VOLCANIC STRATIGRAPHY 

OF THE 

QUATERNARY 

RHYOLITE PLATEAU 

IN YELLOWSTONE NATIONAL PARK 

Volcanic Stratigraphy of 

the Quaternary Rhyolite 

Plateau in Yellowstone 

National Park 

By ROBERT L. CHRISTIANSEN and H. RICHARD BLANK, JR. 

GEOLOGY OF YELLOWSTONE NATIONAL PARK 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 729-B 

Nomenclature for Quaternary volcanic rocks 

of the Yellowstone rhyolite plateau is 

based upon recognition of three successive 

cycles of volcanism, only two of which are 

identified within the national park 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1972 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 72-600023 

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

Washington, D.C. 20402- Price 40 cents (paper cover) 

Stock Number 2401-2105 
Yellowstone National Park, the oldest of the 

areas set aside as part of the national park system, lies amidst the Rocky Mountains in northwestern Wyoming and adjacent parts of Montana 

and Idaho. Embracing large, diverse, and complex 

geologic features, the park is in an area that is 

critical to the interpretation of many significant 

regional geologic problems. In order to provide 

basic data bearing on these problems, the U.S. 

Geological Survey in 1965 initiated a broad program of comprehensive geologic and geophysical 

investigations within the park. This program was 

carried out with the cooperation of the National 

Park Service, and was also aided by the National 

Aeronautics and Space Administration, which 

supported the gathering of geologic information 

needed in testing and in interpreting results from 

various remote sensing devices. This professional 

paper chapter is one of a series of technical geologic reports resulting from these investigations. 

Abstract ........... ---------------------------------------------------------------------

Introduction ......................................................................... . 

General geologic relations .................. -------------------------------

Yellowstone Group .... ---------------------------------------------------------- Huckleberry Ridge Tuff ____________________________________________ _ 

Mesa Falls Tuff.----------------------------------------------------------

Lava Creek Tuff .......................................................... . 

Rocks of the first volcanic cycle ....................................... . 

Junction Butte Basalt ............................................... . 

Rhyolite of Broad Creek .......................................... . 

Lewis Canyon Rhyolite ............... ---··-····--···----···--····--·· 

Sediments and basalts of The Narrows ................. . 

Rocks of the second volcanic cycle .................................. . 

Rocks of the third volcanic cycle ..................................... . 

Mount Jackson Rhyolite ............................................ . 

Undine Falls Basalt .................................................. . 

CONTENTS 

Page 

B1 

1 

2 

3 

3 

5 

7 

9 

9 

10 

10 

11 

11 

11 

11 

12 

Rocks of the third volcanic cycle- Continued 

Plateau Rhyolite ........................................................ . 

Mallard Lake Member ....................................... . 

Upper Basin Member ........................................ . 

Obsidian Creek Member .................................... . 

Central Plateau Member ................................... . 

Shoshone Lake Tuff Member ............................ . 

Roaring Mountain Member .............................. . 

Post-Lava Creek basalts ... ···----···--···---···---···---···---···--

Swan Lake Flat Basalt ..................................... . 

Falls River Basalt ............................................. . 

Basalt of Mariposa Lake .................................. . 

Madison River Basalt.. ...................................... . 

Osprey Basalt ............. ··--·······-····--·····---··----------·--

Sediments associated with the Yellowstone 

rhyolite plateau .............................................................. . 

References cited ................................................................. . 

ILLUSTRATIONS 

Page 

B12 

12 

12 

13 

13 

14 

14 

15 

15 

15 

16 

16 

17 

17 

18 

Page 

FIGURE 1. Index map of region around Yellowstone National Park ..................... ---··----·----····--····-···················-····-·····--····--···· B2 

2. Index map showing major localities in Yellowstone National Park................................................................... 4 

3. Diagrammatic relations between stratigraphic units outlined in table L ................... ·-··---···---···---·-·---··----···---··· 7 

TABLE 

TABLE 1. Summary of stratigraphic nomenclature .............................. -----·---··----·----······-·--.. ·---.. --............................................... .. 

v 

Page 

B6 

GEOLOGY OF YELLOWSTONE NATIONAL PARK 

VOLCANIC STRATIGRAPHY OF THE QUATERNARY RHYOLITE PLATEAU 

IN YELLOWSTONE NATIONAL PARK 

By ROBERT L. CHRISTIANSEN and H. RICHARD BLANK, JR. 

ABSTRACT 

The volcanic sequence of the Quaternary Yellowstone plateau consists of rhyolites and basalts representing three 

volcanic cycles. The major events of each cycle were eruption 

of a voluminous ash-flow sheet and formation of a large collapse caldera. Lesser events of each cycle were eruption of 

precaldera and postcaldera rhyolitic lava flows and marginal 

basaltic lavas. The three major ash-flow sheets are named 

and designated in this report as formations within the Yellowstone Group. The lavas are assigned to newly named 

formations organized around the three ash-flow sheets of the 

Yellowstone Group to represent the volcanic cycles. 

Rocks of the first volcanic cycle comprise the precaldera 

Junction Butte Basalt and rhyolite of Broad Creek; the 

Huckleberry Ridge Tuff of the Yellowstone Group; and the 

postcaldera Lewis Canyon Rhyolite and basalt of TheN arrows. 

Rocks of the second volcanic cycle do not crop out within 

Yellowstone National Park, and only the major unit, the Mesa 

Falls Tuff of the Yellowstone Group, is named here. 

The third volcanic cycle is represented by the precaldera 

Mount Jackson Rhyolite and Undine Falls Basalt; the Lava 

Creek Tuff of the Yellowstone Group; and the postcaldera 

Plateau Rhyolite and five post-Lava Creek basaltic sequences. 

Collapse to form the compound and resurgent Yellowstone 

caldera was related to eruption of the Lava Creek Tuff. The 

Plateau Rhyolite is divided into six members- the Mallard 

Lake, Upper Basin, Obsidian Creek, Central Plateau, Shoshone Lake Tuff, and Roaring Mountain Members; all but the 

Mallard Lake postdate resurgent doming of the caldera. The 

basalts are divided into the Swan Lake Flat Basalt, Falls 

River Basalt, basalt of Mariposa Lake, Madison River Basalt, 

and Osprey Basalt. 

Sediments are intercalated in the volcanic section below the 

Huckleberry Ridge and Mesa Falls Tuffs and within the 

Junction Butte Basalt, sediments and basalts of The Narrows, 

Undine Falls Basalt, Plateau Rhyolite, and Osprey Basalt. 

INTRODUCTION 

This paper summarizes the stratigraphy of volcanic rocks which form the Yellowstone rhyolite 

plateau, a volcano-tectonic feature of Quaternary 

age which lies at an average elevation of about 

2,500 m (meters), largely within Yellowstone N ational Park. Rock units which form parts of the 

plateau sequence but which lie entirely outside the 

national park are not discussed here except for one 

major ash-flow tuff which bears directly on the 

regional stratigraphy. Volcanic rocks of the plateau 

and its outliers cover more than 6,500 square kilometers in Wyoming, Idaho, and Montana. 

Early work on rhyolitic rocks of the Yellowstone 

region, especially that by Iddings (1888; and in 

Hague and others, 1899), attracted wide attention. 

These rocks, however, have received surprisingly 

little modern study, considering their volume, youth, 

and potential regional and volcanological significance. 

The only detailed work of recent years directed 

specifically toward the rhyolite plateau was the excellent study by Boyd (1961). Other modern studies 

that have borne importantly on the volcanic rocks 

of the plateau include those by Howard (1937), 

Love (1956a, b), Hamilton (1960, 1963, 1964), and 

Brown (1961). In addition, detailed mineralogic and 

petrologic studies of single volcanic units include 

those of Bowen (1935), Fenner (1938), and Wilcox 

(1944). 

Potassium-argon ages referred to in this paper 

are based on work still in progress by J. D. Obradovich and are stated approximately only in order to 

aid visualization of the stratigraphy. These ages and 

the K-Ar analyses will be discussed in a later paper. 

Terminology used for the features and relations of 

ash-flow tuffs follows Smith (1960a, b) and Ross 

and Smith (1961). Figures 1 and 2 identify major 

features of the area. Most localities referred to in 

this paper are shown in these figures. Features not 

shown in the figures can be found on the U.S. Geological Survey's topographic map of Yellowstone 

National Park, published in 1961 (scale 1: 125,000). 

Bl 
B2 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

110" 109" 

Livingston 

45" 

44" 

0 50 MILES 

0 50 KILO METERS 

EXPLANATION 

Yellowstone Group: 

LAC Lava Creek Tuff (type area) 

MFT Mesa Falls Tuff (type section) 

MFR Mesa Falls Tuff 

(principal reference section) 

HR Huckleberry Ridge Tuff 

(type section) LEC Lewis Canyon Rhyolite (type area) 

FIGURE 1.-Index map of region around Yellowstone National Park, showing locations of type sections of formations in 

the Yellowstone Group, type area of Lewis Canyon Rhyolite, and features outside the national park referred to in text. 

GENERAL GEOLOGIC RELATWNS 

Most previous workers considered the rhyolites and 

basalts of the Yellowstone plateau to be of Pliocene 

age. Richmond and Hamilton (1960) and Hamilton 

(1960) first called attention to relations which suggested a late Quaternary age for some of the youngest flows. Stratigraphic relations that we have 

worked out, together with extensive and continuing 

potassium-argon dating by J. D. Obradovich, show 

that virtually all of the plateau volcanism was 

younger than about 2 m.y. (million years) and that 

some was as young as about 70,000 years. We consider all of this time span to be within the Quaternary, and this seems to accord with the present 

thinking of most stratigraphers. 

The volcanic rocks of the Quaternary rhyolite 

plateau are virtually all either rhyolites or basalts, 

and the rhyolites are greatly predominant. The rare 

rocks of intermediate composition are mixed lavas 

with rhyolitic and basaltic components. Areally, the 

plateau consists of two parts, as demonstrated by 

Boyd (1961}-an outer ring that consists largely 

of rhyolitic welded tuff but also lesser amounts of 

rhyolitic and basaltic lavas, and an inner area 

covered mainly by rhyolitic lava flows, some of them 

very large (fig. 2). The inner lavas partly filled an 

enormous elliptical caldera, 70 by 45 km (kilometers) across, and spilled over its margins in 

places, especially on its southwest side. 

Although rimmed by mountains on its other sides, 
VOLCANIC STRATIGRAPHY OF THE QUATERNARY RHYOLITE PLATEAU B3 

the Yellowstone rhyolite plateau along its southwestern margin overlooks the eastern Snake River 

Plain, a topographically lower area also covered 

largely by Quaternary volcanic rocks, principally 

basalts but including subordinate rhyolites. The 

transitional Island Park region lies between the 

plateau and the plain and was shown by Hamilton 

(1965) to be a large caldera. Our study extends 

to include the Island Park area, and although we 

do not discuss it here at length, its history is intimately related to that of the Yellowstone rhyolite 

plateau and is partly contemporaneous. 

We have found that the ash-flow tuffs which 

largely constitute the outer part of the Yellowstone 

rhyolite plateau and its outliers comprise three 

major ash-flow sheets, each principally a single cooling unit separated from the others by major unconformities. These three ash-flow sheets form a basis 

for dividing the entire volcanic section into three 

parts, each of which represents a volcanic cycle in 

the evolution of its respective source area. The plan 

of the paper, therefore, is to describe first the three 

major ash-flow sheets and then to discuss the volcanic stratigraphy of the rhyolite plateau in terms 

of these cycles. Table 1 summarizes the stratigraphic nomenclature outlined here. Figure 3 shows 

some of the stratigraphic relations between the 

units outlined in table 1. Because the second cycle 

was completely confined to the Island Park region, 

it will not be discussed at length. 

YELLOWSTONE GROUP 

We recognize three cooling units of ash-flow tuff 

as the major stratigraphic units of the rhyolite 

plateau and establish them here as the Huckleberry 

Ridge, Mesa Falls, and Lava Creek Tuffs of the 

Yellowstone Group. Boyd (1961, p. 393-400) named 

and described the Yellowstone Tuff, but he did not 

provide a formal stratigraphic definition of the unit. 

The two exposures he described in most detail are 

sections of two of the three ash-flow sheets recognized in this paper. Since these three sheets, as 

discussed in the following paragraphs, are separated 

by unconformities representing time spans of 

600,000-800,000 years and are associated in their 

source areas with overlying and underlying lavas, 

each is designated here as a separate formation. The 

name Yellowstone Group now replaces the Yellowstone Tuff of the literature (for example, Boyd, 

1961 ; Brown, 1961 ; Fraser and others, 1969; Witkind, 1969) which clearly has been used to include 

at least two of our three formations. We define the 

Yellowstone Group here as the rhyolitic tuffs which 

constitute the three ash-flow formations designated 

in this paper. No single exposure shows all three formations in unbroken succession, but the stratigraphic 

relations are shown by mapping in the Island Park 

area and in the southern part of the Madison Range, 

or Henrys Lake Mountains. 

The Yellowstone Group lies on rocks as old as 

Precambrian and as young as early Quaternary. In 

many areas, particularly in eastern Yellowstone National Park, the group overlies rocks of the Absaroka Volcanic Supergroup of Eocene age (Smedes 

and Prostka, 1972). 

All three formations of the Yellowstone Group 

consist mainly of welded ash-flow tuff, predominantly 

devitrified, containing phenocrysts of quartz, sanidine, and sodic plagioclase. Common minor phenocrysts are opaque oxides, clinopyroxene, fayalite, 

zircon, hornblende, allanite, and chevkinite. 

HUCKLEBERRY RIDGE TUFF 

We name the Huckleberry Ridge Tuff, the oldest 

ash-flow sheet of the Yellowstone Group, for the 

topographic crest east of the Snake River just south 

of Yellowstone National Park and north of Jackson 

Lake. The type section (fig. 1) is designated as the 

cliff exposure at the head of a large landslide 1.8 

km N. 10° E. of the Snake River bridge at Flagg 

Ranch on the highway about 3 km south of the 

South Entrance of Yellowstone National Park. The 

Huckleberry Ridge Tuff, locally as thick as 300 m, 

is exposed widely around the margins of Yellowstone National Park and in surrounding areas south 

to southern Jackson Hole, north down the valley 

of the Gallatin River as far as the West Fork, and 

west into the Island Park region to the edge of the 

younger lavas of the Snake River Plain. In central 

Yellowstone Park the formation is largely buried by 

younger rocks. The Huckleberry Ridge in various 

areas overlies eroded rocks of virtually every major 

older unit exposed in the Yellowstone region. West 

of Yellowstone Park the formation commonly is 

overlain by the Mesa Falls Tuff, but in most other 

areas it is most commonly overlain by the Lava 

Creek Tuff. 

The type section of the Huckleberry Ridge is 

about 170 m thick and consists mainly of welded 

phenocryst-rich rhyolitic ash-flow tuff forming a 

single compound cooling unit. The tuff overlies 

sandstones and shales of Cretaceous age and is 

eroded at the top, but regional relations indicate 

that no more than a few meters has been removed 

along the present topographic surface. The section 

is divided into three widely mappable informal members on the basis of significant changes in welding 
B4 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

(;ru:H<y Lttkc 

J?('S€1'/JO-ir 

o Flagg Ranch 

0 

0 

5 10 15 MILES 

5 10 15 KILOMETERS 

( 

("' 

.J 

FIGURE 2.-Index map showing major localities in Yellowstone National Park referred to in text. 

\ . 

. ~ 

/ 

/ 
VOLCANIC STRATIGRAPHY OF THE QUATERNARY RHYOLITE PLATEAU B5 

EXPLANATION 

l______L__j 

~ 

Plateau Rhyolite Other rocks of third 

volcanic cycle 

Rocks of first volcanic cycle 

Plateau Rhyolite: 

Roaring Mountain Member: 

RCS Crystal Spring flow 

RO Obsidian Cliff flow 

RCC Cougar Creek flow 

RR Riverside flow 

Obsidian Creek Member: 

OGR Mixed lavas of Gardner River 

OGL Mixed lavas of Grizzly Lake 

OW Willow Park dome 

OA Apollinaris Spring dome 

OL Landmark dome 

OG H Gibbon Hill dome 

OGC Geyser Creek dome 

OPH Paintpot Hill dome 

OS Osprey Basalt (type section) 

MR Madison River Basalt (type area) 

SLF Swan Lake Flat Basalt (type area) 

LAC Lava Creek Tuff (type area) 

SC Upper member; Sheepeater Canyon 

(principal reference section) 

PM Lower member; Purple Mountain 

(principal reference section) 

UF Undine Falls Basalt (type locality) 

and minor variations in phenocryst content. The 

lowest member, about 45 m thick, consists of a few 

centimeters of nonwelded to partially welded glassy 

ash-flow tuff overlain successively by about 1 m of 

black vitrophyric welded tuff, about 2 m of darkgray microspherulitic densely welded tuff, and a 

brown-weathering devitrified zone. The basal part 

of the member contains abundant (40-50 percent) 

phenocrysts. About 30 m above the base, phenocrysts become less abundant; from there upward 

they decrease in size and abundance for about 15 

m. This widely mapped zone of relatively sparse 

phenocrysts contains a thin zone near the top in 

which the tuff is less densely welded than that below or above, and this zone is used to divide the 

lower two members. The middle member, about 30 

m thick, has a thin basal phenocryst-poor zone ; 

above, it has abundant large phenocrysts, and at 

the top it contains conspicuous dark-gray to brown 

crystallized pumice along with light-colored crystallized and collapsed pumice typical of the member 

as a whole. This two-pumice, large-phenocryst zone 

is overlain sharply by the third member, about 95 m 

thick, which is characterized by moderately abundant phenocrysts that are distinctly smaller than 

those in the underlying units. The upper member 

shows marked lineation of stretched pumice and 

alined phenocrysts and minor flowage folding of the 

compaction foliation. 

Basal air-fall tuffs are as much as about 2 m thick 

north of central Yellowstone Park but are thin or 

absent to the south. The lower two members of ashflow tuff of the type section of the Huckleberry 

Ridge, separated by a relatively phenocryst-poor 

zone and a partial welding break (a devitrified but 

less densely welded zone between two devitrified 

more densely welded zones), have been traced virtually everywhere the formation is found. The commonly lineated, upper member occurs only south 

of central Yellowstone Park. Although this upper 

member is demonstrably an ash-flow tuff, in many 

exposures it superficially resembles a rhyolitic lava 

flow. This characteristic of the member, which constitutes a large proportion of the highly altered 

exposures in the Red Mountains in southern Yellowstone National Park, in part led Boyd (1961) to 

regard the rocks there as mainly rhyolite flows 

which he designated the Red Mountains Rhyolite. 

The base of this upper member in many places overlies a thin crystal ash and grades southward from 

the type section into a separate cooling unit; the 

Huckleberry Ridge Tuff as a whole, therefore, is a 

composite sheet. 

Potassium-argon dating of sanidine from the 

Huckleberry Ridge Tuff by J. D. Obradovich (written commun., 1970) indicates an age of 2 m.y. 

MESA FALLS TUFF 

The Mesa Falls Tuff, the middle ash-flow sheet 

of the Yellowstone Group, does not crop out within 

Yellowstone National Park, but we name and briefly 

describe it in order to provide a complete summary 

of the Yellowstone Group and to help outline the 

Quaternary volcanic evolution of the Yellowstone 

region. The formation is named here for Upper 

Mesa Falls on the Henrys Fork of the Snake River 

in the Island Park area of eastern Idaho. The type 

section (fig. 1) is designated as the roadcut on U.S. 

Highway 20 about 6.8 km north of Ashton, Idaho. 
B6 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

TABLE 1.- Summary of stratigraphic nomenclature 

Approximate Volcanic age cycle Rhyolites Basalts and sediments 

(million years) 

Sediments of late Bull Lake 

through Holocene age 

0.07 

Osprey Basalt 

Roaring Central Shoshone (includes gravels) 

Mountain Plateau Lake Tuff 

Member Member Member 

Madison River Basalt Sediments of Potts Hot Springs, 

till of Bechler Meadows, 

sediments of Seven mile Hole, Plateau sediments of Upper Falls, 

Rhyolite ~ ro sediments of Lower Falls, 

Obsidian ~ c;; <J> 

Upper ro <J> 0 sediments of Red Rock, u:: ro 0. 

Creek Basin ~ sediments of Inspiration Point 

~ Q):!: Ill 

Member Member -"' ro .!:: ro V> ~ :2:~ 1- ...J ro > 4- ro 

"Ill 0:: 0 _J 

ro !!!. ~ 

;;: ro (/) ro <J> 

Mallard Lake Member LJ._ ro 

Ill 

0.6 Lava Creek Tuff of Yellowstone Group 

Undine Falls Basalt (includes gravels) 

<0.8 Mount Jackson Rhyolite 

'0 

t: 

1.2 0 Mesa Falls Tuff of Yellowstone Group () 

Q) 

(/) 

1.6 Sediments and basalts of The Narrows 

Lewis Canyon Rhyolite 

-- ~ 

2.0 ~ Huckleberry Ridge Tuff of Yellowstone Group ;:;: 

<2.4 Rhyolite of Broad Creek 

The Mesa Falls Tuff varies greatly in thickness but 

commonly is more than 100 m thick. It is present 

mainly in the Island Park area and on the lower 

slopes of the Centennial Mountains and the southern 

Madison Range. The Mesa Falls overlies the Huckleberry Ridge Tuff and older units, and in the Island 

Park area it overlies lavas of the second volcanic 

cycle that are not discussed in this report. The Mesa 

Falls most commonly is overlain by the Lava Creek 

Tuff, but in the Island Park area second-cycle volcanic units also overlie the Mesa Falls. 

The type section of the Mesa Falls is shown in 

a photograph by Hamilton (1965, fig. 3). At that 

locality a thick basal air-fall pumice and a thin 

distal ash-flow tuff overlie a buried loess and a 

Junction Butte Basalt Gravel of Mount Everts (includes gravels) 

deeply weathered outcrop of the lower member of 

the Huckleberry Ridge Tuff. The basal 5-6 m of the 

type section of the Mesa'Falls Tuff consists of wellsorted and well-bedded air-fall pumice and crystals, 

commonly several millimeters to a centimeter or 

more in grain size. This basal air-fall pumice is 

overlain by phenocryst .. .rich (about 40 percent) ashflow tuff about 15 m thick with conspicuously large 

feldspar phenocrysts (some greater than 1 em). 

The ash-flow tuff forms a simple cooling unit, nonwelded and glassy in the basal 11!2 m, overlain by a 

pale-pink vapor-phase zone. The top of the section 

is eroded, but regional relations show that only the 

presumed original glassy nonwelded top of the cooling unit, probably never more than a few meters 
VOLCANIC STRATIGRAPHY OF THE QUATERNARY RHYOLITE PLATEAU B7 

EXPLANATION 

PRM Roaring Mountain Member } 

PCP Central Plateau Member 

PSL Sho~hone Lake Tuff Member Plateau Rhyolite 

POC Obs1d1an Creek Member 

PUB Upper Basin Member 

PM L Mallard Lake Member 

Third volcanic cycle OS Osprey Basalt 

M R Madison River Basalt 

SLF Swan Lake Flat Basalt 

FR Falls River Basalt 

M L Basalt of Mariposa Lake 

LAC Lava Creek Tuff 

U F Undine Falls Basalt 

Second volcanic cycle 

MJ Mount Jackson Rhyolite 

M F Mesa Falls Tuff 

First volcanic cycle 

{ 

TN Sediments and basalts of The Narrows 

LEC Lewis Canyon Rhyolite 

H R Huckleberry Ridge Tuff 

BC Rhyolite of Broad Creek 

J B Junction Butte Basalt 

FIGURE 3.- Diagrammatic relations between stratigraphic units outlined in table 1. Not all possible stratigraphic relations 

discussed in text are shown. 

thick, is missing. The Mesa Falls has been traced 

continuously along the ridges and roadcuts to more 

densely welded tuff about 5 km ·north, where the 

formation is thicker and has a conspicuous pinkishbrown devitrified zone. 

The type section is the only easily accessible 

good exposure of virtually the entire ash-flow sheet. 

Nevertheless, because the ash-flow tuff there is only 

a thin distal part of the sheet, the section is not 

representative in gross lithology of the formation 

as a whole. For this reason we designate the cliffs 

along the gorge of the Henrys Fork at Upper Mesa 

Falls as the principal reference section (fig. 1). 

Here the characteristic lithology of thicker sections 

of the formation is seen. A thick pinkish devitrified 

zone with abundant large phenocrysts forms the 

wall of the inner gorge of the river at the level of 

the falls. The base is not exposed but probably is 

not far beneath the level of exposures. A thickness 

of about 140 m is exposed. The devitrified zone 

grades upward into a vapor-phase zone that is poorly exposed on the east side of the upper canyon wall 

in roadcuts above the falls. Just below the rim of 

the upper canyon wall near the top of the road to 

the falls, the Mesa Falls Tuff is overlain by the 

Lava Creek Tuff, but the contact is not exposed. 

Basalts cap the section where the road crosses the 

canyon rim. A section of the Mesa Falls (complete 

except for the base) is well exposed on the nearly 

inaccessible west wall of the gorge at Upper Mesa 

Falls. 

Sanidine from the Mesa Falls Tuff has been dated 

by the K-Ar method by J. D. Obradovich (written 

commun., 1970) as 1.2 m.y. old. 

LAVA CREEK TUFF 

The Lava Creek Tuff, the upper ash-flow sheet 

of the Yellowstone Group, is named here for a 

stream which enters the Gardner River about 3 km 

southeast of Mammoth. The formation is divided 

throughout its extent into two major informal members. They are exposed overlying the Huckleberry 

Ridge Tuff in a 210-m-thick section in the walls of 

the upper canyon of Lava Creek and its tributary, 

Arrow Canyon. This is designated the type area 

(figs. 1 and 2). Arrow Canyon and nearby parts of 

Lava Creek canyon are more than 13 km from the 

nearest roads or trails ; there is no readily acces-
B8 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

sible exposure of the Lava Creek Tuff in which both 

of its members as well as its base and top are well 

exposed. We therefore designate a more accessible 

principal reference section for each of the members. 

The Lava Creek Tuff is more than 300 m thick in 

several areas. The formation occurs widely in Yellowstone National Park and has been traced south 

to Jackson Lake, westward through much of the 

Island Park area to the southern foothills of the 

Centennial Mountains, and southwestward along the 

margin of the Snake River Plain to the south side 

of the Snake River about 25 km northeast of Idaho 

Falls. The Lava Creek in various areas overlies most 

of the older rock units exposed in the region. 

Around the margins of the Yellowstone plateau, the 

Lava Creek commonly overlies the Huckleberry 

Ridge Tuff. The Lava Creek is overlain by most of 

the younger units of the third volcanic cycle in and 

near Yellowstone National Park. 

The principal reference section of the lower member is the south-facing cliff of Purple Mountain 

above an old gravel pit about 1 km east of Madison 

Junction (fig. 2). The section overlies the Mount 

Jackson Rhyolite and is overlain a few kilometers 

northeast by the upper member of the Lava Creek 

Tuff; a few patches of Madison River Basalt lie on the 

lower member on benches in the cliff southwest of 

Purple Mountain. In its basal 300 m the lower member has somewhat fewer and distinctly smaller phenocrysts and is less densely welded than above. 

Collapsed pumice in this less densely welded basal 

portion, though not particularly more abundant, is 

more conspicuous than above; rhyolitic lithic inclusions (including welded tuffs) are especially abundant. Boyd (1961, p. 393) called this basal part the 

Purple Mountain Pumice Breccia, but we regard it 

only as a unit of the lower member of the Lava 

Creek Tuff and do not believe that it warrants a 

separate formal name. This basal unit is overlain 

sharply by devitrified densely welded tuff with 

especially abundant large phenocrysts. This densely 

welded tuff, about 180 m thick here, is typical of 

the member in most localities and generally weathers 

to crumbly dark-brown crudely columnar outcrops 

or to steep grussy slopes. The lower member of the 

Lava Creek Tuff is the only part of the Yellowstone 

Group in which hornblende is more abundant than 

pyroxene. 

Boyd (1961, p. 393) correlated outcrops of partially welded tuff containing abundant lithic inclusions in the Stonetop Mountain area, north of 

Yellowstone Lake, with the Purple Mountain Pumice 

Breccia. Our mapping, however, shows that these 

outcrops are in the upper, not the lower, member of 

the Lava Creek. 

The principal reference section of the upper member is on the east wall of Sheepeater Canyon on the 

Gardner River 1 km northeast of Osprey Falls (fig. 

2). This section was described by Boyd (1961, p. 

395) as the only complete section of the Yellowstone 

Tuff, but it is not a stratigraphically complete section of either the Yellowstone Group or the Lava 

Creek Tuff. It is a well-exposed section 140 m thick 

with exposures of both the basal and the upper contacts of the upper member of the Lava Creek. The 

Lava Creek overlies the Quaternary Undine Falls 

Basalt, which in turn lies on lower Tertiary intrusive dacite and Cretaceous sedimentary rocks. The 

member is overlain by the Swan Lake Flat Basalt, 

which was erupted soon enough after emplacement 

of the Lava Creek to preserve the unconsolidated 

ashy and pumiceous top of the member. The basal 

few meters of air-fall pumice and ash is overlain 

by about 2 m of phenocryst-rich glassy welded tuff 

-nonwelded and gray tuff at the base but densely 

welded black vitrophyre at the top. Above this 

vitrophyre is about 4% m of dark-gray microspherulitic densely welded tuff alternating with 

vitrophyre. Farther above is devitrified generally 

pinkish brown densely welded tuff with abundant 

large phenocrysts (a few of them nearly 1 em) that 

increase slightly in abundance upward, conspicuous 

large white collapsed crystallized pumice lenticles, 

and abundant reddish-brown rhyolitic lithic inclusions. The devitrified zone is almost 90 m thick. It 

is overlain by an orange-pink vapor-phase zone about 

40 m thick. The vapor-phase zone is overlain in turn 

by 3% m of unconsolidated nearly white glassy ashflow tuff of the original top of the cooling unit. This 

is one of the few known exposures of any of the 

three ash-flow sheets of the Yellowstone Group in 

which this frothy glassy top, presumed to have been 

present initially over the entire area of each ashflow sheet, is preserved. Locally, where the overlying basalt does not have a rubbly basal breccia, 

the top of the frothy ash and pumice is reddened 

and even fused to a depth of a few centimeters. 

Where the upper member overlies the lithologically very similar lower member, several meters of 

relatively phenocryst-poor welded tuff occur in the 

base of the upper member. This phenocryst-poor 

zone grades downward from densely welded into 

partially welded at the contact between the two 

units, and locally a sorted and bedded crystal ash 

several centimeters thick forms a parting between 

the two. A section containing this contact is ex-
VOLCANIC STRATIGRAPHY OF THE QUATERNARY RHYOLITE PLATEAU B9 

posed in roadcuts just downstream from Virginia 

Cascade on the loop road south of the main highway between Norris Junction and Canyon Village, 

although constant spalling of the cuts generally 

covers the contact itself. 

The Yellowstone caldera formed by collapse of 

the magma-chamber roof as a result of eruption of 

the Lava Creek Tuff. The two members were 

erupted in immediate succession from adjacent but 

distinct source areas, each of which formed a discrete caldera segment with its own central resurgent dome. 

Sanidine from the Lava Creek Tuff has been 

dated by the K-Ar method by J. D. Obradovich 

(written commun., 1970) as 600,000 years old. 

Richmond (1970b, p. 8, 21) noted that ash beds probably derived from the Lava Creek eruptions are 

slightly younger than deposits of the Cedar Ridge 

Glaciation (Rocky Mountains) and of the Kansan 

Glaciation (Great Plains). 

ROCKS OF THE FIRST VOLCANIC CYCLE 

The Huckleberry Ridge Tuff and the rhyolitic and 

basaltic lavas associated temporally and spatially 

with it are discussed here as the first volcanic cycle 

of the rhyolite plateau. The record of this cycle is 

quite incomplete because much of the source area 

of the Huckleberry Ridge and associated rocks was 

disrupted by later deformation and is now covered by 

younger rocks. Little else than marginal parts of 

the first-cycle volcanic fields can be observed within 

Yellowstone National Park. However, parts of a 

large caldera related to eruption of the Huckleberry 

Ridge Tuff are preserved in the southern Island 

Park area and between Grassy Lake Reservoir and 

the Red Mountains (figs. 1, 2). 

JUNCTION BUTTE BASALT 

The basalts of the Yellowstone rhyolite plateau, 

although not nearly as voluminous as the rhyolites, 

occur widely around the plateau margins. None of 

them, however, crops out within the source areas 

of the major ash-flow tuffs. Thus it is somewhat 

difficult to assign individual basaltic units confidently to the different volcanic cycles in a genetic 

sense. One can reasonably say that basaltic volcanism was intermittently active around the margins of the plateau before, during, and after each 

of the major rhyolitic cycles. Nevertheless, it is possible to show that certain basaltic units are time 

and stratigraphic relatives of the major rhyolitic 

units, and it is in this sense that the Junction Butte 

Basalt is discussed with the first volcanic cycle. 

The name Junction Butte Basalt is given here to 

basalts in northern Yellowstone National Park that 

conformably underlie the Huckleberry Ridge Tuff. 

The name comes from the prominent butte between 

the Yellowstone and Lamar Rivers, but the type 

locality is designated as the west side of the Grand 

Canyon of the Yellowstone opposite the mouth of 

Deep Creek (fig. 2). Outcrops of Junction Butte 

Basalt occur in the valley of the Yellowstone River 

and on adjacent uplands from near Deep Creek to 

Blacktail Deer Creek and Mount Everts. The basalts 

unconformably overlie the Absaroka Volcanic Supergroup and Paleozoic and Precambrian rocks. The top 

of the Junction Butte commonly is an erosionally 

stripped surface, but locally the Huckleberry Ridge 

Tuff, the Mount Jackson Rhyolite, or the Lava 

Creek Tuff overlies it. 

At its type locality the Junction Butte lies in a 

paleovalley eroded in Eocene volcanic rocks of the 

Absaroka Supergroup and is overlain unconformably 

by the Wapiti Lake flow of the Mount Jackson 

Rhyolite and by the Lava Creek Tuff. The basalt 

section at the type locality is about 90 m thick and 

consists of three or four thick flows. A few kilometers downstream, below the mouth of Tower 

Creek, only a single flow of the Junction Butte Basalt 

is exposed. This flow, the Overhanging Cliff flow, is 

about 30 m thick where it forms the spectacular 

roadside cliff just north of Tower Creek. In this 

cliff and in other good exposures along the canyon 

rims downstream, the flow is characterized by welldeveloped two-tiered columnar jointing. The lower 

colonnade is commonly about 2%-3 m thick, and 

small blocks from the upper zone, or entablature, 

of irregularly intersecting prismatic joints generally form conspicuous black talus cones beneath cliff 

exposures. The basalt is black and aphanitic in hand 

specimen and contains only sparse plagioclase 

phenocrysts. In the valley of Tower Creek the Overhanging Cliff flow and a lithologically similar overlying flow lie topographically below the Huckleberry 

Ridge Tuff. Although the contact is not well exposed, parallelism of the respective outcrop bands 

along 2 km of a narrow canyon suggests that the 

basalts are concordant beneath the ash-flow tuff. 

A few meters of gravel and associated sand and 

silt commonly lies at the base of the Junction Butte 

Basalt. Inasmuch as earlier workers had difficulty in 

determining age relations between certain outcrops 

of the Overhanging Cliff flow and the Yellowstone 

Tuff, we carefully searched the underlying gravels 

for clasts from ash-flow tuffs of the Yellowstone 

Group. We have failed to find any, although a few 
B10 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

cobbles of welded tuff from the underlying Eocene 

Absaroka Volcanic Supergroup do occur. Howard 

(1937, p. 36-80) similarly noted the presence of less 

than 1 percent rhyolites in the same gravels. These 

gravels contrast with the younger sediments of The 

Narrows which contain moderately abundant (commonly 10-15 percent) welded tuff cobbles of the 

Yellowstone Group- although, for reasons not clear 

to us, Brown (1961, p. 1182-1183) stated that he 

could find no rhyolite clasts in either of the two 

gravels we distinguish in the Tower Creek area. 

These observations support the conclusion that the 

Overhanging Cliff flow is older than the Huckleberry Ridge Tuff. 

Basalt which we regard as Junction Butte crops 

out conformably beneath the Huckleberry Ridge and 

above Cretaceous shales and sandstones on the 

southeast side of Mount Everts, on the steep slopes 

extending about 3 km west from just northwest of 

Blacktail Pond. The basalt ranges in thickness from 

a few meters to about 100 m. Two flows are present 

in the sequence, and the outcrops closest to Blacktail Pond form a buried mound of scoria and cinders, probably representing the source vent for the 

flows. 

The Junction Butte Basalt has not been dated 

isotopically, but it probably is only slightly older 

than the 2-m.y.-old Huckleberry Ridge. Flows in 

the Tower Creek-Grand Canyon area all have stable 

reverse remanent magnetic polarities and, therefore, probably are less than about 2.4 m.y. old 

(Cox, 1969, fig. 4). The two flows on Mount Everts 

are normally polarized and may be older or younger, 

but preservation of a cinder cone conformably beneath the Huckleberry Ridge suggests that the basalt is barely older than the tuff. 

RHYOLITE OF BROAD CREEK 

The rhyolite of Broad Creek underlies the Huckleberry Ridge Tuff and crops out only in the area 

east of the Grand Canyon of the Yellowstone on 

the east side of Broad Creek between about 112 and 

2112 km south of the Wapiti Lake Trail (fig. 2). A 

single flow about 60 m thick forms the unit as exposed. The flow forms the base of the exposure at 

this locality. The rhyolite contains about 30 percent 

quartz, sanidine, and plagioclase phenocrysts, and 

it has the steeply dipping flow layering typical of 

the upper portions of rhyolitic lava flows. 

This early rhyolite of the first cycle has not been 

dated isotopically but appears to be conformable 

beneath the overlying welded tuff. We presume that 

the flow represents some of the earliest rhyolitic 

activity of the Yellowstone rhyolite plateau but that 

it is not much older than the 2-m.y.-old Huckleberry Ridge. 

LEWIS CANYON RHYOLITE 

The Lewis Canyon Rhyolite is named here for the 

excellent exposures in Lewis Canyon between Lewis 

Lake and the Snake River. The formation consists 

of several bulbous, nonstratiform rhyolitic lava 

flows, but we have recognized no more than one in 

any single section. The north side of Glade Creek, 

between the Snake River and the east end of Grassy 

Lake Reservoir, is designated the type area (fig. 1). 

In this area, the Lewis Canyon Rhyolite overlies the 

Huckleberry Ridge Tuff and Cretaceous sedimentary rocks and is overlain by the Lava Creek Tuff. 

The contacts in this area are not well exposed, but 

the relations are clear from mapping. The rhyolite 

is well exposed on the north side of Glade Creek 

along the meadows about 3 km southeast of Grassy 

Lake Reservoir. The flow is more than 200 m thick 

in the type area. 

Exposures of the formation are excellent in Lewis 

Canyon, but the base is not exposed there. As in 

the Glade Creek area, the Lewis Canyon is overlain 

by the Lava Creek Tuff. The base of the Lewis 

Canyon Rhyolite is again poorly exposed above 

Paleozoic and Mesozoic sedimentary rocks about 7 

km east of Lewis Canyon, at the east end of nearly 

continuous exposures along the north side of the 

Snake River between the Lewis River and Red 

Creek. 

The rhyolite has 30-40 percent conspicuous 

phenocrysts, including abundant much-embayed 

sodic oligoclase, generally about 5 millimeters long, 

and less abundant, slightly smaller phenocrysts of 

quartz, sanidine, clinopyroxene, and opaque oxides. 

Despite the abundant plagioclase and relatively 

sparse sanidine, chemical analyses show the rocks 

to be rhyolites. 

Boyd (1961, p. 392) regarded the Lewis Canyon 

Rhyolite as part of his Jackson flows, but we separate the Lewis Canyon from the Mount Jackson 

Rhyolite (as we call the remainder of his Jackson 

flows) on the basis of their different lithology, particularly the phenocryst assemblages, and on what 

appears to us to be a significantly different relation 

to the Huckleberry Ridge and Lava Creek Tuffs, 

despite the occurrence of both rhyolites between the 

two. The Mount Jackson Rhyolite is conformable 

beneath the Lava Creek and was erupted from 

vents around what was to become the third-cycle, 

Yellowstone caldera. By contrast, the Lewis Canyon 
VOLCANIC STRATIGRAPHY OF THE QUATERNARY RHYOLITE PLATEAU Bll 

Rhyolite is overlain on an erosional unconformity 

by the Lava Creek and appears to be closer in age 

to the first-cycle Huckleberry Ridge Tuff, the Lewis 

Canyon filling a segment of the first-cycle caldera. 

The Lewis Canyon is petrographically similar to 

early postcaldera rhyolite lavas (the Upper Basin 

Member of the Plateau Rhyolite) similarly situated 

in the Yellowstone caldera. This suggests parallel 

roles in evolution of the first and third volcanic 

cycles for these similar rhyolites. 

The age of the Lewis Canyon Rhyolite has not 

yet been determined isotopically. 

SEDIMENTS AND BASALTS OF THE NARROWS 

Gravels and basalts near The Narrows of the 

Grand Canyon of the Yellowstone, below Tower 

Creek (fig. 2), were included by Pierce, Christiansen, and Richmond (1970) in the Osprey Formation. K. L. Pierce and we have found in subsequent 

work that the sediments and basalts in the vicinity 

of The Narrows formerly included in the Osprey 

postdate the Huckleberry Ridge Tuff but predate 

the Lava Creek Tuff. The correlation with the 

Osprey, therefore, was in error. We will use informal terminology for this unit until the pertinent 

geologic relations can be discussed more fully in a 

later paper. For the present we note only that four 

basaltic lava flows, most of them very thick and 

with conspicuous two-tiered columnar jointing, are 

interlayered with the sediments of The Narrows. 

The four flows all contain plagioclase phenocrysts, 

and the upper flow is particularly coarse grained. 

The sediments and basalts of The Narrows lie in 

a paleovalley cut through the Huckleberry Ridge 

Tuff and the underlying Junction Butte Basalt, and 

the gravels contain abundant clasts of the Huckleberry Ridge. The Lava Creek Tuff has not been 

found directly overlyip.g these sediments and basalt 

but is judged to be younger on the basis of the 

reverse paleomagnetic polarities of all four basalt 

flows and on the basis of a K-Ar age of 1.6 m.y. 

for sanidine from an ash bed interlayered in the 

unit. 

ROCKS OF THE SECOND VOLCANIC CYCLE 

Because rocks related to the second eruptive 

cycle of the Yellowstone region are not exposed 

within the national park, we do not introduce formally any of these formations in this paper other 

than the Mesa Falls Tuff, already defined. However, 

in order to emphasize the cyclic nature of Quaternary volcanic activity in the region, we make brief 

reference to them here. The Mesa Falls Tuff was 

erupted from the area of the Island Park caldera, 

and late collapse of that area was related to that 

eruption. Basaltic and rhyolitic lavas were closely 

associated in space and time with this volcanism. 

As in the first and third cycles, some associated 

lavas predate and others postdate the major ashflow tuff. 

ROCKS OF THE THIRD VOLCANIC CYCLE 

Most of the Quaternary volcanic rocks exposed 

in Yellowstone National Park are related to the 

third eruptive cycle of the rhyolite plateau. Eruption and emplacement of these rocks were major 

geologic events of the middle and late Pleistocene 

history of the Rocky Mountain region. The key 

event in this cycle was eruption of the Lava Creek 

Tuff 600,000 years ago and collapse of the roof of 

its source magma chamber to form the gigantic 

compound Yellowstone caldera. The stratigraphic 

succession and petrologic evolution of this volcanic 

cycle must ultimately serve as a model for interpreting the more fragmentary record of the first 

and second cycles. 

MOUNT JACKSON RHYOLITE 

The Mount Jackson Rhyolite is defined here to 

include rhyolitic lava flows essentially conformable 

beneath the Lava Creek Tuff of the Yellowstone 

Group in areas near the wall of the Yellowstone 

caldera. The formation takes its name from the 

summit on the cliffs forming the north wall of the 

Madison Canyon 51f2 km west of Madison Junction 

(fig. 2). The Madison Canyon near Mount Jackson 

is the type area of the formation. The base of the 

Mount Jackson Rhyolite is not exposed in the type 

area, but the formation is overlain by the Lava 

Creek Tuff along the steeply sloping original surfaces of the flows. Little erosion occurred along 

this contact before emplacement of the Lava Creek, 

for the chilled glassy tops of the flows are everywhere preserved at the contact. Two rhyolite flows 

are present in the Madison Canyon area; the section 

there is as much as 450 m thick. The upper flow 

forms the summits and cliffs of Mounts Haynes and 

Jackson and is separated from the underlying flow 

by a thin fused tuff exposed near Harlequin Lake. · 

Rhyolite flows situated similarly with respect to 

the Lava Creek Tuff are exposed in the vicinity of 

Big Bear Lake, south of the Madison Canyon; near 

Wapiti Lake, west of the Mirror Plateau; on Flat 

Mountain, southwest of Yellowstone Lake; and in 

one small outcrop on Stonetop Mountain, north of 

Yellowstone Lake. Each of these flows is correlated 
B12 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

with the Mount Jackson Rhyolite, and Boyd (1961, 

p. 392-393) included most of these localities in his 

Jackson flows. 

The Mount Jackson Rhyolite flows are quite 

varied in general appearance, as is typical of rhyolitic lavas, but all contain 30-50 percent large 

phenocrysts of quartz, sanidine, plagioclase, and 

minor opaque oxides and clinopyroxene. These flows 

lithologically resemble younger rhyolitic lava flows 

of the third volcanic cycle and are distinguished 

mainly by their stratigraphic and structural positions. 

J. D. Obradovich (written commun., 1970) has 

dated sanidine from the two flows in the Madison 

Canyon area as about 790,000 and 640,000 years old. 

This accords with the magnetic polarity time scale 

(Cox, 1969, fig. 4) ; remanent polarities of the two 

flows are, respectively, reverse and normal. 

UNDINE FALLS BASALT 

Basalts underlie the Lava Creek Tuff conformably 

in two areas. These basalts are named here the 

Undine Falls Basalt for the waterfall on Lava 

Creek near the Mammoth-Tower Junction Road just 

south of Mount Everts. The cliff on the north side 

of Lava Creek canyon just west of the falls is designated the type locality (fig. 2). The section there is 

about 6 m thick and consists of several thin basaltic 

flows overlying Cretaceous shales and sandstones 

and underlying the Lava Creek Tuff. Similar basalts, 

regarded as the same sequence of flows, occur as far 

northeast as Oxbow Creek and as far southwest as 

Obsidian Creek south of Winter Creek. 

Probably correlative basalts occur north of Broad 

Creek, east of the Grand Canyon of the Yellowstone. 

Basalt, judged from mapping to be a single flow, 

occurs in that area between a flow of the Mount 

Jackson Rhyolite and the Lava Creek Tuff. The 

source vent is marked by a small scoria and cinder 

cone 3 km north of the Broad Creek crossing of the 

Wapiti Lake Trail. We believe this basalt north of 

Broad Creek to be at least approximately correlative 

with the type Undine Falls, inasmuch as both are 

conformable beneath the Lava Creek Tuff and have 

normal remanent magnetic polarities. We thus presume them both to be younger than 700,000 years. 

PLATEAU RHYOLITE 

Boyd (1961, p. 403-409) called the rhyolitic lavas 

younger than his Yellowstone Tuff, other than a few 

small rhyolite domes and mixed-lava complexes, the 

Plateau flows. He did not define them formally, but 

he clearly regarded them as a formation and named 

them as a composite of the flows of the Madison, 

Pitchstone, Central, and Solfatara Plateaus and nearby areas. We here name and define the Plateau 

Rhyolite in an essentially parallel way to include the 

rhyolitic lava flows of the Yellowstone rhyolite plateau that are younger than the Lava Creek Tuff. 

Inasmuch as we designate six new members of the 

formation, each with its own type area, the Plateau 

Rhyolite is defined here as the rock units that constitute the six named members. 

Mallard Lake Member 

The Mallard Lake Member of the Plateau Rhyolite is defined here to include rhyolite erupted within 

the Yellowstone caldera after it formed but before 

the rise within it of its two resurgent structural 

domes. The member consists of a single rhyolitic 

lava flow at least 120 m thick. It is named for a 

small lake near the center of its outcrop area, about 

5 km northeast of Old Faithful (fig. 2). The area 

around Mallard Lake is designated the type area. 

The base of the member is nowhere seen in outcrop, but a drill hole near Rabbit Creek, just beyond 

the edge of the flow at the northwest end of the 

Mallard Lake resurgent dome, penetrated the underlying Lava Creek Tuff at a depth of only 10 m 

(White and others, 1969). The Mallard Lake flow is 

broken by northwest-trending normal faults that 

form a complex graben along the axis of the dome. 

The Mallard Lake Member and the graben system 

are overlain by the Upper Basin and Central Plateau 

Members of the Plateau Rhyolite. No complete sections of the Mallard Lake Member are exposed, and 

most of the original glassy shell of the flow has 

been eroded away. Most exposures are crystallized 

rhyolite with 20-30 percent phenocrysts of quartz 

and sanidine. Most mafic phenocrysts are altered to 

iron oxides. Flow layering is well displayed, and the 

rocks commonly are thinly flow-laminated. 

The Mallard Lake Member is younger than the 

600,000-year-old Lava Creek Tuff and is overlain by 

a flow about 530,000 years old (J. D. Obradovich, 

written commun., 1970). Structural reasoning suggests that the Mallard Lake is only very slightly 

younger than the Lava Creek. 

Upper Basin Member 

The Upper Basin Member, named here, is defined 

to include early postcaldera rhyolitic lava flows and 

associated pyroclastic deposits that were erupted 

within the Yellowstone caldera after resurgent 

doming. The Upper Basin Member is characterized 

by abundant plagioclase phenocrysts, commonly 

oligoclase that is highly embayed. 
VOLCANIC STRATIGRAPHY OF THE QUATERNARY RHYOLITE PLATEAU B13 

The type area of the Upper Basin Member is in 

the Upper Geyser Basin (fig. 2), where outcrops of 

the member are scattered widely among the sediments and hydrothermal deposits that floor the 

basin. Four drill holes in the Upper Geyser Basin 

all bottomed in rhyolite of the Upper Basin Member. 

These outcrops and drill cores from the Upper Geyser Basin, as well as many outcrops north along the 

Firehole River nearly to Tangled Creek in the Lower 

Geyser Basin, are interpreted to belong to a single 

flow. The steep front of a younger flow of the member forms the south end of the Upper Geyser Basin ; 

the same flow forms the topographic surface around 

Scaup Lake. Both flows in the type area of the member overlie the Mallard Lake Member and underlie 

the Central Plateau Member of the Plateau Rhyolite. 

Other than in the geyser basins of the Firehole 

River, rhyolites of the Upper Basin Member are exposed only in the upper Grand Canyon of the Y ellowstone and in the area southeast from it to Fern 

Lake. Boyd (1961, p. 405-406) discussed the Canyon 

flow, named for the Grand Canyon of the Yellowstone, and relations of that flow along the rim to 

pyroclastic rocks deeper in the canyon. The flow 

and tuffs seem to be gradational from one to the 

other, which led Boyd to suggest tentatively that the 

Canyon flow is an example of a froth flow, an eruptive mechanism proposed by Kennedy (1955, p. 495) 

to explain some features of welded tuffs. We disagree with this explanation, but discussion of the 

mechanisms of eruption is deferred to a later 

paper. In our interpretation, the rhyolite section in 

the upper Grand Canyon area consists of a basal, 

largely agglutinated air-fall tuff overlain successively by the Canyon flow proper and by a younger flow 

exposed near the Dunraven Pass Road. 

The older Mallard Lake Member and the younger 

intracaldera members of the Plateau Rhyolite generally have little or no phenocrystic plagioclase. The 

Upper Basin Member, by contrast, has considerably 

more plagioclase than sanidine phenocrysts, and in 

some flows the feldspar phenocrysts are exclusively 

plagioclase. Clinopyroxene too is considerably more 

abundant in the Upper Basin than in the other 

members. 

Potassium-argon dating, by J. D. Obradovich 

(written commun., 1970), of sanidine, plagioclase, 

and nonhydrated glass from rhyolites of the Upper 

Basin Member shows that most of the flows are 

only slightly younger than the Lava Creek Tuff, 

ranging from about 600,000 to 530,000 years old. 

One flow, however, which we correlate on the basis 

of its phenocryst mineralogy with the Upper Basin 

Member, is about 260,000 years old. 

Obsidian Creek Member 

The Obsidian Creek and Roaring Mountain Mem"" 

hers of the Plateau Rhyolite each comprise a series 

of isolated rhyolitic lava flows and domes that lies 

wholly outside the Yellowstone caldera and is, therefore, particularly difficult to define in traditional 

stratigraphic terms. In a sense they are extracaldera parallels of the intracaldera Upper Basin and 

Central Plateau Members. With this parallel in 

mind, we here define the Obsidian Creek Member 

as early post-Lava Creek rhyolitic domes and flows 

outside the Yellowstone caldera in which plagioclase is a common phenocrystic constituent. In 

terms both of age and of the petrography of their 

rhyolitic components, two lava flows of mixed rhyolite and basalt can reasonably be included in the 

member. The following bodies constitute the member (and thus, by definition, its type area): The 

mixed lavas of Gardner River (Fenner, 1938, 1944; 

Wilcox, 1944; Hawkes, 1945) and Grizzly Lake 

(Boyd, 1961, p. 403) ; and the Willow Park, Apollinaris Spring, Landmark, Gibbon Hill, Geyser 

Creek, and Paintpot Hill eruptive domes (fig. 2). 

The mixed lavas and the domes occur in a northtrending belt between the Norris Geyser Basin area 

and the Gardner River. Obsidian Creek, for which 

they are named, parallels much of this belt. 

The mixed lava of Gardner River lies partly between flows of the Swan Lake Flat Basalt and partly on the Lava Creek Tuff. All the other bodies can 

be observed, or can be inferred with considerable 

confidence from map relations, to lie on the Lava 

Creek. The mixed lava of Grizzly Lake forms the 

top of its local stratigraphic section, but the 

Gardner River, Willow Park, and Apollinaris Spring 

bodies are overlapped by flows of the Swan Lake 

Flat Basalt, and the Landmark, Gibbon Hill, Geyser 

Creek, and Paintpot Hill bodies are overlain by 

younger flows of the Plateau Rhyolite. 

No radiometric age determinations have been 

made on the Obsidian Creek Member. Stratigraphic 

relations just cited, however, suggest that at least 

most of the member is only slightly younger than 

the Lava Creek Tuff. 

Central Plateau Member 

The Central Plateau Member of the Plateau Rhyolite is defined here to include late postcaldera rhyolitic lava flows, some of them very large, erupted 

from vents within the Yellowstone caldera. Eighteen 

separate flows and domes have been mapped as 

parts of the member, but seldom can one find more 

than three flows superposed in a single stratigraphic 
B14 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

succession. The type area is designated as the Central Plateau (fig. 2), located in the middle of the 

Yellowstone caldera between its two separately 

identified collapse segments. The flows of the member form the Madison, Pitchstone, Central, and 

Solfatara Plateaus and a few lower areas nearby. 

There may be other flows in the sequence that are 

entirely buried. The member constitutes by far the 

greatest part of the total volume of the Plateau 

Rhyolite. 

The Central Plateau Member overlies the Obsidian Creek, Upper Basin, and Mallard Lake Members of the Plateau Rhyolite; the Lava Creek Tuff; 

and a few older units. Only surficial sediments of 

late Pleistocene age overlie the Central Plateau 

Member. Glacial and nonglacial sedimentary deposits have been identified locally within the member, as is noted in the final section of this report. 

Some individual flows of the member are 300 m 

or more in thickness, and a few extend more than 

20 km from their source vents. The flows have 

abundant phenocrysts, generally 30-50 percent. 

Most of them have mainly quartz and sanidine with 

no plagioclase; a few have minor plagioclase. All of 

them contain in their glassy portions minor clinopyroxene, opaque oxides, and fayalitic olivine. 

Potassium-argon age determinations on sanidine 

from the Central Plateau Member range from about 

200,000 to about 70,000 years (J. D. Obradovich, 

written commun., 1970). 

Shoshone Lake Tuff Member 

A single rhyolitic ash-flow tuff crops out within 

the Plateau Rhyolite sequence. We name it here 

the Shoshone Lake Tuff Member for exposures on 

the steep slopes west of Shoshone Lake. The type 

section is in the gully north of Fall Creek (fig. 2), 

west of the Shoshone Lake Geyser Basin, where 

the member is about 180 m thick and lies between 

two flows of the Central Plateau Member. 

The member is widespread between West Thumb 

and Shoshone Lake. Elsewhere, the tuff is largely 

covered by younger rhyolite flows, but small exposures occur between various flows of the Central 

Plateau Member in the Bechler River area, near 

Little Firehole Meadows, and in the upper drainage 

of Nez Perce Creek. In the area of Shoshone Lake 

the ash-flow tuff has an irregular distribution which 

is hard to explain by erosion but which suggests the 

possibility that it was emplaced on a lobe of glacial 

ice that occupied the lake basin. The distribution 

and the pattern of welding and crystallization zones 

of the member indicate that its source area was in 

the vicinity of West Thumb, which may, in fact, be 

a caldera lake formed by collapse after eruption of 

the tuff. 

Most of the type section consists of glassy nonwelded to partially welded tuff, but local vaporphase and devitrified zones a few meters thick occur 

in the upper part. The degree of welding varies vertically between more and less densely welded tuff. 

By contrast, the Shoshone Lake Tuff Member near 

West Thumb, the deep western basin of Yellowstone Lake, commonly is more densely welded and 

contains a much thicker vapor-phase zone in proportion to glassy welded tuff. The basal part of the 

member is well exposed in the roadcut above Bluff 

Point, along the northwest shore of West Thumb. 

The basal glassy zone there is less than 5 m thick 

and rests against a flow of the Central Plateau 

Member. A specimen from the vapor-phase zone at 

the Bluff Point locality was illustrated by Ross and 

Smith (1961, fig. 11, p. 27). The upper part of the 

section at Bluff Point probably is repeated by faulting, but exposures are poor and do not allow determination of the stratigraphic thickness. The tuff 

appears to be overlain by a higher flow of the Central Plateau Member. About 21f2 km southwest of 

Bluff Point, in a gravel pit northwest of Potts Hot 

Springs, the Central Plateau Member overlies 

gravels containing boulders and cobbles of the 

Shoshone Lake Tuff Member. 

The Shoshone Lake Tuff Member contains 20-30 

percent phenocrysts of quartz, sanidine, subordinate 

plagioclase, and minor clinopyroxene and opaque 

oxides. It contains more abundant pumice than 

tuffs of the Yellowstone Group and generally has a 

coarser matrix. Inclusions of older rhyolite, mainly 

black vitrophyre, are abundant throughout the 

member. 

The age of the Shoshone Lake Tuff Member is 

bracketed by the ages of two flows of the Central 

Plateau Member, sanidine from which has been 

dated as about 200,000 and 150,000 years old respectively; sanidine from a bedded tuff in the Potts 

Hot Springs gravel pit, probably a part of the Shoshone Lake, is 180,000 years old (J. D. Obradovich, 

written commun., 1970). 

Roaring Mountain Member 

We name the Roaring Mountain Member here for 

four young phenocryst-free or phenocryst-poor 

extracaldera rhyolitic lava flows that occur as isolated bodies north of the Yellowstone caldera. The 

member comprises the Crystal Spring, Obsidian 

Cliff, Cougar Creek, and Riverside flows (which, by 

definition, constitute its type area, fig. 2) ; Roaring 
VOLCANIC STRATIGRAPHY OF THE QUATERNARY RHYOLITE PLATEAU B15 

Mountain is in the vicinity of the Crystal Spring 

and Obsidian Cliff flows, which occur in the same 

belt as the Obsidian Creek Member. The Cougar 

Creek and Riverside flows are in the area west and 

northwest of Madison Canyon. The four flows overlie the Lava Creek Tuff, the Swan Lake Flat Basalt, 

and parts of the Obsidian Creek Member of the 

Plateau Rhyolite. The Cougar Creek and Riverside 

flows are overlain by flows and cut by dikes of the 

Madison River Basalt. The other flows are overlain 

only by surficial deposits of late Pleistocene and 

Holocene age. 

All the flows are phenocryst-free or very phenocryst-poor rhyolites which contain abundant fresh 

black obsidian as well as crystallized and partly 

crystallized material. Phenocrysts, where present, 

are quartz, sanidine, plagioclase, and opaque oxides. 

Obsidian from three of the four flows has been 

dated by the K-Ar method by J. D. Obradovich 

(written commun., 1970) ; the dates seem to indicate a considerable age span for the member. The 

Crystal Spring and Obsidian Cliff flows yielded 

young apparent ages (about 160,000 and 75,000 

years respectively) ; the Cougar Creek flow gave an 

age of 400,000 years. However, the reliability of 

these obsidian dates is uncertain. 

POST-LAVA CREEK BASALTS 

Basalts younger than the Lava ,Creek Tuff occur 

around the margins of the rhyolite plateau surrounding the caldera. Because different relations are 

displayed in several widely separated areas, these 

basalts have been grouped into five named formations. 

Swan Lake Flat Basalt 

Basalts younger than the Lava Creek Tuff but 

older than an unconformity that represents the first 

major period of canyon cutting into the Lava Creek 

occur widely in the area north of Obsidian Cliff, 

nearly to Mount Everts. We here name these lavas 

the Swan Lake Flat Basalt after a valley partly 

surrounded and largely underlain by them about 8 

km southwest of Mammoth. The type area (fig. 2) 

is designated as the Sheepeater Cliffs, on the east 

side of the canyon of the Gardner River southeast 

of Bunsen Peak. Flows of the lower part of the 

sequence are well exposed there and are conformable on the Lava Creek Tuff, whose easily eroded 

nonwelded glassy cap is locally preserved. Flows of 

the Plateau Rhyolite locally overlie the Swan Lake 

Flat Basalt. No single section, however, exposes 

well both the base and the top of the formation. 

East of the Sheepeater Cliffs, basalts form a 

rounded hill that probably marks one of the source 

vents for the flows; the formation totals nearly 200 

m in thickness in that area. Other cinder cones that 

probably mark vents for the Swan Lake Flat Basalt 

occur near Horseshoe Hill, northeast of Obsidian 

Cliff. 

Boyd (1961, p. 402) observed that basalts of two 

ages occur in the Bunsen Peak area, one conformable on the Yellowstone Tuff (our Lava Creek Tuff) 

and the other filling a younger canyon. We recognize 

the former as the Swan Lake Flat Basalt and the 

latter as the Osprey Basalt. 

The Swan Lake Flat Basalt generally is light 

gray to moderate gray and contains sparse to abundant large phenocrysts of plagioclase, as much as 

1 em across, and locally some olivine phenocrysts. 

Outcrops of a lithologically distinctive basalt in the 

area near Geode Creek (Howard, 1937, p. 19-21) 

probably represent a mixed lava dominated by basalt and have therefore been mapped separately. A 

faint streakiness, a few xenocrysts,- and considerable textural heterogeneity show the mixing in 

hand specimen. Extremely large plagioclase phenocrysts, as much as 5 em across, occur rarely in the 

rock. At least one flow near Geode Creek appears to 

have been fed through a vent now preserved as a 

low scoria mound above an exposed dike about midway between the main Mammoth-Tower Junction 

Road and the unpaved Crescent Hill Road, 1lj4 km 

east of Geode Creek. 

The age of the Swan Lake Flat Basalt is suggested by its stratigraphic relations. The Swan Lake 

Flat is conformable on the middle Pleistocene Lava 

Creek Tuff, which probably is somewhat younger 

than the Cedar Ridge (Kansan) Glaciation of the 

Rocky Mountains (Richmond, 1970b, p. 8, 21). The 

Swan Lake Flat is older than canyon cutting that 

predated the Osprey Basalt. Post-Lava Creek canyon cutting in the drainage of the Yellowstone 

River predated a till of probable Sacagawea Ridge 

(Illinoian) age in the Grand Canyon area (Richmond, 1970a; 1970b, p. 21). Thus the episode of 

canyon cutting between the Swan Lake Flat and 

Osprey Basalts probably was contemporaneous with 

the Yarmouth Interglaciation of the Great Plains. 

Richmond (1957) noted that the episode of deepest 

canyon cutting during the Pleistocene occurred in 

many parts of the Rocky Mountains during this 

same interglaciation. 

Falls River Basalt 

The Falls River Basalt is named here to include 

basalts younger than the Lava Creek Tuff and older 

than the Central Plateau Member of the Plateau 
B16 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

Rhyolite in the region of the southwest corner of 

Yellowstone National Park. The formation takes its 

name from the major stream draining that area. 

The type locality is the west rim of the valley of 

Falls River just below Cave Falls (fig. 2), where 

the formation overlies the Lava Creek and is overlain by upper Pleistocene glacial deposits. In the 

drainage area of the Bechler River about 8 or 9 km 

north of its confluence with Falls River, just above 

Cave Falls, the Falls River Basalt is overlain by 

several flows of the Central Plateau Member. 

The Falls River Basalt is very similar in lithology 

to the Swan Lake Flat Basalt, commonly containing 

sparse phenocrysts of plagioclase about %-1 em 

across. The basalts generally are moderate gray and 

form dense thin flows with vesicular tops. Just west 

of the southwest corner of the national park these 

flows make up a low shield, Rising Butte (fig. 2), 

which probably was the source of at least some of 

the flows in the park. 

The age of the Falls River Basalt is bracketed by 

the ages of the 600,000-year-old Lava Creek Tuff 

and the overlying Plateau Rhyolite flows, probably 

about 100,000 years old. Apparent conformity of 

the Falls River and the Lava Creek suggests that 

the basalt is closer in age to the underlying tuff 

than to the overlying lavas. 

Basalt of Mariposa Lake 

A few scattered outcrops of basalt southeast of 

the rhyolite plateau are discussed in this section 

even though their stratigraphic position relative to 

the plateau sequence is unknown. These outcrops of 

basalt all occur on Two Ocean Plateau and are 

underlain by Eocene rocks of the Absaroka Volcanic 

Supergroup. These basalts have been mapped by our 

colleagues H. W. Smedes and H. J. Prostka, and the 

following discussion is based mainly on their observations. The basalts are as thick as 50 m and lie on 

an erosional surface of considerable relief. They are 

overlain by upper Pleistocene glacial deposits. Their 

age is unknown, but their lithologic and general petrographic characteristics are quite unlike those of 

the Absaroka volcanic rocks and much like those of 

upper Cenozoic basalts of the region. They are very 

similar to the Swan Lake Flat and Falls River Basalts in lithology and in their spatial relations to 

the rhyolite plateau except that no ash flows of the 

Yellowstone Group, our regional stratigraphic 

framework, were emplaced on the surface of Two 

Ocean Plateau. These relations suggest the possibility that the basalts of this area are stratigraphic 

relatives of other post-Lava Creek basalts, but the 

basalts on Two Ocean Plateau could be older. 

The upper Cenozoic basalts of Two Ocean Plateau 

are designated here as the basalt of Mariposa Lake; 

they partly fill an old valley in which that lake lies 

(fig. 2). These basalts also occur on the flat ridge 

about 4 km northwest of Mariposa Lake and in two 

patches on the ridge west of the Yellowstone River 

between Badger and Phlox Creeks. Other isolated 

patches may be present beneath the widespread 

mantle of glacial debris in the area. 

Madison River Basalt 

Post-Lava Creek basalts which occur in widely 

scattered patches in the area south and west of the 

Gallatin Range to the vicinity of the Madison Canyon and the eastern part of the Madison Valley 

near West Yellowstone are named here the Madison 

River Basalt. We designate the basalt-covered uplands west of the south end of the Gallatin Range, 

south of upper Maple Creek, the type area. The 

lavas lie on an eroded surface on the Lava Creek 

Tuff but predate deeper canyon cutting in the area. 

Locally they overlie the Cougar Creek and Riverside flows of the Roaring Mountain Member of the 

Plateau Rhyolite. Some outcrops in the type area 

are deeply mantled by upper Pleistocene glacial 

deposits, and till of Bull Lake (early Wisconsin) age 

south of West Yellowstone contains abundant clasts 

of Madison River Basalt. Some flows are so small 

that it is difficult to show them on maps at a scale 

of 1 : 62,500. Other outcrop areas contain two or 

more flows, cover several square kilometers, and 

represent sections more than 50 m thick. Like the 

Swan Lake Flat and Falls River Basalts, the Madison River Basalt contains moderately abundant 

plagioclase phenocrysts and relatively rare olivine 

phenocrysts, and it generally is moderate gray to 

light gray. Chemically, however, these basalts are 

different. Although the chemistry of the rhyolite 

plateau volcanic rocks has not been used in defining 

the formations, it is worthy of note that the other 

basalts of the region are all olivine tholeiites with 

very low potassium contents. The Madison River 

Basalt, however, contains markedly more potassium, 

iron, titanium, phosphorus, fluorine, rubidium, lead, 

and uranium and contains less calcium and magnesium, even when compared with the other basalts 

of the area of similar silica content. 

The Madison River Basalt is at least partly 

younger than the Roaring Mountain Member of the 

Plateau Rhyolite. Relations just noted suggest that 

the Madison River is younger than at least part of 

the Swan Lake Flat and Falls River Basalts, but it 

is entirely older than glacial deposits of early Wisconsin age and may be largely older than the can-
VOLCANIC STRATIGRAPHY OF THE QUATERNARY RHYOLITE PLATEAU B17 

yon cutting contemporaneous with the Y armonth 

Interglaciation. (See discussion of the age of the 

Swan Lake Flat Basalt.) 

Osprey Basalt 

The Osprey Formation was defined by Pierce, 

Christiansen, and Richmond (1970) to include basalts and minor interlayered gravels in the northern 

Yellowstone National Park area that are younger 

than the Swan Lake Flat Basalt. Subsequent work 

by K. L. Pierce and by us has shown inclusion of 

the Tower Creek Gravel Member in the Osprey to 

be erroneous. As a result, we now exclude the sediments and basalts of The Narrows and change the 

name from Osprey Formation to Osprey Basalt. 

The flows and interlayered gravels of the Osprey 

Basalt partly fill deep canyons eroded into the Lava 

Creek Tuff and the Swan Lake Flat Basalt. As discussed in the section on the Swan Lake Flat Basalt, 

the valleys in which the Osprey was deposited may 

be of the same age as the Yarmouth Interglaciation; the formation is overlain by glacial deposits 

of late Pleistocene age. 

SEDIMENTS ASSOCIATED WITH THE YELLOWSTONE 

RHYOLITE PLATEAU 

G. M. Richmond, K. L. Pierce, and H. A. Waldrop 

are studying upper Pleistocene surficial deposits 

which overlie the plateau volcanic rocks as well as 

sediments interlayered with the volcanics. Their 

studies will be reported in forthcoming publications. 

For this reason, the sedimentary stratigraphy of 

the rhyolite plateau is not treated at length in the 

present report; however, deposits intercalated with 

the volcanic units are mentioned briefly in this 

section. 

Gravels and associated minor sands and silts in 

the Junction Butte Basalt, the sediments and basalts of The Narrows, and the Osprey Basalt have already been noted. All of these sediments originally 

were included in the Tower Creek Conglomerate as 

mapped by Hague, Iddings, and Weed in the Y ellowstone monograph atlas (Hague, 1904) and by 

Brown (1961, p. 1182-1183). Deposition of these 

sediments was separated by emplacement of ashflow sheets of the Yellowstone Group and by episodes of valley cutting, and the gravels contain 

different assemblages of coarse clasts. Pierce, Christiansen, and Richmond (1970), therefore, restricted 

the term "Tower Creek" to the canyon-filling sediments which contain Yellowstone Group clasts and 

used the name as a member of the Osprey. Because 

subsequent separation of the type Tower Creek 

Gravel Member from the Osprey introduces further 

confusion into the term, we no longer use it but 

regard the gravels informally as associates of the 

various basalts. Minor gravels also are interlayered 

with the Undine Falls Basalt. 

On the south side of Mount Everts an alluvial 

channel gravel about 2 m thick, composed almost 

entirely of debris of the Absaroka Volcanic Supergroup, underlies the Huckleberry Ridge Tuff and 

overlies Cretaceous sedimentary rocks. A wedge of 

loess lies between the Huckleberry Ridge and Mesa 

Falls Tuffs in the type section of the Mesa Falls. 

Howard (1937) noted and described sediments in 

and near the upper Grand Canyon of the Yellowstone. In the area of the Grand Canyon, the Canyon 

flow is overlain by highly altered well-bedded sandstones and siltstones along the canyon rims ; perhaps the best exposure of these sedimentary rocks 

is in cliffs about one-half kilometer northwest of 

Inspiration Point. Just above Lower Falls, sediments 

including a possible till of pre-Wisconsin age occupy 

a paleovalley cut in the Canyon flow and in the sediments of Inspiration Point and are overlain in turn 

near Upper Falls by clays, silts, sands, and partly 

cemented pumiceous sandstone (Richmond, 1970a; 

1970b, p. 10). Sanidine from pumice in the basal 

beds of the sediments of Upper Falls has a K-Ar age 

of about 290,000 years; the sediments are overlain by 

a flow of the Central Plateau Member, sanidine from 

which has been dated as about 110,000 years (J. D. 

Obradovich, written commun., 1970). Two sedimentfilled segments of a high-level channel cut off from 

the su bseq uen tly deepened main channel are 

stranded on the north wall of the Grand Canyon 

near the trail to the Red Rock overlook and about 

two-thirds kilometer west of Inspiration Point. 

These sediments of Red Rock are similar to those 

of Upper Falls but may be older. Sands, gravels, 

and tuffs in the Grand Canyon at Sevenmile Hole, 

about 8 km downstream from Lower Falls, may be 

partly equivalent in age to those in the area of Upper and Lower Falls and Red Rock (G. M. Richmond, 

oral commun., 1971). 

At a gravel pit two-thirds kilometer northwest 

of Potts Hot Springs a flow of the Central Plateau 

Member is underlain by gravels and sands containing abundant clasts from the Central Plateau and 

Shoshone Lake Tuff Members. The sediments also 

appear to overlie the Shoshone Lake on a steep 

paleoslope. 

A thin till just north of Bechler Meadows in 

southwestern Yellowstone National Park, about 2 

km east of Boundary Creek, lies between the Falls 

River Basalt and a rhyolite flow of the Central 

Plateau Member. 
B18 GEOLOGY OF YELLOWSTONE NATIONAL PARK 

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U. S. GOVERNMENT PRINTING OFFICE : 1972 0 - 459- 592 

Christiansen and Blank, Jr.-VOLCANIC STRATIGRAPHY, QUATERNARY RHYOLITE PLATEAU IN YELLOWSTONE NATIONAL PARK-Geolog