£' I Geologic Setting of the 

Glacier Peak and 

Mazama Ash-Bed Markers in 

West-Central Montana

GEOLOGICAL SURVEY BULLETIN 1395-H

^'°^

Geologic Setting of the 

Glacier Peak and 

Mazama Ash-Bed Markers in 

West-Central Montana

By R. W. LEMKE, M. R. MUDGE, 

RAYE.WILCOX, and H. A. POWERS

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1395-H

Describes how ash beds can be used 

as stratigraphic markers in 

correlating deposits of late 

Pleistocene and Holocene age 

in west-central Montana

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1975
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Main entry under title:

Geologic setting of the Glacier Peak and Mazama ash-bed markers in west-central Montana.

(Contributions to stratigraphy) (Geological Survey Bulletin 1395-H)

Bibliography: p.

Supt.ofDocs.no.: I 19.3:1395-H

1. Geology, Stratigraphic Quaternary. 2. Volcanic ash, tuff, etc. Montana. 3. Geology

 Montana. 

I. Lemke, Richard Walter, 1913- II. Series. III. Series: United States Geological

Survey Bulletin 1395-H. 

QE696.G42 552'.2'0978652 74-28349

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

Washington, D.C. 20402 

Stock Number 024-001-02624-4
CONTENTS

	Page

Abstract __________________________________________ Hi

Introduction ________________'_______________________ 1

Regional late Pleistocene and Holocene geologic history ______________ 3

Identification of the ash beds _____________________________ 9

Glacier Peak ash bed __________________________________ 11

Geologic setting ___________________________________ 12

Descriptions of selected localities ________________________ 14

Locality RWL-13 _______________________________ 14

Locality HS-91 ________________________________ 16

Locality HS-87 ________________________________ 17

Mazama ash bed ____________________________________ 19

Geologic setting ___________________________________ 23

Descriptions of selected localities ________________________ 23

Locality RWL-1 _______________________________ 23

Locality RWL-6 _______________________________ 27

Locality HS-176 _______________________________ 28

Summary _________________________________________ 29

Acknowledgments ____________________________________ 30

References cited _____________________________________ 30

ILLUSTRATIONS

Page

FIGURE 1. Map showing distribution of Glacier Peak and Mazama ash falls in 

Northwestern United States and Canada and location of area 

described in this report ________________________ H2

2. Map showing Glacier Peak and Mazama ash-bed localities and major

elements of glaciation in west-central Montana __________ 4

3. Chart showing glaciation and postglaciation sequence during late

Pleistocene and Holocene time in west-central Montana _____ 6

4. Cross section showing stratigraphic relations of Mazama ash bed with

glacial and postglacial deposits at locality RWL-6 ________ 29

5. Photograph of locality HS-176 showing Mazama ash bed overlying 

lacustrine clays deposited during the last alpine Pinedale glacier in 

the area _________________________________ 30

m
IV

TABLES

Page

TABLE 1. Physical properties and shard habits of three upper Quaternary ash

beds of Northwestern United States _________________ HIO

2. Electron microprobe analyses of glass of upper Quaternary ashes _ 12

3. Glacier Peak ash-bed localities and descriptions ___________ 13

4. Mazama ash-bed localities and descriptions ______________ 20
CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGIC SETTING OF THE GLACIER

PEAK AND MAZAMA ASH-BED MARKERS

IN WEST-CENTRAL MONTANA

By R. W. LEMKE, M. R. MUDGE, RAY E. WILCOX,

and H. A. POWERS

ABSTRACT

The Glacier Peak and Mazama ash beds are informally named stratigraphic markers 

that assist in interpreting the age and environment of deposits of late Pleistocene and 

Holocene age in west-central Montana. There are only three known exposures in the area 

that contain both ash beds. The one bed is distinguished from the other petrographically 

by the habit and range of refractive index of its glass shards and the refractive indices of 

the hornblende phenocrysts, and chemically by the composition of the glass. In the field, 

identification is aided by the younger Mazama ash bed generally occurring higher in the 

exposure, commonly within a few feet of the top, and generally being somewhat thicker 

and more extensive than the Glacier Peak ash bed.

The Glacier Peak ash, dated radiometrically at approximately 12,500 years B.P. (before 

present), occurs as light-gray to tan-gray lentils and pods, mostly in beds of silt that commonly overlie deposits of the middle stade of Pinedale Glaciation and underlie deposits of 

the Altithermal interval. In one exposure the ash bed is interlayered in glacial lake beds of 

Pinedale age. Exposures of this ash bed are not as common as those of the Mazama ash 

bed, and have not been found as far east.

The Mazama ash, radiometrically dated at about 6,600 years B.P., occurs near the base 

of the deposits of the Altithermal interval as distinct but discontinuous light-gray to tan 

beds that range in thickness from 0.1 foot (3 centimetres) to as much as 2.5 feet (76 centimetres). It commonly overlies a distinct fossil soil and locally is overlain by a less distinct fossil soil.

INTRODUCTION

Beds of volcanic ash resulting from eruptions of volcanoes in the 

Cascade Range of Washington and Oregon occur in Quaternary 

deposits over a wide area of the Northwestern United States and 

southwestern Canada. Two of these ash beds (fig. 1), informally named 

the Glacier Peak and Mazama beds, are particularly widespread and 

have distinctive characteristics which make them useful stratigraphic

Hi
H2 CONTRIBUTIONS TO STRATIGRAPHY

120° 110°

50^

40

200 400 MILES 

_I

200

I 

400 KILOMETRES

FIGURE 1. Distribution of Glacier Peak and Mazama ash falls in Northwestern United 

States and Canada and location of area described in this report. Dotted pattern shows 

distribution of Glacier Peak ash fall; radial line pattern shows distribution of Mazama 

ash fall.

markers. The purpose of this paper is to describe briefly the geologic ^W 

settings in which the ash beds are found, how to identify the ash beds, 

and how the beds are useful as stratigraphic markers in upper 

Pleistocene and Holocene deposits in west-central Montana. 

The ash localities described in this paper are shown in figure 2. Some
GLACIER PEAK AND MAZAMA ASH-BED MARKERS, MONTANA H3

additional ash localities in the study area are not shown because their 

stratigraphic relations are essentially the same as other described ones, 

or because the exposures are poor and stratigraphic relations are 

obscure. The known ash localities that are shown are limited chiefly to 

areas where detailed geologic studies have been made or to areas along 

main highways. Therefore, it is likely that many ash localities are yet to 

be discovered. Additional exposures probably occur along the main 

valleys and tributary valleys of the Missouri, Teton, Marias, and Sun 

Rivers.

Only three localities (RWL-3, RWL-13, and HS-91) have been found 

where both the Glacier Peak and Mazama ash beds are present in a 

single exposure. These three localities are particularly important 

because they provide a fairly complete stratigraphic sequence of deposition during the past 12,500 years, or since near the close of the middle 

stade of the Pinedale Glaeiation in the region.

REGIONAL LATE PLEISTOCENE AND HOLOCENE 

GEOLOGIC HISTORY

No glacial deposits older than Wisconsin age have been identified in 

the Plains area of west-central Montana (Lemke and others, 1965). 

Also, pre-Wisconsin drift has not been identified in the mountains immediately to the south and west of the study area, although preWisconsin alpine glacial deposits long have been recognized to the 

northwest in Glacier National Park (Alden and Stebinger, 1913). More 

recently, Richmond (1957, 1960, 1965) described three pre-Bull Lake 

alpine tills from that area, separated by deeply weathered interglacial 

soils.

Most of the Plains area, to about as far south as Great Falls, was 

covered twice by continental glaciers during Wisconsin time (fig. 3). 

The earlier of these ice sheets merged in north-central Montana with 

alpine ice of the Bull Lake Glaciation and the later ice sheet merged in 

the same general area with alpine ice of the Pinedale Glaciation (Richmond, 1965). Because of these established correlations by Richmond 

(1965), the glaciations of the Plains area are referred to as the continental Bull Lake Glaciation and the continental Pinedale Glaciation rather 

than by the less firmly correlated midwestern nomenclature of Frye and 

Willman (1963).

During the continental Bull Lake Glaciation, Glacial Lake Great 

Falls was impounded along the southern edge of the ice front. This lake 

was of wide areal extent, was more than 600 feet (182 metres) deep in 

places, and reached a maximum surface altitude of approximately 

3,900 feet (1,188 m) (Colton and others, 1961). Glacial Lake Choteau 

and Glacial Lake Cut Bank were formed at the same time along the 

western edge of the ice.
H4 CONTRIBUTIONS TO STRATIGRAPHY

113° 112°

RWL-3 

RWL-7

Big Belt 

Mountains

FIGURE 2. Glacier Peak and Mazama ash-bed localities and major elements ^

Lindvall

Ice of continental Pinedale Glaciation is estimated to have advanced 

into the region more than 20,000 years ago and to have persisted until
GLACIER PEAK AND MAZAMA ASH-BED MARKERS, MONTANA H5

111' 110"

EXPLANATION vjj$$>

X Mazama ash bed locality £$&£ 

D Glacier Peak ash bed locality % $$.

 Locality where both Mazama !; ;£ £; 

and Glacier Peak ash beds ii£££ 

are exposed '! ;:$&

20

UNGLACIATED

30 MILES

I I I

0 10 20 30 KILOMETRES

of glaciation in west-central Montana, as modified from Colton, Lemke, and 

(1961).

about 12,500 years ago (end of the middle stade). Glacial Lake Great 

Falls, Lake Choteau, and Lake Cut Bank again were impounded during
H6 CONTRIBUTIONS TO STRATIGRAPHY

SERIES

Holocene

tocene

'a

CL

GLACIATION AND POSTGLACIATION SEQUENCE

Post-Altithermal

(Neoglaciation)

Altithermal

Pre-Altithermal

Pinedale 

Glaciation

Middle

stade

Interstade

Early

stade

-

Interglaciation

Bull Lake

Glaciation

Late

stade

YEARS B.P.

 4,000 

 6,600 

 7,500 

 12,000 

 12,500 

>29,000

(C 14 date)

70,000

(K-Ar date)

VOLCANIC ASH BEDS

FIGURE 3. Glaciation and postglaciation sequence during late Pleistocene and 

Holocene time in west-central Montana. Taken in part from Richmond (1972). Chart 

not to scale below part showing Pinedale Glaciation.
GLACIER PEAK AND MAZAMA ASH-BED MARKERS, MONTANA H7

Pinedale time, south and west of the ice front. In one locality 

(RWL-13), the Glacier Peak ash bed is intercalated with deposits of the 

upper part of Glacial Lake Choteau of Pinedale age.

The Rocky Mountains in west-central Montana were glaciated 

several times during the Wisconsin. Extensive deposits of alpine till and 

outwash were laid down during Bull Lake time in the main mountain 

canyons and in places in the adjacent foothills at the mouths of the 

canyons. The Pinedale interval was marked locally by even more extensive glaciation in the mountains, which persisted until about the end of 

the middle stade. A mature zonal soil developed on the alpine Bull Lake 

deposits during the interglacial stage following the Bull Lake Glaciation, and a somewhat less mature soil was developed on the surface of 

the deposits of the alpine Pinedale Glaciation (Richmond, 1965). 

Gerald M. Richmond and R. W. Lemke are using these zonal soils in 

differentiating the continental drift of the Bull Lake Glaciation from 

the continental drift of the Pinedale Glaciation in current studies of the 

Plains area of north-central Montana. However, nowhere in the Plains 

area has Bull Lake soil been found buried beneath the younger Pinedale 

drift, as can be found in the alpine deposits. Also, no buried soil profile 

has been found in the study area south of the drift borders that separate 

lake deposits of continental Bull Lake Glaciation from those of continental Pinedale Glaciation. However, in exposures along the Sun 

River north of Augusta there is an alpine Bull Lake soil on outwash 

gravel overlain by alpine Pinedale Till.

As used by Richmond (1972) and followed in this paper, the Holocene 

is arbitrarily defined as beginning at the end of the last abrupt 

deterioration of Pinedale glaciers (end of the middle stade) in the 

mountains in the western part of the area and of glacial lake filling 

along the edge of the continental Pinedale ice. This deterioration of 

Pinedale glaciers occurred about 12,000 years ago and is approximately 

marked stratigraphically by the Glacier Peak ash bed.

Holocene deposits in the Plains area commonly consist of an intermixture of alluvial, colluvial, and eolian deposits that were laid down in 

the larger stream valleys and their tributaries. The alluvial deposits are 

mostly fine grained (clay, silt, and sand) and seem to represent, in large 

part, slack water deposits. In places they attain thicknesses of 50 feet 

(15 m) or more, but thicknesses of 10-20 feet (3-6 m) are more common. 

These deposits, particularly those in the secondary and tertiary 

tributaries, are now being actively incised head ward by stream erosion.

A variety of Holocene deposits is present in most mountain valleys in 

the eastern part of the northern Rocky Mountains; the deposits include 

till, outwash gravel, stream alluvium, alluvial fan deposits, terrace 

alluvium, talus, landslide deposits, and rockfall and rockslide 

avalanche deposits (Mudge, 1967, 1972). In general, the materials are 

more coarse than alluvial and colluvial deposits in the Plains. Till and
. H8 CONTRIBUTIONS TO STRATIGRAPHY

rockfall deposits are as much as 200 feet (60 m) thick; other deposits are 

commonly 20-50 feet (6-15 m) thick. Nearly all the deposits have been 

incised by stream erosion.

For purposes of discussion in this paper, the Holocene Epoch is 

divided into three time subdivisions: (1) pre-Altithermal interval, (2) 

Altithermal interval, and (3) post-Altithermal interval. Identification 

of deposits laid down during these three intervals is facilitated by use of 

the Mazama ash bed as a key marker.

The Altithermal interval began in west-central Montana approximately 7,500 years ago and ended about 4,000 years ago (Richmond, 1972). The beginning of the interval was marked by total disappearance of glaciers from the mountains in the area and by distinct 

soil formation in places on the Plains. As pointed out by Haynes (1968) 

and Richmond (1972), an episode of erosion beginning between 5,000 

and 4,000 years ago is believed to be associated with climatic cooling 

that probably marked the end of the Altithermal and led to the onset of 

Neoglaciation about 4,000 years ago.

On the basis of the preceding discussion, the pre-Altithermal deposits 

of Holocene age in the region are those postglacial deposits that were 

laid down between about 12,000 and 7,500 years ago. Although glacier 

ice had largely or completely disappeared from the mountains in westcentral Montana during this interval, it still existed in places in Alberta 

as the late stade of the Pinedale. The alluvial sequence representing the 

pre-Altithermal deposits commonly consists of a clayey silt to a silty 

clay with lesser amounts of sand. Coarser grained clasts, including 

gravel, cobbles, and boulders, commonly constitute many of the 

deposits in areas of high relief, such as in the Rocky Mountains and the 

Belt Mountains, as well as along parts of some main stream channels. 

The finer grained alluvial deposits generally can be distinguished from 

the overlying Altithermal deposits by being somewhat more compact, 

by having a fairly blocky texture, and by commonly being partially iron 

stained or containing .white blebs of calcium carbonate. Indistinct soil 

profiles are discontinuously present, and snails, clams, and other fossils 

are present locally. At locality RWL-1, clams from near the top of the 

interval were dated (W-594) at 8,800±300 years.

In the Plains area the alluvial deposits of the Altithermal interval 

commonly consist of silty clay to clayey silt with lesser amounts of sand. 

The alluvium is generally somewhat less compact than the preAltithermal deposits, contains little or no iron staining and no white 

blebs of calcium carbonate, and commonly has a less blocky texture. A 

fairly conspicuous fossil soil is present in places at the base of the interval. At locality RWL-1, snails in the lower part of this soil were dated 

(W-902) at 7,460±250 years. That the fossil soil is of Altithermal age is 

supported by the studies of Richmond (1965), who found that, 

throughout western Montana, Oregon, and Washington, the Mazama
GLACIER PEAK AND MAZAMA ASH-BED MARKERS, MONTANA H9

7 ash bed occurs in or on soil of Altithermal age. The Mazama ash, where 

present, directly overlies the lowermost Altithermal soil and forms a 

clearly distinguishable marker horizon. Above the ash bed, other less 

distinct soil profiles are present locally. Freshwater snail shells are 

sparse to locally abundant throughout the interval, but freshwater clam 

shells have not been found.

In and near the mountains, terrace alluvium, alluvial fan deposits, 

and lacustrine deposits were laid down during the Altithermal interval. 

The terrace alluvium at localities HS-92, -255, -552, and -808 (table 4) 

consists of silt and clayey silt with some sand and gravel. The alluvial 

fan deposits at locality HS-94a consist of poorly sorted gravel with some 

silt. The lacustrine deposits at localities HS-176, -204, and -206 consist 

mostly of silty clay. In many of the deposits, disseminated calcium carbonate is in beds above and below the Mazama ash bed. Snails from 

lake beds below the Mazama ash bed at locality HS-176 yielded a 

carbon-14 age of 7,870±200 years, and snails above the ash were dated 

at 4,740±250 years. Peat within the Mazama ash at locality 204B-206 

was dated by carbon-14 methods at 7,280±400 years (Mudge, 1967). In 

and near the mountains, a fossil soil is present beneath the Mazama ash 

bed only at localities HS-91 and 69W23. No evidence of a poorly 

developed soil was recognized above the ash bed in the exposures in the 

mountains.

In the Plains area it is not yet possible to distinguish, with confidence, the post-Altithermal deposits from the Altithermal deposits. It 

appears, however, that the post-Altithermal deposits may be thin 

(probably less than 5 feet (1.5 m) in most places) and, in the finegrained alluvial sequence, may be represented in large part by the 

modern soil profile. More studies of fossils, pollen, and radiocarbon 

dating may help to differentiate these deposits.

In the mountainous area there were two periods of glaciation that 

resulted in deposits of till in the upper reaches of some valleys and 

deposits of glacial outwash and terrace deposits in the lower reaches. 

These deposits are dated mostly by their stratigraphic relationships to 

the periods of glaciation. Locally, peat deposits in alluvial fans have 

been dated by carbon-14 methods (Mudge, 1967, 1972). Other types of 

deposits shown by Mudge (1967, 1972) have been dated only by inference or relationship to older deposits. Most deposits are less than 50 

feet (15 m) thick.

^ IDENTIFICATION OF THE ASH BEDS

In early work in Washington State, no petrochemical distinction was 

made between Glacier Peak and Mazama ash beds, and many occurrences of ash now known to be of Mazama origin were erroneously 

assigned to a Glacier Peak source (Rigg and Gould, 1957). A striking
H10 CONTRIBUTIONS TO STRATIGRAPHY

characteristic of the Glacier Peak ash in a zone, located 50-100 miles 

(80-160 km) from the source, is its resemblance to tapioca in color, size, 

and roundness of the particles. The shards are vesicular but not finely 

pumiceous, as are those of the powdery Mazama ash, and they are 

crowded with microphenocrysts and microlites. These distinctions persist downwind to the east and into the area described in this paper, 

where the tapiocalike aspect of the Glacier Peak shards can be seen in 

washed material with the aid of the stereomicroscope. Phenocrysts are 

relatively abundant in the Glacier Peak ash as compared with the 

Mazama ash and include intermediate plagioclase, green-olive 

hornblende, orthopyroxene, and opaque oxide. In the Mazama ash, 

phenocrysts are present only in moderate amounts and include the 

same species as the Glacier Peak plus scattered clinopyroxene and 

apatite. Specific properties of the individual constituents are listed in 

table 1.

TABLE 1.0*- Physical properties and shard habits of three upper Quaternary ash beds of 

Northwestern United States

Mazama Glacier Peak Mount St. Helens "J" 1

Glass shards:

Habit

Dominant

refractive

index.

Phenocrysts:

Plagioclase n x ___

Hornblende n x ___

Orthopyroxene n x -

Clinopyroxene n x _

Opaque oxides

Apatite ________

Mainly finely

pumiceous

stretched "silky"

shards.

1.499-1.511

(bimodal).

1.545-1.553

1.658-1.665

1.688-1.692

2 1. 686-1. 689

Abundant

Sparse

Predominantly

vesicular

"tapioca"

granules.

1.499-1.502

1.545-1.555

1.645-1.653

1.689-1.695

None

Abundant

None

Some vesicular

granules, some

stretched pumice.

(a) 1.501-1.503

(b) 1.503-1.507

(b) 1.548-1.557

(b) 1.646-1.655

(b) 1.693-1.697

^^onc

Abundant.

None.

' Properties for Mount St. Helens "J" based on examination of only four samples of near-source lump pumice: (a) 

three samples, (b) one sample. 

"Only sparsely present.

The Glacier Peak ash is distinguishable from the Mazama ash not 

only by the habit of the glass shards, but also by the range of the refractive indices of the glass and by the refractive indices of the hornblende 

phenocrysts (Powers and Wilcox, 1964). There is, however, another ash 

bed that might possibly be present in the region that closely resembles 

the Glacier Peak ash. This is the equivalent of the "J" pumice, erupted 

from Mount St. Helens volcano, Washington, sometime between 12,000 

and 8,000 years ago, according to Mullineaux, Hyde, and Rubin (1972). 

Preliminary work on samples of this pumice from near the volcano 

shows that the "J" ash downwind should be expected to contain the 

same phenocryst suite as that of the Glacier Peak ash, and that even the
GLACIER PEAK AND MAZAMA ASH-BED MARKERS, MONTANA Hll

refractive indices of the individual constituents should be very similar.

Chemical compositions of the individual constituents, both glass and 

phenocrysts, serve to distinguish the Glacier Peak ash from the 

Mazama, and on the basis of preliminary work, apparently also from 

the Mount St. Helens "J" ash. Major-element analyses by standard 

chemical methods (Peck, 1964) have been published for glass of the 

Mazama ash and the Glacier Peak ash (Powers and Wilcox, 1964). In 

the case of the Glacier Peak ash, however, results by these methods are 

significantly affected by the presence in the shards of abundant 

microphenocrysts and microlites, which are not removable by the usual 

separation procedures. The results by use of the electron microprobe are 

believed to provide values closer to the true amounts, because the 

microphenocrysts can be avoided. The probe results have a further advantage in that they reveal differences in glass composition from shard 

to shard, a characteristic more marked in the Mazama glass than in the 

Glacier Peak glass.

Electron microprobe analyses of the glass of these three ashes are 

given in table 2, which shows that the Glacier Peak glass is higher in Si 

and K and lower in Al, Ti, Fe, Mg, Ca, and Na than the Mazama glass. 

Fewer elements distinguish the Glacier Peak glass from the Mount St. 

Helens "J" glass, but comparison with the two available analyses of "J" 

(John A. Westgate, written commun., 1972) suggests that the distinction may be possible on the basis of the higher K and lower Fe, Mg, and 

Na in the Glacier Peak glass. Obviously, more samples of known "J" 

material must be analyzed.

GLACIER PEAK ASH BED

The Glacier Peak ash bed is the fallout from a major eruption of 

Glacier Peak volcano in the northern Cascade Range, about 50 miles (80 

km) east of Everett, Wash. (fig. 1). Buried remnants of this ash fall 

have been found over a broad fan-shaped area extending eastward some 

500 miles (800 km) from the volcano (Powers and Wilcox, 1964; Fryxell, 

1965; Westgate, 1968); the suggestion has been made (Wilcox, 1969; 

Westgate and others, 1970) that the eruption consisted of two pulses, 

but that the pulses were not separated by any geologically significant 

interval of time.

The age of mollusk shells in the Glacier Peak ash bed at Lower Grand 

Coulee (about 95 miles (153 km) east-southeast of Glacier Peak), 

Wash., was reported by Fryxell (1965; see also Chatters, 1968, p. 494) as 

12,000±310 years and that of snails in the Glacier Peak ash bed at 

Diversion Lake, Sun River, Mont. (loc. HS-87 of this report), was 

reported by Ives, Levin, Oman, and Rubin (1967, p. 517) as 12,750±350 

years (Mudge, 1967). The difference between these two results is not 

considered important for the present purpose, and an arbitrary value of 

12,500 years will be used in this report.
H12 CONTRIBUTIONS TO STRATIGRAPHY

TABLE 2. Electron microprobe analyses of glass of upper Quaternary ashes (recast to

total 100 percent)

Glass Mazama Glacier Peak Mount St. Helens "J"

Area 

Spec. No.

Si02

A12 03

Ti02

Feas 

FeO

MgO

CaO

Na20

K2O

Source 

60-7-29R

73.0

15.7

.38

1.85

.46

1 91

4 17

2.79

Montana 

HS-92

72.7

15.6 

.38

1.87 

.45 

1.67 

4.60 

2.77

Source 

68W48

77.1 

13.7

.17

.95 

.29 

1.15 

3.21 

3.40

Montana Source 

HS-87 68W20 3-22-71-13 8-22-71-15

76.6 

13.2 

.18

.99 

.22 

1.26 

3.94 

3.62

77.1 

12.9 

.20

1.03 

.22 

1.35 

3.50 

3.72

75.9 

13.7 

.19

1.30 

.34 

1.33 

4.79 

2.37

75.3 

14.0 

.21

1.44 

.39 

1.47 

4.97 

2.31

Total __ 100 100 100 100 100 100 100

Other 

(mostly 

H20) _. 3.4 3.2 5.4 5.1 5.3 No data No data

DESCRIPTION OF SAMPLES

60-7-29R. Mazama airfall pumice lump, SWM sec. 5, T. 28 S., R. 8 E., about 20 miles (32

km) NE. of Crater Lake, Klamath County, Oreg. 

HS-92. Mazama ash, NEMSWW sec. 12, T. 21 N., R. 8 W., Ban- Creek quad., Lewis

and Clark County, Mont. 

68W48. Glacier Peak airfall pumice lump, Trinity (abandoned), about 12 miles (19.3

km) E. of Glacier Peak volcano, Holden quad., Chelan County, Wash.

HS-87. Glacier Peak ash, NWWSWW sec. 36, T. 22 N., R. 9 W. (unsurveyed), Sawtooth Ridge quad., Lewis and Clark County, Mont. (Age 12,750±250 C-14

years (Mudge, 1967). 

68W20. Glacier Peak ash, NEWSW'/« sec. 8, T. 24 N., R. 3 W., New Rockport Colony,

Teton County, Mont. 

8-22-71-13. Mount St. Helens "J" airfall pumice lump, upper part of a 3-foot (0.9-m) bed.

Center sec. 25, T. 9 N., R. 5 E., Mount St. Helens quad., Skamania County,

Wash. Analyzed by John A. Westgate. 

8-22-71-15. Mount St. Helens "J" airfall pumice lump, lower part of the 3-foot (0.9-m)

bed. Same locality as -13. Analyzed by John A. Westgate.

GEOLOGIC SETTING

Known exposures of Glacier Peak ash in west-central Montana, 

although widespread (table 3), are not nearly so common as exposures 

of Mazama ash. Only three exposures of Glacier Peak ash (HS-108, 

RWL-3, and RWL-13) have been found on the Plains in the area shown 

in figure 2. The easternmost exposures, localities RWL-3 and RWL-13, 

are at approximate longitude 112°, or about 65 miles (104 km) farther 

west than the easternmost known exposure of Mazama ash in westcentral Montana. Locality RWL-3 is along the Missouri River on the 

north flank of the Big Belt Mountains. Locality RWL-13 is in the Plains 

area along the south wall of the Teton River valley. The western exposures shown in figure 2 are along the Sun River, both within the 

mountains and on the Plains to the east. The two outcrops where the 

stratigraphy is best exposed are at localities HS-91 and HS-87.
TABLES. Glacier Peak ash-bed localities and 

Locality 

No.

RWL-3

RWL-13

HS-86 _

HS-87 _

HS-108

HS-540

HS-91 _

Specimen 

No.

_69W15__

_69W21 __

_ 68W136 _

_68W21

and

68W1.-V7.

_68W141 _

68W147

-69W19 __

Location Thickness 

of ash

(ft) (cm)

SW'/iNWVl sec. 21, 0-0.1 0-3.0

T. 16 N., R. 2 W., 5

miles (8 km) NW.

of Craig, Cascade

County.

NW'/lSE'/l sec. 8, T. 0- .05 0-1.5

24N..R. 3W., New

Rockport (Hutterite) site, Teton

County.

SEWNWW.sec. 3, T. 0- .2 0-6.1

21 N., R. 9 W.,

Teton County.

NE'/iSE'/, sec. 35, T. 0- .2 0-6.1

22 N., R. 9 W.,

Lewis and Clark

County.

SE'/iNW'/4sec. 27, T. .2 6.1

21 N., R. 6 W.,

Lewis and Clark

County.

SEWSW/4 sec. 27, T. .2 6.1

21 N., R. 10 W.,

Lewis and Clark

County.

SW4NW/4 sec. 3, T. .2 6.1

21 N., R. 9 W.,

Lewis and Clark

County.

Type of deposit 

in which ash is 

present

Alluvial gravel fan

along Missouri

River.

Lacustrine deposits of

Glacial Lake Great

Falls.

Alluvial fan, silt __

do

Colluvium, silt

do

do

Underlying Overl

material mate

Gravel ____ Gravel 

Lacustrine Lacust

clay. clay

Fine-grained Silt 

sand.

Gravel Gravel 

Silt Silt 

Siltyclay do 

Gravel _ do 
H14 CONTRIBUTIONS TO STRATIGRAPHY

DESCRIPTIONS OF SELECTED LOCALITIES

Three Glacier Peak ash-bed localities will be described in detail to 

show their stratigraphic relations and to show how the ash bed can be 

used as a marker bed. These are localities RWL-13, HS-91, and HS-87.

LOCALITY RWL-13 

New Rockport (Hutterite) site in the NW'/tSE'A sec. 8, T. 24 N., R. 3 W., Teton County

[Measured and sampled at north end of conspicuous landslide scarp by R. W. Lemke, M. R. Mudge, and Ray E.

Wilcox, 1969]

Unit Thickness 

No. (feet) (cm)

15. Windblown(?) deposits. Clay, silty, medium-dark-gray; 

massive; no apparent bedding except at south end of scarp, 

which contains foreset beds that have a NE. apparent dip. No 

pebbles ______________________________ 5.1 155

14. Mazama ash (specimen No. 69W20), light-gray, silty; fairly distinct lens of impure ash that thickens and thins laterally. Rodent burrows ___________________________ 0.2-0.5 6-15

13. Windblown(?) deposits. Clay, silty. Same as unit 15_____ 2.3 70

12. Glacial lake beds, continental Pinedale. Clay, silty. Laminae 

0.05-0.1 ft (about 1 cm) thick, alternating light gray with dark 

gray; distinctly laminated ___________________ 2.5 76

11. Glacial lake beds, continental Pinedale. Clay, medium-gray; 

prismatic structure; dark-brown iron stains on fracture surfaces ___________________________ .7 21

10. Glacier Peak ash (specimen No. 69W21), white, pure, silt-size,

in a distinct lens ________________________ 0.05 1.5

9. Glacial lake beds, continental Pinedale. Clay. Same as unit 11 .8 24 

8. Sand, medium-fine to silt-size, clean, light-gray; distinct bed 0.15-.4 4.6-12.2

Unconformity.

7. Glacial lake beds, continental Pinedale(?). Clay, light-gray; upper 1.0 ft (30 cm) iron stained along exposed faces; unit grades 

down into hard gray laminae with some partings 0.05-0.15 ft 

(about 1-3 cm) thick; little oxidation along less well 

developed prismatic surfaces; sand lenses as much as 8 in. (20 

cm) thick and 1.0 ft (30 cm) long extend diagonally up to the 

unconformity __________________________ 7.1 216

6. Sand, silty, medium-grained; thickens and thins laterally; 

partly crossbedded in lower part; a few pebbles 1 in. (2.5 cm) 

long at base, some of which consist of quartzite ______ 0.4-0.6 12-18

Unconformity.

5. Glacial lake beds, continental Bull Lake(?). Clay, megalaminae 

of light-gray clayey silt as much as 1.0 ft (30 cm) thick alternating with dark-gray silty clay laminae 0.1-0.3 ft (2-8 cm) 

thick that have sublaminae of light-gray clayey silt. 

Periglacial(?) convolutions in upper 0.5-1.0 ft (15-30 cm); two 

or three mauve-colored laminae of clayey silt, 0.02 ft (about 1 

cm) thick, in upper 1.0 ft (30 cm) ______________ 8.2 250
GLACIER PEAK AND MAZAMA ASH-BED MARKERS, MONTANA HIS

Unit Thickness 

No. (feet) (cm)

(Section description continued at large slide block west of scarp. Bed

correlation established.)

4. Till, continental Bull Lake(?). Mostly silty clay, light-gray, oxidized; contains subrounded pebbles that are mainly quartzite, l'/2-3 in. (4-8 cm) across _________________ 2.3 70

3. Silt, clayey, light-gray, compact _________________ 1.9+ 58+

(Section continued at another slide block farther to the west. Bed correlation established between exposures.)

2. Reworked shale fragments, dark-gray; fissile, with some pebbles 1.0 30 

1. Till, continental Bull Lake(?); clayey, bluish gray, with a few 

pebbles, grading down into yellowish-brown water-reworked 

till (indistinct banding) ____________________ 2.3 70

Approximate total thickness _______________ 35.8+ 1,091 +

Locality RWL-13 is important, because the two well-exposed ash 

beds provide key markers in helping to unravel the rather complex 

Pleistocene stratigraphy of the area. The site lies about 1 mile (1.6 km) 

northeast of the drift border of the continental Pinedale Glaciation. 

Lacustrine deposits of Glacial Lake Choteau of that glaciation constitute the uppermost glacial deposits at the site. The observation of the 

Glacier Peak ash near the top of these lacustrine deposits establishes for 

the first time a close date for the end of the continental Pinedale Glaciation in the region. Likewise, it allows the inference to be made that the 

lowermost exposed lacustrine deposits are of Bull Lake age and that the 

till exposed in the basal part of the section (units 1 through 4) also can 

be assigned to the continental Bull Lake Glaciation. Some or all of this 

till may have been water reworked or ice rafted and, therefore, may not 

have lateral continuity. Certainly unit 2, consisting of shale fragments, 

has been reworked by glacial melt-water streams from nearby outcrops 

of Cretaceous shale. However, outcrops along the Teton River, about a 

quarter of a mile (400 m) downstream, expose 35-40 feet (10-12 m) of 

hard, compact till believed to belong to the continental Bull Lake 

Glaciation.

If, as seems likely, the glacial lake beds of unit 5 belong to the continental Bull Lake Glaciation, the question then arises as to whether 

the unconformity at the top of this unit or the unconformity at the top 

of unit 7 marks the end of deposition of the Bull Lake lake beds. Inasmuch as it is believed that there was more than one interval of alpine 

Pinedale Glaciation in the region, it seems reasonable to assign the upper unconformity to one of the time breaks in the Pinedale.

Lake beds in units 8 through 12 are assigned to the continental 

Pinedale Glaciation. The fact that only about 3 feet (90 cm) of lake beds
H16 CONTRIBUTIONS TO STRATIGRAPHY

overlies the Glacier Peak ash, dated at approximately 12,500 years, indicates that the ice-marginal lakes of the region were in existence only a 

short time (perhaps tens of years or at most a few hundred years) after 

that date. Also, by 12,500 years ago major recession of the continental 

Pinedale ice was well underway in the region. However, continental ice 

still was in existence 12,000 years ago in southeastern Alberta, where 

Glacier Peak ash has been found on ablation deposits of the ice sheet 

(Westgate, 1972).

The presence of the relatively thick and very homogeneous silt 

deposits (units 13 and 15) that overlie the lacustrine deposits and that 

appear to be eolian in origin is puzzling. The occurrence of the dated 

Mazama ash in the lower third of this silt sequence suggests uniform 

climatic conditions of deposition during at least the last 6,600 years, 

and if there is no time break between the silt deposits and lake beds 

(between units 12 and 13), then uniform climatic and depositional conditions prevailed during approximately the last 12,000 years. This does 

not appear to accord with the environmental conditions indicated from 

other described localities in the area.

LOCALITY HS-91

Borrow pit on the south side of the Sun River in the SWMNWM sec. 3, T. 21 N., R. 9 W.,

Lewis and Clark County

[Measured by M. R. Mudge and R. W. Lemke, 1969. Color designations are from "Rock-Color Chart" (Goddard and

others, 1948)]

Unit Thickness 

No. (feet) (cm)

6. Talus and colluvium topped with a 4-in. (10-cm) humic-rich 

soil; pebble- to cobble-size angular fragments of carbonate 

and chert with a light-grayish-brown silt matrix ______ 2.2 67

5. Colluvium, with Mazama ash matrix (specimen no. 69W18). 

Angular fragments of carbonate and chert. Light-gray ash 

constitutes about one-third of unit ______________ 2.0± 60±

4. Colluvium, angular fragments of carbonate and chert, unsorted;

finer grained than above units ________________ 1.6 49

Unconformity.

3. Clay, silty (B horizon of paleosol), moderate-reddish-brown 

(5YR 4/4), granular; few angular pebbles; lower half contains 

disseminated CaCOa blebs _____________-__--- -7 21

2. Silt (C? horizon of paleosol), calcareous; with disseminated 

CaCOs. Pale yellowish brown; sparse pods of Glacier Peak ash 

1 in. (2.5 cm) thick and 2 in. (5 cm) long, mainly silt size 

(speciman no. 69W19), occur near middle of unit. Angular to 

subrounded pebbles near base; some blocks 10 in. (25 cm) 

across _______________________________ 2.0 61
GLACIER PEAK AND MAZAMA ASH-BED MARKERS, MONTANA H17

Unit Thickness 

No. (feet) (cm)

1. Gravel (alpine Pinedale outwash), poorly bedded; fragments 

less than 4 in. (10 cm) across; rounded to subrounded carbonate rocks and Precambrian sandstone __________ 3.0 91

Covered by talus.

Total amount exposed ___________________ 11.5± 350±

NOTE: At west end of exposure, units 1, 2, and 3 are overlain unconformably by unit 4.

The Glacier Peak ash at locality HS-91 was deposited on alluvium, 

and its occurrence is very similar to that of locality HS-87, about 2 

miles (3.2 km) downstream. The ash occurs as small pods in a bed of 

silt, about 1 foot (0.3 m) above outwash gravel of the alpine Pinedale 

Glaciation. Here and at locality HS-87 the silt bed beneath the ash and 

above the outwash gravels represents alluvium deposited by the Sun 

River after the Pinedale alpine glacier had completely retreated to the 

cirque areas. The thickness of the silt bed indicates a very short period 

of deposition prior to the Glacier Peak ash fall. Thus, these data further 

substantiate the dating of the end of a major deterioration of Pinedale 

glaciers (about 12,000 years ago), as cited by Richmond (1972) and 

Lemke, Laird, Tipton, and Lindvall (1965). At this locality, the 

Altithermal soil was developed on the overlying silty clay (unit 3), and 

part of the B horizon is preserved. The overlying colluvial deposit (unit 

4) lies unconformably on the fossil soil in the center of the exposure, but 

at the west end of the exposure it overlies units 1, 2, and 3 on an angular 

unconformity. The talus and colluvial deposit in the upper part of the 

exposure contains the Mazama ash.

LOCALITY HS-87

South shore of Diversion Lake, 100 yards (91 m) west of Madison Limestone exposure in 

NE'/4SE'/4 sec. 35, T. 22 N., R. 9 W.

[Measured by M. R. Mudge. 1959. Fossils identified by D. W. Taylor (written commun., 1959). Color designations are 

from "Rock-Color Chart" (Goddard and others, 1948)]

Unit Thickness 

No. (feet) (cm)

Alluvial fan:

11. Soil, grass-root zone, silt with clay, noncalcareous; dusky yellow 

brown (10YR 2/2); massive, with many fragments of chert, 

limestone, and shale; many burrows, one filled with the 

following land snails: (USGS Cenozoic loc. 21825) Papilla and 

Vallonia cyclophorella Sterki _________________ 1.6 49

10. Silt, sandy (oxidized zone), calcareous; moderate brown (5YR

4/4); composition same as above _______________ .8 24
HIS CONTRIBUTIONS TO STRATIGRAPHY

Unit Thickness 

No. (feet) (cm)

Alluvial fan Continued

9. Silt, sandy (caliche zone), very calcareous, with disseminated 

carbonate; many rock fragments of shale, limestone, and 

chert; pale yellowish brown (lOYft 6/2); massive ______ 2.6 79

8. Silt, calcareous, with small nodules and stringers of disseminated carbonate; contains smaller rock fragments than 

above; pale yellowish brown, darker than above; massive. 

Land snails (USGS Cenozoic loc. 21824) Papilla, Vallonia 

gracilicosta Reinhardt, and Oreohelix ____________ 2.4 73

7. Gravel, mostly shale and limestone fragments, up to pebble

size; calcareous; same color as above; silty matrix _____ .7 21

6. Silt, calcareous, with stringers of shaly gravel; moderate yellow 

brown (IOYR 5/4); massive. Insect burrows. Land snails 

(USGS Cenozoic loc. 21823) Papilla muscorum (Linnaeus), 

Vallonia gracilicosta Reinhardt, cf. Succinea, Euconulus 

falvas (Muller), and Oreohelix ________________ .8 24

5. Gravel, mainly small limestone and shale fragments, less than

pebble size; moderate yellow-brown; oriented ________ .1-.5 3-15

4. Silt, with thin 0.1- to 0.3-ft (3- to 8-cm) lenses of Glacier Peak 

ash (specimen nos. 68W21 and 68W137) mainly silt size, 

calcareous, lighter than moderate yellow brown (WYR 5/4). 

Land snails (USGS Cenozoic loc. 21822) Papilla mascorum 

(Linnaeus), Vallonia gracilicosta Reinhardt, and cf. Succinea. Carbon-14 age, 12,750±350 years (specimen no. W1644) _________________________ .7 21

3. Gravel, limestone and shale fragments up to pebble size, 

flattened and subangular; oriented; calcareous; moderate 

yellow brown ___________________________ .7 21

Unconformity.

Terrace deposits (alpine Pinedale outwash):

2. Silt, grading down into very fine grained sand; calcareous; same 

color as unit 4 except slightly darker in sandy part. Land 

snails (USGS Cenozoic loc. 21821) Papilla muscorum (Linnaeus), Vallonia cyclophorella Sterki, and cf. Succinea __ .6 18

1. Gravel, mostly angular to subangular fragments of limestone 

and shale with some pebbles of erratics, poorly oriented, some 

angular cobble-size limestone and sandstone; calcareous; 

moderate yellow brown _____________________ 4.0± 122±

Waterline of Diversion Lake.

Total section exposed __________________15.0-15.4± 457-469±

The Glacier Peak ash bed at locality HS-87 is within an alluvial fan 

deposit that was formed by an unnamed stream which is a southern 

tributary to the Sun River. The ash-bearing silt bed contains small 

snail shells that were dated by Carbon-14 methods at 12,750±350 years 

B.P. (Mudge, 1967; Ives and others, 1967). The silt bed overlies a thin 

gravel bed of limestone and shale fragments (unit 3), derived locally
GLACIER PEAK AND MAZAMA ASH-BED MARKERS, MONTANA H19

from the north-flowing unnamed tributary of the Sun River. The 

gravelly bed represents the first development of the alluvial fan on the 

alpine Pinedale terrace deposits; therefore, an unconformity exists at 

the base of the bed. The silt bed (unit 2) beneath the gravel was very 

likely deposited after the Pinedale glacier had melted back upvalley, 

and probably correlates with the lower part of unit 2 of locality HS-91. 

The underlying coarse outwash gravel was deposited during the retreat 

of this glacier, and is present in almost all exposures of known Glacier 

Peak ash within and near the mountains.

An Altithermal soil above the ash is not recognized at this locality. If 

it formed on the fan, it was subsequently eroded, possibly prior to the 

deposition of gravel unit 5 or 7.

MAZAMA ASH BED

The Mazama ash bed is the fallout from the cataclysmic eruption 

that resulted in the formation of the Crater Lake collapse caldera, 

located in the Cascade Range some 50 miles (80 km) north-northwest of 

Klamath Falls, Oreg. (Powers and Wilcox, 1964). Like many eruptions 

of this type, it consisted of an initial stage of repeated explosive eruptions (from which came the major part of the Mazama ash fall), then a 

culminating eruptive stage in which the predominant action was the effusion of copious vesiculating and fragmenting magma that flowed 

down the flanks and valleys, terminating with the caldera collapse. The 

ash bed called Mazama, therefore, actually consists of several ash falls 

spaced at intervals of weeks to perhaps hundreds of years, carried by 

winds into different quadrants from Crater Lake but generally in the 

eastern 180° semicircle. The occurrences of Mazama ash described in 

the present report are some 500-600 miles (800-965 km) from the 

source. They are part of a major ash-fall lobe which extends northeast 

from Crater Lake into Alberta and Saskatchewan (fig. 1).

In view of the complex and possibly prolonged nature of the Mazama 

eruption, the downwind ash identified as Mazama may be of slightly 

different ages in different areas, and this may account for part of the 

range in the ages that has been inferred from radiometric determinations on carbonaceous materials in deposits associated with the 

ash. Among the occurrences of Mazama ash in the area of this report, 

age determinations have been made on deposits above and below the 

ash at locality HS-176 (see table 4 and description of stratigraphic 

section), but these deposits only serve to bracket the age of the ash at 

sometime between 7,850±200 and 4,740±250 years. Many determinations have been made on peat associated with the Mazama ash in 

Oregon, Washington, and British Columbia; age limits for the peat 

range between 7,000 and 6,000 years. Several workers have determined 

the age of charcoal lumps in the ash that flowed west from Crater Lake
H20 CONTRIBUTIONS TO STRATIGRAPHY

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RWL-7 _69VV130 _

RWL-81 _ 68W131 _

RVVL-91 -68W132 _

RWL-11_68W134 _

RWL-13_69W20 __

RVVL-14_69W105 _

HS-91 __68VV138

and 

68W18.

HS-92 __68W139 _

HS-94a _68W140 _

See footnotes at

NWWNWW sec. 29, 

T. 16 N., R. 2 W., 

4'/j miles (7.2 km) 

NE. of Craig, Lewis 

and Clark County.

SE'/4NW'/4 sec. 14, T. 

22 N., R. 1 W., 10 

miles (16 km) NW. 

of Vaughn, Teton 

County.

SWWNEW sec. 27, T. 

21 N., R. 1 E., 2 

miles (3.2 km) SW. 

of Vaughn, Cascade 

County.

NEViNEW sec. 30, T.

23 N., R. 3 E., 16 

miles (25.6 km) 

NW.of Great Falls, 

Choteau County. 

NW'/4SE'/4 sec. 8, T.

24 N., R. 3 W., New 

Rockport (Hutterite) site, Teton 

County.

SE'/4 sec. 17, T. 21 N., 

R. 9 E., 4 miles (6.4 

km) S. of Shonkin, 

Choteau County.

SW'/4NW'/4 sec. 3, T. 

21 N.. R. 9 W2 

Lewis and Clark 

County.

NW/4SW/4 sec. 12, T 

21 N., R. 8 W., 

Lewis and Clark 

County.

SW/4SW/4 sec. 13, T. 

21 N.. R. 9 W.2 

Lewis and Clark 

County, 

end of table.

0.2-0.5

0-0.3

0-30 Alluvial gravel fan Gravel ____ Gravel _. 

along Missouri , 

River.

15 Stream alluvium __ Mixed gravel Silty clay 

and clay.

12 Deltaic deposits Fossil soil __ Silty clay to 

related to Glacial silty sand

Lake Great Falls.

3 Alluvium _______ ---do ___ Modern soi

6-15 Eolian-colluvium 

deposits.

Silty clay __ Silty clay 

3 Stream terrace ___ Clayey silt __ Fine gravel 

(fossil soil?).

9 Colluvium, silt ___ Silt and Silt 

gravel.

0-9 Alluvium, silt ____ Gravely silt _ Silt with le

ses of grav

6 Alluvial fan, silt __ ---do -__ Gravelly si
H22 CONTRIBUTIONS TO STRATIGRAPHY

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GLACIER PEAK AND MAZAMA ASH-BED MARKERS, MONTANA H23

down Rogue River, Oreg., and oddly enough, these results, too, range in 

age from 7,010±120 to 6,453±250 years. In the present paper, a value of 

6,600 years will be used arbitrarily for the age of the Mazama ash.

GEOLOGIC SETTING

As discussed previously, the Mazama ash is widespread in postglacial 

deposits in the earlier part of the Altithermal interval. It is found in 

alluvial, colluvial, lake, and bog deposits. In alluvial deposits it commonly is found from about 3 feet (90 cm) to as much as 18 feet (5.5 m) 

below the surface, whereas in colluvial or lakebed deposits it is 

generally less than 4 feet (1.2 m) below the surface. It commonly occurs 

in distinct but discontinuous beds ranging in thickness from less than 

0.1 foot (3 cm) to 2.5 feet (76 cm), and in short lenses 1-2 feet (30-60 

cm) long and as much as 1 foot (30 cm) thick. Contacts commonly are 

sharp to moderately sharp. Where pure, the ash is white to grayish 

white; where impure (mixed with silt) it commonly is tan. Size analyses 

were made of five ash samples (localities RWL-1, RWL-2, RWL-3, 

RWL-4, and RWL-5). Although the analyses were made on field 

samples and some of the grains represent the non-ash fraction, it is evident that most of the ash fragments are less than 300 mesh.

The Mazama ash was deposited near the beginning of the Altithermal interval, during a time when a prominent thick soil was being 

formed. The best section, containing fossil soil above and below the ash, 

is at locality RWL-1. Here a well-developed soil is present on the silty 

clay below the ash. Soil also directly overlies the ash bed. At locality 

HS-91, a fossil soil underlies the ash bed. At locality 69W23, lenses of 

Mazama ash overlie a possible fossil soil on a silty clay unit 1.5 feet (45 

cm) thick. These few localities indicate that the position of the ash bed 

in reference to the Altithermal soil varies, depending on the environmental conditions at each locality.

DESCRIPTIONS OF SELECTED LOCALITIES

Three localities of the Mazama ash bed will be described in detail to 

show the stratigraphic relations between ash and enclosing sediments 

and to show how the ash bed can be used as a marker bed. These 

localities are RWL-1, RWL-6, and HS-176.

LOCALITY RWL-1

In undercut bank of the Missouri River, 2'/2 miles (4 km) northeast of Cascade, Mont., in 

SE'UNEW sec. 24, T. 18 N., R. 1 W.

[The section described here is based upon descriptions made in 1956 by R. W. Lemke, E. K. Maughan, and E. B. 

McKee, with additions by R.W. Lemke in 1957,1958, and 1963. When examined again in 1969 by R. W. Lemke, M. R. 

Mudge, and Ray E. Wilcox, the bank had been further undercut and the basal part of the exposed section was 

somewhat different from that described herein]
H24 CONTRIBUTIONS TO STRATIGRAPHY

Unit Thickness 

No. (feet) (cm)

11. Soil, modern profile. Upper 0.5 ft (15 cm) is dark-gray silty clay; 

remainder is light-gray clayey silt with a porous texture 

(probably largely eolian), except that the basal part is slightly 

reddish brown. Contains snail shells, mostly broken 

fragments ____________________________ 4.0 122

10. Soil, weak fossil profile. Silty clay, dark-gray, calcareous. 

Locally abundant freshwater snails identified by D. W. 

Taylor (written commun., 1963, USGS Cenozoic loc. 23362) 

as follows: Lymnaea caperata Say, Promenetus umbilicatellus (Cockerell), Helisoma trivolvis (Say) ______ 1.0 30

9. Mazama ash (ash Nos. 68W124 and 69W11), grayish-white. Size 

analysis: 6.1 percent held on No. 200 sieve, 3.7 percent held on 

No. 300 sieve, and 90.2 percent passing No. 300 sieve. Wedges 

out along eastern part of exposure. Sharp undulating lower 

contact. Upper contact less sharp; core of bison horn found at 

this contact. Noncalcareous in lower 0.4 ft (12 cm), where ash 

is pure; upper 0.6 ft (18 cm) is impure, with much silt, is 

calcareous, and contains freshwater snails, identified by D. 

W. Taylor (written commun., 1963, USGS Cenozoic loc. 

23363) as follows: Lymnaea elodes Say, Nasonia cockerelli 

(Pilsbry and Ferriss), Gyraulus parvus (Say), Helisoma 

trivolvis (Say), Promenetus exacuous (Say), Promenetus umbilicatellus (Cockerell), Physa gyrina Say, Physa skinneri 

Taylor _________________________ 1.0 30

8. Soil, strongly developed; early Altithermal. Silty clay, dark 

yellowish brown grading to moderate yellow brown toward 

base; blocky texture with nodules of impure selenite. Sharp 

upper contact. Snails, fairly abundant in basal part, submitted for radiocarbon determination (W-902), gave age of 

7,460±250 years (Meyer Rubin, U.S. Geol. Survey, written 

commun., Nov. 22, 1960). Shells identified by D. W. Taylor 

(written commun., 1963, USGS Cenozoic loc. 23364) as 

follows: freshwater clam, Pisidium; freshwater snails, Valvata 

lewisi Currier, Lymnaea elodes Say, Nasonia cockerelli 

(Pilsbry and Ferriss), Gyraulus parvus (Say), Helisoma 

trivolvis (Say), Promenetus exacuous (Say), Promenetus umbilicatellus (Cockerell), Physa gyrina Say; land snail, cf. Succinea _______________________________ 1.6 48

7. Clay, silty, grayish-brown; may be an immature fossil soil. Contains a few snails ________________________ .8 24

6. Clay, silty, pale-yellowish-brown. Very lenticular. Contains 

freshwater snails identified by D. W. Taylor (written commun., 1963, USGS Cenozoic loc. 23365) as follows: Lymnaea, 

Helisoma trivolvis (Say), Promenetus umbilicatellus 

(Cockerell), Physa gyrina Say _________________ 0.0-0.8 0-24

5. Sand, silty, light-gray _______________________ .0-.5 0-15

4. Silt, sandy, indistinctly bedded. Clam shells, 3 in. (8 cm) long, 

submitted for radiocarbon determination (W-594), gave an 

age of°8;8QO±300 years (Meyer Rubin, U.S. Geol. Survey, 

written commun., 1957). Shells identified by D. W. Taylor 

(written commun., 1963, USGS Cenozoic loc. 23366) as
GLACIER PEAK AND MAZAMA ASH-BED MARKERS, MONTANA H25

Unit Thickness 

No. (feet (cm)

follows: freshwater clams, Sphaerium striatinum (Lamarck),

Pisidium compressum Prime; freshwater snails, Lymnaea,

Gyraulus paruus (Say); land snail, cf. Succinea. A skull

collected from middle portion of unit (at same horizon as

clam shells for radiocarbon determination) was identified by

G. E. Lewis (U.S. Geo. Survey, written commun., 1957) and

C. B. Schultz and L. G. Tanner (Univ. Nebraska, written

commun., 1968) as Bison antiquus Leidy __________ 1.7-2.0 52-60

Unconformity(?)

3. Silt and sand, interbedded. Very calcareous. Grades eastward 

within 150 ft (45 m) to mostly gray silty clay (lacustrine), 

faintly laminated. Contains clam and snail shells and bone 

fragments in upper 1-2 ft (30-60 cm). Sharp but irregular upper contact ____________________________ 3.5 107

Unconformity.

2. Sand, medium to coarse, clean, well-bedded; conspicuously 

crossbedded. Thickens westward in the exposure. Contains 

about 5 percent black, green, and red grains representing 

minerals with a specific gravity greater than 2.85. Includes 

some silt interbeds about 0.1 ft (3 cm) thick ________ 3.0-10.5 90-320

Unconformity(?)

1. Glacial Lake Great Falls deposits, continental Pinedale(?). Clay 

(70 percent) and silt (30 percent); well laminated, locally distorted; noncalcareous. (Base of section is at normal river 

level; glacial lake deposits extend at least 3 ft (90 cm) below 

water level.) ___________________________ 3.0+ '90+

Approximate total thickness of described section ______ 26.0 790

The fairly complete alluvial and fossil soil sequence exposed in relation to the dated Mazama ash bed at locality RWL-1, coupled with 

supporting radiocarbon dates and identification of fossils from different 

horizons, makes this exposure a particularly valuable one for interpreting late Pleistocene and postglacial events.

Although some uncertainty exists as to whether unit 1 represents 

deposits of Glacial Lake Great Falls laid down during Bull Lake time or 

during Pinedale time, the topographic position of the deposits indicates 

one of the latest glacial lake levels in the area. Therefore, unit 1 is likely 

of Pinedale age.

Unit 2 is clean sand, probably representing regular stream deposits of 

the Missouri River. An erosional unconformity marks the top of the unit 

and probably also the base. If, as supposed, the underlying lacustrine 

deposits are of Pinedale age, then the unconformities mark only short 

hiatuses of time.

It is not clear whether unit 3 represents lacustrine deposits of
H26 CONTRIBUTIONS TO STRATIGRAPHY

Holocene age or of Pinedale age. The sharp but irregular upper contact 

suggests a time break between unit 3 and unit 4. The faintly laminated 

gray silty clay into which unit 3 grades laterally may be either 

lacustrine deposits of a large glacial lake or those of a local postglacial 

pond.

Unit 4 is assigned to the pre-Altithermal interval of the Holocene on 

the basis of clam shells in the unit dated (W-594) at 8,800±300 years 

B.P. A nearly complete skull, identified as Bison antiquus Leidy by G. 

E. Lewis of the U.S. Geological Survey (written commun., 1957) and 

later by Lewis with C. B. Schultz and L. G. Tanner of the University of 

Nebraska (written commun., 1957), also was collected from the middle 

part of the unit. In answer to an inquiry concerning the late age for this 

type of bison in the region, G. E. Lewis (written commun., 1968) stated: 

"We would still retain the original identification, i.e. Bison antiquus. In 

the range 4,500 to 7,500 B. P. we get Bison remains which are intermediate between B. bison and B. antiquus, but of course we should 

expect this."

As indicated from radiocarbon dates for units 4 and 8, units 5, 6, and 

7 were all deposited within about 1,500 years, just prior to the beginning 

of the Altithermal. D. W. Taylor (written commun., 1963) gave the 

following information on the depositional environment of fossil snails 

and clams found in these three units as well as in units 4 and 8:

These assemblages are all similar in general composition. So far as we know, all of the 

fossils may represent species living locally. There is evidence for a slight change in the immediate environment of deposition through the section sampled. The species of perennial 

stream habitat (Sphaerium striatinum, Pisidium compressum) occur only at the base of 

the section [unit 4]. The freshwater snails indicate perennial water ( Valvata lewisi) [unit 

8] to seasonal ponds (Nasonia cockerelli) [unit 8] and probably come from backwaters 

and sloughs that dried up to varying extents in the summer.

It is likely that the indicated environmental differences between the 

units are more a reflection of immediate local conditions than of 

regional climatic changes during deposition of the units.

The well-developed fossil soil of unit 8 provides nearly as recognizable 

a marker bed as the ash bed. It is particularly valuable where the ash 

bed is absent. The radiocarbon date (W-902) of 7,460±250 years B.P. 

for snail shells collected from the basal 0.5 foot (15 cm) of the unit indicates that soil formation began about 7,500 years ago the date that 

we believe approximately marks the beginning of the Altithermal in the 

region. Thus, the fossil soil forms a convenient marker for the beginning 

of this interval. Inasmuch as the overlying Mazama ash was deposited 

about 6,600 years ago, the fossil soil unit was formed during an interval 

of only about 1,000 years.

On the basis of the age of the Mazama ash of unit 9 and the assumed 

length of the Altithermal, it seems likely that the weak soil of unit 10 

was developed during the latter part of the Altithermal.
GLACIER PEAK AND MAZAMA ASH-BED MARKERS, MONTANA H27

Unit 11 probably was formed mostly or entirely during Neoglaciation 

time. If so, the deposits, including the modern soil profile, apparently 

were formed under cooler climatic conditions than prevailed during the 

Altithermal (Richmond, 1972).

LOCALITY RWL-6

The Mazama ash bed at locality RWL-6, 0.6 mile (1 km) west of 

Galata in the NWV4 sec. 4, T. 31 N., R. 3 E., was first described by 

Horberg and Robie (1955). They designated it the type section for 

several ash-bed localities in southern Alberta and northwestern Montana, having similar stratigraphic relations, and named it the Galata 

ash. The cross section constructed by them to show the stratigraphic 

relations at the Galata site was redrawn and redescribed by R. W. 

Lemke in 1956 (fig. 4).

The till (unit 1) at the base of the section probably was laid down 

during the close of continental Pinedale Glaciation in the region. 

Sometime after the deposition of the till (presumably about 12,000 

years B.P.) and before deposition of the Mazama ash bed (approximately 6,600 years B.P.), the channel in which the ash is found 

was cut. Inasmuch as several beds (units 2, 3, 4, and 5) were deposited 

on the till before channel cutting occurred and the uppermost of these 

beds shows a faint soil horizon, the indication is that channel cutting 

did not occur immediately after the close of continental Pinedale 

Glaciation. On the other hand, some evidence suggests that channel 

cutting did not immediately precede deposition of the ash bed: (1) 

several beds (units 6, 7, 8, and 9) underlying the Mazama ash bed were 

deposited after channel cutting started, and (2) the uppermost of these 

beds appears to be the fossil soil that we interpret to be at the base of 

the Altithermal. These interpretations suggest that the cutting 

probably occurred in the pre-Altithermal interval of the Holocene, 

between approximately 10,000 and 7,500 years ago. Pronounced 

channel cutting during this interval has not been recognized elsewhere 

in the region.

The Mazama ash bed (unit 10) attains the greater than normal 

thickness of 2.5 feet (76 cm) in the filled channel. The bed is fairly clean 

and is characterized by undulating but sharply defined upper and lower 

contacts.The fact that the bed is relatively thick in the channel but that 

it thins to extinction a short distance outside the channel suggests: (1) 

the ash was concentrated in the channel largely by fluvial and eolian 

processes probably in standing water or in a very slowly moving 

stream that would not have eroded away the ash, and (2) the ash that 

was deposited originally on the flat surfaces outside the channel was 

removed by wind and water erosion. These assumptions help explain 

why the ash beds in most of the described localities are found in alluvial
H28 CONTRIBUTIONS TO STRATIGRAPHY

deposits adjacent to streams and are not found in the uplands of the 

region. The fine-grained nature of the ash probably makes it particularly susceptible to removal by wind on exposed dry surfaces.

LOCALITY HS-176

Located in streambank in northwesterly tributary of Willow Creek, just northwest of Sawtooth Ranch in the SW'ANE'A sec. 27, T. 21 N., R. 7 W., Lewis and Clark County.

[Carbon-14 age determination by Meyer Rubin (written commun., 1959 and 1964)]

Unit Thickness 

No. (feet) (cm)

3. Clay, silty, calcareous, blocky; medium gray with dark-gray 

lenses; massive; A horizon, 0.2 foot (5 cm) thick; no B horizon. 

Gravel lens locally at base. Mazama ash bed, 0-8in. (0-20 cm) 

thick, is present about 6 in. (15 cm) below ground surface. 

Snails and clams (loc. F-209a') collected from below ash bed 

were dated (W-765) at 7,870±200 years B.P. Snails above ash 

bed dated (W-1416) at 4,740±250 years B.P. Bison bones common in lower part ________________________ 2.6 79

2. Clay, calcareous, blocky, tough; medium gray mottled with 

moderate yellow brown; a 0.7-ft (21-cm) dark-gray slightly 

calcareous clay bed locally overlies this unit; beneath a darkgray lens there is a disseminated CaCOs zone about 0.4 ft (12 

cm) thick __________________ _ _ 1.7 52

1. Clay (lacustrine beds), very tough and plastic; calcareous, dark 

gray; wood (F-210-C) collected from this locality contained 

modern roots which gave it an erroneous carbon-14 age 

(W-774) of 3,060±200 years ______ __________ 3.0+ 90+

Total section exposed ___________________ 7.3 222

Base of stream.

'The mollusks from F-209a were identified by D. W. Taylor (written commun., 1959) as: 

Freshwater clams:

Pisidium casertanum (Poli)

Pisidium obtusale Pfeiffer 

Freshwater snails:

Valuata lewisi Currier

Stagnicola palustris (Muller)

Stagnicola caperata (Say)

Lymnaea stagnalis jugularis Say

Fossaria dalli (Baker)

Fossaria parua (Lea)

Gyraulus circumstriatus (Tryon)

Gyraulus paruus (Say)

Armiger crista (Linnaeus)

Helisoma subcrenatum (Carpenter)

Promenetus exacuous (Say)

Promenetus umbilicatellus (Cockerell)

Physa gyrina Say

Physa skinneri Taylor

Aplexa hypnorum (Linnaeus) 

Land snail:

Ory/omo sp.
GLACIER PEAK AND MAZAMA ASH-BED MARKERS, MONTANA H29

NORMAL STREAM LEVEL

HORIZONTAL SCALE 1 IN. = 20 FT (1 CM = 240 CM) 

VERTICAL SCALE EXAGGERATION X 2

Unit Description

14 Soil, modern

13 Clay, silty, dark-gray, indistinctly bedded

12 Silt, sandy; bedding approximately parallel to the contacts

11 Sand, fine; bedding approximately parallel to the contacts

10 Mazama ash bed; sharp but irregular contacts

9 Fossil soil; clay, silty, faintly bedded

8 Silt, sandy, to silty sand; obscure bedding; probably eolian-colluvial in 

	origin

7 Sand, medium, clean, crossbedded, stream-deposited

6 Gravel, fine, stream-deposited

Unconformity

5 Fossil soil; very poorly developed

4 Clay, silty, medium-gray; probably eolian-colluvial in origin 

3 Clay, silty, dark-gray; faint soil horizons; very incomplete bison skull

found near base

2 Sand, fine, thinly bedded 

1 Till; silt, clay, and sand intermixed with a few pebbles

FIGURE 4. Stratigraphic relations of Mazama ash bed with glacial and postglacial 

deposits at locality RWL-6. Located 0.6 mile (1 km) west of Galata, Toole County, in 

the NWV4 sec. 4, T. 31 N., R. 3 E. Galata ash locality of Horberg and Robie (1955).

The Mazama ash at locality HS-176 was deposited in the upper part 

of lacustrine beds that range in age from middle Pinedale time to about 

4,500 years ago. The lake formed behind a dam created by the end 

moraine of the second advance of Pinedale glaciers. The lake clays are 

more than 90 feet (27 m) thick in the middle of the deposit, as determined from a hole drilled by a power auger. Although most of the 

deposit accumulated during the latter part of middle Pinedale time, 

locally 2-4 feet (60-120 cm) of the upper part accumulated during the 

Holocene (fig. 5).

SUMMARY

As examples given in this paper illustrate, the Glacier Peak and 

Mazama ash beds are useful Stratigraphic markers in subdividing 

deposits of late Pleistocene and Holocene age in west-central Montana. 

Using these Stratigraphic markers as a basis, the capability to make
H30 CONTRIBUTIONS TO STRATIGRAPHY

FIGURE 5. Locality HS-176, showing Mazama ash bed (head of pick) overlying 

lacustrine clays deposited during the last recession of the alpine Pinedale glacier in the 

area. Alluvium overlies the ash bed. Radiocarbon age determinations of snail shells by 

MeyerRubin (written commun., 1959 and 1964) are 7,870±200 years on shells below the 

ash, and 4,740±250 years on shells above the ash.

further subdivisions by recognition and regional correlation of 

associated beds should be enhanced. This, in turn, should provide a real 

impetus for future stratigraphic, ecological, climatic, and other related 

studies.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the contributions of Gerald M. 

Richmond to this paper. Ideas and information exchanged with him in 

the field and office, particularly those related to the Pinedale and Bull 

Lake Glaciations, were of great value. Grateful acknowledgment also is 

made to David Ramaley for careful preparation of gravity fractions of 

the many volcanic ash samples.

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