 1028-C, p. 45-67.

 1956b, Geology of northern Kanaga Island, Alaska: U. S. Geol. Survey; 

Bull. 1028-D, p. 69-81.

 1956c, Reconnaissance geology of some western Aleutian Islands,

Alaska: 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. Foram. Research Contr., v. 23, pt. 3, no.

297, p. 60-72. 

Gates, 0., Fraser, G. D., and Snyder, G. L., 1954, Preliminary report on the

geology of the Aleutian Islands: Science, v. 119, p. 446. 

Gates, 0., and Gibson, W., 1956, Interpretation of the configuration of the

Aleutian Ridge: Geol. Soc. America Bull., v. 67, p. 127-146.
DELAROF AND WESTERNMOST ANDREANOF ISLANDS 245

Gibson, W., and Nichols, H., 1953, Configuration of the Aleutian Ridge: Geol.

Soc. America Bull., v. 64, p. 1173-1188.

Hills, E. S., 1949, Shore platforms: Geol. Mag., v. 86, no. 3, p. 137-153. 

Murray, H. W., 1945, Profiles of the Aleutian Trench: Geol. Soc. America

Bull., v. 56, p. 767-782. 

Rittmann, A., 1952, Nomenclature of volcanic rocks: Bulletin Volcanologique,

1'Association de Volcanologie. Serie II, tome XII, p. 75-102. 

Ross, C. S., Foster, M. D., and Myers, A. T., 1954, Origin of dunites and of

olivine-rich inclusions in basaltic rocks: Am. Mineralogist, v. 39, p. 693-

737. 

Snyder, G. L., 1957, Ocean floor structures, northeastern Rat Islands, Alaska:

U. S. Geol. Survey Bull. 1028-G, p. 161-167. 

Umbgrove, J. H. F., 1947, The pulse of the earth, The Hague, Martinus

Nijhoff, 358 p. 

Vening Meinesz, F. A., 1955, Plastic buckling of the earth's crust: the origin

of geosynclines: Geol. Soc. America Spec. Paper 62, p. 319-330. 

Williams, Howel, 1932, The history and character of volcanic domes: Calif.

Univ., Dept. Geol. Sci. Bull., v. 21, no. 5, p. 51-146. 

 1941, Calderas and their origin: California Univ., Dept. Geol. Sci. Bull.,

v. 25, no. 6, p. 239-346. 

Williams, Howel, Turner, F. J., and Gilbert, C. M., 1964, Petrography: San

Francisco, W. H. Freeman, 406 p.

INDEX

Page 

Acknowledgments.. ... __. 212

Adak Island, Finger Bay volcanics._____ 224 

Paleozoic fossils_.._ __ _...... 216,,224

Adak Strait, submarine topography........ 235,237

Agattu Island Tertiary fossils________ 216 

Aleutian Ridge........__..__ ... ... 235

Aleutian Trench_...._.................... 232

Alluvium......... . ... .. 226

Amatignak Island, geology.________ 215-216 

glaciation...________________ 241,244 

intrusions and boulders...___.___ 231-232 

landforms.-__ _ .... ..__ 215

location....___...______.__... 233

quartz diorite..__________.__ 230 

submarine topography..___ ....__ 237

Tertiary fossils..._____________ 216 

Amchitka Island, location__________ 233 

Amchitka Pass, structure..... .-......--... 233

submarine topography___.__. __ 235 

Amchitka Seavalley_______._...__ 237 

Andesite........... 216,223,224,228-229,230-231, 234

Andreanof Islands, chemical composition of

rocks, table._____.___._ 222 

Anorthite.-..-.._____...__....__... 227

Argillite.................................... 215,217

Ash deposits, Kanaga Island....___.__ 227

Atka Island, Mesozoic fossils..____.__ 216 

Attu Island, Tertiary fossils..--...-.-....... 216

Augite...................................... 223,23C

Barghoorn, E. S., pollens and spores identified

by.............................. 218-221

Basalt dikes............................. 227-228

Bowers Bank, significance.-.__ _.__. 233 

Brown, R. W., fossil wood identified by.... 218-221

Calderas.... _.___. . -............... 234

Cape Chlanak, dikes......... . ........ 224

Chemical composition of rocks, table.... __ 222

Chromite.. ................................ 227

Climate,.. 213-214

Clinopyfoxene...... ... ... ......... 227,228

Cloud, P. E., algae identified by........... 218-221

Cooke, C. W., echinoids identified by...... 218-221

Delarof Islands, chemical composition of rocks,

table... 222

layered rocks. ______________ 217-224

physiography....._ . ... 241

Dikes-.. .. . 216,222,223,224,227-232,243

Drainage-.. . . 238-240

Dunite inclusions-.._.. . . ... 231

Eocene formations_______'..._____ 216

Faults. - 232-234

Finger Bay volcanics..__ . ... 224 

Flows. 217

Page 

Folds . ... ....... ... 233-234

Fossils, Kanaga and Tanaga Islands_____ 224

Qareloi Island, landforms___________ 214

location.,_________________ 213

Qareloi Volcano, geology_________ 225-226

geologic history.._______..__.... 244

position..______.___________ 233

Geography... .... ... .... 213-215

Geologic history___________.___ 243-244

Glaciation. 214,226,241

Granite inclusions_______________ 231

Hypersthene. -________________ 230

Dak Island, chemical composition of rocks,

table-.. 222 

dikes 230 

location....________________ 213 

pluton.............................. 230,231-232

Intrusive rocks, coarse-grained_.___.. 230-232 

fine-grained________________ 227-230

Kanaga Bay, dikes-. ...._______.. 224,230

structural control_ _____;______ 240

Kanaga Island, andesite___________ 228

andesiticash...______________ 227

area......._____..___...._____ 213

calderas.._______________._ 234 

chemical composition of rocks, table__ 222 

dikes. 224,227,231

drainage__________________ 238-240 

fossils ... ... 218-221,224 

inclusions-...__..._ . .-... 231 

landforrns.._________________ 214 

lava flows..... _____ ________ . 225

layered rocks. _______________ 217-224 

physiography.....___...__._..... 237-241

planed surfaces.________.__-. 238-241

rhyodacite._________________ 229

sills. . 228-229

volcanic ash deposits...________ 226-227

Kanaga Pass, submarine topography.__ . 237

Kanaga Volcano. 225-226,234,237,238

Kanaton Ridge...__..._________... 238,240

Kavalga Island, folds____________ 234

location...__________.._____ 213

submarine topography_..__......... 235-237

volcanic ash deposit......______._ 226

Knob Point, sill 230

Labradorite-augite andesite..___.__ . 223 

Landforms... 214-215

Lava flows................. 225,230/231,243

Layered rocks.... 215-227

Limburgite... ............. ... 228

Location of area............ ...... 213

247
248 INDEX

Page

MacNeil, F. S., megafossils identified by... 218-221 

Mesozoic fossils_____________.- 216 

Microlite 224

Mineralogy, definition of terms_____ 223 

Miocene formations-..-..___......_... 216,224

Miocene geologic history________ . 243 

Mount Kanaton, lavaflows..._.. 225,226

Naga Point, dikes.. . . . 224,230

Ogliuga Island, chemical composition of rocks,

table....... .... 222

gravel and volcanic deposits.. 226,227 

location....... 213

submarine topography _ ' 235 

Ollvine. . . . - 227,228 

Orthopyroxene.__________.. 227,228 

Oxyhornblende . ...... 227,228

Paleozoic formations...- 216,224

Physiography............. .. 237-243

Pillow lavas- .. 216,217,243

Pleistocene formations________.. 224 

Pleistocene geologic history -- 244 

Pliocene formations .. .. 224 

Plutons, age. 231-232

Precipitation........... 213

Quartz diorite, Amatignak Island..___.. 230

Quaternary dikes_ ___________ 230

Quaternary volcanoes___________...- 226

Rat Islands, stratigraphy_._______ 216 

Recent as'i deposits..__________ 227 

Recent volcanoes.____________ 225-226,244 

Rhyodacite..... ..-...... . 229-230

Rittman classification._______.____ 223

Sand dunes...___.______.__.. 226

Sills 216,222,243

Skagul Island, boulders.. . 231 :

chemical composition of rocks, table..__ 222.

location________________..... 213

submarine topography_.____.__.. 235

volcanic ash deposit.. __...... 226.

Page 

Skagul Pass, submarine topography.....__ 237

Structure, geologic...... .. ___.. 232-237

Surficial deposits..____.___.....__ 226-227

Tanaga Island, calderas. ________'._..... 234

chemical composition of rocks, table. __ 222 

faults... 234

folds...-. . . 234

fossils... 218-221,224

glaciation___________________ 226 

landforirs.---____.___.._____ 214 

layered rocks..___._......_.,.__ 217-224

location.... . . ... . 213

physiography...... ... 241

Tanaga Pass, formed by fault....-._____ 233

submarine topography.........____ 235-237

Tanaga Volcano_____._ __.__ 225-226 

Tertiary formations..___.-..____ 224,230,231 

Tertiary fossils..._____._________ 216 

Tertiary geologic history..... .......... 243

Todd, Ruth, Foraminifera identified by__ 218-221 

Tomasi, E. J., analyses by..._________ 222 

Topography, submarine______.____ 235-237 

Trend lines... 232,237

Ulak Island, faults. ................... 234 '

landforms.---____________-__ 215 

layered rocks________________ 216-217 

submarine topography_.____..._ 237

Unalaska Is'.and, foliated rocks. _______ 231 

Tertiary fossils. _______________ 216

Unalga Island, chemical composition of rocks,

table.-. ................... 222

location.___________________ 213 

submarine topography.________ 235-237 

volcanic ash deposit. _.___._____ 226

Vegetation.....................__... 213-214,226

Volcanic ash deposits...___________ 226-227

Volcanoes, early shield. ____________ 225

geologic history___.__........___ 244

Recent......... .................. 225-226

Wave planation.._.__....._._....._ 244

Western Point, topography__________ 228

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