Prepared in cooperation with the National Park Service

Surficial Geologic Map of the Gates of the Arctic National 

Park and Preserve, Alaska

By Thomas D. Hamilton and Keith A. Labay

Pamphlet to accompany

Scientific Investigations Map 3125

2011

U.S. Department of the Interior

U.S. Geological Survey

Glaciers and rugged alpine topography of Arrigetch Peaks (Survey Pass quadrangle). Photo by T.D. Hamilton, August 30, 1979
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i

Contents

Introduction ....................................................................................................................................................1

Regional Setting ............................................................................................................................................1

Previous Studies ...........................................................................................................................................3

The Glacial Record .......................................................................................................................................4

Quaternary Tectonics and Drainage Evolution ........................................................................................5

Alpine Features ..............................................................................................................................................6

Acknowledgments.........................................................................................................................................7

Description of Map Units..............................................................................................................................8

Surficial Deposits .................................................................................................................................8

Fan Deposits ................................................................................................................................8

Alluvium ........................................................................................................................................8

Terrace Deposits .........................................................................................................................9

Other Gravel Deposits ................................................................................................................9

Colluvial Deposits ......................................................................................................................10

Sand, Silt, and Organic Deposits ............................................................................................11

Lacustrine and Glaciolacustrine Deposits ...........................................................................12

Glacial Drift and Ice-Contact Gravel .....................................................................................13

Late Holocene Glaciation (Neoglaciation) ..................................................................13

Itkillik Glaciation ...............................................................................................................13

Sagavanirktok River Glaciation .....................................................................................14

Anaktuvuk River Glaciation ............................................................................................14

Gunsight Mountain Glacial Interval ..............................................................................15

Glacial Outwash and Inwash ..................................................................................................15

Bedrock Surface Forms ....................................................................................................................16

References Cited .........................................................................................................................................16

Figures

Figure 1. Northern Alaska, showing locations of Gates of the Arctic National Park and Preserve, 

and Noatak National Preserve.......................................................................................................1

Figure 2. Central Brooks Range, showing map area and areas of previously published surficial 

and bedrock geologic mapping .....................................................................................................2

Figure 3. Shaded-relief map of central Brooks Range and adjoining foothills...................................3

Tables

Table 1. Glacial advances in the central Brooks Range (modified from Hamilton, 2009) ..................4

Table 2. Physical characteristics of Pleistocene drifts, central Brooks Range (where 

differentiated, southern valleys shown in brackets). From Hamilton, 1994 .........................6
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1

Introduction

The Gates of the Arctic National Park and Preserve 

(GAAR) is centered over the central Brooks Range of northern Alaska (fig. 1). To the west, it abuts the Noatak National 

Preserve; its eastern boundary is the transportation corridor 

occupied by the Dalton Highway and the Alyeska Pipeline. 

The GAAR extends northward beyond the northern flank of the 

Brooks Range into the southern Arctic Foothills (Wahrhaftig, 

1965). Its southern boundary lies beyond the south flank of the 

Brooks Range within foothills and depositional basins of interior Alaska. The accompanying surficial geologic map covers 

all of the GAAR with the addition of a 10-km (6.2-mi) belt or 

“buffer zone” beyond its boundaries. A narrower (5-km) buffer 

zone is employed where the GAAR extends farthest north and 

south of the Brooks Range, in the north-central and southwestern parts of the map area, respectively.

The surfical geologic map incorporates parts of ten surficial 

geologic maps previously published at 1:250,000 scale (fig. 2). 

In addition, a small part of the buffer zone mapped in the southwest corner of the map area was compiled from unpublished 

surficial geologic mapping of the Shungnak 1:250,000-scale 

quadrangle. Each of those individual maps was developed from 

(1) aerial and surface observations of morphology and composition of unconsolidated deposits, (2) tracing the distribution 

and interrelation of terraces, abandoned meltwater channels, 

moraines, abandoned lake beds, and other landforms, (3) stratigraphic study of exposures along lake shores and river bluffs, 

(4) examination of sediments and soil profiles in auger borings 

and test pits, and exposed in roadcuts and placer workings, and 

(5) analysis of previously published geologic maps and reports. 

The map units used for those maps and employed in the present 

compilation are defined on the basis of their physical character, 

genesis, and age. Relative and absolute ages of the map units 

were determined from their geographic locations and from their 

stratigraphic positions and radiocarbon ages (see reports listed 

on figure 2 for radiocarbon age data).

C h u k c h i

S e a

B e a u f o r t S e a

C o l v i l l e

R i v e r

CANADA

A R C TI C

FO OT H I L LS

H i g h w a y

Kobu k Ri v e r

N o a t a k R i v e r

NOAT

B R O O KS

G AAR

K o y u ku k

R i v e r

R A N G E

D a l t o n

Yu k o n

R i v e r

200 250 Miles

0 100 200 300 400 500 Kilometers

0 50 100 150

168° 162° 156° 150° 144°

70°

68°

66°

Figure 1. Northern Alaska, showing major drainages and locations of Gates of the Arctic National Park and Preserve (GAAR) and Noatak National Preserve (NOAT).

Regional Setting

The central Brooks Range is dominated by rugged, glacierabraded peaks and ridges that rise to 1,800-2,100 m (6,000-

7,000 ft) altitude and are indented by deep cirque basins. Broad, 

glacier-carved troughs extend north and south to both flanks of 

the range and, in the westernmost part of the map area, trend 
2

westward forming the upper Noatak River valley. The eastwest-trending Arctic Foothills (fig. 1) north of the range are 

dissected by north-flowing drainages and by broad gaps carved 

by former glaciers. The major valleys followed by those northflowing streams are spaced fairly regularly at intervals of about 

100 km (fig. 3). Terrain beyond the south flank of the Brooks 

Range is dominated by west-flowing rivers and streams of the 

Kobuk and Koyukuk drainage systems and by the structural 

basins that control their courses. Isolated foothills south of 

the Brooks Range generally rise no higher than 900–1,200 m 

(3,000–4,000 ft).

Climate within the map area varies from arctic in the north 

to subarctic in the south. Winters are long and cold throughout 

the map area, but summers vary from short and cool in the north 

to longer and much warmer near and beyond the south flank of 

the Brooks Range (Shulski and Wendler, 2007, p. 38). Precipitation increases through the summer, reaching maximum values 

in August (Shulski and Wendler, 2007, p. 62). Both rainfall and 

snowfall decrease northward and eastward across the map area 

(Ellis and others, 1981). This gradient is reflected in depth of 

winter snow cover and in the distribution of modern and Pleistocene glaciers.

Summer temperature gradients are reflected also in the 

vegetation of the map area. Boreal forest, dominated by birch, 

spruce, and cottonwood trees, occupies all but the highest 

hilltops south of the Brooks Range, and extends into the range’s 

southern valleys (fig. 3). Beyond tree limits, tundra vegetation 

covers all but the higher and steeper rock slopes, ridge crests, 

and peaks.

Permafrost is present at shallow depth in much of the 

Brooks Range and throughout the adjoining Arctic Foothills. 

Depth of its upper surface ranges from 15 to 25 cm in poorly 

drained deposits beneath thick moss and sod cover to about 

half a meter in permeable coarse-grained sediments and several 

tens of meters beneath the larger lakes and rivers. Permafrost is absent beneath some of the larger water bodies near 

HOWARD PASS PHILIP SMITH MTS KILLIK RIVER CHANDLER LAKE

AMBLER RIVER CHANDALAR SURVEY PASS WISEMAN

SHUNGNAK

BETTLES

HUGHES

159° 156° 153° 150° 147°

69°

68°

67°

66°

Hamilton, 1978b, 1979b

Hamilton, 1978a, 1979a

Hamilton, 1979c, 1980a

Hamilton, 1979d, 1980b

Hamilton, 2002b

Hamilton, 1981a, Hamilton

and Brubaker, 1983

Hamilton, 1980c,1982a

Hamilton, 2002a

Hamilton, 1984a, 2009

Hamilton, 1984b,

2009

Hamilton, unpublished

0 50 100 150 200 250 300 Kilometers

Mull and others, 1994 Kelley, 1990 Brosgé and others, 1979

Di Brosgé and Reiser, 1964 llon and others, 1986

Nelson and Grybeck, 1980 Mayfield and

Tailleur, 1978

Patton and others, 1968

Patton and Miller, 1966

Patton and Miller, 1973

0 25 50 75 100 125 150 Miles

Figure 2. Central Brooks Range, showing Gates of the Arctic National Park and Preserve (in green) and its buffer zone (in gray). Gray stippled pattern designates park inholdings. Area within the buffer zone covers parts of 

eleven 1:250,000-scale quadrangles. The first reference cited beneath each quadrangle name is the surficial geologic map from which the park map was compiled. The second reference listed for eight of the quadrangles is a 

compilation of stratigraphic sections and radiocarbon age determinations. Listed in italic is the bedrock geologic 

map available for ten of the quadrangles.
3

the southern flank of the range and in its southern foothills. 

At those southern localities, depth of its upper surface ranges 

from 15 to 25 cm in muskeg areas to 0.5 m or more in permeable coarse-grained sediments. Although thicknesses generally 

are unknown, mining, water-well, and other records (Ferrians, 

1965; Williams, 1970, p. 31–32; Brown and others, 1997; 

Kurtak and others, 2002) suggest that the base of permafrost 

may lie at depths of 150–300 m through much of the map area, 

shallowing to 50–150 m near and beyond the south flank of the 

Brooks Range.

Bettles

159° 156° 153° 150° 147°

69°

68°

67°

66°

Kill i k

R

C o l v i l l e R

C h a n d a l a r R

K o y u k u k R

K o b u k R

0 50 100 150 200 250 300 Kilometers

Yu k o n R

No atak R

N igu R

C h

a

n

d l e r R

A n a k t u v u k

R

S a g a v a n ir k t o k R

A lat na R

o h

n

R

Continental

Continental

Divide

Divide

0 25 50 75 100 125

Atigun

Pass

150 Miles

S. Fork Koyukuk R

N. Fork Koyukuk R

M. Fork Koyukuk R

Figure 3. Shaded-relief map of central Brooks Range and adjoining foothills, showing principal drainages, position of arctic treeline (red), and Continental Divide (brown). Boundary of Gates of the Arctic National Park and 

Preserve is shown in green; black line outlines map area.

Previous Studies

Early accounts of explorations, mining activities, and 

native settlements in the central Brooks Range are reviewed by 

W.E. Brown (2007), who provides an extensive bibliography of 

pertinent publications.

The basic sequence of Brooks Range glaciations was 

initially determined by Detterman (1953; Detterman and others, 

1958), and later modified by Porter (1964), Hamilton and Porter 

(1975), and Hamilton (1978c, 1979e, 1986). Detailed mapping of surficial geologic units within the range was carried 

out initially by Porter (1966) in the Anaktuvuk River valley of 

the Chandler Lake quadrangle and by Hamilton (1969) in the 

Alatna River valley of the Survey Pass quadrangle. Later reconnaissance mapping along the proposed Trans-Alaska Pipeline 

corridor was accomplished by U.S. Geological Survey personnel (Ferrians, 1971; Kachadoorian, 1971). Subsequent surficial geologic mapping of the ten 1:250,000-scale quadrangles 

from which the GAAR map is derived was carried out by U.S. 

Geological Survey field parties during 1975–1987, with resulting publications shown on figure 2. Surficial geology of the 

small part of the Shungnak quadrangle at the extreme southwest 

corner of the map area was mapped from field observations and 

airphotos, but was not published previously.

Bedrock mapping of the central Brooks Range and neighboring foothills also has been carried out by numerous field 

parties, as reviewed by Smith and Mertie (1930) and later by 

Moore and others (1994) and Patton and others (1994). Geologic maps published for each of the 1:250,000-scale quad-
4

rangles in the GAAR region are cited on figure 2. A subsequent 

compilation by Till and others (2008) reviews the largely metamorphic rock units of the south-central Brooks Range and their 

history of investigations.

Construction of the Trans-Alaska Pipeline and haul road 

(later renamed the Alyeska Pipeline and the Dalton Highway, 

respectively) provided unprecedented access to the eastern 

part of the map area. This has led to a wide range of geologic, 

hydrologic, and permafrost studies (for example, Brown and 

Kreig, 1983; Ellis and Calkin, 1979; Ellis and others, 1984; 

Onesti, 1983; Onesti and Walti, 1983; Sloan and others, 1976; 

Yoshikawa and others, 2007). The Dalton Highway has also 

provided a route into the Brooks Range and Alaska’s Arctic 

Slope for permafrost-related field trips (Brown and Kreig, 1983; 

Walker and others, 2009), and for other field trips and commercial tours (Diel and Banet, 1993).

The Glacial Record

Pleistocene glaciers originated in cirques along the continental divide that separates north-flowing from south-flowing 

drainages of the central Brooks Range; and more locally near 

the north flank of the range, on resistant igneous intrusions 

south of the divide, and on some of the higher uplands south 

of the range. Most of those localities remained active centers 

of snow accumulation and frost action during the Holocene, as 

attested by concentrations of modern glaciers, active and inactive rock glaciers, cirque moraines, and talus aprons. A small ice 

cap may have formed on uplands around the heads of the Alatna 

and Nigu River valleys, where high-altitude erosion surfaces 

(cross-hatched pattern) are common. These surfaces bear a thin 

cover of slightly weathered felsenmeer and erratic stones; they 

evidently were eroded by glaciers during the last glacial maximum (Itkillik II glaciation), and probably also during earlier 

glaciations as well. Some mountain valleys within the map area 

lacked high-altitude source areas, and those valleys probably 

were unglaciated during Itkillik II time but were glaciated 

earlier. The lower courses of many of those unglaciated valleys 

were filled with ice from main-valley glaciers, which extended 

as much as 10 km into each of them. In addition to thick 

moraine dams near their mouths, the unglaciated valleys contain 

inactive alpine fans and thick colluvial aprons that formed under 

severe periglacial conditions during Itkillik II time.

Although some glaciers radiated from local uplands south 

of the Brooks Range, most glaciers flowed north and south 

through deeply incised valley systems to terminate at and 

beyond both flanks of the range. Outwash trains were deposited 

along streams that issued from the ice fronts, and lakes formed 

behind moraine dams and in other localities blocked by glacier 

ice. Loess derived from outwash, drained lake basins, and other 

glacial deposits formed thick and extensive blankets across 

upland surfaces that lay beyond the limits of the younger glacial 

advances.

Drift of five major glacial intervals (table 1) is recognized 

within northern valleys and adjoining foothills of the map area. 

Drift and erratic boulders of the Gunsight Mountain glacial 

interval represent one or more glaciations of probable late 

Tertiary age in the Arctic Foothills region (Hamilton, 1979c, 

1979e; 1986). Extensive alluvial terraces of Tertiary (?) age 

(unit Ttg) originate or are truncated at the northern limits of 

Gunsight Mountain drift and erratic boulders; the terraces may 

in part be contemporaneous with Gunsight Mountain glacial 

advances and incorporate Gunsight Mountain outwash. The 

oldest Pleistocene terraces (unit tg1) contain scattered residual 

boulders derived from Gunsight Mountain drift; these terraces 

therefore postdate the glacial advances of Gunsight Mountain time. The northern limits of Gunsight Mountain residual 

boulders incorporated in younger terrace deposits lie beyond 

the north margin of the map. Probably correlative erratic limits 

Table 1. Glacial advances in the central Brooks Range (modified from Hamilton, 2009).

Glacial Event Age Assignment Age (yr)1 Reference

Neoglaciation Late Holocene

-------------------------------------------------------------------- 5,000 Hopkins, 1975

Early Holocene

-------------------------------------------------------------------- 10,000 Hopkins, 1975

Late Itkillik readvance | | | Late Pleistocene | 12,800 Hamilton, 1986

Itkillik II | (Itkillik glaciation) | | | 26,000 Hamilton, 1986

Itkillik I | | ------------------------------------------------------------------- 122,000 Richmond and Fullerton, 1986

Sagavanirktok River Middle Pleistocene

------------------------------------------------------------------- 780,000 Baski and others, 1992

Anaktukuk River Early Pleistocene

---------------------------------------------------------------- 2,600,000 Mascarelli, 2009

Gunsight Mountain Late Tertiary

1Ages to 26,000 yr given in radiocarbon years before present; older ages in calendar years
5

were mapped near Anaktuvuk Valley by Detterman and others 

(1963).

The Anaktuvuk and Sagavanirktok glaciations of Detterman (1953) are currently termed Anaktuvuk River and Sagavanirktok River glaciations to avoid confusion with previously 

named rock-stratigraphic units (Keroher and others, 1966, p. 91 

and 3379). In the Arctic Foothills region, drift sheets of Anaktuvuk River age have been modified by stream erosion, forming 

dissected surfaces that extend downvalley into alluvial terraces 

of early Pleistocene age (unit tg1). The terraces stand at the 

same general level as outwash of Anaktuvuk River age; therefore they may in part be contemporaneous with the Anaktuvuk 

River glaciation. Drift and outwash of Sagavanirktok River 

age show the same relation to younger Pleistocene terraces in 

the Arctic Foothills region (unit tg2), which are believed to be 

in part younger and in part contemporaneous with this glacial 

event. Sagavanirktok River drift locally forms two units that 

contrast in postglacial modification (Hamilton, 2003; Keller and 

others, 2007) and possibly were separated by a nonglacial interval. Pediment glacial deposits of probable Sagavanirktok River 

age at the north flank of the range beyond the map area (crosshatched map pattern in Hamilton, 1980c) have been cut into 

glacial deposits of probable Sagavanirktok River age. A major 

erosion interval appears to have separated the Sagavanirktok 

River glaciation from ice advances of Itkillik age.

Hamilton and Porter (1975) divided the Itkillik glaciation into Itkillik I and II phases (hereafter termed Itkillik I 

and Itkillik II); its drift is designated id1 and id2, respectively. 

Porter (1964) had defined four stades of Itkillik glaciation based 

on type localities within the Anaktuvuk Valley, but restudy of 

these localities indicates that drifts of his Banded Mountain and 

Antler Valley stades correspond to the separate Itkillik I and 

II glacial advances and that deposits of his Anayaknaurak and 

Anivik Lake Stades do not represent separate glacial readvances (Hamilton, 1979e). Drift of late Itkillik II age (unit id3) 

represents deposits formed during final events of the waning 

Itkillik glaciers near their source areas along the Continental 

Divide (fig. 3) and close to the south flank of the Brooks Range. 

That drift formerly had been assigned to the Alapah Mountain 

glaciation (Detterman and others, 1958; Porter, 1964), but was 

later reinterpreted as alpine moraines that formed during a late 

readvance of Itkillik II glaciers (Hamilton, 1979c). Unstable 

kettles with actively caving gravel rims along the north flank 

of the Brooks Range and within some of its northern valleys 

suggest that drift of Itkillik II age may still be partly ice cored in 

those localities (Hamilton, 1982b).

Radiocarbon ages on glacial deposits across the central 

Brooks Range (Hamilton, 1979a, b; 1980b; 1982a, 1986, 

1994; Hamilton and Brubaker, 1983; Hamilton and others, 

1987) indicate that the Itkillik I glaciation occurred largely or 

wholly before 53,000 14C years B.P.1

, and that Itkillik II glacial 

advances began about 29,000 yr B.P. and culminated about 

20,000 years ago. Late Itkillik readvances to positions at or near 

the north flank of the Brooks Range took place about 12,500 to 

11,500 yr B.P. (Hamilton, 1986, 2003). Post-Itkillik basin-filling 

1All numerical ages given in this section are in radiocarbon years B.P. 

(before present).

had commenced by at least 10,500 yr B.P. in some valleys 

(Hamilton, 1986, 2009). The Itkillik II glaciation clearly lies 

within the time range of late Wisconsin glaciation in the standard North American glacial succession. Because of their significant age differences, drifts of Itkillik I and II ages differ in 

their degrees of postglacial modification by weathering, erosion, 

and mass wastage. Therefore, the drifts can be differentiated by 

use of such criteria as moraine morphology, boulder weathering, and soil development (table 2), and they show distinctive 

differences in soil geochemistry (Munroe and Bockheim, 2001; 

Keller and others, 2007).

Glacier advances of Itkillik II age were extensive in the 

eastern and central parts of the map area, but became progressively more restricted westward because of lower lying potential source areas at valley heads. Some mountain valleys near 

the western part of the map area did not support glaciers of that 

age. Those valleys differ from their neighbors in containing 

inactive fans and thick colluvial aprons that presumably formed 

on ice-free surfaces during Itkillik II time.

The Fan Mountain I and II glacial advances of Porter 

(1964) are equated with the late Holocene neoglacial interval as 

described by Porter and Denton (1967) for the North American 

Cordillera, and Porter and Denton’s terminology is followed 

here. Cirque-glacier activity and correlative episodes of talus 

formation and rock-glacier motion have been dated provisionally by Calkin and Ellis (1980, 1981), Ellis (1978, 1982), and 

Ellis and Calkin, 1984) using lichenometry, and by Hamilton 

(1981b) on the basis of 14C-dated episodes of alluviation in 

cirque-headed stream valleys. Older Neoglacial drift (unit nd1) 

may have formed mainly between about 4,000 and 2,000 yr 

B.P., with some moraines enclosed within the older drift perhaps 

as young as 800 years. More recent Neoglacial deposits (unit 

nd2) reflect widespread “Little Ice Age” glacial readvances 

of the past 450 years. Nearly all moraines of this age occur in 

cirques occupied by existing glaciers. Neoglacial moraines and 

other cirque deposits are abundant in eastern and central parts 

of the map area but become increasingly rare westward as the 

altitude of valley heads decreases.

About 234 modern cirque glaciers have been identified in 

the map area. Most glaciers are small in size, having surface 

areas of 1 km2 or less. Most have undergone rapid retreat during 

the past century (Hamilton, 1965; Ellis and Calkin, 1979; Ellis 

and others, 1981), and many present-day glaciers are stagnating and becoming unrecognizable under thick accumulations of 

ablation debris.

Quaternary Tectonics and Drainage 

Evolution

Although the eastern Brooks Range has been tectonically 

active during the Quaternary (Grantz and others, 1983; Moore 

and others, 1994), the central part of the range has been largely 

inactive during that interval. Glacial deposits of the eastern 

Brooks Range show the impact of severe drainage disruption 

between successive glacial advances owing to crustal movements. Beyond the north flank of the central Brooks Range, in 
6

contrast, glacial deposits of early to late Pleistocene age nest 

symmetrically within each other; thereby indicating long-term 

persistence of drainage courses. However, the differences in 

relative extent of glacial advances between adjoining valleys 

during that interval record the progressive development of 

master valleys that are spaced about 100 km apart as the result 

of piracies within their upper drainage courses (Hamilton, 2003, 

p. 4).

Whereas most of the central Brooks Range exhibits little 

evidence for Quaternary tectonism, a belt of probably active 

faulting along the south flank of the Brooks Range trends eastward up the Kobuk River valley and continues farther east into 

the Koyukuk drainage system (Bettles quadrangle). The western 

part of this belt, termed the Kobuk fault zone, was mapped by 

Patton and Miller (1966), and has a history of earthquake activity (Gedney and Marshall, 1981). More recent surficial geologic 

mapping (Hamilton, 1984c, 2002a, b) has demonstrated fault 

offsets of deposits dating from the middle and late Pleistocene 

(Sagavanirktok River and Itkillik glaciations). Aligned faultrelated geomorphic features include 1) elliptical pingos and 

sand extrusions, 2) elongate sag and thaw ponds, 3) fault scarps, 

4) uplifted lake shores, 5) offset drainages, and 6) anomalously 

deep, narrow, and straight gully systems. Those features, forming single and multiple fault strands, could be traced nearly 

continuously for nearly 40 km up the floor of the Kobuk River 

valley (southwest corner of map area) then intermittently for 

another 120 km across the Hughes and Bettles quadrangles into 

the Koyukuk River valley near Bettles (just beyond southern 

boundary of the map area, fig. 3). Right-lateral offsets evident at some stream crossings are consistent with the dextral 

offsets indicated by recent seismicity along the fault (Gedney 

and Marshall, 1981). The Kobuk fault zone may form part of a 

major regional structure that extends east-west across much of 

northwest Alaska (Patton and others, 1994; Avé Lallemant and 

others, 1998).

Near the south flank of the Brooks Range within the Wiseman quadrangle, lineaments (straight linear features) are evident 

in unconsolidated deposits near the south flank of the Brooks 

Range and south of the Koyukuk River near the confluence of 

its North and Middle Forks. These features are expressed as 

aligned and unusually straight swales, gullies, and vegetation 

lines, associated in some places with pingos or pingo clusters 

(Hamilton and Obi, 1982), stream deflections, or abrupt linear 

boundaries of surficial map units. No vertical or horizontal 

offsets of unconsolidated deposits could be detected in the 

field, however, and a fault origin for these linear elements is 

unproven. They may represent true faults, perhaps caused by 

isostatic readjustments at the time of deglaciation (for example, 

Pelletier, 2004; Hampel and others, 2007); alternatively, they 

may merely be surface expressions of underlying bedrock 

structures.

Alpine Features

In addition to cirques, their glaciers, and associated 

moraines, other distinctive alpine deposits and landforms are 

characteristic of the higher parts of the central Brooks Range. 

Many of these features are related to rapid production of angular 

rock debris by frost action and other weathering processes on 

the walls of upper mountain valleys and on other steep slopes 

oversteepened by erosion of former glaciers. Talus cones and 

aprons, steep alpine fans, rock glaciers, snow-avalanche tracks, 

and other alpine features have been studied most intensively 

(Brown and Kreig, 1983; Walker and others, 2008) near the 

Dalton Highway in the Atigun Pass area (fig. 3).

Talus (unit tr) consists of angular rock rubble that accumulates where rockfall is common. It forms talus cones at the bases 

of rugged ravines or chutes and broader aprons along the bases 

of steep cliffs (Washburn, 1980, p. 231–234). Where active 

today, talus accumulations stand at steep (30°–35°) angles, with 

unstable unweathered surfaces. Inactive talus rubble is weathered and lichen-encrusted, and may support vegetation cover. In 

southern valleys of the Brooks Range, inactive talus is commonly relict from late Pleistocene intervals of more severe frost 

climate.

Steep alpine fans (unit af), as described by Walker and 

others, 2009, p. 17), “....occur at the bases of chutes that commonly broaden upward into larger source areas. They are composed of coarse angular rubble mixed with variable amounts of 

finer debris. Their surfaces typically exhibit one or more channels bordered by ridgelike levees of rock rubble. Slope angles 

(12°–25°) are intermediate between those of talus cones and 

alluvial fans. Although formed commonly by slushflows during 

Table 2. Physical characteristics of Pleistocene drifts, central Brooks Range (where differentiated, data 

from southern valleys shown in brackets). From Hamilton, 1994.

Glaciation Width of Maximum Boulders per Maximum Maximum

 moraine flanking 250 m2 boulder depth of

 crests (m) slopes (°) protusion (cm) oxidation (m)

Itkillik II 3–5 18–23 12–215 40–80 0.3

[2–3] [16–22] [0–191] [20–30]

Itkillik II 5–20 15–20 9–213 25–50 1.0–1.2

[5–15] [14–20] [0–135] [≤20]

Sagavanirktok 75–200 2–3.5 1–5 100 >8

River

Anaktuvuk 500 1.2 0–1 10 >30

River
7

spring snowmelt period, large increments of debris are added at 

longer intervals during periods of exceptionally heavy summer 

rainstorms.”

Alluvial fans (unit f) in upper mountain valleys appear 

much like alluvial fans elsewhere, but their upper surfaces 

commonly are littered with dispersed angular rock rubble up 

to boulder size (as illustrated in Walker and others, 2008, figs. 

20 and 21). Shredded plant remains may also be evident. These 

debris are carried across the fan surface by slushflows during 

the spring snowmelt period (Onesti, 1983, 1989). As detailed 

by Onesti, the floors of steep gorges tributary to upper valleys 

fill deeply during winter with snow blown by strong winds 

from adjoining upper slopes and ridge crests. These thick snow 

accumulations absorb great amounts of meltwater runoff during 

the spring snowmelt period, finally becoming water-saturated. 

Highly erosive slush then flows rapidly down the steep floor of 

the gorge and out across the adjoining alluvial fan.

Rock glaciers (unit rg) generally are fed by rockfalls, and 

coarse, blocky, angular talus dominates their outer surfaces. 

These features move by internal deformation (creep) of interstitial or underlying ice, and are classified as either lobate or 

tongue-shaped (White, 1976). Tongue-shaped rock glaciers are 

longer than wide, with maximum lengths as great as 2500 m 

(Ellis and Calkin, 1979). They commonly form on the floors of 

cirques, and may be cored by stagnant glacier ice. Lobate rock 

glaciers, which typically are broader than long, develop below 

talus cones along the bases of steep valley walls (White, 1976). 

Their constituent debris is ice-cemented. Active rock glaciers 

are recognizable by their steep and unweathered frontal slopes, 

which meet their upper surfaces at sharp angles. Inactive rock 

glaciers have weathered and lichen-covered frontal slopes, 

which rise at more gentle angles and round back more gradually 

to merge with their upper surfaces.

A distinctive variant of rock glacier found along the walls 

of southern valleys of the central Brooks Range is here termed 

debris glaciers (unit dg). These features differ from true rock 

glaciers in their location, mode of nourishment, composition, and mechanism of movement. Debris glaciers typically 

originate in shallow drainage basins along the upper walls of 

southern valleys, where they are fed by solifluction and shallow 

debris flows rather than by rockfall (Ronald Daanen, University of Alaska Fairbanks, written commun.). Several streams of 

flowing debris commonly merge into a single elongate lobe that 

may flow 1 km or more down the valley side, with some lobes 

extending into and continuing to move through dense forest. 

In addition to rock rubble, debris glaciers contain decomposed 

rock (regolith), blocks of sod, soil, spruce logs and other large 

wood fragments, and lenses or irregular masses of ground ice. 

The rock rubble typically consists of tabular slabs of schist, 

phyllite, siltstone, or shale; these slabs generally dip downslope 

in orientations that reflect movement by internal sliding. Shear 

planes visible at the termini of some debris glaciers provide 

further evidence that internal sliding is likely to be an important component of their movement. The coarse debris occurs in 

an abundant ice-rich matrix of generally micaceous sand and 

silt that contains little clay, but small wood fragments, humic 

soil material, and other organic detritus commonly are present (Daanen, 2009). Measurements by Ronald Daanen (written 

commun.) show that rates of debris-glacier movement commonly average nearly 1 cm per day.

Talus cones, steep alpine fans, and alluvial fans form a 

continuum in which intermediate forms are common between 

each end member. Talus cones and rock glaciers show similar 

intergradation, with some talus cones exhibiting incipient rockglacier creep on their distal slopes. Rock glaciers and debris 

glaciers may intergrade in places, but generally these two end 

members are distinct.

Although snow avalanches are a severe hazard in many 

upper valleys of the central Brooks Range (for example, Brown 

and Kreig, 1983; Walker and others, 2008), they commonly 

leave little trace after melting and in those cases do not constitute mappable surficial geologic units. However, snow-avalanche tracks and deposits (unit av) are conspicuous on forested 

valley walls, where the passage of snow avalanches destroys or 

severely damages tree limbs and trunks and associated avalanche-runout zones are littered with woody debris. Avalanche 

tracks mapped around the heads of the Kobuk and Noatak River 

valleys in the Survey Pass quadrangle probably reflect unusually high snowfall around the heads of those valleys, which 

serve as conduits for moist air masses moving inland from the 

Bering Sea (Ellis and others, 1981).

Acknowledgments

My initial studies of glaciation in the central Brooks 

Range were supported by research grants from the Arctic 

Institute of North America, the Geological Society of America, 

and the National Science Foundation. Subsequently, a more 

comprehensive program of surfical geologic mapping of that 

region was supported by the Arctic Environmental Studies Program of the U.S. Geological Survey, administered by Oscar J. 

Ferrians Jr. The National Park Service (NPS) contributed to the 

final compilation and publication of the last two maps (Bettles 

and Hughes) in the 1:250,000 surficial geologic map series. 

The NPS, through the Arctic Network Inventory and Monitoring Program, also funded the compilation of the present map 

and this report.

Most of the surficial geologic mapping of the map area 

was carried out during 14 expeditions into the central Brooks 

Range during 1975–1987. The field assistants, helicopter pilots 

and mechanics, and cooks that participated in each of those 

expeditions and the pilots that flew fuel, food, and gear in to us 

are too numerous to mention, but my thanks go to them all. Of 

particularly valued assistance over many years were David and 

Tam Ketscher, owners of Sourdough Outfitters and The Bettles 

Trading Post, who provided food, lodging, and other assistance 

in Bettles and also flew most of our resupply flights.

Digital representation of the map was developed by Keith 

A. Labay, who also assisted with unit designations and GIS 

databases. The map and report also benefited from thorough 

reviews by Robert B. Blodgett, Bruce A. Giffen, and Frederic 

H. Wilson, who pointed out errors, omissions, and ambiguities 

in a previous draft. Discussions with Ronald Daanen helped to 

clarify my thinking about the newly named debris glaciers and 

their differentiation from true rock glaciers.
8

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS

[Nearly all surficial deposits shown on the map are Quaternary in age. Those of inferred Tertiary age are 

designated by the prefix T.

Map units shown in parentheses, for example, (id), indicate thin and generally discontinuous deposits over 

near-surface bedrock. Map units shown with slashes, such as al/sa, indicate deposits of the first unit above 

known or inferred subsurface deposits of the second unit (color represents upper unit). Units of either type 

are described below only where additional explanation is necessary. Units queried where uncertain.]

FAN DEPOSITS

af Steep alpine fan deposits (Holocene and late Pleistocene)—Coarse, very poorly sorted, 

nonstratified to weakly stratified, subangular to subrounded silty sandy gravel at mouths 

of avalanche chutes and steep canyons. Common in upper mountain valleys. Upper segments generally channeled, with levees of angular to subangular coarse debris. Subject to 

snow avalanches during winter, slushflows during spring snowmelt, and debris flows during 

summer. Surface gradients generally 12°–25°, intermediate between those of alluvial fans 

and talus cones

afi Steep alpine fan deposits, inactive (late Pleistocene)—As described in unit af. Generally weathered and covered with sod and vegetation. Commonly are periglacial relics that 

formed beyond limits of ice advances of last major (Itkillik II) glaciation under climate more 

rigorous than that of present day

f Fan deposits (Holocene and late Pleistocene)—Range from poorly sorted, weakly stratified, 

subangular, silty, sandy coarse gravel at mouths of steep canyons to moderately sorted and 

stratified subrounded sandy gravel at mouths of large tributary valleys with relatively gentle 

gradients. Locally subject to icings (aufeis) during winter

fi Fan deposits, inactive (Holocene and late Pleistocene)—As described in unit f. Generally 

weathered and covered by 0.3–0.5 m organic silt to stony silt beneath sod and vegetation. 

Differentiated only on large compound fans

fs Silt fans (Holocene and late Pleistocene)—Unusually fine-grained fan deposits consisting of 

poorly sorted silt and sandy silt. Present only in valley of Kugukpak Creek (Killik River 

quadrangle), where associated with widespread solifluction deposits and outcrops of Hunt 

Fork Shale (as mapped by Brosgé and others, 1979)

fsa Sand fans (Holocene and late Pleistocene)—Fan deposits in which sand dominates. Mapped 

only near northeast corner of Shungnak quadrangle

fd Fan-delta deposits (late Pleistocene)—Compound units that consist of alluvial-fan facies, as 

described in unit f, near valley walls that grade distally into well sorted and generally well 

stratified sand, silt, and fine gravel of deltaic and lacustrine facies. Commonly associated 

with present-day or former lakes dammed behind end moraines at or near mouths of mountain valleys

fda Fan-delta deposits, active (late Holocene)—Alluvial fan and delta deposits, as described in 

unit fd. Mapped only along west side of Galbraith Lake (Philip Smith Mountains quadrangle)

fdi Fan-delta deposits, inactive (Holocene and late Pleistocene)—Alluvial fan and delta 

deposits, as described in unit fd. Form eroded remnants around unit fda along west side of 

Galbraith Lake (Philip Smith Mountains quadrangle) and in Hughes quadrangle

ALLUVIUM

al Alluvium, undivided (Holocene and late Pleistocene)—Varies from poorly sorted, moderately 

well stratified, subangular coarse gravel near heads of mountain valleys to moderately well 

sorted gravelly sand and sandy fine gravel along slow-flowing stretches of major streams. 

May contain local beds and lenses of sand and sandy silt. Muddy fine gravel and gravelly 

mud locally present in some streams south of Brooks Range. Along smaller streams, unit 

includes fan, floodplain, and low terrace deposits that are too small to be designated separately

al2 Modern alluvium (late Holocene)—Gravel to gravelly mud, as described in unit al. Generally 

unvegetated and subject to annual flooding. Commonly subject to aufeis formation (Yoshikawa and others, 2007). Differentiated only along principal streams within major valleys
9

al1 Low alluvial-terrace deposits (Holocene)—Gravel to gravelly mud, as described in unit al. 

Mantled with 0.3–1 m of silt, sand, turf, and peat, and generally vegetated. Forms terraces 

generally within 3–4 m of modern stream levels. Differentiated only along principal streams

alsa Alluvium, sand facies (Holocene and late Pleistocene)—Moderately sorted to well-sorted, fine 

to medium sand, parallel bedded to slightly crossbedded, commonly with thin interbeds of 

sandy peat or organic silty fine sand. Deposited by slow-flowing streams within basins partly 

dammed by end moraines near mouths of mountain valleys. Upper 1–2 m locally reworked 

by wind into sand sheets and dunes. Commonly grades downward into lacustrine deposits. 

Readily dissected by streams, leaving low (3–5 m) paired terraces

TERRACE DEPOSITS

td2 Terrace deposit, low-level (late Pleistocene)—Compound deposit with terrace-like upper surface. Consists of outwash gravel and silty flood-plain deposits of Itkillik II advance underlain by laminated clay and silt (lacustrine) and sandy fine gravel (deltaic) of Itkillik I age. 

Mapped only in Hughes quadrangle, where deposit stands 8 m above Reed River near its 

mouth

td1 Terrace deposit, high-level (middle Pleistocene)—Compound deposit with alluvial surface 

53–60 m above modern river level. Consists of coarse and fine gravel, strongly oxidized 

gravel, some sand, silt, and clay; and generally thick (up to 24 m) till deposits of Sagavanirktok River age. Capped by silt up to 7 m thick. Mapped only along Kobuk River in 

Hughes and Shungnak quadrangles

tg Terrace gravel, undivided (middle and early Pleistocene; late Tertiary?)—Coarse gravel to 

sandy fine gravel, commonly consisting of rounded to subrounded pebbles and cobbles in 

sandy matrix. Forms alluvial terraces of uncertain age or origin

tg3 Pleistocene terrace gravels, younger (middle Pleistocene)—Oxidized gravel, as described in 

unit gr, forming terraces generally 12–15 m high that are inset with within higher alluvial 

surfaces. Forms lower terraces or valley floors along some smaller streams that originate 

north of Brooks Range. Bears thick (4–8 m) cap of ice-rich organic silt in some localities. 

Composed of undifferentiated nonglacial alluvium plus distal outwash of Sagavanirktok 

River age. Designated only in Killik River quadrangle

tg2 Pleistocene terrace gravels, older (early Pleistocene)—Oxidized gravel, as described in unit 

gr, along major drainages north of Brooks Range. Forms terraces 30–45 m above rivers in 

Killik River quadrangle; 45–65 m above rivers in Chandler Lake quadrangle. Commonly 

contains residual erratic boulders derived from former deposits of Gunsight Mountain drift. 

Bears thick (5–10 m) cap of ice-rich silt in most localities, and commonly overlies 10–20 m 

of bedrock exposed by downcutting. Composed of undifferentiated nonglacial alluvium plus 

distal outwash of Anaktuvuk River age in Killik River quadrangle; commonly caps erosion 

surfaces formed within drift sheets of Anaktuvuk River age in Chandler Lake quadrangle. 

Lies beyond outer moraines of Sagavanirktok River Glaciation and above outwash terraces 

of Sagavanirktok River age

tg1 Pleistocene terrace gravels, oldest (early Pleistocene; latest Tertiary?)—Oxidized sandy 

gravel, as described in unit gr. Forms terraces 80 to 110 m above modern stream levels north 

of Brooks Range in Chandler Lake quadrangle. Generally caps erosion surfaces that formed 

within drift limits of Gunsight Mountain glacial interval but lie beyond outer moraines and 

outwash of Anaktuvuk River Glaciation

Ttg Tertiary terrace gravels (late Tertiary)—Strongly oxidized gravel of Tertiary age, containing 

rounded stones to large cobble size in sandy matrix, capped by ice-rich silt as thick as 10 m 

containing thaw basins. Forms erosion remnants, many terracelike in form, 60–150 m above 

modern stream levels. Streams in some cases have downcut through gravel and then through 

30–50 m of underlying bedrock. Mapped only north of Brooks Range along north margin of 

map area

OTHER GRAVEL DEPOSITS

gr Gravel, undifferentiated (middle and early Pleistocene; Late Tertiary?)—Gravel and sandy 

gravel of diverse origins and composition. Generally applied to isolated, gravelly erosion 

remnants of uncertain origin and composition

grfn Fine gravel (middle and early Pleistocene)—Rounded pebbles and small cobbles in matrix of 

slightly oxidized sand that commonly has high quartz content. Forms terrace-like erosion 
10

remnants and broader alluvial surfaces about 18–23 m above modern drainage levels in lowlands south of Norutak Hills (Hughes quadrangle). Queried where gravel present but grain 

size uncertain

pg Piedmont gravel (middle and early Pleistocene; late Tertiary)—Moderately well sorted, 

rounded to subrounded pebbles of schist and quartz in abundant matrix of medium to coarse 

sand containing schist chips, commonly interbedded with medium to coarse sand; generally oxidized yellowish-brown (10YR 4/6) to dark yellowish-brown (10 YR 3/4). Grades 

laterally into fan deposits consisting of platy pebbles, small cobbles, and some large cobbles 

of schist and quartz in coarse sand-granule matrix. Forms hummocky erosion remnants in 

trough between Brooks Range and foothills to the south in Survey Pass and Hughes quadrangles. Commonly overlain by erratic cobbles and boulders of Itkillik I age, but abuts 

moraines of this age in places and may in part be contemporaneous with them

COLLUVIAL DEPOSITS

av Avalanche tracks and deposits (late Holocene)—Angular unsorted unstratified loose rock 

rubble, commonly with intermixed woody plant debris. Forms tongues and fans along lower 

walls of mountain valleys. Associated with tracks and chutes where soil and vegetation are 

generally absent and that commonly are bordered by zones of battered trees or shrubs from 

which bark and branches have been partly stripped (Luckman, 1978). Recognized only in 

deep mountain valleys in southwestern part of map area (Survey Pass, Ambler River, and 

Hughes quadrangles)

c Colluvium, undivided (Holocene and Pleistocene)—Mixed solifluction deposits (unit s) and 

talus-rubble deposits (unit tr) in sheets and aprons more than about 1–2 m thick. Most extensive on moderate to steep slopes above and beyond limits of ice advances of Itkillik age. 

Also common on upper slopes below exposed or near-surface bedrock

dg Debris glaciers (Holocene and late Pleistocene)—Tabular rock rubble in abundant ice-rich 

matrix of generally micaceous sand and silt with wood fragments, humic soil material, 

and other organic detritus commonly present (Daanen, 2009). Commonly highly elongate, 

extending down valley walls below forest limits. Subject to slow and perhaps intermittent 

downslope motion. Common in southern valleys of central Brooks Range, where they are 

most numerous on phyllite, siltstone, shale and schist bedrock. Also present on limestone 

with siltstone or phyllite interbeds

fl Flow deposits (Holocene and late Pleistocene)—Very poorly sorted rock debris in abundant 

muddy matrix. Typically develop below arcuate detachment scars resulting from thaw of 

ice-rich permafrost. Collapse of those headwalls results in continual input of debris and 

meltwater into flow mass. Forms distinctive steep-fronted lobes that are subject to slow 

and continuous movement, especially during summer thaw season. These features have 

been variously termed tundra earthflows, retrogressive flow-slides, bimodal slope failures, 

or active-layer detachment slides (Brown and Kreig, 1983; Burn and Lewkowicz, 1990; 

French, 2007, p. 232–233). Common in Killik River quadrangle

ls Landslide deposits (Holocene and late Pleistocene)—Unsorted, nonstratified, coarse to fine 

angular rubble forming tongues and lobes associated with detachment scars and slide tracks 

on high, steep walls of mountain valleys. Subject to episodes of rapid downslope motion 

and long periods of relative stability. Most common in upper mountain valleys that supported active glaciers during late Itkillik II time. Although some recent landslides have been 

reported (for example, Yeend, 1972), most landslide activity probably took place on oversteepened slopes soon after deglaciation. 

Subunit (ls) designates incipient landslides marked by fractures and sagging rock masses on 

mountain slopes. Recognized only in Ambler River and Survey Pass quadrangles

pr Protalus rampart deposits (late Holocene)—Unsorted, unstratified, coarse angular rock debris 

forming arcuate low ridges. Associated with persistent snowbanks in shaded sites, most 

commonly at bases of cirque headwalls. Subject to rockfalls during spring thaw

rg Rock-glacier deposits, undifferentiated (Holocene and late Pleistocene)—Very poorly sorted, 

unstratified, coarse angular rock debris with matrix of silt and fine rubble; Active and 

inactive components (described separately below) either undetermined or too small to be 

mapped separately. Fed by talus cones and aprons, which commonly are too small to show 

on map. Form lobate deposits along bases of valley walls and tongue-shaped deposits within 

cirques
11

rga Rock-glacier deposits, active (late Holocene)—Coarse angular rock debris, as described 

in unit rg, but containing abundant interstitial ice. Upper surfaces generally unvegetated, 

unweathered to moderately weathered; with lichen cover sparse. Frontal slopes barren, steep 

(35°–38°), and highly unstable; they meet upper surfaces at abrupt angle. Tongue-shaped 

deposits commonly overlie stagnant glacier ice. Subject to slow downslope motion

rgi Rock-glacier deposits, inactive (Holocene and late Pleistocene)—Coarse angular rock 

debris, as described for unit rg, but generally lacking interstitial ice or underlying stagnant 

glacier ice. Upper surfaces and frontal slopes weathered, covered by lichens, and commonly 

partly covered by sod and vegetation. Frontal slopes grade into upper surfaces without 

abrupt angles

s Solifluction deposits (Holocene and late Pleistocene; middle Pleistocene?)—Very poorly 

sorted, unstratified to weakly stratified, stony silt and organic silt; forms smoothly graded, 

gently to moderately sloping sheets and aprons more than 1–2 m thick. Platy to elongate 

stones generally oriented parallel to slope. Forms widespread thick deposits beyond limits 

of Anaktuvuk River drift. Successively thinner and more locally present on successively 

younger drift surfaces. Within Brooks Range, unusually thick and abundant deposits overlie 

shale-rich formations and phyllitic bedrock, and appear to be associated with some fault 

zones. 

Subunit (s) designates thin (less than about 1.5 m) but widespread and generally continuous 

solifluction blankets above near-surface bedrock

tr Talus rubble (Holocene and late Pleistocene)—Angular, unsorted, unstratified rock debris, 

forming cones and aprons more than 2 m thick and generally sloping 30°–33° along lower 

flanks of mountain valleys and on lower parts of cirque headwalls. Also forms thinner and 

generally discontinuous sheets over many uplands mapped as bedrock. Active and inactive 

components (described separately below) either undetermined or too small to be mapped 

separately.

 Active talus rubble (designated by symbol only; see Symbols) is generally unvegetated, 

unweathered to slightly weathered, with lichen cover sparse to absent. Subject to rockfalls 

from slopes above, especially during spring thaw

 Note: Active talus rubble and associated active rock glaciers in cirques of Arrigetch Peaks 

(Survey Pass quadrangle) are too numerous to label separately and are shown by a special 

symbol (see Symbols)

tri Talus rubble, inactive (Holocene and late Pleistocene)—Angular rock debris, as described in 

unit tra. Generally weathered and lichen covered, and with partial sod cover at some localities. Thin (less than 1–2 m) blankets of stabilized talus occur on many uplands beyond limits 

of Itkillik glaciation

SAND, SILT, AND ORGANIC DEPOSITS

ds Dune sand (Holocene)—Moderately well sorted medium to fine sand commonly containing 

shale chips and with thin interbeds of sandy peat; grass rootlets may be abundant. Forms 

extensive parabolic and longitudinal dunes along Killik River near north flank of Brooks 

Range. Also present in Survey Pass quadrangle, where it forms series of subparallel ridges 

as much as 6.5 m high on Alatna River floodplain south of Takahula Lake

sa Sand deposits (late Pleistocene)—Moderately sorted silty fine sand to medium sand, horizontally bedded to slightly crossbedded, commonly with thin interbeds of sandy peat or organic 

silty fine sand. Deposited initially by slow-moving streams within sedimentary basins partly 

dammed by end moraines near north and south flanks of Brooks Range. Upper 1–10 m 

locally reworked by wind into sand sheets and dunes. Commonly grades downward into 

lacustrine deposits (see stippled map pattern). Generally dissected by postglacial streams, 

forming terraces 5–15 m high that border stream channels and modern floodplains of sandy 

alluvium (unit alsa). 

 Composite unit al/sa is distinguished in many glacial valleys of the southern Brooks Range 

(Survey Pass and Wiseman quadrangles), where fluvial reworking of sand into sandy alluvium is more widespread across narrow valley floors

rs Retransported silt deposits (Holocene and Pleistocene)—Thick (up to 15 m) deposits of nonstratified to weakly stratified silt and organic silt containing local lenses of stony to sandy silt. 

Typically ice-rich. Derived from eolian silt (loess) that was eroded by debris flows, solifluction, gulley-incision, and other slope processes, and then redeposited on lower slopes and 
12

valley floors. Most deposits presently are stable and vegetated, but some in northern part of 

map area are subject to present-day solifluction activity. Most common in Hughes quadrangle

si Organic silt deposits (Holocene and late Pleistocene)—Weakly stratified sandy silt, organic 

silt, and silty peat, containing abundant ice in form of lenses, wedges, and interstitial grains. 

Formed largely of loess, with admixed organic and solifluction deposits. Fills thaw basins, 

valleys of underfit streams, and other poorly drained depressions beyond limits of Anaktuvuk River drift north of Brooks Range in Killik River and Chandler Lake quadrangles. 

Forms smaller and more localized deposits on younger surfaces farther south, but generally 

is absent from deposits of Itkillik or younger age. High ice content may be largely due to 

Holocene ice-wedge growth

us Upland silt deposits (Holocene and Pleistocene)—Poorly to moderately sorted generally 

unstratified silt, organic silt, and slightly clayey, sandy or stony silt on uplands of low to 

moderate relief beyond oldest drift limits and above highest terrace levels both north and 

south of Brooks Range. Formed from loess mixed by frost action with local organic matter, 

rock rubble, and other weathering products. Generally bears tussock cover broken by frost 

boils at sites north of Brooks Range; bears continuous forest cover at sites south of range. 

Grades laterally into solifluction deposits on slopes steeper than about 1–2 degrees

m Muskeg deposits (Holocene and Pleistocene)—Laminated peat composed of sedges and 

mosses, with scattered leaves, twigs, and other plant fragments. Occurs beneath Sphagnum

and black spruce in poorly drained depressions beyond Itkillik II drift limits at and beyond 

south flank of Brooks Range. Most extensive across basins formerly occupied by lakes of 

Pleistocene age. Grades laterally into retransported silt deposits near bases of solifluction 

slopes. Generally contains abundant ice as lenses, wedges, and interstitial grains

LACUSTRINE AND GLACIOLACUSTRINE DEPOSITS

b Beach deposits (Holocene)—Moderately well sorted, coarse to medium sand containing schist 

chips, interbedded with platy fine gravel; ranges to poorly sorted, gravely sand and sandy 

fine gravel where mixed by ice shove during spring breakup period. Mapped within Survey 

Pass quadrangle around Iniakuk Lake and at south end of Walker Lake, where series of 

raised beach ridges rise to heights of 12–16 m and 18–24 m, respectively. Also mapped 

around shores of lakes or former lakes in Bettles, Chandler Lake, Hughes, and Philip Smith 

Mountains quadrangles

dt Deltaic deposits (Holocene and late Pleistocene)—Generally well stratified sand and sandy fine 

gravel deposited by streams at lake margins. Commonly build outward into lake, and overlie 

fine-grained lacustrine deposits. Large deltas currently are forming at north ends of Narvak 

Lake and Lake Minakokosa near north margin of Hughes quadrangle. Other deltas in map 

area are too small or too intermixed with other deposits to designate individually

dti Deltaic deposits, inactive (Holocene)—Composition uncertain. Mapped upstream from active 

delta at north end of Lake Minakokosa in Hughes quadrangle. May be compound deposit 

related to multiple lake stages

l Lacustrine deposits (Holocene and late Pleistocene)—Well stratified clay, silt, and silty fine 

sand, grading into sand and gravelly sand near former shorelines and sandy fine gravel near 

former stream mouths. Include beach deposits too small to be designated separately

igl Glacial-lake deposits of Itkillik age (late Pleistocene)—As described in unit l, with dispersed 

dropstones commonly present. Extensive thick deposits occur behind Itkillik-age moraines 

along floors of most major valleys of central Brooks Range. Shown only by stippled pattern 

where buried beneath younger alluvium, sand sheets, or solifluction and fan deposits

igl3 Glacial-lake deposits of late Itkillik II readvance (late Pleistocene)—As described in units l

and igl. Associated with moraine dams or drift deposited during late Itkilllik II time

igl2 Glacial-lake deposits of Itkillik II age (late Pleistocene)—As described in units l and igl. Associated with moraine dams or drift deposits of Itkillik II age

igl1 Glacial-lake deposits of Itkillik I age (late Pleistocene)—As described in units l and igl. Associated with moraine dams or drift deposited during Itkillik I time. Most common in lower 

mountain valleys of Wiseman quadrangle

idt Deltaic deposits of Itkillik age (late Pleistocene)—As described in unit dt. Associated with 

glacial lakes of Itkillik age in Hughes quadrangle

sgl Glacial-lake deposits of Sagavanirktok River age (middle Pleistocene)—As described in units 

l and igl, generally with silt and (or) muskeg cover up to several meters thick. Mapped only 

in Hughes quadrangle
13

sdt Deltaic deposits of Sagavanirktok River age (middle Pleistocene)—As described in unit dt. 

Associated with glacial lakes of Sagavanirktok river age in Hughes quadrangle

GLACIAL DRIFT AND ICE-CONTACT GRAVEL

Late Holocene Glaciation (Neoglaciation)

nd Drift of neoglacial age (late Holocene)—Unsorted unstratified coarse to fine angular rubble; 

forms lobes and arcuate ridges with moderate to steep frontal slopes. Clasts unweathered to 

slightly weathered; generally unvegetated except by lichens. Generaly restricted to cirques 

at higher altitudes, commonly near valley heads. Designates drift remnants of uncertain 

neoglacial age or composite drift bodies too small for subdivision

nd2 Drift of late neoglacial age (late Holocene)—Angular rubble, as described in unit nd; forms 

lobes and arcuate ridges of ice-cored drift with steep, unstable frontal slopes. Unvegetated, 

unweathered to slightly weathered, and with lichens sparse to absent. Restricted to cirques, 

and generally associated with active glaciers

nd1 Drift of early neoglacial age (late Holocene)—Angular rubble, as described in unit nd. Forms 

more subdued lobes and ridges with stable frontal slopes; generally eroded by axial streams. 

Generally in cirques, but locally may extend into upper valleys. Weathered and lichen 

encrusted, with partial sod cover in some localities. Ice cores generally absent

Itkillik Glaciation

id Drift of Itkillik age, undifferentiated (late Pleistocene)—Unsorted to poorly sorted generally nonstratified compact till, ranging in composition from muddy sandy boulder gravel to 

clayey stony silt, with local stratified ice-contact deposits consisting of moderately sorted 

sand and sandy gravel. Contains faceted and striated stones up to boulder size. Designates 

thick (greater than 3 m) drift deposits, usually within mountain valleys, that cannot be 

assigned to a specific Itkillik moraine system

id3 Drift of late Itkillik II readvance (late Pleistocene)—Till and stratified ice-contact deposits, 

as described in unit id. Forms sharp-crested end moraines, irregular ground moraine, and 

steep-sided ice-contact stratified drift deposits in many upper mountain valleys of southern 

Brooks Range. Forms usually subdued moraines of silty till (probable redeposited lacustrine 

sediments) near mouths of large mountain valleys along north flank of Brooks Range. Loess 

cover generally absent, and exposed stones very slightly weathered; oxide penetration to 

only 20–30 cm depth in most permeable deposits

id2 Drift of Itkillik II age (late Pleistocene)—Till and stratified ice-contact deposits, as described 

in unit id, with ice-contact deposits very abundant in most valleys. Forms prominent end 

moraines and associated glacial deposits north of Brooks Range in major valleys, along both 

range flanks in other valleys, and in upper Noatak River valley (Ambler River quadrangle). 

Drift lobes sharply defined, with narrow (generally 1–3 m) morainal ridges, prominent 

knob and kettle morphology, and conspicuously channeled outwash trains. Crests and upper 

slopes lack loess and solifluction cover, and exposed boulders and cobbles exhibit slight to 

moderate weathering; oxidation has penetrated 30–50 cm into better drained deposits. Most 

swales lack solifluction deposits, and abandoned meltwater channels commonly are floored 

with lichen-covered coarse gravel. Unstable kettles with actively caving gravel rims beyond 

north flank of Brooks Range indicate that residual glacier ice may persist locally

id1 Drift of Itkillik I age (late Pleistocene)—Till and stratified ice-contact deposits, as described 

in unit id. In northern valleys, forms closely nested concentric end moraines with flanking 

slopes up to 20° and subdued knob and kettle topography; associated with outwash trains 

partly obscured by solifluction. Moraine crests generally 3–10 m wide and partly bare of 

loess; upper slopes are blanketed by 0.5 to 2 m of stony organic silt (loess and colluvium). 

Cobbles and boulders exposed at surface are moderately to heavily weathered; stones in soil 

profiles are etched, pitted, and oxidized to depth of about 1 m. Swales partly filled with 1–3 

m of ice-rich organic silty solifluction deposits. Shallow earthflows common on steep slopes.

 At and beyond south flank of Brooks Range, forms broad, heavily forested piedmont lobes 

with large kettle lakes and extensive outwash terraces. Eroded arcuate end moraines and lateral embankments extend from glaciated main valleys of southern Brooks Range into lower 

courses of unglaciated tributaries. 

Subunit id1A designates outer moraine belt in drift sheets where conspicuous inner moraines 
14

(designated id1B) represent local readvances of Itkillik I glaciers during interval of general 

ice wastage. 

Subunit (id1) designates thin deposits of Itkillik I drift above bedrock. On hillslopes or lower 

valley walls, these deposits commonly are mixed with silt, rock rubble, and organic detritus, 

and hence become a compound (glacial-colluvial) unit

ik Kame and kame-terrace deposits (late Pleistocene)—Unusually extensive and thick (generally 

greater than 30 m) deposits of moderately well to well sorted sand, gravelly sand, and sandy 

gravel within undifferentiated drift of Itkillik age. Forms generally steep-sided knobs or knob 

complexes on drift sheets of Itkillik age and sharply defined terraces along their margins

ik3 Kame and kame-terrace deposits (late Pleistocene)—Unusually extensive and thick deposits 

of sand and sandy gravel, as described in unit ik, usually with less than 0.2 m cover of silt, 

organic silt, and sod. Formed within and marginal to drift of late Itkillik II readvance

ik2 Kame and kame-terrace deposits (late Pleistocene)—Thick and extensive gravel deposits, as 

described in unit ik, within and marginal to drift of Itkillik II age

ik1 Kame and kame-terrace deposits (late Pleistocene)—Unusually thick and extensive sand and 

gravel deposits, as described in unit ik, within and marginal to drift of Itkillik I age. Occurrence within outwash train associated with Itkillik I glacial advance west of Anaktuvuk 

River (Chandler Lake quadrangle) suggests persistence of residual glacier ice from Sagavanirktok River glaciation at time of Itkillik I glacial advance

im Meltwater deposits (late Pleistocene)—Extensive complexes of kames, kame terraces, and 

eskers consisting generally of sandy gravel that formed in contact with stagnating glacier ice 

of late Itkillik II age on floor of Anaktuvuk River valley south of range front and on adjoining valley floor of John River (Chandler Lake and Wiseman quadrangles)

Sagavanirktok River Glaciation

sd Sagavanirktok River drift, undifferentiated (middle Pleistocene)—Poorly sorted nonstratified till ranging in composition from silty sandy boulder gravel to clayey stony silt, with 

local deposits of moderately well sorted ice-contact gravel; generally oxidized and strongly 

jointed. Erratic boulders sparse; they commonly protrude less than 0.15 m above ground 

surface. Forms distinct but subdued end moraines and ground moraine with most crests and 

flanks covered by continuous blanket of organic silt (loess and solifluction deposits). 

 In northern valleys, surface boulders are composed only of a highly indurated quartzite 

and conglomerate of Kanayut Conglomerate (Nilsen and Moore, 1984; Moore and others, 

1989). Swales and kettles generally contain more than 5 m of ice-rich organic silt (colluvial and lacustrine deposits). Some ridge crests locally lack silt cover, exposing weathered 

subrounded gravel of resistant lithologies. Broadly dissected (to 2–3 km width) along major 

rivers, with depth of erosion 25–40 m. 

 At and beyond south flank of Brooks Range, forms broad morainal ridges and hummocky 

till plains; isolated drift remnants occur beyond Itkillik drift limits elsewhere within southern mountain valleys. Generally covered by thick (more than 3 m), nonstratified to weakly 

stratified blanket of silt, stony silt, and organic silt (loess, solifluction, and muskeg deposits). 

Crests of some ridges and knolls yield limited exposures of weathered gravel consisting of 

subrounded pebbles, cobbles, and small boulders of resistant lithologies. 

Subunit (sd) designates thin (less than 3 to 5 m) and generally discontinuous drift deposits 

on bedrock beyond Itkillik ice limits

sd2 Drift of younger Sagavanirktok River age (middle Pleistocene)—Till and ice-contact gravel, 

as described in unit sd. Forms subdued end moraine and ground moraine with many ridge 

crests bare of loess and solifluction cover. Swales and kettles more abundant and less modified than on older deposits of Sagavanirktok River age

sd1 Drift of older Sagavanirktok River age (middle Pleistocene)—Poorly exposed glacial deposits 

of composition probably similar to unit sd, forming distinct but very subdued and dissected 

moraines beyond limits of sd2 drift. Ridge crests and flanks bear generally continuous cover 

of organic silt

Anaktuvuk River Glaciation

ad Drift of Anaktuvuk River age (early Pleistocene)—Bouldery glacial deposits of uncertain 

composition overlain by continuous cover of organic silt (loess and solifluction deposits) 

generally more than 2–3 m thick. Erratic boulders very sparse (generally <1 per km2); they 
15

typically protrude less than 0.2 m above ground surface and consist of only most resistant 

(thick-bedded and nonferruginous) facies of Kanayut Conglomerate. Forms subdued till 

plains and low broad morainal ridges with gentle (1°–2°) flanking slopes except where 

steepened by postglacial erosion. Former swales and kettles generally filled with ice-rich, 

silty, organic colluvial and lacustrine deposits more than 5 m thick. Deeply and broadly dissected by minor as well as major steams, with depth of dissection 45–60 m along most major 

valleys and to width of 6 km and depth of 100 m in Killik River valley. In smaller valleys, 

widths and depths of dissection are typically about 1 km and 40–60 m, respectively. North of 

Brooks Range, occurs beyond drift limits of Sagavanirktok River age and forms oldest and 

northernmost continuous drift sheets. South of Brooks Range, forms patchy drift remnants 

south of Kobuk River valley

Gunsight Mountain Glacial Interval

Tgmd Drift of Gunsight Mountain age (late Tertiary)—Highly eroded bouldery glacial deposits of 

unknown initial composition, lacking primary relief and overlain by continuous cover of 

organic silt generally more than 2–3 m thick. Mapped only north of Brooks Range beyond 

limits of Anaktuvuk River drift. In Killik River quadrangle, former distribution commonly 

is marked by northern limits of erratic boulders incorporated in terrace deposits of early 

Pleistocene (tg1) age. Recognized in Chandler Lake quadrangle near Nanushuk River and 

near Gunsight Mountain (north of map margin), where it is associated with abundant erratics 

of resistant Kanayut Conglomerate facies on postglacial erosion surfaces and with rock-cut 

channels and benches probably eroded by glacial meltwater streams. Deeply and broadly 

dissected, with depth of erosion 60–80 m along most valleys. Probably eroded to depth of 

about 100 m along Killik River and to about 300 m along range front east of Kurupa Lake 

(Killik River quadrangle)

GLACIAL OUTWASH AND INWASH

no Outwash of neoglacial age (late Holocene)—Moderately well sorted and stratified sandy coarse 

gravel forming modern floodplains and low (1–3 m) vegetated terraces that extend downvalley from modern glaciers and from end moraines of neoglacial age. Forms mappable unit 

only in some higher valley heads of Killik River and Survey Pass quadrangles; too small to 

be designated separately elsewhere 

io Itkillik outwash, undifferentiated (late Pleistocene)—Moderately well sorted and stratified 

sandy gravel forming aprons and valley trains in front of moraines of Itkillik age that extend 

into terrace remnants farther downvalley. Largest stones decrease in size from subrounded 

cobbles and very small boulders near moraine fronts to rounded to subrounded pebbles and 

granules farther downvalley

io3 Outwash of late Itkillik II readvance (late Pleistocene)—Sandy gravel, as described in unit io, 

generally without loess or peat cover and oxidized to only 20–30 cm depth. Forms valley 

trains beyond end moraines of late Itkillik II age. Terraces near moraine fronts commonly 

12-15 m high; they generally are continuous downvalley and merge distally with outwash 

terraces of Itkillik II age

io2 Outwash of Itkillik II age (late Pleistocene)—Sandy gravel, as described in unit io, generally 

with thin (0.3 m or less) cover of silt and sod. Exposures in northern valleys show stones 

etched, fractured, and pitted to 30–40 cm below surface; oxidized to depths of 30–45 cm. 

Depths are slightly greater in southern valleys (30–50 cm and 40–50 cm, respectively). 

Forms extensive aprons and valley trains in front of Itkillik II moraines. Terraces near 

moraine fronts are up to 40 m high in major valleys; they generally are continuous downvalley, decreasing progressively in height to about 3–5 m. 

Subunits io2A and io2B designate outwash trains associated with outer and inner moraines 

west of Kobuk River at south end of Walker Lake (Survey Pass quadrangle). Subunit io2B is 

lower than io2A, and is inset within it

io1 Outwash of Itkillik I age (late Pleistocene)—Sandy gravel, as described in unit io, generally 

with thin to moderate (0.3–3 m) loess and solifluction cover that contains frost-churned 

stones with vertical orientations. In northern valleys, upper 1–1.5 m oxidized, with silt illuviation and weathered stones. In valleys at and beyond south flank of Brooks Range, cover 

of silt and organic silt (loess and solifluction deposits) may be up to 4 m thick. Forms aprons 

and valley trains in front of Itkillik I moraines. Terraces are up to 40 m high near moraine 
16

fronts, and decrease in height progressively downvalley. Commonly incised within drift of 

Sagavanirktok River age and dissected in turn by Itkillik II outwash. 

Subunits io1A and io1B designate outwash associated with outer and inner moraines of Itkillik I age (units id1A and id1B) at several localities beyond south flank of Brooks Range

so Sagavanirktok River outwash, undifferentiated (middle Pleistocene)—Moderately well 

sorted and stratified oxidized sandy gravel, with largest stones generally decreasing in size 

from cobbles and small boulders near moraine fronts to pebbles and cobbles further downvalley. Generally overlain by 1–4 m of organic silt (loess and solifluction deposits). Commonly associated with underfit or abandoned stream courses and dissected to depths as great 

as 30–40 m in some valleys

so2 Younger outwash of Sagavanirktok River age (middle Pleistocene)—Sandy gravel, as 

described for unit so, forming outwash trains originating at outer limits of younger moraines 

of Sagavanirktok River age or formed within outer moraines of that age

so1 Older outwash of Sagavanirktok River age (middle Pleistocene)—Sandy gravel, as described 

for unit so, forming outwash trains originating at or near outer limits of older moraines of 

Sagavanirktok River age

ao Outwash of Anaktuvuk River age (early Pleistocene)—Oxidized gravel of uncertain composition forming terrace remnants 50–60 m high that originate at outer limits of drift lobes of 

Anaktuvuk River age. Generally overlain by 3–5 m of organic silt (frost-churned loess and 

solifluction deposits). Mapped north of range front in Killik River and Chandler Lake quadrangles

ii Inwash of Itkillik age (late Pleistocene)— Well sorted to moderately sorted and stratified gravelly sand and sandy fine gravel, commonly grading upvalley into fan deposits and downvalley into lacustrine beds. Loess, sod, and silt cover generally thin (less than 0.2 m) to absent. 

Deposited near mouths of nonglaciated tributaries blocked by Itkillik-age glaciers in main 

valleys, forming benches and terraces that abut outer flanks of lateral moraines or outer faces 

of end moraines

ii3 Inwash of late Itkillik II readvance (late Pleistocene)—As described in unit ii. Forms deposits 

that abut drift deposited during late Itkillik II time

ii2 Inwash of Itkillik II age (late Pleistocene)—As described in unit ii. Forms deposits that abut 

drift of Itkillik II age

ii1 Inwash of Itkillik I age (late Pleistocene)—As described in unit ii. Forms deposits that abut drift 

of Itkillik I age

BEDROCK SURFACE FORMS

B Bedrock, undifferentiated—Generally unweathered within Brooks Range, where glacial erosion has created steep valley walls, sharp-crested ridges, and deep cirques. More weathered 

and subdued in appearance in northern and southern foothills, where it generally is covered 

by thin sheets of windblown silt (loess) and frost-shattered rock rubble. Lithologies as 

described by Moore and others, 1994, and Till and others, 2008

Bg Bedrock, glacier-scoured—Bedrock smoothed and abraded by overriding glacier ice. Rock 

surfaces generally well exposed, streamlined in direction of glacier flow (shown by arrows), 

and channeled by meltwater. Erratic boulders and cobbles commonly dispersed across rock 

surfaces

Bs Bedrock, silt-covered—Bedrock with cover of airborne silt (loess), 0.5 m or more thick over all 

but the highest and steepest slopes. Common beyond limits of late Pleistocene glaciation in 

foothills north and south of Brooks Range

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