Bx
AD012
Undifferentiated
May include minor amounts of talus and colluvium. In areas away from Mount Moffett and Mount Adagdak, most of the bedrock consists of Finger Bay Volcanics (Coats, 1956). These rocks are extensively fractured and faulted, and locally exhibit some weatheri
Quaternary and Tertiary
Adak
Igneous
Qal
AD002
Alluvial and other deposits
Unconsolidated beach, delta, lagoon, and alluvial deposits
Quaternary
Unconsolidated
Qs
AD004
Alluvium
Alluvium, dune and beach sands
Qas
AD006
Ash, soil, and tundra mantle
AT004
Alluvium and beach deposits
Attu
KK002
Surficial deposits
Alluvium, colluvium, landslide deposits, and beach deposits
Holocene
Kiska
RI002
Alluvium, eluvium, and talus
Rat Islands
RI003
Alluvium - Sand, gravel, and boulders.
GI003
Ash and soil cover, Kavalaga Island
Ash and soil cover
Gareloi Island
Surficial deposits, ash and soil cover, Ogliuga and Skagul Islands
RI005
Alluvial deposits, Amchitka Island
Unconsolidated material in valley floors
Quaternary, Holocene
Qt
RI006
Talus, alluvium, and landslide deposits, Segula Island
Talus, alluvium, and landslide deposits, primarily landslide deposits
Qtsc
AD009
Tephra, soil and colluvium
Accumulations of organic-rich soil, peat and tephra mainly on the southern part of the island. Thickness ranges between 1 and 5 meters. Tephra deposits consist of light colored, fine silt-size ash beds to pebble and granule size lapilli beds mostly deriv
Qhpt
AD007
Peat, tephra, and colluvium
Accumulations of peat and tephra on the slopes of ancestral Mount Kanaton typically between 1 and 5 meters thick. Tephra deposits consist of light-colored, fine silt-sized ash beds to lapilli beds mostly derived from eruptions of Kanaga volcano. Colluvia
al
GI006
Ash and soil cover, Tanaga volcano
AD013
AT002
Valley-fill aluvium; includes old beach deposits in lower parts of some valleys
Qa
AD003
Alluvial deposits
AD005
Alluvium, beach gravel, and wqater-sorted pumice deposits.  Rocks younger than caldera.
KK003
Sand and silt
Alluvial of modern streams
Well to moderately sorted angular to rounded gravel, sand and minor silt along present day streams and creeks. Generally less than 1 m thick and forms longitudinal bars, point bars, and transverse bars along stream channels. Generally stable, but may bec
Quaternary (Holocene)
Qf
Alluvial fan deposits
Qaf
Moderately to poorly sorted subangular cobble gravel, sand, and minor silt in small fan-shaped bodies at the mouths of some streams with steep (more than 20 percent) gradients. May contain variable amounts of debris-flow sediment and talus, and usually m
Qc
Colluvium
Colluvium - Boulders and bouldery debris in slope wash, talus, landslide, and mudflow deposits.
ALT
Colluvium - Boulders and bouldery debris in slope wash, talus, landslide, and mudflow deposits.  Contains areas of kaolinized, silicified, and pyritized rock.
Qct
Talus and coarse colluvial deposits
Accumulations of poorly sorted, angular rock debris on hillslopes below bedrock outcrops.
Qhtc
Qcc
Coarse-grained colluvial deposits
Poorly sorted to fall-sorted, angular blocks, boulders, and gravel on steep hillslopes. Includes all forms of talus, rockfall avalanche debris, and may include some debris-flow deposits. Extremely porous and permeable, but generally not water bearing.
Qcf
Fine-grained colluvial deposits
Reworked volcanic ash, eolian sediment, and other fine-grained deposits on hillslopes. Somewhat porous and permeable and locally water bearing.
Colluvial deposits, undifferentiated
Reworked, unconsolidated sediment of all particle sizes on hillslopes. Typically unsorted to poorly sorted and forms a thin blanketing layer on bedrock or other surficial deposits. Porosity and permeability dependent on composition. May be water bearing.
Qcd
Debris-flow deposits
Poorly sorted, matrix and clast-supported, angular bouldery gravel. May consist of reworked lahar deposits, colluvium, and talus. Deposits usually exhibit lobate fronts and levee-like ridges along their margins.
co
Colluvium, Tanaga volcano
Qmf
Mudflow
Qls
Landslide
Landslide deposits
Poorly sorted accumulations of gravel, sand and silt in slumps, and localized debris-flow deposits.
Qhls
Poorly sorted accumulations of gravel, sand, and silt in slumps and debris-flow deposits.
Qcl
Discrete bodies of reworked unconsolidated sediment formed by slumping, sliding, or short-distance debris flow. Consists of poorly sorted to intact blocks of sediment that are generally found a short distance from their source. Common on the lower slopes
Qat
Alluvial terrace deposits
Moderately to well-sorted gravel, sand, and silt in low benches and unpaired terraces along streams and creeks. Usually mantled by tephra. Generally of limited extent in the map area, and most common along large drainages. Porous and permeable and may be
Qlk
Deposits of former lakes, ponds, and bogs
Consists of varying amounts of sand, silt, clay, peat, and reworked or primary tephra. Almost always overlain by tephra deposits. Somewhat porous and permeable and generally water bearing.
Qobb
Organic deposits over beach gravel
Peat, silty peat, and sand peat developed on uplifted beach gravel. Usually consists of 1 to 3 m thick sequences of compact, fibrous peat resting on well-rounded beach gravel. May contain multiple thin, fine-grained tephra layers. Common along the Clam L
Qb
Beach deposits
Includes some narrow stretches of bare rock shore platform
Surficial beach deposits
Qb2
Beach deposits - Sand, pebbles, gravel, and boulders.
Qb1
Beach deposits - Boulders and boulder conglomerate.
Surfical deposits, beach deposits, Ogliuga and Skagul Islands
Beach deposits, Amchitka Island
Beach deposit of sand, cobbles, boulders, and shells of marine fauna characteristic of warmer water.  Inferred late interglacial age
Quaternary, Pleistocene
Beach deposits, Segula Island
Primalrily wave rounded boulders and and minor fine gravel.  Beach at head of cove on north coast is only one that contains appreciable sand.
Qbe
Beach and coastal eolian deposits
Pocket beaches and steep berms of rounded cobble and boulder gravel, and sand. Eolian deposits consists mostly of well-sorted medium to coarse sand.
Qhbe
Beach and eolian deposits
Beach deposits consist of sandy pocket beaches to steep berms of rounded cobble and boulder gravel. Eolian deposits consist mostly of well-sorted medium to coarse sand.
Sand, gravel, pebbles, and boulders
Beach sand
Fine to medium-grained, well-sorted sand found on modern and former beaches. Contains variable amounts of heavy minerals (for example, magnetite) that form discrete beds and lenses (placer deposits). Porous, permeable, and water bearing.
Qbb
Boulder beaches
Moderately sorted, rounded to well-rounded boulder and cobble gravel on high energy beaches. Common in the Clam Lagoon and Andrew Bay area where deposits are found on uplifted wave-cut terraces. Porous and permeable, but not water bearing.
Qbr
Beach ridges and beach-ridge complexes
Moderately to well-sorted coarse to fine sand, and minor fine gravel in nested sets of ridges that are generally parallel to the present coastline. Deposits probably form during storms but are no longer affected by wave action. Indicative of recent tecto
Qbbo
Older boulder beaches
Moderately sorted, rounded to well-rounded boulder and cobble gravel with a sand matrix on uplifted wave-cut platforms and terraces. Former zone of high-energy wave action now isolated from the coastline by tectonic uplift. Overlain by as much as 2.5 m o
Qbby
Younger boulder beaches
Moderately sorted, rounded to well-rounded boulder and cobble gravel with a sand matrix on uplifted wave-cut platforms and terraces. Forms prominent benches in the area between Clam Lagoon and Lake Andrew. Former zone of high-energy wave action now isola
Q
Qlg
Lagoonal deposits
Qtg
Marine terrace boulder gravel
Qmt
Sand dunes
Qd
Surficial deposits, sand dunes
Dune sand
Dune sand - Chiefly in beach ridges.
Qes
Eolian deposits
Dune sand.
Surficial deposits, dunes, Ogliuga and Skagul Islands
Dunes
Eolian deposits, Amchitka Island
Cliff top dunes
Eolian sand
Fine to medium-grained, well-sorted and locally cross-bedded, wind-deposited sand. Usually found in dunes and sand sheets along the coastline. May contain multiple thin (less than 5 cm thick) volcanic ash and peat layers. Porous and permeable and commonl
Glacial till - ground moraine
Ground moraine deposits
Qm
Glacial till - lateral and end moraine
Terminal or lateral moraine deposits
Glacial drift
Glacial drift, including till, lateral moraines, and mudflows
Qmo
Glacial moraine
Qg
Glacial deposits, Amchitka Island
Till, outwash gravel, and silt with varve=like bedding that contains constituents not of local origin.
Qgm
Morainal deposits (till)
Unsorted, massive, matrix-supported, cobble and boulder gravel. Matrix consists of silt, clay, and fine sand. Contains abundant faceted, soled, and striated subangular to subrounded clasts. Typically found as irregular, discontinuous layers and lenses ov
Quaternary (also see description)
gh
Glacial deposits, Tanaga volcano
Glacial deposits
Qld
Delta deposits
Moderately sorted sand and gravel deposits at the mouths of small streams and creeks that enter standing bodies of water. Usually mantled by 1 to 2 m of tephra and are indicative of higher lake levels. Porous and permeable, and may be water bearing.
Tc
Bouldery conglomerate
Bouldery conglomerate, mapped as Tertiary by Coats (1956), is probably Quaternary, certainly Pliocene or younger.
Tertiary
Sedimentary
QTmb
Ts
Fossiliferous marine sandstone
Soft yellow-to-gray sandstone containing lenses of dark pebble conglomerate
Tmr
QTg
Gravel of hornblende andesite, Amchitka Island
Sand and gravel, nonindurated, bedded, composed of fragments of hornblende andesite.  Inferred beach and nearshore marine deposit
Quaternary or Tertiary
QTs
Tilted sedimentary rocks at South Bight, Amchitka Island
Sand, silt, and gravel in tilted beds, truncated at 90-foot altitude, overlain by fossiliferous beach deposit (Qb).  Consist, from base, of 200 feet of layers in random order, a few inches to 2-feet-thick, of carbonaceous sandy silt, fine- to medium-sand
Fragments of carbonized wood common in silt layers
QTf
Faneto Formation
Coarse red sandstone and conglomerate
early Pleistocene or Late Tertiary
Qdl
Disturbed land
Areas affected by human disturbance. Includes filled or reclaimed land, present and former housing developments, and other facilities that locally obscure primary surficial deposits. May be porous and permeable and may be water bearing.
Tgs
Great Sitkin Volcanics - Basalt flows
Basalt flows on Sitkin Creek
Tb
Tgc
Great Sitkin Volcanics - Composite cone
Composite cone of Great Sitkin Volcano.  Principally basalt and andesite flows, but includes some tuff layers.
Tvu
Qkv
Older volcanic rocks of Kanaga Volcano
Basalt and andesite flows, scoria, and tuff breccia.   Rocks younger than caldera.
Qv
Qva
Vent agglomerate and minor intrusive rocks of later stage.  Rocks younger than caldera.
Qk
Rocks of Kiska Volcano
Lava flows and interbedded pyroclastic rocks forming Kiska Volcano.
Pleistocene
Qbt
Basaltic tuff
Basaltic tuff  breccia and tuffaceous sand with minor basaltic flows.
GI005
Composite volcanic cones
Composite volcanic material, Segula Island
Interbedded lava and pyroclastic material.  Lavas are in lenticular tongues and sheets from a few to about 100 feet thick.  Pyroclastic layers are comparable in thickness but cover much wider areas
Composite volcanic material, Davidof and Khvostof Islands
Qcbf
Cone building lava flows and volcaniclastic rocks
Medium to dark grey, massive, and thickly bedded, andesite and basaltic andesite lava flows, interbedded with thin accumulations of pyroclastic and laharic debris. Makes up the remaining sector of the ancestral edifice that was partially destroyed by fla
Quaternary (Pleistocene and Holocene )
Qpv
AD008
Precaldera glaciated or dissected remnants of volcanic cones
Qtbv
Buldir Volcanics
Olivine basalt flows and pyroclastic rocks
sop, sol, tol, tko
Older volcanic rocks, Sajaka, Takawangha, and Tanaga volcanoes
Undivided older volcanic rocks on informally named Sajaka, Tanaga, and Takawangha volcanoes
Pre-Tanaga volcanic rocks
Undivided older volcanic rocks that predate Tanaga volcano
Qvo
Older volcanic rocks, Gareloi volcano
Undivided older volcanic rocks associated with Gareloi volcano
Tmb
Basalt flow, Moffett Volcano
Hv
Tyc
Younger composite cone, Mt. Adagdak Volcano
Younger composite cone, Mt. Adagdak Volcano, principally hornblende andesite tuff breccia, but includes some hornblende andesite flows.
Qkb
Basalt flows
Historic and late prehistoric basalt flows from Kanaga Volcano.  Rocks younger than caldera.
Qkr
Lava flows and interbedded pyroclastic rocks forming Kiska Volcano.  The most recent lava flows, Qkr, are mapped separately.
Qbc
Basalt
Mount Cerberus :  Basalt - Lava flows in part separately mapped and numbered in probable order of eruption.
Qbc1
Qbc2
Qbc3
Qbc4
Qbc5
Qbc6
Qbl
Lakeshore Cone:  Basalt - Lava flows in part separately mapped and numbered in probable order of eruption.
Qbl1
Qbl2
Qbs
Sugarloaf Peak:  Basalt - Lava flows in part separately mapped and numbered in probable order of eruption.
Qbs1
Qby
GI002
Younger basalt flows
Olivine basalt flows, ranging from fine-grained to porphyritic
Ql
Lava of 1929 eruption
Black to dark-reddish brown basaltic lava and scoria and viscous andesitic flows
Surface flows of porphyritic feldspathic basalt, Segula Island
Flows are generally dark-gray, glassy or fine-grained, compact or diktytaxitic, all porphyritic and feldspathic.  Flows are 2-pyroxene andesite and olivine basalt and have uneroded flow forms
Qld74
Lava dome emplaced during 1974 eruption
Dark gray to black, basaltic andesite with highly fractured and blocky carapace. Occupies most of the summit crater.
Qld45
Lava dome emplaced during 1945 eruption
Dark gray to black, glassy, porphyritic basalt. Largely removed by 1974 eruption.
Qllf
Lava flows of Holocene age with well developed lateral levees
Medium to dark gray andesite and basaltic andesite aa lava flows on the northwest flank of the volcano. Characterized by well developed, steep, nearly linear lateral flow levees of blocky aa lava.
Qmlf
Massive lava flows of Holocene age
Medium to dark gray, massive, thickly bedded, andesite and basaltic andesite lava flows erupted from the summit crater on the north flank of the volcano.
Qlf
Younger lava flows of Holocene age erupted from flank vents
Medium to dark grey andesite and basaltic andesite aa lava flows erupted from flank vents on the west flank of the volcano.
Qolf
Older lava flows of Holocene age erupted from flank vents
Qhlf1994
Lava flows of 1994 eruption
Two-pyroxene porphyritic andesite; light to dark gray, unvegetated, fresh.
Phenocrysts of plagioclase, clinopyroxene, orthopyroxene, and Fe-oxides with trace amounts of olivine and hornblende in a glassy groundmass. Abundant mafic inclusions. Forms block
Qhlf1906
Lava flows of 1906 eruption
Qhlfh
Lava flows formed by historical eruptions
Two-pyroxene porphyritic andesite; light to dark gray, unvegetated, fresh.
Phenocrysts of plagioclase, clinopyroxene, orthopyroxene, and Fe-oxides with trace amounts of olivine and hornblende in a glassy groundmass. Abundant mafic inclusions. Overlapping
Qhlf
Lava flows of Holocene age
Two-pyroxene porphyritic andesite; light to dark gray, unvegetated to sparsely vegetated, fresh. Phenocrysts of plagioclase, clinopyroxene, orthopyroxene, and Fe-oxides with trace amounts of olivine and hornblende in a glassy groundmass. Abundant mafic i
Composite volcanic zone
Composite volcanic zone, currently or recently active.
Qtecd
Volcanic dome on flank of East Cape Volcano
Hornblende basalt
Qtecv
East Cape Volcanics
Hornblende basalt and basaltic andesite
Young volcanic rocks, Sajaka, Takawangha, and Tanaga volcanoes
Undivided young volcanic rocks on informally named Sajaka, Tanaga, and Takawangha volcanoes
Younger volcanic rocks, Gareloi volcano
Undivided younger volcanic rocks associated with Gareloi volcano, including historic rocks derived from 1929 eruption
Qgd
Basalt dome
Basalt dome extruded in crater of Great Sitkin Volcano in 1945, as exposed in September 1946.
Qbd
Lava domes of Holocene age
Medium to dark gray andesite and basaltic andesite that form resistant knobs and hills on the northwest and east flanks of the volcano. Retain little original surface morphology, and blocky exteriors largely removed by erosion.
Toc
Older composite cone, Mt. Adagdak Volcano
Principally hornblende andesite lapilli tuff, but includes some hornblende andesite flows.
Qpd
Qga
Ash
Ash, Great Sitkin Volcanics - Probably from flank vent northwest of crater.
Andesite tuff
Andesite tuff - Rocks formed at time of caldera.
Qlp
Lava flows and pyroclastic rocks
Lava flows and pyroclastic rocks - Later than or interbedded with dacite pumice of caldera eruption, largely from Anvil Peak, but also from 914-foot cone near Tuman Head and Threequarter Cone on west rim of caldera.
Late Pleistocene
Qp
Patterson Point Formation
Dacitic ash and pumice and dacitic and andesitic pyroclastic debris.
Qpf
Pyroclastic-flow deposits
Moderate to poorly sorted, non-welded, pumice-flow deposits with rounded to subrounded pumice clasts up to 75 cm in diameter. Ash-rich matrix and abundant angular to subangular lithic clasts up to 25 cm in diameter. Deposits in Glacier Creek valley conta
Talus, avalanche debris and tephra
Poorly sorted, loose rock rubble derived from fragmentation of lava flows and ejecta. Also includes variable amounts of reworked lapilli tephra.
Qhts
Talus, avalanche debris, and tephra
Loose, accumulations of poorly sorted rock rubble derived from fragmentation of lava flows and ejecta on upper slopes of Kanaga Volcano. Also includes variable amounts of reworked  lapilli tephra. Lower contact gradational.
Qhpf
Moderate to poorly sorted, pumice- and lithic-bearing gravelly deposits with ash-rich matrix.
Ql(p)
Lava of 1929 eruption, pyroclastic deposits
Pyroclastic apron derived from 1929 eruption
Surface pyroclastic deposits, Segula Island
A pyroclastic mantle forms constructional surfaces on much of Segula and possibly Davidof and Khvostof Islands.  Consists of light-gray debris at least 150 feet thick.  Fragments range from fine sand to large blocks of lava several feet across, fragments
Qptg
Tephra mantling glacial deposits
Deposits of tephra that overlie morainal deposits. Tephra deposits are the same as those mantling bedrock (Qptb).
Qptb
Tephra mantling bedrock
Deposits of tephra that overlie bedrock, but generally do not obscure bedrock topography and structure. Tephra deposits are usually 1.5 to 3.0 m thick and consist of thin beds of fine-grained (mostly silt and clay size particles) ash and 3 to 5 beds of l
Qptc
Tephra mantling colluvial deposits
Deposits of tephra that overlie undifferentiated colluvial deposits. Tephra deposits are the same as those mantling bedrock (Qptb).
Qptbc
Tephra mantling beach deposits
Deposits of tephra that overlie beach deposits. Tephra deposits are the same as those mantling bedrock (Qptb).
Qptl
Tephra mantling lacustrine deposits
Deposits of tephra that overlie lacustrine deposits. Tephra deposits are the same as those mantling bedrock (Qptb).
Qpta
Tephra mantling alluvial deposits
Deposits of tephra that overlie undifferenciated alluvial deposits. Tephra deposits are the same as those mantling bedrock (Qptb).
Qptbbo
Tephra deposits mantling older boulder beach deposits
Qptbby
Tephra deposits mantling younger boulder beach deposits
Lahar deposits
Poorly sorted gravel, sand, silt, and boulders in fan-shaped deposits. Present in most major valleys. One to five meters thick. Products of eruptive activity and likely formed when pyroclastic flows and lava flows encountered and mixed with snow or ice o
Qhla
Poorly-sorted gravel, sand and silt in fan-shaped deposits mainly on the southeast flank of Kanaga Volcano. Products of eruptive activity and likely formed when pyroclastic flows and lava flows encountered and mixed with snow or ice on the upper flanks o
Pumice-rich lahar deposits
Massive, 10-to-20 meter-thick deposit on southwest side of volcano. Very coarse with cream-colored dacitic pumice clasts to 1 meter diameter and lava clasts 2 to 4 meters diameter. In places, deposit is composed almost entirely of pumice clasts and sandy
Qpl
Poorly sorted, matrix-supported, angular, cobble and boulder gravel of volcanic origin found on the slopes of Mount Moffett and Mount Adagdak. Matrix consists primarily of silt and fine sand. Coarse clasts are angular to subangular and often exhibit brea
Debris-avalanche and lahar deposits
Poorly sorted, massive accumulations of volcanic rock rubble, sand and gravel. Associated with the older edifice.
Sitkin Point Formation
Waterlaid pyroclastic material; tuffaceous graywacke, dacite-boulder tuff-breccia, andesitic tuff, and pumiceous tuff.
Qvs
Waterlaid pyroclastic material; tuffaceous graywacke, dacite-boulder tuff-breccia, andesitic tuff, and pumiceous tuff.  Contains areas of kaolinized, silicified, and pyritized rock.
Moderately well-bedded volcanic detritus, Segula Island
Unconsolidated, rudely sorted, moderately well-bedded volcanic detritus at least 500 feet thick.  Fragments are compositionally similar to other rocks on the island and range from fine sand to blocks several feet across.  Fragments between one-eighth inc
Tmt
Tuff-breccia cone of Mt. Moffett Volcano
Qcs
Qcv
Surface flows of porphyritic feldspathic basalt, central vent, Segula Island
Central vent form of flows, which are generally dark-gray, glassy or fine-grained, compact or diktytaxitic, all porphyritic and feldspathic.  Flows are 2-pyroxene andesite and olivine basalt and have uneroded flow forms
Qrv
Surface flows of porphyritic feldspathic basalt, rift vent, Segula Island
Rift vent flows, which are generally dark-gray, glassy or fine-grained, compact or diktytaxitic, all porphyritic and feldspathic.  Flows are 2-pyroxene andesite and olivine basalt and have uneroded flow forms
Qsd
Spatter and agglutinate deposits
Poorly sorted, oxidized, scoriaceous ejecta, bombs and bomb aggregates on the rim of the summit crater.
Qhs
Tephra and spatter from recent eruptions
Mostly coarse near-vent ejecta, including volcanic bombs, blocks, and scoriaceous debris on and around the rim of the modern crater.
Tmc
Composite cone of Mt. Moffett Volcano
Composite cone of Mt. Moffett Volcano, principally basalt flows and tuff breccia
Tpc
Composite cone of Parasitic cone of Mt. Moffett Volcano
Composite cone of Parasitic cone of Mt. Moffett Volcano, principally basalt lava flows, but includes subordinate tuff breccia.
Tls
Interbedded basalt lava flows and tuffaceous sandstone
Qyb
Basalt lava flow
Short, stubby flow of dark grey basalt erupted from flank vent at the head of Sitkin Creek.
Qe
East Point Formation
Sequence of andesite and basalt flows.
Qholf
Lava flows of the cone-building sequence
Two-pyroxene porphyritic andesite; light to dark gray, mostly vegetated but not extensively weathered. Phenocrysts of plagioclase, clinopyroxene, orthopyroxene, and Fe-oxides with trace amounts of olivine and hornblende in a glassy groundmass. Abundant m
Qlw
Little Sitkin dacite
Sequence of low-silica dacite flows; West Cove member.
LIttle Sitkin dacite
Rhyodacite flow unit.  Low-silica dacite flows with minor amounts of associated pyroclastic material.
Qvda
Low-silica dacite flows associated with youngest major eruption.  Pratt Point member.
Double Point dacite
Sequence of high-silica dacite flows containing many miarolitic inclusions of labradorite andesite; minor amounts of associated pyroclastic material.
Sequence of low-silica dacite flows; West Cove member.  Contains areas of kaolinized, silicified, and pyritized rock.
Rhyodacite flow unit.  Low-silica dacite flows with minor amounts of associated pyroclastic material.  Contains areas of kaolinized, silicified, and pyritized rock.
Sequence of high-silica dacite flows containing many miarolitic inclusions of labradorite andesite; minor amounts of associated pyroclastic material.  Contains areas of kaolinized, silicified, and pyritized rock.
QTgp
Great Sitkin Volcanics - Basalt (?) plugs
QTb
Basalt flows and tuff breccia
Basalt flows and tuff breccia from ancient vent near west coast.  Rocks older than caldera of Mt. Kanaton
AT003
Basalt porphyry, Shemya Island
Basalt porphyry consisting of two basalt plugs of fine-grained basalt having excellent columnar joints
Tertiary or Quaternary
Basalt flows and tuff beds
Basalt flows and tuff beds from vent older than Mt. Kanaton near north coast.
Qbo
Older basalt flows
Olivine basalt flows and beds of scoria
Basalt flows and tuff beds older than Mt. Kanaton volcano
Basalt flows and tuff beds older than Mt. Kanaton volcano (Coats, 1956)
Tmtv
Andesite vent filling, tuff-breccia cone of Mt. Moffett Volcano
Qanp
Td
Basalt and andesite domes, Mt. Adagdak Volcano
Basalt and andesite domes
Qhld
Lava domes
Two-pyroxene silicic andesite lava domes; highly fractured, massive, slab-like, slightly weathered. Round mafic inclusions up to 10 cm across are common. Exposed at west end of intracaldera lake and on island in the lake. Alteration and fractured charact
Tmd
Basalt domes of Mt. Moffett Volcano
Tpd
Basalt dome of Parasitic cone of Mt. Moffett Volcano
Tpv
Basalt vent agglomerate of Parasitic cone of Mt. Moffett Volcano
Basalt vent agglomerate associated with fine-grained olivine gabbro plug of Parasitic cone of Mt. Moffett Volcano
Tpp
Olivine gabbro of Parasitic cone of Mt. Moffett Volcano
Fine-grained olivine gabbro plug of Parasitic cone of Mt. Moffett Volcano
QTm
Massacre Bay Formation
Basaltic and andesitic lava and pyroclastic and coarse clastic rocks
QTv
Tav
Rocks of Andrew Bay volcano
Flows and tuff-breccia of olivine-, hypersthene-, and hornblende-bearing andesite
Tsa
Sand Bay Volcanics - Agglomerate
Agglomerate.  Includes a few thin flows.
Tkb
Olivine and hypersthene basalt
Olivine and hypersthene basalt and andesite flows of ancient Mt. Kanton.  Rocks older than caldera of Mt. Kanaton.Tkb of Coats
Tva
Agglomerate
Agglomerate in vent older than Mt. Kanaton near north coast.
Precaldera glaciated and dissected remnants of volcanic cones
Tsbv
Sand Bay Volcanics
Undifferentiated agglomerate, pyroclastic rocks, and andesite and basaltic lava flows. The lower part of the sequence consists mostly of pyroclastic rocks with a few thin flows. The upper part of the sequence contains mostly lava flows. These two sequenc
TQKv
Mt. Kanaton volcanic rocks
Olivine and hypersthene basalt andesite flows of ancestral Mt. Kanaton volcano (Coats, 1956). Erupted from flank vents on the west flank of the volcano.
Quaternary-Tertiary
GI004
Late Tertiary and Quaternary basaltic rocks
Basaltic rocks
Quaternary and Late Tertiary
AD010
Tsl
Sand Bay Volcanics-- Lava flows
Principally andesite, but includes a few basalt flows.
Ta
QTw
Williwaw Cove Formation
Sequence of andesite and bandaite flows with minor amounts of basalt and dacite flows and major amounts of pyroclastic material.  Dikes of andesite and basalt shown in some areas.
Sequence of andesite and bandaite flows with minor amounts of basalt and dacite flows and major amounts of pyroclastic material.  Dikes of andesite and basalt shown in some areas.  Contains areas of kaolinized, silicified, and pyritized rock.
QTa
QTob
Olivine basalt flows
Olivine basalt flows from ancient vent near Round Head.  Rocks older than caldera of Mt. Kanaton.
Olivine basalt of Round Head
Olivine basalt of Round Head (Coats 1956)
QTp
Pyroclastic rocks and lava flows
Pyroclastic rocks and lava flows - Crystalline vent plugs of cones.
Quaternary or Tertiary (?)
QTi
QTqk
Quartz keratophyre dikes
Quartz keratophyre dikes occur on widely separated parts of Attu Island.  The dikes consists of chalky white euhedral feldspar (albite) crystals and glassy quartz in a bluish-gray to light-gray groundmass, which is a felt of fine-grained albite and quart
Early Quaternary or Late Tertiary
QTha
Hornblende andesite and dacite, Attu Island
Porphyritic dikes and stocks of hornblende andesite and dacite
QTad
Hornblende porphyry, Shemya Island
Hornblende porphyry - Includes some porphyry rich in plagioclase phenocrysts.
Qtad
QTpo
Pochnoi Volcanics
Pochnoi Volcanics - Tuff-breccia, lava flows, and agglomerate, largely basaltic, of shield-shaped volcano.  Locally sandstone from reworked pyroclastic deposits.
Tertiary (?) or Quaternary
QTvb
QT
Interbedded flows, pyroclastic deposits, and sedimentary rocks
Interbedded flows, pyroclastic deposits, and sedimentary rocks, Unalga, Ogliuga, and Skagul Islands
Interbedded flows, pyroclastic deposits, and sedimentary rocks. Includes fine-grained dikes and sills.
QTt
Andesitic and basaltic tuff and tuff-breccia, Shemya Island
Andesitic and basaltic tuff and tuff-breccia
QTva
QTpl
Pyroclastic rocks and lava flows - Listed clockwise from north: North Head Double Cone, Perret Ridge, north tip Fenner Lake, Ragged Top, 1405-foot cone, 1389-foot cone, Tuman Head younger, Tuman Head older.
Quaternary or Tertiary(?)
QTk
Kiska Harbor Formation
Water-laid coarse- to fine-grained pumice and detritus derived from volcanic rock, interbedded with breccia, and flow rock.  Abundance of flow rock increases northward in the area.  One breccia layer, QTkb, and one lava flow, QTkf, are mapped as members
QTkb
Kiska Harbor Formation, breccia member
Breccia layer contained within the Kiska Harbor formation.
QTkf
Kiska Harbor Formation, flow member
One lava flow within the Kiska Harbor formation.
Tch
Chuniksak Formation
Fine-grained laminated siliceous, argillitic, and limy marine sedimentary rocks
Tertiary -- Miocene(?)
Mollusks and plants.  Also diatoms and foraminifera.  Best inferred ages are probably Miocene
Tta
Basement rocks, Shemya Island
Basement rocks - Siliceous and limy argillite and conglomerate; includes some tuff-breccia.
TMzf
Chirikof Formation
Conglomerate, sandstone, carbonaceous shale, and possibly one lava flow; small outcrop on Chirikof Point.  May be same age as Nevidiskov Formation
middle Tertiary -- Miocene(?)
Plants -- leaves in coaly beds, similar species to Chuniksak Formation, hence probably Miocene
Tu
Tn
Nevidiskov Formation
Coarse graywacke and conglomerate
Tertiary -- early Miocene(?) or late Oligocene(?)
Single indeterminate Pecten
Th
Tg
RI004
Gunners Cove Formation
Tuffaceous conglomerate and sandstone, crystal-vitric basaltic tuff, thin basalt flows, and basaltic dikes.  Unit age revised to late Eocene or Oligocene, (Carr and others, 1970, RI007)
Tertiary, Oligocene or early Miocene
Barnacles, crinoids, echinoids, and pectens.  Crinoid Isocrinus aff. I. oregonensis (Moore and Volkes) and pecten Chlamys aff. C. washburnei Arnold of probable middle Tertiary age.
Tbp
Banjo Point Formation
Sandstone, conglomerate, tuffaceous shale, and lapilli tuff of basaltic composition.  Bedded nearshore marine deposits containing sparse fauna of middle Tertiary age, Amchitka Island. Unit definition and age revised to late Eocene or Oligocene, (Carr and
36 species of Formas and poorly preserved mollusks suggest a late Oligocene or early Miocene age.
RI007
Bedded marine sandstone, conglomerate, tuffaceous shale, and lapilli tuff of basaltic composition.  Carr and others (1970) slightly revised the unit and the defined outcrop area and indicated that minor pillow lava was present in the unit
Tertiary, Oligocene or late Eocene
Altered tuffaceous sedimentary beds with some coarse pyroclastic deposits
Altered tuffaceous sedimentary beds with some coarse pyroclastic deposits with fine-grained dikes and sills and coarse-grained dikes.  Rocks are composed of volcanic debris; are dark-gray, greenish-gray, olive-green, bluish-gray, and light brown tuffaceo
Tal
AD011
Andrew Lake Formation
Tuffaceous sandstone and siltstone and siliceous and cherty shale interbedded with  mafic flows or penecontemporaneous sills.  "Graded sandstone beds as much as 0.3-m-thick are common and alternating beds of sandstone and shale, ***, form graded sequence
Tertiary, late Eocene
Fossils reported to be Annularia stellata of Pennsylvanian or Permian age along with mollusks, fish skeletal remains and scales, foraminifera, dinoflagellates, and pectens, all others of Eocene age
MZg
Graywacke and graywacke breccia
Mesozoic
Ti
Dikes and intrusive rocks
Dikes and small intrusive bodies are related to both the Kiska Harbor and Vega Bay Formations.  Relation not differentiated.
Thf
Tp
Dacite porphyry
d
Andesite and dacite
Andesite and dacite, probably includes basalt in small dikes and sills.  Generally altered.  Includes some dikes older than unit gd
Tertiary?
Tbd
Basalt dome.  Rocks older than caldera of Mt. Kanaton.
Thm
Porphyritic andesite sills
Tab
Andesite and basalt dikes
Andesite or basalt dikes, Kavalaga Island
Andesite or basalt dikes
Intrusive rocks, andesite or basalt dikes, Ogliuga and Skagul Islands
Tpi
Intrusive rocks, Ogliuga and Skagul Islands
Intrusion of porphyritic andesite, irregular shape, with feldspar and hornblende conspicuous
QTc
Chitka Point Formation, Amchitka Island
Flows, flow breccias, and dikes of porphyritic andesite and feldspathic basalt, in part interbedded with marine conglomerate, in part with subaerial deposits, Amchitka Island.  Thickness exceeds 1000 feet.  Conglomerate confined to lower part of unit.  U
Quaternary(?) or middle or late Tertiary
Conglomerate contains carbonized frangments of woody material
Chitka Point Formation
Subaerial hornblende- and pyroxene-andesite flows, breccia, tuff, and conglomerate on the northwestern 2/3 of Amchitka Island
Tertiary, Miocene, probably late to middle
Andesite porphyry domes
Includes andesite porphyry, dacite porphyry, hornblende basalt porphyry, quartz-hornblende basalt porphyry, and hornblende-biotite basalt porphyry
Tt
Extrusive rocks, tuff dominant
Ttb
Extrusive rocks, tuff-breccia dominant
Tvl
Extrusive rocks, lava dominant
Lavas and pyroclastic deposits, predominantly lava and flow breccia
Predominantly lava and flow breccia, but includes minor pyroclastic deposits on Ogliuga and Skagul Islands
Tvp
Lavas and pyroclastic deposits, predominantly pyroclastic deposits
Predominantly pyroclastic deposits; tuff and breccia, but includes minor sedimentary rocks, lava and flow breccia, Ogliuga and Skagul Islands
Lavas and pyroclastic deposits, undifferentiated
Lavas and pyroclastic deposits, undifferentiated, along untraversed coastline on Ogliuga and Skagul Islands
Altered pyroclastic deposits and pillow lavas with minor sedimentary beds
Altered pyroclastic deposits and pillow lavas with minor sedimentary beds, with fine-grained dikes and sills, Ulak Island
Predominantly lava and flow breccia, but includes minor pyroclastic deposits on Kavalga Island
Predominantly pyroclastic deposits; tuff and breccia, but includes minor sedimentary rocks, lava and flow breccia, Kavalga Island
Lavas and pyroclastic deposits, undifferentiated, along untraversed coastline on Kavalga Island
Tv
Vega Bay Formation
Marine deposits of volcanic breccia, tuff, and a few pillow lava flows, all of basaltic composition, with a few interbeds of conglomerate and sandstone of similar material.
Middle Tertiary
Tpb
PPaf
Finger Bay Volcanics
Finger Bay Volcanics of Adak and Kagalaska Islands (Fraser and Snyder, 1959).  Unit mapped as Paleozoic by Coats (1956), since revised.  Consists of basalt and hornblende basalt tuff, flow breccia, agglomerate, and basalt dikes, and subordinate rhyolite
Permian and Pennsylvanian
Tvb
fb
Finger Bay Volcanics of Adak and Kagalaska Islands (Fraser and Snyder, 1959).  Unit mapped as Paleozoic by Coats (1956), since revised.  Consists of altered andesite and basaltic pyroclastic deposits and lavas: minor amounts of pillow lava, volcanic wack
HFS
mr
Finger Bay Volcanics, mixed rocks
Mixed rocks of the Finger Bay Volcanics intimately injected by Miocene(?) granodiorite, quartz diorite, and gabbro on Kagalaska Island (Fraser and Snyder, 1959).
PlPf
Finger Bay Volcanics, undifferentiated
Finger Bay volcanics - Undifferentiated flows, flow breccia, tuff, and dikes.
Permian (?) or Pennsylvanian (?)
Tr
Rat Formation
Rat Formation - Flows and flow breccias of porphyritic andesite, and minor amounts of conglomerate composed of andesitic debris.
a
Amchitka Formation, Amchitka Island
Volcanic agglomerate, tuff-breccia, tuff, and pillow lave flows of andesitic to latitic composition.  Tilted, jointed, and slightly metamorphosed in many areas.  Thickness several thousand feet
early Tertiary or older
Tfbv
Undifferentiated andesite and basalt lava flows, flow breccia, tuff and dikes. Deformed and altered to greenstone. See Coats, 1956 for additional information.
Early Tertiary basaltic rocks
Early Tertiary
Basaltic rocks, argillite, and graywacke
Paleozoic
Tak
Amchitka Formation, pillow lavas and breccias of Kirilof Point
Glassy, generally monolithologic breccia, and tuff breccia, subordinate pillow lava, and very minor bedded volcsanic sedimentary rocks probably deposited in a submarine environment.  Less mafic than other rocks on Amchitka Island, as felsic as latite
Tao
Amchitka Formation, older breccias
Coarse to fine-grained sedimentary breccia and minor interbedded sandstone, siltstone, and claystone composed of volcanic debris; generally propolitically altered.  May be as much as 4,000 feet thick, although estimated is considered high.
TMzkb
Krugoli Formation - lava
Lava; chiefly basaltic and spilitic.
Late Mesozoic or Early Tertiary
TMzkt
Krugoli Formation, tuff and tuff-agglomerate.
TMza
TMzv
Basement rocks -- volcanic rocks
Volcanic rocks, including basaltic, spilitic, and keratophyric lava and tuff.  Pillow lava very common.
Early Tertiary or Late Mesozoic
A series of radiometric dates were done on samples from the original mapping in the mid-1970's.  Ages typically were no older that 30 Ma, Oligocene
TMzl
Basement rocks - Agattu
Basement rocks - lava
TMzt
Basement rocks - tuff and tuff-agglomerate
Tgr
Albite granite
Albite granite.  It occurs in irregular masses at Steller Cove and as a small stock east of Mikhail Point.  The granite is gray, pale-pink, or purplish and has widely varying grain size.  Locally euhedral gray-green feldspar (albite) phenocrysts up to 5
Tagr
Diabase and gabbro dikes, sills and small plutons
Diabase and gabbro dikes, sills and small plutons, Attu Island
Tgb
PPag
Gabbro
Gabbro intruding Finger Bay volcanics of Adak Island (Fraser and Snyder, 1959).  Unit mapped as Paleozoic by Coats (1956), since revised.  Consists of gabbro, locally including hornblende gabbro and augite-quartz syenite
Diabase and gabbro
Diabase and gabbro, Agattu Island
Medium- to coarse-grained intrusive gabbro and related rock types.
gd
Granodiorite, quartz diorite, and gabbro
Granodiorite, quartz diorite, and gabbro in stocks, sills, and dikes.  Younger plutons
K-Ar dates 10-13 Ma
Granodiorite and quartz diorite
Granodiorite and quartz diorite, Ilak and Amatignak Islands, includes andesite dikes on Ilak Island
Tqd
Quartz diorite, Amchitka Island
Medium-grained quartz diorite and related rock in dikes, sillls, and a small stock.  Intrudes Amchitka and Banjo Point Formations
Reported K-Ar age on biotite of 15.8 Ma (Marlow and others, 1973)
gr
Granite?
Granodiorite, quartz diorite, and gabbro in stocks, sills, and dikes.  Older plutons
K-Ar dates in the range of 30-33 Ma
Toem
TMzk
Krugoli Formation, undifferentiated - Agattu
Undifferentiated; includes bedded argillite, siltstone, and chert.
TMzkc
Krugoli Formation - conglomerate Agattu
Conglomerate.
TMzc
TMzu
Basement rocks - undifferentiated
Basement rocks - fine-grained banded sedimentary rocks - Attu
Fine-grained thin-bedded, sedimentary rocks, chert silicious siltstone, argillite, limestone, and fine tuffaceous graywacke.  Some graded beds.  Turbidite characteristics, evidence of soft-sediment deformation
Undetermined Foraminifera, diatoms, and radiolaria
Basement rocks - coarse-grained and conglomeratic sedimentary rocks - Attu
Coarse graywacke and conglomerate, interbedded with pillow lavas, pyroclastic rocks, and fine siliceous rocks.
Basement rocks - conglomerate, argillite, siltstone, and chert.
TMzs