Neogene Tectonics and

Geomorphology of the

Eastern Uinta Mountains in

Utah, Colorado, and Wyoming

FRONTISPIECE. View north up Lodore Canyon toward Browns Park from rim of Douglas Mountain, showing 

downward steepening of the cross-canyon profile 245-300 m above river level, presumably caused by Pleistocene reentrenchment of the Green River. Canyon here is about 700 m deep. Even-topped Cold Spring 

Mountain is on distant skyline, below Diamond Peak at upper right.
Neogene Tectonics and 

Geomorphology of the 

Eastern Uinta Mountains in 

Utah, Colorado, and Wyoming

By WALLACE R. HANSEN

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1356

The relationship of a widespread erosion surface 

and its capping of conglomerate to the subsequent 

structural and drainage history of a classic region

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1986
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Hansen, Wallace R., 1920-

Neogene tectonics and geomorphology of the eastern Uinta Mountains in Utah, Colorado, and Wyoming.

(Geological Survey professional paper ; 1356)

Bibliography: p.

Supt. of Docs. No.: I 19.16:1356

1. Geology, Stratigraphic Tertiary. 2. Geology Uinta Mountains Region (Utah and Wyo.) 3. Geology 

Colorado. 4. Geomorphology Uinta Mountains Region (Utah and Wyo.) 5. Geomorphology Colorado. 

I. Title. II. Series. 

QE691.H36 1986 551.7'86 85-600166

For sale by the Branch of Distribution

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425 

Denver, CO 80225
CONTENTS

Abstract ....................

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

Regional setting ..............

Acknowledgments .............

Gilbert Peak erosion surface .........

Extent in the Eastern Uinta Mountains

Page

1

2

2

12

Wild Mountain upland surface ................. 14

Bishop Conglomerate ...................... 17

General features and distribution ............. 17

Lithology and thickness .................. 21

Sandstone ........................ 22

Tuff ........................... 23

Physical geography at the onset of deposition of the Bishop

Conglomerate ..................... 24

Altitude and relief ................... 27

Rates of uplift ..................... 27

Browns Park Formation .................... 28

Post-Bishop, pre-Browns Park change in drainage direction 29 

Age of the Browns Park Formation ........... 31

Bishop Conglomerate beneath the Browns Park Formation . 31

Critical relationship of the Browns Park Formation to the

Bishop Conglomerate near Lily Park ........... 32

Random relationship of the Browns Park Formation to the Gilbert Peak erosion surface ................ 34

Deformation ........................... 39

North flank ......................... 41

South flank ......................... 45

Nonuniform tilting on the south flank ........ 45

West-to-east tilting on the south flank ....... 47

Cliff Ridge area .................. 47

Mud Springs Draw and Wolf Creek ...... 47

Southeast end of the range ................ 48

Northeast end of the range ................ 50

Summary of post-Browns Park deformation ....... 51

Diamond Peak and Browns Park .......... 51

Yampa fault ...................... 51

Deerlodge monocline .................. 53

Drainage development ...................... 53

Adjustments on the south flank in the Pot Creek area . 54

Adjustments on Blue Mountain .............. 55

Adjustments on the north flank .............. 57

Flaming Gorge area .................. 57

Vermillion Creek and Irish Canyon ......... 60

Physiographic setting and development of Lodore Canyon . 62

Yampa River ........................ 64

Postulated capture of the master drainage of the Green

River basin ...................... 67

Relationship of capture to fish faunas ........ 70

Quaternary reentrenchment of the Green River ..... 74

References Cited ......................... 74

ILLUSTRATIONS

FRONTISPIECE. View north up Lodore Canyon toward Browns Park. 

FIGURES 1-5. Maps showing:

1. Regional setting of the Uinta Mountains .................................... 3

2. Pertinent features in the Eastern Uinta Mountains region .......................... 4

3. Structural setting of the Uinta anticline .................................... 5

4. Generalized structure of the Uinta Mountains ................................. 6

5. Distribution of Browns Park Formation, Bishop Conglomerate, and Gilbert Peak erosion surface .... 7

Photographs depicting character and extent of Gilbert Peak erosion surface ..................... 9

Composite profile of erosion remnants between Gilbert Peak and Green River airport ............... 10

Photographs showing:

8. Aerial view, Davis Draw-Hoy Flat area .................................... 11

9. Remnants of the Gilbert Peak erosion surface in the easternmost Uinta Mountains ............ 13

10. Gilbert Peak erosion surface forming east rim of Lodore Canyon ...................... 14

11. Wild Mountain as seen from "Harpers Plateau" ................................ 15

12. Felsenmeer on the Madison Limestone, Douglas Mountain .......................... 16

6.

7. 

8-12.

13. Map showning generalized distribution of Bishop Conglomerate and Browns Park Formation in southeastern Uinta 

Mountains ......................................................... 17
VI CONTENTS

FIGURES, 14-16. Photographs of the Bishop Conglomerate:

14. At south end of Little Mountain ......................................... 19

15. At southwest end of Little Mountain ...................................... 20

16. Coarsening-upward sequence at Sand Wash ................................... 21

17. Diagram showing directions of deltaic foreset bedding in the Laney Shale Member of the Green River Formation

and transport directions of the Bishop Conglomerate ................................. 26

18-20. Photographs showing:

18. Eolian crossbedding in the Browns Park Formation .............................. 28

19. Water-laid tuffs in the Browns Park Formation ................................ 30

20. Bishop Conglomerate resting on Morrison Formation ............................. 33

21. Sketch section showing relationship of Bishop Conglomerate to Browns Park Formation at canyon of Little Snake

River ............................................................ 33

22. Photograph of basal conglomerate of Browns Park Formation resting on truncated Glen Canyon Sandstone ... 34

23. Aerial photograph over Browns Park looking north across Cold Spring Mountain .................. 36

24. Diagram showing how a fault that initially controlled drainage can shift away from drainage during degradation . 37

25. Generalized tectonic map of the Eastern Uinta Mountains ............................... 38

26. Cross section drawn to explain middle to late Tertiary deformation of the Gilbert Peak erosion surface ..... 42

27. Map showing general extent of faults on north flank of Uinta Mountains between Flaming Gorge and Browns Park . 43

28. Cross section showing displacement of Gilbert Peak erosion surface in the Goslin Mountain area ......... 44

29. Geologic section showing northerly tilt of Gilbert Peak erosion surface on Blue Mountain ............. 45

30. Structure map of base of Bishop Conglomerate showing post-Bishop warping and faulting of Gilbert Peak erosion

surface, Diamond Mountain-Island Park-Blue Mountain area ............................. 46

31. Profiles along both flanks of Uinta Mountains, showing eastward decline in altitude of Gilbert Peak erosion surface . 47

32. Photograph of Bishop-capped berm on the south flank of Stuntz Ridge ....................... 48

33. Structure map of the Bishop Conglomerate in the Wolf Creek area ......................... 49

34. Photograph from rim of Johnson Draw west-northwest along the Yampa fault ................... 52

35-37. Maps showing:

35. Development of Green River from late Oligocene time to present ...................... 54

36. Barbed tributaries in Cart Creek area ..................................... 55

37. Distribution of Bishop Conglomerate in Pot Creek area ........................... 56

38. Diagram demonstrating effect of tilt on the low point of a concave-upward profile ................. 57

39. Drainage map of the Blue Mountain area ........................................ 58

40. Sketch map of hills, drainages, and tectonic features of northeast flank of Uinta Mountains ........... 59

41. Diagram showing subsidence of Bear Mountain and reversal of gradient of Gilbert Peak erosion surface .... 60

42. Geologic map of Vermillion Creek area affected by beheading of Irish Canyon ................... 61

43. Photograph of Klauson Pasture, showing terrace incised below Bishop Conglomerate ................ 65

44. Photograph of Yampa River emerging from Cross Mountain Canyon ......................... 66

45. Geologic map and section showing large, retrogressive landslides in Yampa Canyon ................ 68

46. Block diagram of a retrogressive landslide ........................................ 69

47. Photographs of Quaternary terraces in the Eastern Uinta Mountains area ...................... 73

TABLES

Page

TABLE 1. Distance from nearest possible sources of Bishop Conglomerate and maximum observed sizes of clasts ......... 18

2. Summary of Tertiary and Quaternary tectonic, geomorphic, and sedimentologic events .................. 40
NEOCENE TECTONICS AND GEOMORPHOLOGY OF THE

EASTERN UINTA MOUNTAINS IN 

UTAH, COLORADO, AND WYOMING

By WALLACE R. HANSEN

ABSTRACT

A recent reevaluation of the Bishop Conglomerate in the Eastern 

Uinta Mountains region helps clarify how the conglomerate relates 

to the Browns Park Formation and, in turn, how both formations 

relate to middle Tertiary tectonic activity and late Tertiary and 

Quaternary drainage adjustments. Field relations indicate widely divergent histories for the two formations: They are separated in their 

type areas by great topographic discontinuity, but east of the Uinta 

Mountains, between Cross Mountain and Maybell, Colo., the Browns 

Park lies directly on the Bishop.

The Bishop Conglomerate is roughly coextensive with the subjacent Gilbert Peak erosion surface; the Bishop was deposited on this 

broad pediment soon after the surface formed, perhaps while the surface was still forming. In many places, however, mountainward parts 

of the surface are bare rock. These places and the adjacent mountains 

were the sources of the conglomerate. The site of Browns Park, 

which lies in the axial region of the Uinta anticline, was part of the 

crest of the Uinta Mountains when the Gilbert Peak surface was 

forming.

On the high proximal margins of the pediment, especially on the 

south slope of the range, headward parts of the surface penetrate 

deep into the range as flat-bottomed, alluviated remnants of dendritic 

valleys. Basinward, these valleys coalesced, and at one time they 

must have formed a continuous bajada around the north, east, and 

south margins of the range. A long period of crustal stability was 

needed to develop so broad and uniform a plain.

Nearly flat to rolling uplands 180 m or more above the Gilbert 

Peak surface and the Bishop Conglomerate at various places in the 

Eastern Uinta Mountains are regarded as remnants of a truncation 

surface higher and older than the Gilbert Peak surface. Remnants 

of this surface, here called the Wild Mountain upland surface, are 

preserved on Wild, Diamond, Douglas, Blue, and Cross Mountains 

and perhaps on summits farther west. They consist of resistant bare 

rock, and they truncate subjacent formations without regard for rock 

structure. The Wild Mountain upland surface probably is Eocene in 

age. It probably formed in the mountains while the thick Eocene 

alluvial and lacustrine deposits of the Wasatch, Green River, Bridger, 

and Uinta Formations were accumulating in the adjacent basins.

The Gilbert Peak erosion surface is regarded as Oligocene. Biotite 

and hornblende separates from tuffs high in the superjacent Bishop 

Conglomerate have recently been dated by the K-Ar method at 29 

m.y. (Oligocene). These tuffs overlie most of the coarse conglomerate, 

but they are interbedded with and overlain by loose, poorly sorted 

pebbly sandstone that resembles some sandstone in the Browns Park 

Formation. This similarity has led to past correlations of Bishop with 

Browns Park in the Eastern Uinta Mountains. Some part of this 

sandstone sequence might be equivalent to the Browns Park Formation, but differing physical properties suggest that it is not.

With Laramide uplift of the Uinta Mountains, initial drainage was

away from the mountains, and coarse erogenic deposits accumulated 

at the flanks. By middle Eocene time, however, during deposition 

of the Laney and Parachute Creek Members of the Green River Formation, a hydraulic connection probably existed between Lake 

Gosiute, on the north side of the range, and Lake Uinta, on the 

south side, as suggested by several workers, but the connection was 

blocked by the rise of the Axial Basin anticline at the end of Eocene 

time. The Gilbert Peak erosion surface was cut only after strong 

erogenic and epeirogenic uplift had warped the floors of the Tertiary 

basins enough to produce slope gradients sufficient to start erosion 

and pedimentation. From a late Eocene height of about 300 m above 

sea level, the basin floors rose to about 1,200 m by middle Oligocene 

time, at a rate of about 20 cm per thousand years. The mountains 

were perhaps 3,000-3,700 m above sea level. At that time, the Green 

River Basin probably had an outlet east toward the North Platte 

River.

Renewed deformation, beginning in late Oligocene time, terminated deposition of the Bishop Conglomerate and initiated far-reaching drainage changes. The south flank and the crest of the Eastern 

Uinta Mountains were tilted northward at a rate of about 28 m/km, 

lowering the crest about 1.6 km and reversing the directions of 

streams that formerly flowed south; the tilting also had an easterly 

component of about 9-11 m/km. The north flank was warped and 

tilted to varied attitudes, mainly by movements of blocks between 

the Uinta-Sparks fault zone and another fault zone along the north 

and northeast sides of Red Canyon and Browns Park. Much of this 

tilting was southward. The inward tilting of both flanks triggered 

eastward drainage through the Red Canyon-Browns Park area along 

the future course of the Green River. As a result of tilting and warping, the Gilbert Peak erosion surface was lowered 520-975 m on the 

north flank by gravitative movement along the Uinta fault. New tributary drainage on the north flank was directed south where previous 

drainage had flowed north.

Meanwhile, subsidence in the easternmost Uintas and the western 

part of Axial Basin lowered the Bishop Conglomerate sufficiently 

over wide areas to produce a topographic and structural trough in 

which the Browns Park Formation soon began to accumulate. In that 

area the Browns Park Formation was deposited directly on the subsided Bishop, but in other places, such as Red Canyon and Browns 

Park, it accumulated in canyons and valleys tens to hundreds of 

meters below nearby Gilbert Peak and Bishop remnants. Regionally, 

the Browns Park Formation thus has a random relationship to the 

Bishop Conglomerate. Deformation continued during and after the 

deposition of the Browns Park Formation, but at a reduced and diminishing rate. Most of the tributaries of the Green River 

downstream from Flaming Gorge were essentially in their present 

courses before the Browns Park Formation was deposited.

The Browns Park Formation continued to accumulate in Browns 

Park until the old valley was overtopped at the site of Lodore Canyon

1
NEOGENE OF EASTERN UINTA MOUNTAINS

in late Miocene or early Pliocene time. This concept calls for a muchthickened Browns Park Formation, but the evidence seems irrefutable. Drainage that had flowed toward the downwarp at the east end 

of the Uintas now spilled south toward the Uinta Basin, eroding 

through the Bishop Conglomerate on the south flank of the range 

and superimposing itself into the underlying older rocks, where it 

ultimately cut its canyons. Because of the wide distribution of the 

Bishop on the rims of the canyons, there can be no doubt that the 

drainage was superimposed through it. Upstream, drainage reinvigorated by its diversion to a steeper course to the Uinta Basin, eroded 

rapidly downward and headward, eventually capturing all the runoff 

of the Green River Basin.

Drainage, meanwhile, was evolving in the Yampa River area. Like 

the course of the Green, the course of the Yampa in the Uinta Mountains is younger than the Bishop Conglomerate. Upstream from the 

Uinta Mountains, however, an ancestral Yampa River was transporting pebbly gravel from the Park Range into the Lay-Maybell area 

before the main body of the Browns Park Formation was deposited. 

This gravel is the true basal conglomerate of the Browns Park Formation, and it is younger than the Bishop Conglomerate. With the 

tectonic subsidence of the Eastern Uinta Mountains following deposition of the Bishop, the Lay-Maybell-Browns Park area became a 

stilling basin for the accumulation of the dominantly fine-grained 

lacustrine and eolian deposits of the Browns Park Formation. These 

deposits probably had varied sources, but much of the sand might 

have come from the Bishop, winnowed out by deflation. The abundant tuffs of the Browns Park Formation probably came from volcanic centers far to the west.

Drainage through the Yampa Canyon area must have existed when 

the basal conglomerate of the Browns Park Formation was accumulating, but the canyon at that time was shallow and juvenile. 

The canyon deepened later when runoff was reinvigorated; the time 

of this deepening can only be surmised, but downcutting and entrenchment would have been greatly enhanced by the spillover of 

the Green River into Lodore Canyon and the resultant rejuvenation 

of the drainage system. The present course of the Yampa through 

Cross Mountain Canyon and Juniper Canyon is an accident of 

superimposition following deposition of the Browns Park Formation, 

as first postulated by E. T. Hancock many years ago.

The most significant postulated drainage change in the Uinta region, in terms of the total budget of the streams involved, and probably also the most recent, was the diversion of the Upper Green River 

from an ancestral easterly course across the Rock Springs uplift in 

middle Pleistocene time. The river was flowing about 200 m above 

its present level at Green River, Wyo., at that time and only about 

210 m lower than the present Continental Divide at Tipton, Wyo. 

Some post-diversion uplift of the Continental Divide, therefore, is 

required warping up to the east at a rate of about 1.8 m/km, or 

about 3.1 mm/km/103 yrs. Similarly, the Continental Divide at Tipton, 

since the postulated diversion, has risen at a rate of about 475 mm/103 

yrs).

Canyon profiles in the Uinta Mountains indicate reentrenchment 

of the Green River since the initial cutting of its canyons. Downcutting had slowed, and the river had begun to widen the valley floor 

after Lodore Canyon was cut to a depth of about 460 m. Rejuvenated, 

the river then cut an additional 245-300 m to its present level. Crosscanyon profiles now show a downward-steepening slope break about 

245-300 m above the river. Regional uplift, perhaps in response to 

unloading, could have rejuvenated the entire drainage system 

through the mountains, or the capture of the Upper Green River 

in middle Pleistocene time could have caused reentrenchment by 

greatly increasing the discharge and competence of the system. Both 

uplift and capture may have been involved. Continued episodic 

downcutting and presumed uplift are indicated by Quaternary terINTRODUCTION

A recent reevaluation of the Bishop Conglomerate in 

the eastern part of the Uinta Mountains of Utah and 

Colorado helps clarify how the conglomerate relates to 

the Browns Park Formation and, in turn, how both formations relate to Tertiary tectonic activity and late 

Tertiary and Quaternary drainage adjustments. Field 

relations indicate divergent geomorphic histories for 

the two formations, yet over broad areas they are 

nearly coextensive and, as will be shown, the Browns 

Park locally rests directly on the Bishop. This latter 

relationship has led to the widely held but contentious 

belief that the Bishop is merely the basal conglomerate 

of the Browns Park Formation, a hypothesis first advanced by Sears (1924a, p. 289) but later vigorously 

disputed by his associate Bradley (1936, p. 182) and 

subsequently by others. Although Sears abandoned the 

idea, it has been revived in recent years, and it now 

appears that Sears was at least partly correct correct 

in identifying as Bishop the thick conglomerate beneath 

much of the Browns Park Formation but incorrect in 

assigning it to the Browns Park. Some of the conglomerate beneath the Browns Park Formation surely is 

Bishop, but a hiatus is indicated between the two formations. Sears, moreover, showed that the Bishop is 

stratigraphically below the Browns Park and, hence, 

is older. The Browns Park is now well dated as largely 

Miocene; the Bishop is Oligocene. Earlier geologists 

had thought the Bishop to be the younger, and some 

of Sears' contemporaries had even believed that the 

Browns Park was a facies of the Eocene Bridger Formation, despite the fact that Powell (1876, p. 168) had 

described the unconformity between the Browns Park 

and the Bridger many years before. In their type areas 

the Bishop and the Browns Park are separated by 

great topographic discontinuity, the Bishop capping 

high mesas and the Browns Park filling deep adjacent 

valleys. Because of its wide geographic extent and relatively uniform character, the Bishop is an excellent 

datum for use in reconstructing tectonic and geomorphic events in the Uinta region.

REGIONAL SETTING

The area discussed in this report centers around the 

eastern part of the Uinta Mountains here called simply "the Eastern Uinta Mountains" and includes parts 

of the adjacent Tertiary basins in Utah, Colorado, and 

Wyoming (figs. 1 and 2). For a clear understanding of 

regional geographic relationships in this broad area, the 

l:250,000-scale topographic maps of the Vernal and 

Rock Springs quadrangles are invaluable aids. These 

maps are culturally obsolescent, but they portray well
INTRODUCTION

42°

flaming Gorge V£ Little 

Reseruoir \f Mounts

MaybelL/ Mountain

 v_T I /O,

5 -j2 i-j< _i£t- o «? <

100 

_l_

150 KILOMETERS

I

100 MILES

FIGURE 1. Regional setting of the Uinta Mountains and principal rivers. Tint indicates mountainous areas.
5! 

C|o c|

§

FIGURE 2. Locations of pertinent features in the Eastern Uinta Mountains area.
INTRODUCTION

East border of

Wyoming-Utah

Thrust Belt EXPLANATION

ANTICLINE Showing plunge

THRUST FAULT-Sawteeth 

on upper plate

150 KILOMETERS

100 MILES

FIGURE 3. Regional structural setting of the Uinta anticline.

most of the larger scale topographic features. More detailed maps of local areas appear in the following text 

from place to place as needs arise.

The Uinta Mountains trend generally eastward from 

a junction with the Wasatch Range east of Salt Lake 

City. The crestline of the range is broadly arcuate, concave to the south; from the Wasatch Range junction 

it trends roughly N. 80° E. for about 90 km, then 

swings gradually around to about S. 60° E. at the Colorado State line.

Structurally the range is a large compound anticline 

whose axis extends west beyond the Uinta Mountains 

through the Wasatch Range (Butler and others, 1920, 

p. 251; Billingsley, 1933, p. 19), emerging at the 

Wasatch Front as the axis of the Cottonwood uplift 

(Crittenden and others, 1952, 1973; Eardley, 1968). In 

the opposite direction the axis projects southeast toward the White River uplift in Colorado, merging into 

a plateau region of multiple but smaller folds (Tweto, 

1976), a total axial length of at least 350 km (fig. 3).

Although the term Uinta arch, or even Uinta anticline, is sometimes applied to the Cottonwood uplift, 

a clear distinction should be drawn, as these features 

are structurally distinct, even though they are collinear. The Cottonwood uplift is a semicircular, eastplunging half dome, cored by Precambrian rocks and 

middle Tertiary intrusives, truncated on the west by 

the Wasatch fault, and separated from the Uinta anticline by a deep, north-trending synclinal valley partly 

filled with Tertiary volcanics (Boutwell, 1912, p. 43; 

Hintze, 1913, p. 130; Bromfield and others, 1977). The 

Uinta anticline proper is about 260 km long. Its west 

plunge at the town of Kamas, Utah, is well defined

topographically as well as structurally. Its east plunge, 

equally well defined, is near the mouth of the Little 

Snake River a few miles west of Maybell, Colo.

In a general way this large fold coincides with the 

mountainous mass of the range, although some of the 

outer hogbacks and cuestas of the fold extend several 

miles out from the mountains. Near the east end of 

the range the anticlinal axis passes beneath the thick 

basinal fill of the Browns Park Formation, which buries 

i much of the northeast limb. Just east of the Little 

Snake River, the short but abrupt Cross Mountain anticline straddles the Uinta axis. Curiously, its crestline 

trends nearly north. Farther southeast the Uinta axis 

reemerges at the Axial Basin anticline (fig. 3; see also 

fig. 25).

The range is flanked in many places by high plateaus 

or mesas, mostly capped by remnants of the once more 

extensive Bishop Conglomerate. In the western part 

of the range many of these plateaus exceed 3,000 m 

in altitude, and some even reach 3,660 m, but toward 

the east they are progressively lower, finally dropping 

below 2,000 m at Elk Springs Ridge just west of Elk 

Springs, Colo.

The eastern part of the range is modified on the 

south flank by several large subsidiary anticlines that 

increase its breadth and complexity. These folds center 

around Dinosaur National Monument, and their impressive exposure lends a dramatic dimension to the local 

scenery. The main Uinta anticline itself is compound 

in that it consists of two elongate domes, nearly equal 

in size, alined on a single east-west axis (Hansen, 

1957b; 1965, p. 137). These domes are expressed structurally by strike and dip changes in the Uinta Mountain 

Group and by inward bowings of the flanking forma-
111°

NEOCENE OF EASTERN UINTA MOUNTAINS

110° 109°

REATER GREEN RIVER M

WYOMING

UTAH JL~^ COLORADO

X

oVernal f /JP^ rpH«f

*s IN

100 KILOMETERS 

I

i

50 MILES

EXPLANATION

PRECAMBRIAN UINTA MOUNTAIN GROUP j SYNCLINE

FAULT j MONOCLINE

THRUST FAULT- Sawteeth on upper plate; FORM LINE ON UINTA MOUNTAIN GROUP

dashed where concealed 

ANTICLINE-Showing plunge

FIGURE 4. Generalized structure map of the Uinta Mountains showing compound form of Uinta anticline. Modified from Hansen (1957b, 

fig. 1; 1965, fig. 45.) Concealed thrust faults in Uinta Basin after Ritzma (1969). A similar concealed thrust fault may border the 

range on the north.

tions (fig. 4). They are thus separated by a shallow 

structural swale across the fold axis, roughly in line 

with the towns of Manila and Vernal. Both domes are 

also outlined by relative positive gravity anomalies 

(Behrendt and Thiel, 1963), which probably result from 

dense Precambrian rock at shallow depth beneath the 

range and from density contrasts between the Uinta 

Mountain Group and the younger rocks of the basins 

(Behrendt and Thiel, 1963; Cook and others, 1975). The 

Laramide history of the two domes was roughly the 

same, but the eastern dome rose higher and has been 

more deeply dissected, exposing the Early Proterozoic 

or Archean Red Creek Quartzite. Starting in late Oligocene time, the histories of the two domes took separate 

turns as the eastern dome began to tilt and subside. 

In consequence, the summits of the Eastern Uinta 

Mountains are about 1,200-1,500 m lower than their 

western counterparts. The highest point in the Eastern 

Uintas is Diamond Peak, an outlier off the north flank,

which is capped by Bishop Conglomerate and reaches 

an altitude of 2,960 m.

Though most maps show the Uinta Mountains as a 

single, continuous mountain range, they actually comprise two structurally and topographically distinct 

parts, which are differentiated here as the "Eastern 

Uinta Mountains" and the "Western Uinta Mountains." 

I draw the boundary between the two at a pass north 

of Vernal drained by Cart Creek on the north and 

Reader Creek, a tributary of Little Brush Creek, on 

the south (Hansen, 1969a, p. 13). Utah Highway 44 

traverses the pass, joining Vernal to Manila. Within 

a few kilometers of this pass, the altitude, physiography, climate, and vegetation change drastically from 

the lofty, well-watered Western Uintas to the lower, 

more arid Eastern Uintas. The purpose of this report 

is to evaluate the Eastern Uinta Mountains, their Tertiary physiographic development, their Neogene structural history, and the ensuing drainage adjustments.
INTRODUCTION

Gosiin________~ WYQM1N£_ 

ourrtain UTAH

DINOSAUR 

NATIONAL 

MONUMENTJS

O

50 KILOMETERS

_J

3D MILES

EXPLANATION

BROWNS PARK FORMATION

:-:'i BISHOP CONGLOMERATE

GILBERT PEAK EROSION SURFACESlighty to moderately dissected

FIGURE 5. Generalized distribution of the Browns Park Formation, Bishop Conglomerate, and bare-rock Gilbert Peak erosion surface in

the Eastern Uinta Mountains and adjacent areas.
NEOGENE OF EASTERN UINTA MOUNTAINS

ACKNOWLEDGMENTS

I am pleased to express my thanks to the many 

people who helped make this report possible. Peter D. 

Rowley and Paul E. Carrara shared geologic-mapping 

responsibilities in the Eastern Uinta Mountains in the 

late 1970's, and they prepared and reviewed maps and 

articles that bear on my conclusions. Bruce Bryant, 

who recently remapped the Western Uinta Mountains 

at a scale of 1:250,000, shared many stimulating discussions with me about the Uinta Mountains in general 

and the Bishop Conglomerate in particular. Bryant and 

Carrara both critically reviewed this report and offered 

many useful suggestions. Peter L. Martin carefully 

edited the entire report and ferreted out many weaknesses. John R. Dyni, Glen A. Izett, and Stanley J. 

Luft contributed valuable information from their expert 

knowledge of the Browns Park Formation and its regional geologic setting. M. Dean Kleinkopf gave helpful 

advice on structural and geophysical interpretations. 

Henry W. Roehler cleared up many of my uncertainties about the lower Tertiary rocks. Richard B. 

Taylor and Raymond L. Parker arranged for photoreconnaissance flights in fixed-wing aircraft over the 

Eastern Uinta Mountains, flights piloted by Parker and 

Robert D. Miller. Karen S. Bailey supervised numerous 

iterations of the manuscript. Bernard W. Hawkins 

drafted the illustrations. Charles B. Hunt, down 

through the years, has generated a flood of provocative 

ideas that have greatly influenced my thinking. All of 

the above people are or have been affiliated with the 

U.S. Geological Survey. The National Park Service furnished logistical assistance in the rugged canyon country of Dinosaur National Monument. Andrew E. Godfrey, of the U.S. Forest Service, provided enthusiastic 

intellectual and physical support in the field, including 

the use of a helicopter.

GILBERT PEAK EROSION SURFACE

The Gilbert Peak erosion surface is the pediment on 

which the Bishop Conglomerate accumulated, named by 

Bradley (1936, p. 170) for extensive remnants on the 

north and west slopes of Gilbert Peak on the north 

flank of the Western Uinta Mountains. This surface is 

an excellent datum for the reconstruction of Neogene 

events, having been modified in many places by warping, faulting, and dissection clues to neotectonic activity in the area. Bradley described in detail the origin 

and history of this surface as it exists on the north 

flank of the range, and he discussed its geomorphic relationship with the Bishop Conglomerate. He noted its 

presence on the south flank, also, but restricted his observations to the north. Powell (1876, atlas) and King

(1878, atlas) in a general way and Schultz (1918, pi. 

5) in more detail had earlier depicted the extent of the 

Bishop Conglomerate (and hence the underlying Gilbert 

Peak surface) on both flanks, but they did not map the 

extensive and geomorphically critical exposures in the 

Blue Mountain-Yampa Plateau area or the deposits 

along the crest of the range near Pot Creek (fig. 5). 

The Blue Mountain-Yampa Plateau deposits are critical 

in that their source was the crestal part of the range 

across the present canyons of the Green and Yampa 

Rivers. These rivers, therefore, could not have existed 

at the time of Bishop deposition, and PowelPs concept 

of antecedence, which called for a Green River older 

than the mountains, is thus invalidated.

Rich (1910, p. 615, 619) first described the Gilbert 

Peak surface but did not name it. Rich regarded it as 

a peneplain, probably formed under humid climatic conditions, and he inferred that the overlying Bishop Conglomerate was deposited in response to renewed uplift 

of the Uinta Mountains accompanied by a shift from 

humid to arid conditions. Rich also toyed with the idea 

of planation by wind erosion under an arid climate. The 

pediment concept at that time had hardly yet emerged, 

and Rich attributed the concavity of the Gilbert Peak 

surface profile to renewed uplift of the mountains. 

Sears (1924a, p. 302) also identified the surface as a 

peneplain, but he correctly sensed its basic origin and, 

in later years, privately referred to it as a pediment. 

Sears (1924a) suggested that it perhaps merged with 

a similar plain on the southern flank of the Wind River 

Range, described by Blackwelder (1915).

At the time of its formation, the Gilbert Peak erosion 

surface was an extraordinarily broad, monotonously flat 

plain, better depicted by photographs than by words 

(figs. 6, 8, 9, and 10). Much of what remains today 

survives because it is covered and protected by a capping of Bishop Conglomerate. Mountainward parts of 

the surface on hard rock, however, lack the gravel cap 

and may have been stripped or may never have been

FIGURE 6. Two views of the Gilbert Peak erosion surface.

Top. A few kilometers downstream from Green River, Wyo., and 

65 km north of the Uinta Mountains. Looking southeast across 

the Green River toward the Gilbert Peak erosion surface at Flattop Mountain on the skyline. Surface truncates the Wilkins Peak 

Member of the Green River Formation, which dips right (west) 

off the Rock Springs uplift.

Bottom. Red Canyon and north flank of the Uinta Mountains as 

seen from Bear Mountain, right foreground. Gilbert Peak erosion 

surface truncates Uinta Mountain Group on Dowd Mountain in 

the middle distance. Note that the surface here slopes southward 

(left) toward the mountains. Photographed in 1951 before construction of Flaming Gorge dam, which now backs water into 

the canyon in the foreground. Compare with views in figure 9.

10 NEOCENE OF EASTERN UINTA MOUNTAINS

covered. These areas and the higher mountains were 

the sources of the gravel. The smoothly graded, slightly 

concave surface sloped gently away from the mountains 

into the Green River Basin to the north (fig. 7), the 

Sand Wash Basin to the northeast, the Piceance Creek 

Basin to the southeast, and the Uinta Basin to the 

south. In all probability it also extended southeastward 

some distance across the present Yampa River toward 

Axial Basin, where it may have merged with a similar 

surface extending down from The Flat Tops of the 

White River uplift, although the evidence of such a link 

has been erased by erosion. According to Bruce Bryant 

(written commun., 1983) the oldest dated Neogene deposits on The Flat Tops are about 24 m.y. old and thus 

equate with the older part of the Browns Park Formation rather than the Bishop. The subjacent erosion surface, however, might still be about the same age as 

the Gilbert Peak surface.

Several even-topped ridgelines far out in the Tertiary 

basins accord rather closely in altitude with the Gilbert 

Peak erosion surface, and though they lack a Bishop 

Conglomerate cap rock, I regard them as likely correlatives of the Gilbert Peak surface. They owe their present prominence to resistant bedrock. Among them are 

the crestline of White Mountain west of Rock Springs; 

Kinney Rim and Laney Rim bounding the Washakie 

Basin on the west and north, respectively; the crest 

of the Citadel Plateau south of Maybell; Pinyon Ridge 

south of Elk Springs; Calamity Ridge east of Rangely; 

and the Cathedral Bluffs rim west of the Piceance 

Creek Basin. Aspen Mountain, on the Rock Springs uplift, is a monadnock that stood above the old pediment 

surface. So is Wilkins Peak, southeast of Green River. 

All these features are shown on figure 1.

The Gilbert Peak surface truncates hard and soft 

rocks alike, with little regard for lithology or structure, 

although resistant rocks stand well above the surface 

locally as hogbacks or monadnocks. The Madison Limestone (Mississippian) in particular has resisted 

pedimentation more stubbornly than has the Uinta 

Mountain Group (Middle Proterozoic); its limestone outcrops rise above the truncated Uinta Mountain Group 

in many places. Limestone Ridge, on the northeast side 

of Cold Spring Mountain, is just one example. Locally, 

the Round Valley Limestone (Pennsylvanian) stands 

above the surface also, especially on the south slope 

of the range near Vernal. On the higher flanks of the 

pediment on the south slope of the range, scattered 

monadnocks or inselbergs (island mounts) surrounded 

by Bishop Conglomerate merge gradually with the old 

mature topography of the mountains. In such places 

the headward parts of the Gilbert Peak surface penetrate deep into the range as a complex of graded or 

partly graded, flat-bottomed, alluviated, dendritic valSOUTHWEST NORTHEAST

METERS 

4000 -

3000 -

2000

Gilbert Peak

Poison Mountain/Cottonwood Mountain

Mickey 

Mountain

Cedar 

Mountain B|ack

Mountain Green River 

Airport

20 40 60 

KILOMETERS 

VERTICAL SCALE x 20

80 100

FIGURE 7. Composite profile of remnants of Gilbert Peak erosion 

surface between Gilbert Peak, Utah, and Green River airport, 

Wyoming. Much of the profile as shown depicts the top of the 

Bishop Conglomerate, which reaches a thickness of about 60 m 

at the south end of Cedar Mountain. The remnant at the Green 

River airport is near the low point of the Gilbert Peak surface 

and may, in fact, be a slightly lower, younger surface; it supports 

a gravel deposit that contains pebbles from sources to the north 

as well as south. The high, steep southern part of the profile near 

Gilbert Peak is on bare bedrock. The profile, which about coincides 

with the Hickey Mountain profile of Bradley (1936), is derived from 

the new l:100,000-scale topographic maps of the area. Profile 

dashed where eroded.

leys; basinward, these valleys coalesced with the flanking pediment.

Many such valleys still exist on the south slope of 

the Uinta Mountains, little changed from their middleTertiary form (fig. 8), especially in the area between 

Ashley Creek on the west and the Canyon of Lodore 

on the east. (For example, see Kinney, 1955; Rowley 

and others, 1979; Hansen and others, 1980, 1981, 1983.) 

Bradley (1936, p. 171) noted places in the western half 

of the range where ramplike remnants of an old surface 

(Gilbert Peak?) extend completely across the crest, 

joining the north flank with the south in a smooth, unbroken profile. The process through which two opposed 

concave ramps have merged at the crest into a continuously graded convex divide has not been addressed and 

remains unclear. I suspect that the remmants cited are 

parts of the Wild Mountain upland surface (p. 14) 

rather than the Gilbert Peak.

As visualized by Bradley (1936, p. 174), the Gilbert 

Peak erosion surface was formed in a semiarid environment by laterally corrading streams. Borrowing from 

Rich (1910, p. 614), Bradley cited as evidence the fresh, 

unweathered character of the truncated rock and the 

lack of a buried soil beneath the overlying Bishop Conglomerate, and he noted the thinning of the Bishop toward the mountains and toward the basins from a 

thicker section in between. In an arid climate, mostly 

intermittent streams flowing away from relatively well-
GILBERT PEAK EROSION SURFACE 11

Pine Mountain Diamond Peak

M O U N T A I N

BROWNS PARK

3*,

FIGURE 8. Aerial view, Davis Draw-Hoy Flat area, one of several anomalous old dendritic valleys floored with Bishop Conglomerate 

at the crest of the Uinta Mountains south of Browns Park in Moffat County, Colo., and Daggett County, Utah. Davis Draw, at 

an elevation of 2,130 m, drains north into Browns Park 500 m below. Note barbed tributaries. Hoy Flat drains south, toward the 

foreground, and the drainage divide is in the middle distance. Pine (formerly Bishop) Mountain is type locality of Bishop Conglomerate. 

Width of view at bottom edge of photograph is about 4.8 km.

watered sources in the mountains would have lost volume and competence to infiltration and evaporation in 

the basins. They would, therefore, have dropped most 

of their loads en route at intermediate places in the 

profile. Here, as their channels became blocked by their 

own debris, they would have been forced to shift laterally, constantly diverting into new courses across the 

growing plain.

A long period of crustal and climatic stability following earlier crustal and climatic unrest must have 

been required to develop so broad and uniform a plain. 

As will be discussed further, the time at the onset 

probably was early Oligocene. Then, when the climate 

became drier, the Bishop Conglomerate was deposited 

across the medial part of the plain, thickest in the 

deepest concavity of the profile (Bradley, 1936, p. 179).
12 NEOCENE OF EASTERN UINTA MOUNTAINS

Most of the runoff flowed in ephemeral but occasionally 

energetic, highly competent streams, and most of it 

probably failed to reach the centers of the basins. One 

effect would have been to slightly flatten the depositional profile. At the time of its deposition the Bishop 

Conglomerate must have formed a nearly continuous 

bajada around the north, east, and south flanks of the 

range.

Botanic evidence suggests widespread cool, dry conditions during the latter part of the Paleogene Subperiod, in contrast with warmer, more humid earlier 

climates (MacGinitie, 1969; Leopold and MacGinitie, 

1972). This climatic change has been attributed to widespread explosive volcanism in western and southern 

North America (Axelrod, 1981), but the tectonic, 

epeirogenic rise and topographic diversification of the 

Cordilleran region at that time must have affected the 

climate also. (See p. 24.)

EXTENT OF THE GILBERT PEAK

EROSION SURFACE IN THE 

EASTERN UINTA MOUNTAINS

The bare-rock surface of the Gilbert Peak pediment 

is preserved in many places, though slightly to moderately eroded. In most places, however, the surface is 

mantled, and protected from erosion, by the Bishop 

Conglomerate, which rather faithfully reflects its form 

and extent. (See figs. 9 and 36.)

On the north flank of the range, north of Red Canyon 

of the Green River, well-preserved but warped remnants of the gravel-free surface truncate the Uinta 

Mountains Group from Flaming Gorge to Browns Park 

and include, among others, Bear Mountain (Bare Top), 

Dutch John Bench, and Goslin Mountain (Hansen, 1965, 

p. 115, 163, 171). All these places are identified on figure 20. North of Browns Park extensive remnants include Head of Cottonwood, 0-Wi-Yu-Kuts Mountain, 

and Cold Spring Mountain, the latter made classic by 

Bradley's study (1936). The tops of these mesas are 

slightly dissected by differential erosion, mostly along 

shaly zones, but viewed from a distance, most of them 

appear as almost perfectly flat plains. Cold Spring 

Mountain is especially noteworthy, but Dutch John 

Bench alone is almost pristine, virtually unaltered by 

erosion since middle Tertiary time. Being bare of 

gravel, except locally, all these remnants must have 

been near the mountainward limit of the pediment, 

close to or above the depositional margin of the Bishop 

Conglomerate, although they are now lower than conglomerate-capped remnants to the north and northeast 

at Little, Pine, and Middle Mountains and Diamond 

Peak, having been lowered by subsequent faulting. 

Local gravel patches were once more extensive than

now, having been partly stripped by erosion, but much 

of the bare-rock surface may never have supported 

gravel. Bouldery gravel deposits high on the north 

flank of Cold Spring Mountain above Talamantes Creek 

(figs. 5 and 40) are remnants of Bishop Conglomerate 

in my opinion, although some geologists have mapped 

them as Browns Park. These remnants contain boulders as large as 1.2 m across.

Farther southeast, on Douglas Mountain, bare-rock 

remnants of the Gilbert Peak surface also truncate the 

Uinta Mountain Group. The dissection of these remnants has gone further than that above Red Canyon 

and Browns Park, but seen in profile they, too, present 

remarkably even skylines. Remnants of the old surface 

skirt the northeast slope of Zenobia Peak, along a 

rough dirt road called Douglas Mountain Boulevard, 

then are lost in the dissection near Lodore Canyon. 1 

Remnants form the canyon rim farther south in the 

Zenobia Creek-Pot Creek area, however, and in places 

they pass under patches of Bishop Conglomerate and 

younger colluvium (fig. 10; Hansen and others, 1983). 

At the Gates of Lodore, the flat tops of both canyon 

rims are isolated remnants of the Gilbert Peak erosion 

surface also.

On the south side of Douglas Mountain the Gilbert 

Peak surface has largely been destroyed by dissection. 

No flat area of any great extent remains, but the form 

of the restored surface is vaguely preserved in the accordance of ridgelines truncating the Morgan, Weber, 

and Park City Formations along the north rim of

'The shorter name "Lodore Canyon," generally used locally in preference to the more 

formal "Canyon of Lodore," will be used in this report.

FIGURE 9. Remnants of the Gilbert Peak erosion surface in the easternmost Uinta Mountains area, Moffat County, Colo.

Top. View across East Cactus Flat, a stripped surface, toward 

West Cactus Flat in distance and Tanks Peak at upper left. Escarpment of Yampa fault in middle distance at left. Light-colored 

cliffs in middle distance mark the wall of Yampa Canyon, there 

about 300 m deep but mostly out of view.

Middle. View eastward across Thanksgiving Gorge toward East 

Cactus Flat in the middle distance and Cross Mountain on the 

skyline. East Cactus Flat truncates the Weber Sandstone. Cross 

Mountain is topped by a remnant of the Wild Mountain upland 

erosion surface truncating the Madison Limestone. The walls of 

Yampa Canyon catch the sun and shadows in the middle distance.

Bottom. View east across valley of Little Snake River toward 

Cross Mountain. Flat bench flanking Cross Mountain in middle 

distance is capped by Bishop Conglomerate about 45 m thick 

resting on Mancos Shale in the center of the picture and on Glen 

Canyon Sandstone at the far left. Note the gentle tilt of the 

conglomerate to the left (north). Compare with views in 

figure 6.
rss*
14 NEOCENE OF EASTERN UINTA MOUNTAINS

FIGURE 10. Gilbert Peak erosion surface, forming the east rim of Lodore Canyon downstream from Jack Springs Draw, at left. In the 

middle distance the surface is mostly bare rock, but in the background it is covered by Bishop Conglomerate and Quaternary alluvium. 

Canyon wall here is about 715 m high.

Yampa Canyon in Dinosaur National Monument. Seen 

in profile, these ridgelines describe a well-formed surface. Near the confluence of the Yampa River with the 

Green, the restored surface is at about 2,190 m elevation and slopes gently northward at a rate of about 

9.5 m/km. It accords nicely, therefore, with the base 

of the Bishop Conglomerate at Harpers Corner to the 

southwest, slightly higher at about 2,285 m, and with 

the benchland along Pot Creek to the northwest, a bit 

lower at about 2,130 m. The restored surface also 

slopes gradually east at a rate of about 5-6 m/km and 

truncates bedding at an angle of about 6-10 degrees.

THE WILD MOUNTAIN UPLAND, 

AN OLDER EROSION SURFACE

Flattish to rolling uplands at various places in the 

Eastern Uinta Mountains are regarded as remnants of 

a truncation surface higher and older than the Gilbert 

Peak erosion surface (fig. 11). This surface, here called 

the Wild Mountain upland surface, may have been

shaped by altiplanation, as its remnants generally lack 

the smooth gradients of stream-cut profiles. Abundant 

coarse gravels in the Wasatch and Green River Formations near the mountains indicate intensive early degradation at the flanks of the range and alluviation of the 

basins, with the added implication that a degradation 

surface was forming in the mountains at the same time. 

Some remnants of the Wild Mountain upland surface 

stand 180 m or more above the Bishop Conglomerate. 

In general they consist of resistant bare rock, such as 

limestone or quartzite, with no surficial cover, and they 

truncate the subjacent formations without regard for 

rock structure. Likely candidates include the top of 

Wild Mountain (figs. 5, 11) in the northwest part of 

Dinosaur National Monument, for which the surface is 

named, and Hoy Mountain to the north of Wild Mountain, both at altitudes of 2,620-2,680 m. The top of Offield Mountain east of Hoy Mountain may be a remnant 

also. Wild Mountain is topped by the Madison Limestone and limestone beds in the Morgan Formation. 

Hoy and Offield Mountains are composed entirely of
WILD MOUNTAIN UPLAND SURFACE 15

FIGURE 11. Wild Mountain, on skyline, as seen from "Harpers Plateau," looking north. "Harpers Plateau" is the local name of the Bishopcapped mesa southwest of Harpers Corner. Whirlpool Canyon is out of view in the middle distance in front of Wild Mountain. Rolling 

upland surface of Wild Mountain here truncates the Morgan Formation (Pennsylvanian).

the Uinta Mountain Group. West or north from Hoy 

Mountain, in the highlands south of Browns Park, no 

remnants of an upland surface remain, but summit 

levels there are in good general accordance despite 

rather complete dissection. These summits are generally highest to the south and west, diminishing in altitude to the north and east. Bradley (1936, p. 171) 

speculated about the possible existence of a pre-Gilbert 

Peak surface in more westerly parts of the range, 

where accordant ridgelines flank many of the higher 

peaks and locally form the crestline itself. The rolling 

upland at the head of Ashley and Brush Creeks is regarded here as a remnant of this surface, as it stands 

300 m or more above nearby deposits of the Bishop 

Conglomerate.

East of Wild Mountain, across Lodore Canyon, the 

Madison Limestone forms a continuous outcrop about

40 km long, reaching to the east end of the range at 

Lone Mountain; most of the summits on this outcrop 

are nearly accordant. Here, there is no broad platform 

either, just a general accordance, declining gently from 

west to east. Some points, such as Zenobia Peak and 

Allred Peak, rise well above the general level. Limestone Ridge,2 5 miles south-southwest of Greystone, 

has an even crestline mantled by a felsenmeer of coarse 

subangular blocks (fig. 12), a product perhaps of long 

exposure to a harsh, cold climate. Limestone Ridge is 

about 2,380 m above sea level, though it must have 

stood much higher before the tectonic subsidence of the 

Eastern Uinta Mountains. Both it and Wild Mountain 

are visible in profile from the Harpers Corner road several miles to the south in Dinosaur National Monument.

2The name "Limestone Ridge" is applied locally to at least three separate ridges in the 

Eastern Uinta Mountains.
16 NEOGENE OF EASTERN UINTA MOUNTAINS

5f ^<* " NP " """" '

: ,

i*"* m. '*?£ r^J"' - ^J^'SSf-

. I * TT-^-wAJrj ic "1 4 '

,J*&, W > ^:"«S»% *f "^ &, '3^5" ^P '? -**^ * "^ ?" " iW*"*1' ' __ ^B| *'irg "**-- ^-^St'^K . ^<&- ~f ^i i^^^^fe^^^^^sjg;^,^^

FIGURE 12. Felsenmeer on the Madison Limestone at Limestone Ridge, Douglas Mountain, about 8 km south-southwest of Greystone, 

Colo. The larger rock fragments are about 30 cm across. The crest of Limestone Ridge is a slightly dissected remnant of the Wild 

Mountain upland surface.

Along the south flank of the Eastern Uinta Mountains, south of Yampa Canyon, is the extensive Blue 

Mountain highland. Blue Mountain basically consists of 

two west-to-east summit ridges separated medially by 

the drainages of Wolf Creek and tributaries of Hells 

Canyon. These drainages are partly controlled by old 

downwarps and faultlines and are partly floored with 

Bishop Conglomerate. Several broad, nearly accordant 

summits along Blue Mountain may be remnants of the 

Wild Mountain upland surface. Noteworthy among 

these, on the northern ridge, are (1) Round Top Mountain, altitude 2,620 m, topped by limestones of the Morgan Formation; (2) the high ridge between Marthas 

Peak and Serviceberry Gap (fig. 33), altitude about 

2,600 m, also topped by the Morgan Formation; (3) 

Tanks Peak, altitude 2,657 m, topped by Morgan; and 

(4) the long ridge north of Bear Valley (fig. 33), topped 

by the Madison Limestone. This latter ridge extends 

east from Tanks Peak to Thanksgiving Gorge, sloping 

gradually from an altitude of about 2,380 m on the west 

to about 2,270 m on the east, a slope of about 13 m/km. 

All these summits stand hundreds of meters above

nearby deposits of Bishop Conglomerate. Tanks Peak 

stands even higher, well above the general level, and 

might be regarded, therefore, as an ancient monadnock 

on the old Wild Mountain surface.

South of Wolf Creek and Hells Canyon the ridgelines 

are supported by much younger rock but attain comparable heights. Buckwater Ridge, elevation 2,560 m, and 

Lazy Y Point, north of the town of Dinosaur and of 

Skull Creek (fig. 13), are underlain chiefly by the 

Dakota Sandstone and the Glen Canyon Sandstone respectively. Together they form a rather even-topped 

ridgeline about 24 km long, sloping gently eastward and 

standing 790 m or so above the valley floor to the 

south. At the east end is Skull Creek Rim, altitude 

about 2,330 m.

It is significant that the profiles of all these ridgelines 

slope eastward. As will be further discussed later, the 

south flank of the Eastern Uinta Mountains was tilted 

northward and eastward after the Bishop Conglomerate was deposited, and the eastward tilt is reflected 

in the slopes of the profiles. The northerly tilt component on the Wild Mountain surface is too fragmentary
WILD MOUNTAIN UPLAND SURFACE 17

to reconstruct, but subsequent northerly tilt is clearly 

shown by the attitude of the nearby Bishop Conglomerate.

Cross Mountain, a sharp, fault-bounded local uplift, 

is separated from the Eastern Uinta Mountains by the 

Lily Park syncline (Dyni, 1968). Cross Mountain has 

a truncated crestline that may be a remnant of the Wild 

Mountain surface. If so, it too has been tilted and in 

a complex fashion. Unlike the Uinta crestline, however, 

the crestline of Cross Mountain slopes southward, although the Bishop Conglomerate on the west flank of 

the mountain shows northward tilting of a later time 

and at a lesser magnitude. Along the limbs of the Lily 

Park syncline the Bishop Conglomerate has a northerly 

tilt of about 9 m/km.

109W

BISHOP CONGLOMERATE 

GENERAL FEATURES AND DISTRIBUTION

The Bishop Conglomerate was named by Powell 

(1876, p. 44, 169) for its occurrence on Bishop Mountain 

(now called Pine Mountain), just north of the Utah-Colorado State line in southern Wyoming (fig. 2), where 

it is well exposed. Exposures are even better a few 

miles to the northwest, along the southeast side of Little Mountain and on Black Mountain, west of Flaming 

Gorge Reservoir in southern Wyoming. PowelPs original name, "Bishop Mountain Conglomerate," was shortened by later geologists. The same formation was called 

"Wyoming Conglomerate" by the geologists of the 40th

108"3D'

25 KILOMETERS

I

0

EXPLANATION

BROWNS PARK FORMATION

\ 

25 MILES

*l\:?jf:.-f BISHOP CONGLOMERATE

FIGURE 13. General distribution of Bishop Conglomerate and Browns Park Formation in southeastern Uinta Mountains and some of the 

principal named geographic features. Formation boundaries generalized from Rowley and others (1979).
18 NEOCENE OF EASTERN UINTA MOUNTAINS

Parallel Survey (King, 1878, atlas, maps 1 and 2). Its 

distribution along the north flank of the Uinta Mountains is about as shown by Bradley (1936, pi. 34). Its 

distribution in the Eastern Uinta Mountains shown on 

figure 13 is generalized from the detailed map of Rowley and others (1979). Its distribution and character in 

the Western Uinta Mountains have been under study 

recently by Bruce Bryant of the U.S. Geological Survey. Remnants of the Bishop Conglomerate are more 

extensive on the south flank of the mountains than on 

the north but are widespread on both (figs. 5, 13). In 

the higher, western part of the range, the distribution 

is somewhat obscured locally by glacial deposits.

Although the Bishop Conglomerate is very widespread on the flanks of the Uinta Mountains, its present distal limit is erosional, and it must have once extended considerably farther from the mountains than 

now. Presumably it reached nearly to the centers of 

the adjacent basins, where it may have merged with 

deposits from other rimming mountain ranges, though 

it must have been much thinner there than closer to 

the mountains. Both the Duchesne River and White 

River flow to the Green in courses that are roughly 

parallel to the Uinta Basin axis (fig. 1> the Duchesne 

all the way from its confluence with the Strawberry, 

which continues the parallelism westward, and the 

White downstream from its crossing of the Douglas 

Creek arch. These rivers likely were established in consequent courses on the old basin floor following the disappearance of Eocene Lake Uinta, but their courses 

now are well south of the basin axis. Perhaps they were 

displaced southward by alluvial fans building out from 

the mountains as the Bishop Conglomerate spread 

southward, or by a basinward equivalent of the Starr 

Flat Member (of Andersen and Picard, 1972) of the 

Duchesne River Formation, or by both formations. At 

this writing (1982) the possibility seems good that the 

Bishop and the Starr Flat are equivalent, a view shared 

with me by Bruce Bryant (written commun., 1982).

In its most characteristic form the Bishop Conglomerate consists of rather loosely cemented bouldery, cobbly conglomerate and coarse, poorly sorted, pebbly, 

friable sandstone. Locally, in deposits that probably 

originated as debris flows, the clasts are largely matrix 

supported. Clasts tend to be subangular to subrounded, 

but some are very well rounded. The firmness of 

cementation is varied, and surface exposures are 

mostly loose and gravelly. The conglomeratic character 

of the rock is most evident on the sides of steep hills 

and bluffs where the calcareous cement has not been 

dissolved away and where the rock may form low cliffs 

and overhanging ledges. On Black Mountain, west of 

Flaming Gorge Reservoir (fig. 2), resistant conglomerate forms nearly continuous cliffs more than 2.2 km 

long.

The coarseness of the rock varies laterally and vertically. In most places the upper meter or so is finer 

grained than the rock at depth commonly as fine as 

pebble gravel but with scattered cobbles and boulders. 

Figure 14 depicts this characteristic very well. Except 

in slopeside exposures, the formation may appear to 

be deceptively fine grained. Away from its source, 

which was the crestal part of the Uinta Mountains, the 

rock is progressively less coarse, just as one would expect, owing to attrition of the clasts and to reduced 

competence of the ancient streams. Along a given 

drainage line the size (volume) of the largest boulders 

diminishes exponentially with distance. Regionwide it 

tends to do the same (table 1). The median grain size 

of all clasts probably does also, but the formidable sampling required for proof has never been attempted. On 

any exposures, the maximum clast size greatly exceeds 

the median. Boulders 2 m or more in longest diameter 

are occasionally found several kilometers from the 

nearest possible source. On the slopes of Diamond Peak 

in Colorado (not be confused with Diamond Mountain 

in Utah), where a possible source was only 3-5 km 

away, many boulders exceed 1 m across, and 2-m boulders are not uncommon. Weber (1971, p. 168) noted 

an angular block of red quartzite "over 8 feet long"

TABLE 1. Maximum observed sizes ofclasts in the Bishop Conglomerate and distances from nearest possible sources in the Uinta 

Mountains

Locality

Near Brush Creek, Utah 1 ............

Little Mtn., Wyo. ...................

Wapiti Peak, Colo. ..................

Ruple Ridge, Utah ..................

Elk Springs Ridge, Colo. ............

Miller Mtn., Wyo. ...................

Black Mtn., Wyo. ...................

Unnamed mesa N. of Sage Crk., Wyo . . .

Flat Top Mtn. N. of Baggs, Wyo. 3 .....

Size 

(m)

2.4

3.2

7.6

1.5

1.5

2

1.2

2

.8

.5

.6

.2

.18

.05

Distance 

(km)

3

10

11

12

14

19

19

24

30

32

39

48

56

85

Probably transported by mudflow.

The Elk Springs deposit may have had a slightly nearer 

source at Cross Mountain, possibly 18 km.

Flat Top Mountain supports no conglomerate only a 

scattering of resistant pebbles, mostly cherts and quartzites, including a distinctive red chert common in the Round 

Valley and Morgan Formations of the Eastern Uintas.
BISHOP CONGLOMERATE 19

- sss/^-* fe

FIGURE 14. Bishop Conglomerate capping south end of Little Mountain, a high mesa south of Rock Springs, Wyo., elevation 2,788 m. 

Surface rises toward the south (left) at a rate of 24 m/km. Conglomerate at surface is deceptively fine grained; for comparison with 

more typical rock, see figure 15, which was taken to left of automobile and just below rim of mesa. Uinta Mountains in background 

have been lowered by post-Bishop subsidence.

(>2.4 m) near the top of Diamond Peak. Such a block 

must have been transported by mass wastage, perhaps 

by a debris flow.

Rich (1910, p. 610) observed that boulders <1.5 m 

in diameter were common at the southern end of Miller 

Mountain 24 km from the nearest possible source, although cobbles and pebbles predominate. I measured 

one boulder there that was 2 m across. At Lookout 

Mountain in Sand Wash basin, 19-24 km from the 

nearest possible source, pebbles and cobbles predominate, although occasional boulders exceed 0.76 m. Thirteen to fourteen kilometers farther distant, just south 

of Powder Wash gas field, a gravelly bench that is 

geomorphically continuous with Lookout Mountain contains very few boulders larger than 46 cm, and most 

of the clasts are less than 20 cm across. This deposit 

is about 32 km from the nearest possible source. At 

Elk Springs Ridge, near the east end of the Uintas, 

boulders 1.2 m in diameter must have travelled 18-19 

km. Here, also, pebbles and cobbles predominate. At
20 NEOCENE OF EASTERN UINTA MOUNTAINS

Wapiti Peak, which is near the east end of Elk Springs 

Ridge, Dyni (1980) noted boulders as much as 1.5 m 

across, 14 km from a nearest possible source. At the 

shoulder of Aspen Mountain, south of Rock Springs and 

48 km from the nearest possible source, pebbles predominate, and the coarsest material consists of small 

cobbles (about 8-20 cm). Size gradations are similar on 

Cedar Mountain, west of the Green River in Wyoming 

(Bradley, 1936, p. 172).

On the south flank of the range, from Ashley Creek 

eastward to Little Brush Creek, occasional groups of 

large angular quartzite blocks, "some individuals 

weighing several tons," were noted by Kinney (1955, 

p. 115), who attributed their transport to mudflows. 

Just west of Utah Highway 44, I measured one tabular 

boulder that was 7.6 m long and saw others that may 

have been larger. The very poorly sorted, matrix-supported texture of the conglomerate in other places also 

suggests mass transport. In the Yampa Plateau-Ruple 

Ridge area of Dinosaur National Monument I have seen 

2-m boulders 19 km from their nearest possible source, 

but boulders this size in some places are clast supported and appear to be fluvial rather than colluvial. 

Figure 15, for instance, shows three very coarse, inversely graded, clast-supported bedding units passing upward from pebble conglomerate at the base to boulder 

conglomerate at the top. Bedding thicknesses are about 

1-2V2 m. Upward coarsening of flood deposits has been 

noted by previous workers. In a tractive load under 

high-energy transport, the coarsest particles tend to 

rise toward the top (Bagnold, 1954; Scott and Gravlee, 

1968). This bedding habit, however, may also result 

from torrential flooding, rising to a gradual peak of intensity, then declining rapidly. If normally graded

FIGURE 15. Bishop Conglomerate at the southwest rim of Little Mountain, Sweetwater County, Wyo. Three coarse, inversely graded 

bedding units grade from pebble conglomerate at the base to boulder conglomerate at the top. Clasts are subrounded to well rounded. 

The hammer is about 33 cm long.
BISHOP CONGLOMERATE 21

FIGURE 16. A coarsening-upward sequence of beds at the base of the Bishop Conglomerate at Sand Wash, Moffat County, Colo. Dipping 

to the right, the Bishop rests unconformably on the Bridger Formation. About 30 m of section is visible in this view, but the upper 

part is greatly foreshortened by perspective. Boulders near the top of the picture are as much as 1.2 m across.

sedimentation accompanied the decline, as seems likely, 

its deposits may have been stripped by the next succeeding flood, inasmuch as the tops of the bedding units 

show evidence of scour.

Coarsening upward on a sequential scale, rather than 

a bedding scale, is well exposed at Sand Wash (fig. 

13), where the Bishop rests unconformably on the 

Bridger Formation (fig. 16). Several bedding units are 

involved in a gradual upward coarsening over a stratigraphic distance of 20 m or more, from pebbly silty 

sandstone at the base to boulder conglomerate at the 

top. Similar, but cyclothemic, gradations elsewhere 

(Steel and others, 1977, for example) have been attributed to tectonic activity. Increased aridity, accompanied by more intense but less frequent flooding, 

might have achieved the same end at Sand Wash.

LITHOLOGY AND THICKNESS

Local sources of the Bishop Conglomerate are apparent in the varied lithology of the formation. Red quartzite or sandstone derived from the Uinta Mountain 

Group is the most abundant clast in general, but in 

some places gray Paleozoic limestone clasts outnumber 

those of all other lithologies, including red quartzite. 

At Cedar Mountain in Wyoming the abundance of limestone cobbles and the paucity of red quartzite suggest 

that limestone was more widely exposed in the mountains south of there when the Bishop was deposited 

than it is now, much of it having since been stripped 

away. Limestone predominates on Black Mountain also. 

The chief sources were the Madison and Round Valley 

Limestones, although some limestone may have been
22 NEOCENE OF EASTERN UINTA MOUNTAINS

reworked from Tertiary conglomerate at the flank of 

the mountains. Much of the Uinta Mountain Group in 

the source area at that time must have been concealed. 

In the Eastern Uinta Mountains also, the Round Valley 

and the Morgan (Pennsylvanian) were important 

sources for the Bishop Conglomerate along the south 

slope of Blue Mountain and in local remnants along 

Douglas Mountain; these formations yielded distinctive 

clasts of red chert as well as limestone. Along Wolf 

Creek, Bear Valley, and Klauson Pasture, limestone 

cobbles greatly outnumber red quartzite because of the 

wide extent of the outcropping limestone and the distance to a source of the Uinta Mountain Group. South 

of Rock Springs on Little, Miller, Pine, and Aspen 

Mountains, but not on adjacent Diamond Peak, distinctive clasts of white to pale green metaquartzite, amphibolite, and other metamorphic rocks were derived 

from sources to the south in the Red Creek Quartzite 

(Early Proterozoic or Archean). The transporting 

streams simply picked up and redeposited whatever 

rocks were in their drainage paths, and the more abundant, more resistant rock types selectively became predominant.

The thickest sections of Bishop Conglomerate are on 

the south flank of the Uinta Mountains. Near the Uinta 

River at Jefferson Park, Kinney (1955, p. 115) noted 

a thickness of 244 m consisting mostly of light-tan to 

red boulders in a matrix of coarse sandstone. Bruce 

Bryant (oral commun., 1981) suspected that at least 

part of this section should be assigned to the Starr Flat 

Member (Oligocene?) (of Andersen and Picard, 1972) 

of the Duchesne River Formation. Kinney (written 

commun., 1959) expressed a similar suspicion years before. The Bishop may thus merge downward and basinward in the Uinta Basin with the Duchesne River Formation, although the contact relationship remains unclear, and a sizable hiatus could exist. Bryant (written 

commun., 1982) recently obtained fission-track dates 

very close to the age of the Bishop from tuff beds that 

he considers to be Starr Flat: 30.0, 30.6, and 34.0 m.y.

East from Jefferson Park the Bishop Conglomerate 

thins rather uniformly, according to Kinney (1955), to 

about 90 m near the west end of Diamond Mountain 

Plateau. Near the southeast end of the plateau, however, a magnificently exposed section about 200 m thick 

is nearly all conglomerate. In other places on Diamond 

Mountain Plateau, coarse, poorly sorted, light-gray 

sandstone predominates over conglomerate. The sandstone is very calcareous, and some of it is tuffaceous. 

Kinney (written commun., 1959) suspected that all 

these rocks correlate in time with the White River Formation (Oligocene) of the High Plains region and the 

Wyoming Basin, a correlation since confirmed by

radiometric dating. (See "Tuff' section, below.) Tuff 

beds on Diamond Mountain, the Yampa Plateau, and 

Blue Mountain include air laid(?) tuff that contains 

phenocrysts of fresh black biotite and hornblende.

Near Jones Hole, the thickness of the Bishop Conglomerate, including tuff and sandstone, may reach 150 

m (Hansen, 1977b). No such thickness exists anywhere 

farther east on either flank of the Uinta Mountains. 

At the type locality on Pine Mountain the thickness 

is 60-75 m (Roehler, 1972a, b). On the Yampa Plateau 

and on Blue Mountain, thicknesses range from as much 

as 60 m to as little as a thin skin, depending largely 

on topographic irregularities on the local substrate and 

on random post-Bishop erosion (Hansen and Rowley, 

1980a, b). At Klauson Pasture at the east end of the 

Uinta Mountains, the conglomerate is about 60 m thick 

(Rowley and others, 1979). Bradley (1936, p. 172) noted 

that, on the north flank of the range, the formation 

attains its maximum thickness near the Utah-Wyoming 

State line (generally less than 60 m) and thins irregularly to the north and south. It rises in altitude southward and thins against the bare Gilbert Peak erosion 

surface. Northward the conglomerate "thins very 

gradually and before dissection * * * presumably extended far out into the basin" (Bradley, 1936, p. 172). 

Bruce Bryant (written commun., 1983) has observed 

stratigraphic sections as thick as 150 m just north of 

the North Flank fault zone in the Blacks Fork drainage, west of where Bradley did most of his work.

SANDSTONE

Much of the upper part of the Bishop Conglomerate 

as mapped on the south flank of the Uinta Mountains, 

and locally the lower part, is friable calcareous sandstone, light gray to pale pink, coarse grained, poorly 

sorted, and poorly bedded. This sandstone resembles 

somewhat the sandstone in the Browns Park Formation 

and might in part be stratigraphically equivalent, but 

my impression is that it is much less well sorted and 

that its individual grains, which commonly are pink, 

are generally subangular, whereas most of the sandstone in the Browns Park is fine grained, is moderately 

well sorted and well bedded, and has well-rounded 

grains (Luft and Thoen, 1981). The sandstone in the 

Bishop also generally contains abundant small rock 

fragments, mica, and scattered dark grains. Individual 

beds in both formations, however, depart widely from 

the perceived norm, and they range through a wide 

spectrum of color, grain size, and sorting. The Bishop 

contains nothing comparable to the striking eolian sand 

that is so abundant in the Browns Park Formation in
BISHOP CONGLOMERATE 23

the Maybell-Elk Springs area or to the fresh vitric, 

crystal-poor, mafic-poor tuffs of the Browns Park Formation in the type area. These tuffs are much younger 

than any known in the Bishop.

I suspect that deposition continued without much interruption on the south flank of the Uinta Mountains, 

generally fining upward, while the sands were accumulating above the conglomerate. Locally steep dips 

in the Pot Creek area suggest eolian deposition. Deposition ended when tilting and faulting changed the hydrologic regime and initiated dissection. At the east end 

of the range, however, large-scale subsidence produced 

a structural sag that provided catchment for sediments 

throughout most of succeeding Browns Park time. In 

Browns Park itself, several million years elapsed after 

deposition of the Bishop Conglomerate before the 

Browns Park Formation began to accumulate, and a 

great deal of geomorphic history intervened.

TUFF

Tuff was introduced at times into the Bishop Conglomerate, explosive volcanism being widespread during that period in the Western Interior (Axelrod, 1981, 

fig. 2). Datable tuffs on Diamond Mountain and the 

Yampa Plateau provide clues to the minimum age of 

the conglomerate. Southern Utah and central Nevada 

are possible sources (Rowley and others, 1975, p. B9; 

Burke and McKee, 1979, p. 183). Biotite and 

hornblende from a sample collected above the main 

body of conglomerate by Glen A. Izett and me in the 

SWVfc sec. 13, T. 2 S., R. 23 E., have radiometric potassium-argon ages of about 29 m.y. (biotite, 29.50 + 1.08 

m.y.; hornblende, 28.58 + 0.86 m.y.) as determined by 

Harald H. Mehnert of the U.S. Geological Survey, 

using the following constants:

40K\£ =0.581xlO-10/yr \p =4.962xlO-10/yr 40K/K= 1.167x10^

The height of this tuff above the base of the conglomerate can be estimated at about 90 m if the dip (3° NNE.) 

is projected northward to the outcrop from the basal 

contact at the rim of Diamond Mountain Plateau and 

if corrections are added for about 30 m of displacement 

on intervening faults.

This tuff bed and others overlie most of the conglomerate, but they are interbedded with and overlain by 

loose, coarse-grained pebbly sandstone. Some of the 

sandstone might be appreciably younger than the dated 

tuff, therefore, and might be temporally equivalent to 

some part of the Browns Park Formation. The lower, 

coarsely conglomeratic part of the formation, however, 

could be appreciably older than the tuff; its maximum

possible age is limited by the subjacent Duchesne River 

Formation. Emry (1981) has suggested that the vertebrates of the Duchesnean land mammal age are partly 

Eocene Uintan (Brennen Basin Member of Andersen 

and Picard, 1972) and partly Oligocene Chadronian 

(Dry Gulch Creek and Lapoint Members of Andersen 

and Picard, 1972). The Lapoint Member underlies the 

Starr Flat, which in turn may be equivalent to the 

Bishop Conglomerate.

The dated tuff is light gray, compact, firm, and 

flecked with abundant euhedral biotite, much of which 

is coarser than 0.5 mm across (Hansen, 1965, table 3). 

The tuff is further distinguished by abundant crystal 

fragments of feldspar and quartz aggregated with clay, 

by abundant euhedral hornblende, and by euhedral pyroxene and magnetite commonly adhering to glass 

shards. The euhedra are deeply etched, and the shards 

are partly altered to clay. Zircon forms inclusions in 

the feldspar. What might be the same tuff bed was 

sampled by Winkler (1970) and dated by Damon (1970, 

p. 52) at 26.2 + 0.7 m.y. (biotite). A tuff near the Harpers Corner road in Dinosaur National Monument is 

finer grained and rather more altered but is similar in 

most other respects.

These tuffs resemble the tuffs in the lower part of 

the Browns Park Formation in the Little Snake RiverMaybell area (Izett and others, 1970, p. C151). This 

resemblance is the chief reason why the Bishop on the 

south flank of the Uinta Mountains had been correlated 

with the Browns Park Formation in recent years (Kinney and others, 1959; Hansen and others, 1960; Untermann and Untermann, 1965).

A tuff collected by Winkler on the Yampa Plateau, 

about 37 m above the base of the Bishop, was dated 

by Damon (1970, p. 52) at 41.3±0.8 m.y. from a biotite 

separate. The biotite was brownish black and slightly 

resinous (Damon, p. 53), which suggests incipient alteration (Mauger, 1977, p. 23). I am inclined to discount 

this date because of its wide disparity with the ages 

from the nearby Diamond Mountain localities and because of contradictory geomorphic evidence for the age 

of the Bishop. This date lies within the age of the saline 

facies of the Uinta Formation at Duchesne, Utah 

(Damon, 1970, p. 51; Mauger, 1977, p. 19, 32), and is 

close to the age of the Green River-Uinta Formation 

boundary in the western Uinta Basin. A correlation of 

the Bishop with these rocks is improbable. The Eocene 

rocks on both flanks of the Uinta Mountains are deeply 

truncated by the Gilbert Peak erosion surface, including the Wasatch, Green River, and the Bridger Formations and probably the Duchesne River Formation, and 

an enormous amount of post-Green River erosion took 

place before the overlying Bishop was deposited.
24 NEOCENE OF EASTERN UINTA MOUNTAINS

PHYSICAL GEOGRAPHY AT THE

ONSET OF DEPOSITION OF THE

BISHOP CONGLOMERATE

When the Eocene lakes disappeared from the basins 

bordering the Uinta Mountains 45-40 m.y. ago 

(Mauger, 1977, p. 37), they were supplanted by alluvial 

plains. A detailed discussion of this complex transition 

is outside the scope of this report, but a brief summary 

might help place into perspective the middle Tertiary 

events that followed. Ryder and others (1976, p. 511) 

aptly described the extinction of Lake Uinta south of 

the Uinta Mountains in just two sentences:

Following the deposition of strata containing the richest oil shale of 

Lake Uinta, the water became hypersaline, and the remaining oilshale and mud-supported carbonate units are therefore associated 

with sodium-rich evaporite and Magadi-type chert beds. By late Eocene and early Oligocene time, the last vestiges of Lake Uinta were 

buried by coarse alluvial sediments (Uinta and Duchesne River Formations) derived mainly from the Uinta uplift.

West of the present Uinta Basin, Lake Uinta persisted locally, as a freshwater lake, into Oligocene time 

(Weiss, 1982), but as the basin gradually filled with 

sediment, its rim was overtopped to the south, and 

drainage was southward out of the basin from that time 

on (Hunt, 1969, p. 94). The disappearance of the lakes 

was accompanied by climatic cooling that Roehler 

(1974, p. 58) has suggested was triggered by volcanism, 

a concept recently emphasized in a broad sense by 

Axelrod (1981) for the whole of the Western Interior 

during Cretaceous and Tertiary time. Roehler (written 

commun., 1983) cites evidence of worldwide cooling 38 

m.y. ago. Tectonism probably was a factor also. A continued rise of the Uintas and other Rocky Mountain 

uplifts surely would have altered the local climate.

The extinction of Lake Gosiute north of the Uinta 

Mountains in middle Eocene time was caused by 

sedimentation in a lake that was a playa most of the 

time, in the view of most investigators (Eugster and 

Surdam, 1973; Bradley, 1973; Eugster and Hardie, 

1975; Surdam and Wolfbauer, 1975; Surdam and Stanley, 1979), although Desborough (1978) and Boyer 

(1982) have recently raised doubts about the playa-lake 

model. At any rate, the last major stage of Lake 

Gosiute (the Laney stage) was predominantly fresh 

until its disappearance and must, therefore, have had 

an outlet, presumably south into Lake Uinta (Bradley, 

1929, p. 89, 1964, p. A2; Roehler, 1965, p. 147; Surdam 

and Stanley, 1979, p. 101, 105) by way of the Sand 

Wash Basin. A physical connection at times between 

the two lakes, moreover, is indicated by their common 

fish faunas.

One or the other or both of the lakes probably 

drained east at times to the Mississippi Valley. If Lake

Uinta received the overflow of Lake Gosiute, it in turn 

may have drained east across Colorado during its freshwater stages via some unknown outlet to the Mississippi Valley, where the affinities of the well-known fish 

faunas of the Green River Formation chiefly lay. The 

numerous gars, catfish, perches, bowfins, paddlefish, 

mooneyes, herrings, and perhaps sunfish (Priscacaridae) (Grande, 1980) were all members of eastern 

American families (Miller, 1958; Uyeno and Miller, 

1963), none of which is indigeneous to the Colorado 

River system, and most of which are unknown in the 

Eocene faunas of the Pacific Northwest (Grande, 1980). 

If these fishes entered Lake Uinta from Lake Gosiute, 

rather than vice versa, the chief high-water outlet of 

Lake Gosiute might have been east across Wyoming 

to the Mississippi. Such easterly overflow might have 

followed a rise of both lakes to a common high water 

level. In either event, the fish faunas suggest that the 

Continental Divide in Eocene time was west of the 

Green River lakes and the Uinta Mountains, a likelihood that is compatable with Eocene paleogeography. 

The Colorado River system at that time did not yet 

exist. None of the 14 species of fishes native or endemic 

to the present Upper Colorado River system (Behnke 

and Benson, 1983), moreover, had progenitors in the 

Green River lakes except, perhaps, the wide ranging 

suckers (Catostomidae), which are Nearctic in their distribution. The indigenous Salmonidae (trouts) and Cottidae (sculpins) of the modern Upper Green River and 

the Columbia are known in North America from the 

Eocene (Miller, 1965; Patterson, 1981, p. 276) but are 

not present in the fish fauna of the Green River Formation.

A contributing cause of the disappearance of both 

lakes was a decrease in tectonic downwarping, which 

enabled depositional filling to overtake subsidence. The 

Uinta Mountains, however, probably continued to rise, 

as shown by the accumulations of coarse conglomerate 

above the Green River Formation close to the south 

flank of the mountains, in the Duchesne River Formation (Andersen and Picard, 1972). In most places this 

conglomerate is gently tilted, locally to as much as 30 

degrees. On the north flank at Phil Pico, a thick pile 

of conglomerate contains rocks of Wasatch, Green 

River, and Bridger age (Bradley, 1964, p. A53; Rowley 

and others, 1979). Downwarping continued in the Uinta 

Basin while most of the Duchesne River Formation was 

being deposited, followed by regional uplift, probably 

starting in early Oligocene time. This uplift terminated 

the long depositional cycle that had begun in the basin 

with the Laramide orogeny. To the north in the Green 

River Basin, reelevation of the Rock Springs uplift 

steepened dips locally to as much as 13 degrees after 

the Bridger Formation had been deposited but before
BISHOP CONGLOMERATE 25

the Bishop Conglomerate had (Bradley, 1964, p. A14). 

The stage was being set for the cutting of the Gilbert 

Peak erosion surface.

Before the onset of pedimentation the gradient between the foot of the Uinta Mountains and the central 

parts of the adjoining basins had to be increased 

enough to change the drainage regimen in the basins 

from a depositional to an erosional mode. This gradient 

was achieved by deformation and concomitant erosion, 

the extent of which has been described by Bradley 

(1964, p. A16) as follows:

Without departing from its original depositional dip, the Bishop Conglomerate bevels the tilted Bridger and Green River Formations and 

the more steeply inclined Upper Cretaceous rocks of the Rock 

Springs uplift.

The extensive erosion surface on which the Bishop Conglomerate 

was deposited indicates qualitatively that the post-Eocene deformation must have occurred appreciably earlier than the cutting of this 

erosion surface. In the Green River Basin this extensive surface bevels beds high in the Bridger Formation, and farther east it cuts 

progressively lower in the Tertiary; at Pine Mountain and Diamond 

Peak (near the junction of Wyoming, Colorado, and Utah) it bevels 

rocks that belong to a tongue of the Wasatch Formation which overlies the basal unit of the Green River Formation. These formations, 

therefore, must have been deformed long enough prior to the deposition of the Bishop Conglomerate to permit the removal of several 

thousand feet of beds over a large area and to allow for the reduction 

of the terrain to a remarkably smooth surface.

To erode all that rock, the drainage needed a new 

base level, and strong uplift was needed to provide the 

gradient. At the close of the Green River epoch the 

floors of the basins were only 150-300 m above sea level 

(Bradley, 1929, p. 89; MacGinitie, 1969, p. 52, 73), and 

the gradient of the basin floors was only about 0.2-0.4 

m/km (Bradley, 1964, p. A16). But the graded relief 

on the Gilbert Peak erosion surface is hundreds of 

meters. The surface truncates rocks of Precambrian to 

Eocene age, and it slopes as much as 7-19 m/km across 

the Tertiary basin sediments alone. The relief is more 

than 600 m across the southeastern part of the Green 

River Basin between Pine Mountain on the south and 

Pilot Butte on the north. This profile, which cuts 

deeply into the Cretaceous of the Rock Springs uplift, 

may have been warped, according to Bradley (1936, p. 

186), but the relief is just as great on undeformed profiles 100-115 km to the west, cut entirely on the 

Bridger Formation.

The Gilbert Peak erosion surface thus could not have 

been cut without strong prior uplift and dissection of 

the floors of the basins, without a drastic change in 

base level and local physiography. There also seems to 

be no way for the Gilbert Peak surface to have been 

graded to a master stream flowing out of the Green 

River, Washakie, and Sand Wash Basins into the Uinta 

or the Piceance Creek Basin, as will be pointed out, 

although a hydraulic connection across the Uinta axis

probably had existed during the deposition of the 

Laney and the Parachute Creek Members of the Green 

River Formation (Bradley, 1929a, p. 89; Roehler, 1965, 

p. 147; Surdam and Stanley, 1979, p. 101, 105). This 

connection would have formed a narrow strait across 

the axis at Axial Basin and perhaps would have had 

an appreciable current. Surdam and Stanley (p. 105) 

have described a dispersal of volcanic lithic detritus into 

the Piceance Creek Basin at that time from the Lake 

Gosiute Basin, an event that would have signaled the 

filling and demise of Lake Gosiute. Other sources of 

volcanic detritus are plausible also, as noted further on, 

and the likelihood that north-to-south drainage persisted across Axial Basin into Gilbert Peak time, at any 

rate, is remote. Axial Basin was relatively high structurally and topographically by the end of Eocene time 

and in Gilbert Peak time (before post-Bishop collapse 

of the eastern Uinta Mountains), and the threshold or 

strait across Axial Basin would have been correspondingly high.

If early southward drainage ever crossed Axial 

Basin, it probably was halted before or by the time 

of the sharp anticlinal rise of Axial Basin at the end 

of Eocene time. Sears (1924b, p. 300) was first to point 

out that the early Tertiary rocks of that area (the Fort 

Union, Wasatch, and Green River Formations) rest on 

the Cretaceous with no discordance of dip, and, therefore, that Axial Basin was uplifted after their deposition. This time of uplift probably marked the beginning 

of the disturbance that deformed the Eocene rocks in 

the adjacent basins and terminated their deposition.

The Washakie Formation of Wyoming and the Uinta 

Formation of Utah and Colorado are partly equal in 

age, but "there is no evidence to suggest that they ever 

were physically connected across the Uinta uplift" 

(Roehler, 1973, p. 9). Roehler noted lithologic and color 

differences between the Washakie and the Uinta that 

indicate independent sources for their clastic fractions. 

Much of the Washakie, according to Roehler, consists 

of arkose derived from the Sierra Madre to the east, 

whereas the Uinta Formation had local sources in the 

Uinta Mountains. Volcanic ash constituents of the two 

formations could have had a common source but need 

not have. Volcanism was widespread at that time (Lipman and others, 1972), and many centers could have 

contributed. The source of both ash and volcaniclastics 

in the Washakie appears to have been the Absaroka 

volcanic field of northwestern Wyoming (Ebens, 1963; 

Roehler, 1973), but sources in Idaho are possible also 

(the Challis Volcanics, for example; Armstrong, 1978; 

Axelrod, 1981, p. 5). The Rattlesnake Hills of central 

Wyoming (Carey, 1959; Love, 1970) could have contributed, given favorable winds aloft during an eruption. 

Absaroka ash could have settled in the Uinta Basin,
26 NEOCENE OF EASTERN UINTA MOUNTAINS

and probably did, but ash may also have traveled from 

igneous centers in western Nevada (Armstrong, 1963) 

if intrusive bodies there vented to the surface. Tuffs 

younger than 45 m.y. may have entered the Uinta 

Basin from sources in the Sevier orogenic belt (Dyni, 

1981, p. 14).

Detrital volcanic material in the Uinta Formation 

may have had multiple sources also, possibly including 

nearer sources in Middle Park, Colo., or even the Front 

Range, where lower Tertiary volcanic and intrusive 

rocks are abundant (Izett, 1968, p. 22-25; Tweto, 1976, 

1979) and where the volcanic cover then must have 

been wider than now. This detritus might even have 

reached the Uinta Basin by way of the ancestral Colorado River, which, according to Hunt (1969, p. 67, 

70), once flowed west into the Uinta Basin via the 

White River.

By late Eocene time, beginning about 41 m.y. ago, 

and lasting generally into the Miocene, airborne and 

perhaps stream-deposited volcanic material might have 

entered the Uinta Basin from extensive new igneous 

centers in the Great Basin and adjacent areas, including 

the Wasatch, Oquirrh, Tintic, and Marysvale areas of 

Utah, where intrusive and volcanic activity overlapped 

spatially and temporally (Bassett and others, 1963; 

Moore and others, 1968; Laughlin and others, 1969; 

Moore and Lanphere, 1971; Crittenden and others, 

1973; Bromfield and others, 1977). Volcanism started 

in the Marysvale district somewhat later than elsewhere, about 31 m.y. ago, and persisted intermittently 

into the Quaternary (Rowley and others, 1975). Once 

basin-and-range faulting was underway, however, 

stream-carried detritus no longer could have reached 

the Uinta Basin from the down-faulted sources in or 

beyond the Wasatch Range. (For example, see Stokes, 

1976; Hunt, 1982.) By various estimates, regional extension, which led to the faulting, could have started 

as early as 30 m.y. ago (Ingersoll, 1982) or locally as 

late as 17 m.y. ago (Christiansen and McKee, 1978, Steven and others, 1978).

The strongest evidence that drainage did not cross 

Axial Basin in Gilbert Peak or Bishop time is in the 

physiographic form of the Bishop Conglomerate itself 

and its transport direction. Stanley and Surdam (1978) 

and Surdam and Stanley (1979, fig. 19) reported evidence of south to southeast flow of water from Lake 

Gosiute toward the Piceance Creek Basin in middle Eocene time. This direction, however, is 100°-180° from the 

later transport direction and slope of the Bishop Conglomerate on the Gilbert Peak surface, which was north 

to northeast from the Uinta Mountains across the Rock 

Springs uplift and the Sand Wash Basin (fig. 17). Any 

southerly drainage across Axial Basin into the Piceance 

Creek Basin, therefore, should have ended long before

111° 109°

FIGURE 17. Directions of foreset bedding and cross stratification in 

the Laney Member of the Green River Formation (small arrows; 

from Surdam and Stanley, 1979) and transport directions of Bishop 

Conglomerate (large arrows) greater Green River Basin area.

the Gilbert Peak erosion surface was formed. Modern 

south-flowing drainages (such as Sand Wash, the Little 

Snake River, and Vermillion Creek) postdate the deformation of the Gilbert Peak erosion surface. For example, the Bishop Conglomerate east of Sand Wash had 

to come from sources to the west or south, opposite 

or across the present drainage direction; Vermillion 

Creek flows southwest, but Bishop remnants there 

slope northeast. Similarly, the course of the Green 

River farther west is directly opposite the old pediment 

slope of Hickey, Cedar, Black, Little, Miller, Flattop, 

and Pine Mountains.

An alternative and more likely direction for a baseleveling stream in the Green River Basin in Gilbert 

Peak time, therefore, was eastward toward the North 

Platte River, as first suggested by Bradley (1936, p. 

177) and as elaborated by Hansen (1965, 1969b). Ritzma 

had endorsed the idea in 1959; Sears had hinted at it 

in 1924 (1924a, p. 304), but it had never received serious attention until my reconnaissance studies of the 

1960's. Such a stream would have provided a geomorphically credible base level for the Bishop Conglomerate. It may have crossed the present Continental 

Divide at the Great Divide Basin near Tipton, Wyo., 

skirting the north side of the Rawlins uplift, and 

perhaps merging with the Sweetwater River. (See section on the capture of the master drainage of the Green 

River Basin, p. 67.) According to Denson and Chisholm 

(1971, p. C125) the main drainage framework in central 

and eastern Wyoming was in existence at the close of
BISHOP CONGLOMERATE 27

the Eocene, including the courses of the Sweetwater 

and the North Platte, and persists today with only 

minor modifications. The Tertiary history of central 

Wyoming is complex (Love, 1970, 1971), and the exact 

pattern of drainage is problematical, but it is not incompatible with easterly escape of drainage from the Green 

River Basin. Even now the Continental Divide at Tipton is 245 m lower (altitude about 2,070 m) than the 

base level of the Gilbert Peak erosion surface near the 

center of the Green River Basin (altitude about 2,315 

m).

ALTITUDE AND RELIEF

To get some idea of regional altitude and relief at 

the onset of deposition of the Bishop Conglomerate, one 

may use the top of the Tipton Member of the Green 

River Formation as a datum and a basis for speculation, since this horizon has wide areal extent. Along 

with the overlying Bridger and Washakie Formations, 

the Green River Formation was deformed at the end 

of the Eocene, and deposition was supplanted by erosion. From the center of the Green River Basin west 

of Rock Springs to the flank of Pine Mountain about 

80 km to the south, the difference in altitude of the 

top of the Tipton is about 760 m, judging from maps 

by Bradley (1964) and Roehler (1972, a, b). This figure 

essentially represents pre-Bishop differential uplift following deposition of the Bridger and Washakie Formations. Perhaps 150 m should be subtracted from the 

760 m total to account for an initial dip of 0.2-0.4 m/km 

on the Tipton, for stratigraphic transgression of the 

top, and for an unknown but possibly appreciable later 

Tertiary and Pleistocene differential uplift of the mountains. Thus, between the end of the Eocene and the 

deposition of the Bishop Conglomerate, the northeast 

flank of the Uinta Mountains may have risen some 600 

m relative to the centers of the adjacent basins.

Differential uplift, however, was accompanied and 

reinforced by regional (epeirogenic) uplift, which raised 

the centers of the basins as well as the mountains high 

enough perhaps 900 m to change the climate from 

the subtropical one of the middle Eocene to the more 

temperate climate of the post-early Oligocene, as indicated by floristic and faunal studies (Dorf, 1959; MacGinitie, 1969; Love, 1971; Leopold and MacGinitie, 

1972). Nine hundred meters of regional uplift would 

also provide a reasonable gradient for drainage out of 

the Green River Basin to an ultimate base level far 

to the east, a gradient somewhat less than but not too 

different from that of the present Sweetwater and 

North Platte Rivers across central Wyoming. The center of the Green River Basin, therefore, may have been 

about 1,200 m above sea level when the Gilbert Peak

erosion surface began to form. The mountains were 

more than 1,600 m higher, because the surface itself 

has 1,600 m or more of relief, and the peaks stand 300- 

600 m above it. Perhaps they stood 3,000-3,700 m 

above sea level. The crestline was much degraded during Gilbert Peak and Bishop time, and the coarseness 

of the Bishop suggests that local relief was greater then 

than now.

RATES OF UPLIFT

Figures in the previous paragraph provide a means 

of estimating minimum uplift rates in the Lake Gosiute 

area between the end of the Eocene and the deposition 

of the Bishop Conglomerate. The maximum elapsed 

time was about 9 m.y., inasmuch as the Eocene ended 

about 38 m.y. ago, and a tuff well up in the Bishop 

is about 29 m.y. old, as dated by H. H. Mehnert (Hansen and others, 1981). Based on these dates and on 

the altitude differences of the Tipton datum before and 

after uplift, regional uplift in the basin exceeded 10 cm 

per thousand years (900 m/9 m.y.), and the rate of rise 

along the basin rim exceeded 16.6 cm per thousand 

years (1,525 m/9 m.y.). The actual uplift, however, 

probably took much less than 9 m.y., perhaps half as 

much, because a long period of crustal and climatic stability had to follow uplift and accompany the cutting 

of the Gilbert Peak erosion surface to allow so broad 

and featureless a plain to form. This period itself probably lasted millions of years, maybe most of Oligocene 

time. The Rock Springs uplift, for example, had to be 

truncated by the Gilbert Peak erosion surface before 

the Bishop was deposited while, concomitantly, hundreds of square kilometers of hard rock was being bevelled along the Uinta Mountains. Reasonable rates of 

uplift, therefore, perhaps would have been closer to 20 

cm per thousand years near the center of the basin 

and 30 cm per thousand years along the mountain flank. 

By way of contrast, the earlier rate of Laramide uplift 

of the Uinta Mountains was a half to a whole order 

of magnitude higher, in the range of 1-2 m per 

thousand years (about 9,150 m in 5-10 m.y.; Hansen, 

1984, p. 15). This figure compares favorably with a calculated deformation rate for the Wind River Mountains 

in Wyoming 1.3 m/1,000 yrs for the displacement of 

the Wind River fault (Hurich and Smithson, 1982, p. 

1559).

In the time since the Bishop Conglomerate was deposited the average rate of uplift has been very low. The 

center of the basin has risen about 1,100 m in the last 

29 m.y. about 3.8 cm per thousand years (or, in round 

numbers, 4 cm). During that 29-m.y. interval the rate 

probably was uneven and may have varied greatly from 

time to time.
28 NEOCENE OF EASTERN UINTA MOUNTAINS

BROWNS PARK FORMATION

The Browns Park Formation (fig. 13) was named for 

the valley of Browns Park astride the Utah-Colorado 

State line, where it is widespread and is exceptionally 

well exposed (Powell, 1876, p. 44, 186). From Browns 

Park it extends like a blanket east and south to Elk 

Springs and southeast into the Maybell-Lay-Axial 

Basin area, retaining a thickness of more than 490 m 

(Dyni, 1968; McKay and Bergin, 1974). Since Powell's 

time it has been described in considerable detail in 

many published reports, partly because it has been host 

to commercial uranium deposits.

The Browns Park Formation seems to have begun 

to accumulate in some places soon after the Bishop 

Conglomerate was deposited, perhaps with no great 

stratigraphic hiatus; the evidence is inconclusive. In 

most places, however, millions of years intervened. The 

region was in the midst of tectonic deformation, and 

sediments were accumulating in favorable locations 

while rock was being eroded nearby. The lithology of 

the formation, therefore, is varied (Luft and Thoen, 

1981), but it consists chiefly of light-colored to nearly 

white, loosely cemented, generally calcareous sandstone and light-gray to white, vitric and ashy to earthy, 

friable to firm, rhyolitic tuff. Locally the sandstone is

tan, has a pale greenish cast, or is rusty brown. Some 

of the sandstone is tuffaceous, and some of the tuff is 

sandy. Eastward from the Lay-Maybell area the appearance changes, and the formation more resembles 

the North Park Formation or the Troublesome than 

typical Browns Park.

Much of the sandstone in the more easterly area is 

eolian (fig. 18) and is intensely cross-stratified, especially in the area between Elk Springs and Maybell 

(Dyni, 1968, 1980; McKay and Bergin, 1974; Honey and 

Izett, in press). The source of the sand is undetermined, but deflation of nearby loose, sandy, upper 

parts of the Bishop is a likely possibility. The Bishop 

at that time was more widespread than it is now.

In the west, much of the sandstone in the Browns 

Park is cross-laminated or is obscurely bedded and appears to be fluvial. Some of it is too friable to be called 

rock; it is easily crushed between the fingers and would 

be called soil or overburden by engineers. Tuff is abundant in the western part of the area, particularly from 

Vermillion Creek west into Utah. There, more than 35 

percent of the material in measured sections has been 

identified as tuff or tuffaceous sandstone (Hansen, 

1965, fig. 40). The formation also contains bedded chert 

in its lower part near Vermillion Creek and East Boone

FIGURE 18. Eolian crossbedding in Browns Park Formation near U.S. Highway 40 a few kilometers west of Maybell, Colo. Tangentially 

bedded upper set rests on a planar truncation surface on the lower set. Exposure is about 3 m high. Viewed toward the northwest.
BROWNS PARK FORMATION 29

Draw; it contains minor amounts of limestone, especially in the Douglas Draw-Smelter Hill area south of 

Grey stone, some of which is oolitic; it contains siltstone 

and lacustrine clay; and, in the western part of Browns 

Park, it has abundant locally derived conglomerate at 

varied stratigraphic levels. All these rock types are 

mentioned to highlight the lithologic differences between the Browns Park Formation and the Bishop Conglomerate.

In the western Browns Park area and just to the 

west in Red Canyon, the formation has greater 

lithologic diversity than elsewhere, mostly because of 

a varied depositional environment engendered by 

strong topographic relief. I regard patches of wellcemented sandstone and conglomerate along and below 

the rims of Red Canyon as remnants of alluvial fans. 

Most of the constituent material was derived from the 

Uinta Mountain Group, and the resulting deposits 

range from fine-grained sandstone to heterogeneous 

bouldery conglomerate, of which the larger boulders 

exceed 3 m in diameter. In the Dutch John area, sandstone and conglomerate interfinger with tuff. The overall color of the Browns Park at Red Canyon is tawny 

to light gray, exemplified by large exposures near the 

right (south) abutment of Flaming Gorge Dam. It contrasts, therefore, with the somber reds of the adjacent 

Uinta Mountain Group. Although some clasts retain the 

red color of the parent Uinta Mountain Group, most 

of them are partly bleached or mottled. Some have 

bleached shells covering fresh red cores. Calcium carbonate is the chief cementing material, but some sandstone is cemented with silica.

The tuffs are of two general types, vitric and earthy. 

Some of the vitric ones are well bedded or crossbedded, 

and some even contain climbing ripples that suggest 

rapid sedimentation in standing water (fig. 19). Others 

appear to have been reworked by winds into eolian 

crossbeds. Many tuffs, though, are massive or only 

faintly laminated; these perhaps settled quietly from 

still air. All the vitric tuffs have low bulk densities. 

The earthy tuffs are mostly well bedded, commonly 

having laminations that look lacustrine (fig. 19). Such 

a mode of deposition would facilitate alteration of the 

glass to produce the earthy texture. Some beds have 

load casts, ball-and-pillow structures, contortions, and 

broken, disrupted laminations of a sort that should form 

in standing water. As much as 20 percent of the material in several measured sections is lacustrine clay 

(Hansen, 1965, fig. 40), some of which contains diatoms 

and sparse ostracods.

The source of the tuffs is unknown; explosive volcanic 

activity was widespread in the Western United States 

in Miocene time (Izett, 1968; Marvin and others, 1970; 

Christiansen and Lipman, 1972). Most of the nonvolcanic material in the valley of Browns Park was locally 

derived. Broad fans, consisting chiefly of pebbly cobbly 

alluvium derived from the Uinta Mountain Group but 

including Paleozoic limestone and older Precambrian 

metamorphic rocks, spread intermittently from the 

highlands enclosing Browns Park. The fans were buried 

from time to time by falls of volcanic ash, some of which 

was reworked into tuffaceous sandstone. Periodically, 

much of Browns Park was flooded by lake waters that 

deposited blankets of sand and clay. The result is a 

complex interbedding of conglomerate, sand, tuff, and 

clay. The tuffs and clays retain remarkable uniformity 

over considerable distances, but the sands and conglomerate thin from the sides toward the axis of the valley 

(Hansen, 1957a). Parts of the section were exposed to 

erosion at times, but for how long is unknown. Although unconformities have been recognized within the 

Browns Park Formation (Sears, 1924a, p. 296; Ritzma, 

1965b, p. 131; Hansen, 1965, measured sections), their 

significance is undetermined. Sears noted an angular 

discordance near the east end of Browns Park.

Boulders of unusual size, some more than 5 m in diameter, occur in the Browns Park Formation at and 

near the SW corner sec. 21, T. 2 N., R. 24 E., near 

the head of the valley of Browns Park. There the formation fills a steep-sided channel cut into the Uinta 

Mountain Group just north of the existing trench of 

the Green River. These deposits pass into finer grained 

conglomerate which, in turn, interfingers with tuffaceous sandstone (Hansen, 1965, p. 118).

POST-BISHOP, PRE-BROWNS PARK 

CHANGE IN DRAINAGE DIRECTION

Other fanglomerates in the Browns Park Formation 

near the head of Browns Park thicken and coarsen 

markedly from the center of the valley toward the north 

margin and thus provide clear evidence of their source 

and direction of transport. Red quartzite from the 

Uinta Mountain Group is the chief constituent and, on 

lithologic grounds only, could have come from any direction, but abundant pebbles of white metaquartzite, 

amphibolite, and other rock types from the Red Creek 

Quartzite and occasional pebbles of gray Paleozoic limestone must have had sources to the north (Hansen, 

1965, p. 118, 173). The Red Creek material obviously 

came from nearby areas just north of Browns Park. 

The limestone pebbles probably were recycled out of 

conglomeratic beds in the Wasatch Formation, also to 

the north, because all the Paleozoic limestone in that 

part of the Uinta Mountains had been removed by postWasatch erosion and pre-Browns Park movements on 

the Uinta fault. The Wasatch, therefore, is the only 

likely source, and the nearest Wasatch outcrops are
30 NEOCENE OF EASTERN UINTA MOUNTAINS
BROWNS PARK FORMATION 31

north of Browns Park near the Wyoming State line. 

Some cobbles might have been reworked from Bishop 

Conglomerate, but it, too, lay to the north. Thus, the 

direction of drainage across the area was reversed after 

the Bishop Conglomerate was deposited from the south 

and before the Browns Park was deposited from the 

north. The reversal was caused by regional tilting, described in detail later. The tilting and the drainage reversal required a significant interval of time after the 

Bishop was deposited and before the Browns Park Formation was deposited.

AGE OF THE BROWNS PARK FORMATION

Radiometric and fission-track datings indicate a wide 

age span for the Browns Park Formation in the Eastern Uinta Mountains region. The oldest date so far 

measured, 24.8±0.8 m.y., is a potassium-argon age obtained from a biotite separate of chalky white tuff collected by Izett and others (1970; Izett, 1975) from the 

northwest side of the Little Snake River, about 30 m 

above the top of the Bishop Conglomerate, and hence 

very close to the base of the Browns Park Formation. 

This tuff is about 4 m.y. younger than the tuff in the 

Bishop Conglomerate on the Diamond Mountain 

Plateau and is approximately equivalent in age with the 

lowermost Arikaree Formation of the Great Plains and 

the Wyoming Basin. A much younger zircon fissiontrack age of 9.9±0.4 m.y. was obtained by Naeser and 

others (1980, p. 24) from vitric tuff higher in the section 

at Vermillion Creek. Damon (1970, p. 52) determined 

a potassium-argon age of 11.8±0.4 m.y. for glass from 

a vitric tuff collected by G. R. Winkler in Browns Park 

in Utah. These ages thus span about 15 m.y., the latter 

two being equivalent with part of the Ogallala Formation of the Great Plains. Honey and Izett (in press) 

have recently reported an additional age of 11.3 ±0.8 

m.y.from a site between Maybell and Cross Mountain. 

Whether or not deposition was continuous throughout 

that time is unknown, but local unconformities suggest 

episodic pauses of undetermined length. Physiographic 

constraints, discussed further below, argue against 

rocks much older than 12-15 m.y. existing in the 

Browns Park Formation within the valley of Browns 

Park.

FIGURE 19. Water-laid tuffs in the Browns Park Formation near 

Dutch John, Utah (in measured section B-2 of Hansen, 1965, 

p. 121).

Top. Well-stratified, earthy tuffs overlain unconformably by massive, vitric airfall tuff. Scale is in inches (1 in. = 2.54 cm).

Bottom. Climbing ripples, laminae in phase, suggest heavy suspended load and rapid sedimentation in gently oscillating water.

BISHOP CONGLOMERATE BENEATH

THE BROWNS PARK FORMATION IN

THE ELK SPRINGSh-MAYBELL AREA

In many places east of the Uinta Mountains a thick, 

loosely cemented conglomerate, here assigned to the 

Bishop, lies at the base of the Browns Park Formation. 

The existence of this rock has led to most of the speculation regarding the relationship of the Browns Park 

to the Bishop. This speculation arose, perhaps, out of 

the widely held assumption that the conglomerate is 

merely the basal unit of the Browns Park Formation. 

Sears' early conclusion (1924a) that the Bishop and the 

basal Browns Park are one and the same was subsequently challenged by Bradley (1936). This conglomerate has not generally been shown separately on maps, 

nor has it received close scrutiny in the literature; its 

detailed distribution and its relation to the Browns 

Park have largely escaped serious study. McKay (1974) 

assigned it to the Browns Park Formation but noted 

its significant lateral differences and mapped its outcrop separately for about 19 km west of the Little 

Snake River. Dyni (1980) described it in some detail 

but did not differentiate it on the map; most other 

workers in the area did neither.

Evidence now indicates that two separate and distinct conglomerates underlie the Browns Park Formation in the area east from Vermillion Creek to the limit 

of outcrop east of Maybell. One is indeed a basal conglomerate, and it passes upward gradationally into the 

overlying beds (Hancock, 1925, p. 24). The other, I believe, is the Bishop, disconformable with the Browns 

Park and genetically unrelated.

Sears (1924b, p. 295) actually touched close to the 

heart of the problem when, in describing the Browns 

Park Formation, he noted that "almost everywhere the 

base of the formation is marked by a conglomerate, 

which has very different characteristics in the eastern 

and western parts of the field." From Simsberry Draw 

(which is just east of the Little Snake River; fig. 13) 

east toward Juniper Mountain, the basal conglomerate 

of the Browns Park Formation is less than 1 m thick 

and has a light-gray overall tint. North of Sunbeam 

it is locally nonexistent. Just east of Juniper Mountain, 

near the Yampa River, it reaches about 15 m (Hancock, 

1915, p. 186). East from Simsberry Draw to Juniper 

Mountain, it consists of small subrounded to subangular 

pebbles of mafic and felsic igneous and metamorphic 

rocks, red and white quartzite, varicolored chert, and 

milky quartz (McKay, 1974; McKay and Bergin, 1974; 

Sears, 1924b, p. 295). Its lithology indicates a Park 

Range provenance (Izett, 1975, p. 203; oral commun., 

1975).
32 NEOCENE OF EASTERN UINTA MOUNTAINS

From the Little Snake River west toward Vermillion 

Creek, however, the conglomerate changes abruptly in 

thickness, color, and lithology to an aspect much the 

same as that of the classic Bishop Conglomerate: "in 

appearance the two conglomerates are practically identical" (Sears, 1924b, p. 289). Tracing it from the Little 

Snake River to Vermillion Creek, Sears (1924a, p. 295; 

1924b, p. 285) reported a maximum thickness of 90 m. 

McKay (1974) reported 15-90 m. The average might 

be near 45 m. Locally, however, it is nonexistent removed, I believe, by pre-Browns Park erosion. As seen 

at Sand Wash, it is a poorly sorted cobbly conglomerate 

that contains abundant boulders of red quartzite and 

gray limestone, some more than a meter in diameter, 

mostly some distance above the base, and clearly derived from the Uinta Mountains. Its overall color is 

pink. Its dip is southwesterly. At the Little Snake 

River, where it appears to be partly eroded, it is not 

as coarse or as thick as at Sand Wash, but there also 

its provenance is the Uinta Mountains, and it retains 

the overall pink color.

On the southeast side of the Yampa valley, between 

Elk Springs and Maybell, across the Browns Park-Lay 

syncline, the conglomerate reemerges from beneath the 

Browns Park Formation and forms a line of rolling hills 

on and flanking Elk Springs Ridge. There it has a 

northerly dip of several degrees. There, too, the provenance is clearly the Uinta Mountains, although the dip 

is toward the Uintas. Cobbles and small boulders of 

gray limestone predominate, but red quartzite is abundant, and gray chert is common. Notably lacking are 

igneous and metamorphic crystalline rocks from the 

Park Range. The assemblage is typical Uinta Mountains material, and the texture is much coarser than 

that of the Park Range derivatives at the base of the 

Browns Park Formation to the east and northeast. At 

Elk Springs the conglomerate is very well exposed in 

a large gravel pit just south of U.S. Highway 40, where 

it has a dip of about 20 degrees north.

West from Elk Springs, as the conglomerate emerges 

from beneath the white eolian sandstone of the Browns 

Park Formation, it flattens rapidly, then rises gradually westward. To the north the conglomerate slopes 

gently into the Lily Park syncline (Dyni, 1968). Although mapped as Browns Park Formation west and 

north of Elk Springs (Schultz, 1918, pi. 5; Sears, 1924b, 

p. 291; Dyni, 1968, 1980; McKay, 1974), it has the appearance of the Bishop conglomerate where it emerges 

from beneath the Browns Park sandstones. To the west 

it caps flat-topped remnants of a once broader surface 

that merges farther west with the high Gilbert Peak 

erosion surface of the south flank of the Uinta Mountains.

CRITICAL RELATIONSHIP OF

THE BROWNS PARK FORMATION

TO THE BISHOP CONGLOMERATE

NEAR LILY PARK

Although there are thus two distinctly different conglomerates at the base of the Browns Park Formation 

east of the Uinta Mountains, they are not of themselves 

wholly incompatible with Sears' early view (1924a) that 

the Bishop Conglomerate is the basal conglomerate of 

the Browns Park Formation. Exposures in the Lily 

Park area, however, especially in the canyon of the Little Snake River between the east end of the Uinta 

Mountains and Cross Mountain, demonstrate the topographic and sedimentological discontinuity between the 

Bishop Conglomerate and the Browns Park Formation.

West and south of Cross Mountain, at Klauson Pasture, Horse Gulch (on the west flank of Cross Mountain), and Twelvemile Mesa, thick terrace deposits of 

conglomerate stand high above the valleys of the Little 

Snake and Yampa Rivers, lapping across the older formations (fig. 20; see also fig. 9, bottom). These deposits 

have been referred to by earlier investigators as the 

lower conglomerate unit of the Browns Park Formation, but they are here regarded as Bishop. The conglomerate cap at Twelvemile Mesa merges to the south 

with the conglomerate of Elk Springs Ridge, which in 

turn merges westward with the remnants that P. D. 

Rowley, P. E. Carrara, and I mapped as Bishop Conglomerate along the south flank of the Uinta Mountains 

(Rowley and others, 1979). The deposits on Twelvemile 

Mesa and Klauson Pasture have the typical Uinta 

Mountains lithologic affinities and are practically identical; the Horse Gulch deposit is largely local material 

derived from the slopes of Cross Mountain, but all 

three deposits are clearly correlative geomorphically 

and must have once have been a continuous body of 

gravel resting on one surface.

This surface, restored, has a northerly component of 

slope caused by postdepositional tilt. Near the north 

end of Cross Mountain it ends as a terrace, although 

it once may have continued north some distance beneath the Browns Park Formation, just as it still does 

to the south at Elk Springs. Field evidence on that 

point is unclear. However and this relationship is critical the Browns Park Formation extends tongue-like 

down the canyon of the Little Snake River at a level 

lower than the outcropping conglomerate, to and 

beyond the point where Klauson Pasture and the bench 

at Horse Gulch project toward one another (fig. 21). 

Beneath the sandstones of the Browns Park Formation 

at that point, a well-stratified, horizontally bedded 

basal conglomerate about 5 m thick rests on a bedrock 

terrace cut on the Glen Canyon Sandstone about 50 m
STRATIGRAPHIC RELATIONSHIPS 33

FIGURE 20. Poorly sorted, poorly bedded Bishop Conglomerate resting unconformably on truncated Morrison Formation, west flank of 

Cross Mountain, Moffat County, Colo., about 120 m above valley floor. This sequence was considered by earlier investigators to be 

the basal conglomerate of the Browns Park Formation. Boulder at center of photograph is 60 cm across. Compare with figure 22.

above the river (fig. 22). This relationship is well exposed at Wild Cow Draw, 3 km downstream from the 

bridge and gaging station at the head of the Little 

Snake canyon. There, the basal conglomerate is mostly 

subangular pebbles and small cobbles embedded in a 

well-cemented matrix of poorly sorted sandstone. The 

clasts are mostly red quartzite and gray limestone derived from the adjacent mountains. This conglomerate 

is unlike the nearby Bishop, and there can be no mistaking its separate identity. The Bishop here is 60 m 

thick, caps benches 180 m or more above the valley 

floor, and differs from the basal Browns Park in thickness, bedding habit, stratigraphic position, and topographic setting. The Bishop is less well sorted, and its 

bedding is less uniform. One can only conclude that the 

Browns Park Formation was deposited in the valley 

long after the conglomerate on Klauson Pasture and 

on the bench at Horse Gulch was parted by erosion.

Twelvemile Mesa correlates geomorphically across 

the Yampa River with Klauson Pasture (Dyni, 1980), 

and the capping conglomerates on these surfaces are

WEST

Uinta 

Mountains

EAST

Axis of 

Lily Park syncline

Paleozoic and Mesozoic rocks

\\

FIGURE 21. Sketch section across canyon of Little Snake River at 

Lily Park syncline, showing relationship of Bishop Conglomerate 

(Tb) to Browns Park Formation (Tbp). Section is about GVz km 

across. Vertical scale is greatly exaggerated.

virtually identical. But the deposit on Twelvemile Mesa 

passes beneath the Browns Park Formation to the east 

and south, further proving that the Browns Park is 

stratigraphically higher and younger. According to J. 

R. Dyni (oral commun., 1981) the conglomerate beneath 

the Browns Park on Twelvemile Mesa is discontinuous. 

Dyni (1980) suspected pre-Browns Park (pre-"upper
34 NEOCENE OF EASTERN UINTA MOUNTAINS

FIGURE 22. Basal conglomerate of the Browns Park Formation resting unconformably on truncated Glen Canyon Sandstone, in the canyon 

of the Little Snake River at the east end of the Uinta Mountains. Pebbles and small cobbles are embedded in a matrix of well-cemented, 

poorly sorted, coarse-grained sandstone. Exposure is about 50 m above river level. Compare with figure 20.

sandstone unit") folding in the conglomerate. It seems 

clear that the conglomerate on Twelvemile Mesa, as 

well as on Klauson Pasture, was deformed and dissected before the Browns Park Formation was deposited. 

In the broad downwarp between Vermillion Creek and 

Maybell, however, where the Bishop was lowered 

enough tectonically for subsequent burial, the Browns 

Park was deposited directly on it. Elsewhere the 

Browns Park lies in valleys tens to hundreds of meters 

lower than nearby remnants of Bishop Conglomerate.

RANDOM RELATIONSHIP OF THE

BROWNS PARK FORMATION TO THE

GILBERT PEAK EROSION SURFACE

Bradley (1936) was first to evaluate the relationship 

of the Browns Park Formation to the Gilbert Peak erosion surface, largely refuting the earlier conclusions of 

Sears (1924a), who later acceded to Bradley's view. As 

will be shown, this relationship provides further evidence of the disparate ages of the Browns Park Formation and the Bishop Conglomerate. It now appears that 

Sears was correct in identifying as Bishop the conglomerate beneath the Browns Park Formation in the Dry 

Mountain-Sand Wash-Little Snake River area but was 

incorrect in concluding that it also was a basal conglomerate of the Browns Park.

Bradley (1936) did not believe that the conglomerate 

beneath the Browns Park Formation was the Bishop, 

nor that the underlying pediment was the Gilbert Peak 

surface. Specifically, he believed that the pediment 

remnant preserved on Bear Mountain was part of a regional erosion surface (the "Bear Mountain erosion surface") which passed eastward beneath Browns Park and 

was lower and distinctly younger than the Gilbert Peak 

surface. He also supposed that the Browns Park Formation was deposited on this surface shortly after the 

surface formed, and that the conglomerate underlying 

the Browns Park was the basal unit of that formation. 

Remnants of a surface that Bradley correlated with his
STRATIGRAPHIC RELATIONSHIPS 35

Bear Mountain surface are plentiful in the southwestern part of the Green River Basin of Wyoming; they 

generally lie 120-150 m below the Gilbert Peak erosion 

surface and 60-90 m above modern drainage, and they 

are capped with gravel but lack the covering white 

sand of the typical Browns Park (Bradley, 1936, p. 

180). Bradley considered the gravel on these remnants 

to be a basal conglomerate of the Browns Park Formation, but such a correlation between dissimilar 

deposits many kilometers apart is tenuous in the 

absence of typical Browns Park material. In all probability, the gravel on those outlying remnants is Pleistocene or late Pliocene.

I regard the two surfaces Gilbert Peak and type 

Bear Mountain as one, displaced by faulting, for the 

following reasons:

1. In its type area, the Bear Mountain surface at 

Bear Mountain and Dutch John Bench is separated 

from the Gilbert Peak surface at Goslin Mountain by 

faulting.

2. Berm-like remnants or outliers of the Gilbert 

Peak surface might be expected to remain between 

Bear Mountain and the crest of the range if the Bear 

Mountain surface were younger and lower than the Gilbert Peak surface.

3. The Bear Mountain surface as mapped by Bradley is everywhere capped by younger deposits, except 

at the type locality, where it and the adjacent Gilbert 

Peak surface are both bare bedrock.

4. The type Bear Mountain surface rises and 

merges gradually westward with the type Gilbert Peak 

surface (Bradley, 1936, fig. 16).

5. At Red Canyon, the type Bear Mountain surface 

is 360-550 m above modern drainage and stands tens 

to hundreds of meters above remnants of the Browns 

Park Formation, which fill swales and valleys cut into 

the surface. Gravel-capped remnants that Bradley correlated with the surface in southwestern Wyoming are 

only 60-90 m above modern drainage. The type surface 

was deeply dissected before the Browns Park Formation was deposited, and a long period of time, therefore, intervened.

6. The type Bear Mountain surface, like the Gilbert 

Peak surface, projects high above the head of Browns 

Park, rather than passing beneath it as Bradley supposed.

7. Time constraints argue against the development 

of a local but broad surface cut into hard rock and intermediate in age between the Bishop Conglomerate and 

the Browns Park Formation. A long time interval had 

to elapse (a) during pedimentation, (b) during subsequent dissection of the pediment, and (c) before deposition of the Browns Park Formation across the dissected topography.

8. There is only one extensive subsummit pediment 

in the Eastern Uinta Mountains, just as there is only 

one elsewhere in the Southern Rocky Mountains of Colorado (Epis and Chapin, 1975; Epis and others, 1980).

The random relationship of the Browns Park Formation to the Gilbert Peak surface is even more evident 

in the valley of Browns Park (fig. 23). There the Gilbert 

Peak erosion surface forms the broad summits of the 

several high mesas (Goslin Mountain, Mountain Home 

in part, Head of Cottonwood, Bender Mountain in part, 

0-Wi-Yu-Kuts Mountain, and Cold Spring Mountain; all 

on fig. 27) that border the valley and stand 300-600 

m above the valley floor. Filling the valley bottom, the 

Browns Park Formation laps against the lower slopes 

and spurs of these mesas in depositional contact with 

the Uinta Mountain Group (into which the mesas and 

the valley subfloor are carved) and extends into side 

valleys cut by post-Gilbert Peak erosion. The contact 

of the Browns Park Formation with the Uinta Mountain Group is discontinuously faulted along the northeast side of the valley by post-Browns Park movements 

on older faults (Hansen, 1965, plate 1; also fig. 27, this 

report), but the old hard-rock floor of the valley was 

formed chiefly by erosion along the axis of the Uinta 

anticline long after the Gilbert Peak erosion surface had 

formed. Later valley erosion was guided by warping 

and faulting before deposition of the Browns Park Formation.

In many places along the north side of Browns Park 

the Gilbert Peak erosion surface is tilted southward toward the valley, opposite its original slope. This tilting 

created new south-flowing drainages. Earlier, when the 

Bishop Conglomerate was accumulating, the site of 

Browns Park must have been a mountainous source of 

detritus for the conglomerate now exposed in the broad 

area to the north. The Gilbert Peak surface stretched 

northward from these mountains. The reversal of topography came later, following pedimentation, faulting, 

and foundering of the Uinta arch, before the Browns 

Park Formation was deposited, and the resulting 

trough provided a natural stilling basin for the accumulation and preservation of the gravels, sands, tuffs, and 

clays of the Browns Park Formation (Hansen, 1965, p. 

129-130).

The extent of faulting beneath Browns Park is 

obscured by the Browns Park Formation and hence is 

unknown, but it probably is considerable, if the abundance of faults in the valley margin is indicative. Some 

of these faults cut the Browns Park Formation, but 

most of them pass beneath it. The course of drainage 

through Browns Park surely was influenced by such 

faults. A drainage line controlled initially by a fault line 

need not coincide with the present trace of the fault 

if the fault has an appreciable dip. As the land surface
36 NEOCENE OF EASTERN UINTA MOUNTAINS

Miller Mountain Pine Mountain

Pea

-

*

»__ -a, *

FIGURE 23. Aerial view over Browns Park looking north across the Gilbert Peak erosion surface on Cold Spring Mountain, in middle 

distance, toward Pine (Bishop) Mountain, flecked with snow drifts on skyline. Browns Park Formation in the foreground, 760 m below 

top of Cold Spring Mountain, is in depositional contact with the Uinta Mountain Group. Note how the Browns Park Formation extends 

into Spitzie Draw, indicating that the draw which obviously is younger than the Gilbert Peak erosion surface is older than the 

Browns Park Formation, though modified by post-Browns Park erosion. Pine Mountain on the skyline is capped with Bishop Conglomerate (the type locality). The Uinta fault passes between Pine Mountain and Cold Spring Mountain. -~
STRATIGRAPHIC RELATIONSHIPS 37

Initial fault trace

Initial land surface

FIGURE 24. Diagram showing how the trace of a dipping fault that 

initially controlled the position of a drainage line can shift away 

from that line during degradation. As the drainage cuts vertically 

downward (solid arrow) the fault trace shifts laterally (dashed 

arrow).

is lowered by degradation, the fault trace shifts laterally, whereas the drainage is prone to cut vertically 

downward (fig. 24). Examples of this principle are common in the West.

Although Bradley (1936, fig. 16) had thought it flat, 

the hard-rock subfloor of the old Browns Park valley 

apparently is canyonlike. In general its deepest part 

is well north of the present valley bottom, probably 

near the axis of the Browns Park syncline, which 

formed partly by differential compaction of the thick 

sedimentary prism. At Swallow Canyon the Green 

River now impinges on the south wall of the old valley, 

forming a superimposed trench cut into the Uinta 

Mountain Group through a once-buried spur Kings 

Point which has been exhumed and incised. The summit of the spur, 200 m above the river, must have once 

been covered by the Browns Park Formation. The 

buried axis of the old valley lies far to the north.

Just north of the Gates of Lodore, outcrops of the 

Uinta Mountain group limit the position of the old valley bottom to a zone less than 2.4 km wide. There the 

present valley bottom has an elevation of about 1,630 

m, but drill-hole data indicate that the hard-rock floor 

just to the north is at least 180 m deeper (S. J. Luft, 

U.S. Geological Survey, written commun., 1980). The 

buried-valley profile north of the Gates must have sloping sides of at least 10-12 degrees.

It thus is evident that the Browns Park Formation, 

unlike the Bishop Conglomerate, had no genetic relationship to regional pedimentation in the eastern 

Uinta Mountains in general or to the Gilbert Peak surface in particular. The Browns Park Formation filled 

valleys narrow and wide, buried hills and spurs of 

mountains, and spread out locally over broad areas. In 

the Boone Draw-Maybell-Lay area, it was deposited 

across the Bishop Conglomerate after the conglomerate 

and the subjacent Gilbert Peak erosion surface both had 

been partly dissected and had been greatly depressed 

by pre-Browns Park regional warping; locally, the 

Bishop was completely removed before the Browns 

Park was deposited. In the lowest part of its old valley, 

between Simsberry Draw and Jumper Mountain (fig. 

13), the Browns Park has its own basal conglomerate 

of material carried out of the Park Range by an ancestral drainage of the Yampa valley. On the other hand, 

where the Bishop remained above the general depositional level, the Browns Park Formation accumulated 

in depressions topographically lower than the Bishop, 

some much lower.

I know of only one locality in the eastern Uinta 

Mountains where the Browns Park Formation rests 

with apparent conformity on a well-defined but local 

remnant of a pediment. That locality is in Moffat 

County just north of Irish Canyon (fig. 42) in T. 10-11 

N., R. 101 W. Both Sears (1924a, p. 292) and Bradley 

(1936, p. 181-182) described it in some detail because 

of its relationship to Cold Spring Mountain and the 

Uinta-Sparks fault zone. Bradley used this relationship 

to refute the equivalence of the Bishop Conglomerate 

and the basal conglomerate of the Browns Park Formation. A berm-like remnant of a pediment, which Bradley identified as the Bear Mountain surface, cuts into 

the flank of Cold Spring Mountain about 300 m below 

the summit. The berm is capped with a cobbly conglomerate, about 15 m thick, made mostly of limestone cobbles and occasional small boulders in a coarse, poorly 

sorted sandstone matrix. The limestone plainly came 

from the nearby outcrops of Limestone Ridge along the 

north side of Cold Spring Mountain. Cobbles of red 

quartzite from the Uinta Mountains Group are subordinate just a few percent. The conglomerate passes beneath typical white sandstone of the Browns Park Formation and slopes eastward toward Vermillion Creek. 

Although Sears correlated this conglomerate with the 

Bishop, it differs markedly from nearby Bishop Conglomerate in thickness, lithology, and texture, and is, 

in fact, a local basal facies of the Browns Park Formation. On nearby Pine Mountain and Diamond Peak the 

Bishop stands 600 m higher, is nearly 60 m thick 

(Roehler, 1972a, b), and consists mostly of red quartzite 

clasts, some very large.
38 NEOCENE OF EASTERN UINTA MOUNTAINS

40° -

EXPLANATION

NORMAL FAULT Bar and ball on downthrown side

THRUST FAULT Dashed where concealed or inferred.

Sawteeth on upper plate 

ANTICLINE

SYNCLINE 

MONOCLINE

1. Axial Basin anticline

2. Browns Park-Lay syncline

3. Buckwater Ridge syncline

4. Crest fault

5. Cross Mountain anticline

6. Douglas Creek arch

7. Green River basin axis

8. Henrys Fork fault

9. Hiawatha anticline

10. Island Park fault/syncline

11 . Lily Park syncline

12. Moxa arch

13. Mud Springs monocline

14. North flank fault

15. Rangely anticline

16. Red Creek syncline

1 7. Red Wash syncline

18. Rock Springs uplift

1 9. Section Ridge anticline

20. Skull Creek-Willow Creek 

	anticline

21. South flank fault

22. Split Mountain-Yampa-Elk Springs 

	anticline

23. Uinta anticline

24. Uinta Basin axis

25. Uinta Basin boundary fault

26. Uinta-Sparks fault zone

27. Yampa fault

FIGURE 25. Generalized tectonic map of the Eastern Uinta Mountains and vicinity showing principal folds and faults. Dashed lines with 

sawteeth indicate the approximate positions of unexposed north-dipping thrust faults detected or inferred in the subsurface by seismic 

exploration and drilling (Anderman, 1961; Cullins, 1969; Ritzma, 1969). Similar concealed south-dipping thrust faults may border the 

range on the north (Clement, 1977; Cries, 1981). Symbol (D) beside fault indicates downthrow of second movement.
NEOGENE OF EASTERN UINTA MOUNTAINS 39

DEFORMATION

The principal structural features of the Eastern 

Uinta Mountains and adjacent areas are ploted on figure 25. The mountains first arose in latest Cretaceous 

time when the old Western Interior seaway drained for 

the last time. Tectonic events since that time, and corresponding geomorphic and sedimentologic events, are 

summarized in table 2. In a study of the sedimentary 

history of the Uinta Basin, Ryder and others (1976, 

p. 510) noted deformation of the Cretaceous coastal 

plain and closure of the Uinta Basin in latest Cretaceous time, accompanied by the rise of the Uinta Mountains. The easternmost Uinta Mountains region apparently was structurally low as late as Maastrichtian 

time, inasmuch as marine Lewis Shale has been recognized in the No. 1 Raeder-Government test well drilled 

on the Dry Mountain anticline in sec. 29, T. 9 N., R. 

99 W., Moffat County, Colo. (Ritzma, 1965b, p. 135). 

The western part of the range began to rise earlier, 

when the Maastrichtian, orogenic Currant Creek Formation (of Walton, 1944) was deposited on the early 

Campanian, pre-orogenic Mesaverde Formation (Walton, 1944, p. 126). According to Bruce Bryant (written 

commun., 1983), the Currant Creek contains clasts 

from the Uinta Mountain Group. Between these two 

areas, strong early Laramide uplift of the east-central 

part of the range is indicated at Goslin Mountain, 

where the late Campanian Ericson Sandstone was overturned and faulted before the Fort Union Formation 

(Paleocene) was deposited. If the Lewis Shale was ever 

deposited there, it was removed by pre-Fort Union erosion. Obvious chips of Mowry Shale and probable gastroliths from the Morrison Formation or the Cloverly 

are preserved in the Fort Union conglomerates at Goslin Mountain. Their presence indicates 2,400-3,000 m 

of erosion off the Uinta anticline by Paleocene time 

(Hansen and Bonilla, 1954, p. 11; Hansen, 1965, p. 170). 

No really coarse debris was shed from the Eastern Uintas at that time, but little rock hard enough to yield 

coarse debris had yet been exposed.

By Wasatch time (early Eocene) the eastern part of 

the range was eroded to its Precambrian core, as cobbles from the Uinta Mountain Group accumulated in 

the main body of the Wasatch Formation north of Clay 

Basin (Hansen and Bonilla, 1954, p. 11; Wiegman, 1964, 

p. 41) and in the Tipton Tongue of the Green River 

Formation southwest of Vermillion Creek (Sears and 

Bradley, 1925, p. 97; Ritzma, 1955, p. 38). Just north 

of Manila, near the Henrys Fork fault (fig. 25), the 

Wasatch contains very bouldery conglomerate derived 

from most of the resistant formations in the nearby 

mountains down to and including Mississippian rocks. 

The clasts appear in the Wasatch section in the reverse

order of their original stratigraphic positions. The 

coarseness of these deposits suggests a nearby source 

higher than the present Uintas. One limestone boulder 

3.3 m across must have been traveled at least 13 km. 

Major uplift of the Uintas during Tipton time may have 

coincided with movement on the Henrys Fork fault 

(Anderman, 1955, p. 131), with renewed movements on 

the Uinta fault north of Goslin Mountain, where the 

Fort Union is faulted against the Uinta Mountain 

Group (Hansen, 1965, p. 170), and with the flood of 

coarse debris into the Tipton near Vermillion Creek 

(Schultz, 1920, p. 31; Sears and Bradley, 1925; Ritzma, 

1955, p. 39). The Laramide orogeny apparently ended 

in the Eastern Uinta Mountains after the youngest Eocene rocks had been deposited, including the bulk of 

the Duchesne River Formation, whose upper member, 

the Starr Flat Member of Andersen and Picard (1972), 

is conglomeratic near the mountains. The Starr Flat 

Member probably is Oligocene (Andersen and Picard, 

1972, p. 16; Emry, 1981). The range then lapsed into 

a long period of quiescence, during which the Gilbert 

Peak erosion surface began to take form. On the south 

flank of the Uinta Mountains, however, the Starr Flat 

Member(?) rests unconformably on a truncation surface 

that might be the Gilbert Peak erosion surface. If so, 

the surface may have begun to form in late Eocene 

or early Oligocene time. Basinward from the mountains 

the Starr Flat Member apparently intertongues with 

the subjacent Lapoint Member of the Duchesne River 

Formation (of Andersen and Picard, 1972, p. 15-16). 

Perhaps the Starr Flat and the Bishop are isochronous.

Compressive Laramide severing of the Uinta anticline from its root has been suggested by several workers, as noted by Hamilton (1981, p. 90). Stokes (1976) 

and Sears and others (1982, fig. 5) have postulated a 

Precambrian aulacogen, which may subsequently have 

influenced Laramide and later Tertiary structural 

trends. Regional northward tilting in a similar mode 

in Late Proterozoic (?) time (Hansen, 1977a) and stratigraphic thickening along the Uinta Mountains through 

parts of Phanerozoic time (Hansen, 1965) seem to support the aulacogen concept or, at least, the probability 

of an east-trending structural trough bounded by faults. 

Bryant (1985) has summarized evidence of an easttrending continental margin along the site of the Uinta 

Mountains in Archean time. This margin might have 

affected tectonic events through much of subsequent 

geologic time. Bryant's concept does not contradict the 

evidence of a later east-west trough along the Uinta 

trend.

Laramide propagation of fracturing beneath the 

flanks of the range (fig. 26, inset) might best be visualized as underthrusting due to crowding of the thick 

prism of Uinta sediments between the Colorado Plateau
40 NEOCENE OF EASTERN UINTA MOUNTAINS

TABLE 2. Summary of Tertiary and Quaternary tectonic, geomorphic, and sedimentologic events in the Eastern Uinta Mountains

[See figure 25 for locations of tectonic features]

Tectonic events Geomorphic and sedimentologic events

Normal faulting along Diamond Gulch and along Pot

Creek. 

Latest movement along Sparks fault, down to the

northeast.

Rise of Continental Divide at Tipton. 

Eruption of Leucite Hills lavas 1.1 m.y. ago (McDowell,

1971).

Terracing of stream valleys and continued drainage adjustments; superposition and entrenchment of Bull Canyon.

Diversion of Sheep Creek into present subsequent course.

Superposition of Swallow Canyon in Browns Park.

Beheading of Irish Canyon and diversion of Vermillion Creek.

Diversion of Upper Green River at Green River, Wyo., into present course; interior drainage at 

Great Divide Basin; rejuvenation of Green River system.

Diversion of Spring Creek; abandonment of Dutch John Gap.

Continued reexcavation of Browns Park.

Postulated resurgence of Cross Mountain (Hunt, 1969, 

p. 89) and Deerlodge monocline.

Continued northward and eastward tilting of Eastern Uinta 

Mountains accompanied by faulting of Browns Park Formation along northwest margin of Browns Park and 

structural enhancement of Browns Park-Lay syncline.

Superposition of Yampa River at Cross Mountain and Juniper Mountain.

Integration of Yampa River drainage in Uinta Mountains; excavation of Yampa Canyon.

Reexcavation begins in Browns Park valley and Red Canyon.

Entrenchment of Green River begins in Lodore Canyon.

Deformation of Browns Park Formation and growth 

faulting in Elk Springs-Maybell area (Dyni, 1980).

Large-scale tilting of Eastern Uinta Mountains; warping 

and faulting of Gilbert Peak erosion surface; normal 

faulting in Red Canyon-Browns' Park area; subsidence 

and faulting in Maybell-Lay area; renewed movements 

on Island Park and Disaster faults in Dinosaur area (fig. 

30).

Gravitative reversal on Uinta fault.

Overtopping of Browns Park valley with sediment (Browns Park Formation) at Gates of Lodore; 

diversion of Green River southward across site of Lodore Canyon.

Deposition of Browns Park Formation 25-9 m.y. ago and possibly into Pliocene time along collapsed axis of Uinta anticline; initiation of Browns Park-Lay syncline by deposition and compaction; ancestral Yampa River empties into Maybell-Lay basin. Filling of ancestral Red 

Canyon.

Pre-Browns Park pedimentation northeast side Cold Spring Mountain.

Erosional breaching of Uinta anticline; west to east drainage begins along Red Canyon-Browns 

Park alinement; southerly drainage begins off tilted Gilbert Peak erosion surface into Red Canyon and Browns Park (Spring, Goslin, Red, Willow, Beaver, and Vermillion Creeks); beheading 

of Wolf Creek by Hells Canyon; initial drainage Pot Creek and Diamond Gulch; initial drainage 

of Little Snake River.

Crustal stability.

Regional uplift; continued differential rise of Uinta Mountains; warping of Colorado Plateau (Hunt, 1969).

End of subsidence of Tertiary basins.

West-flowing consequent drainage along site of Yampa Canyon.

Consequent drainage down slopes of Gilbert Peak erosion surface and across Bishop Conglomerate on both flanks of Uinta Mountains.

Deposition of Bishop Conglomerate (beginning about 30 m.y. ago); increasing aridity.

Pedimentation of Gilbert Peak erosion surface; first exposure of Red Creek Quartzite since 

Precambrian time. Truncation of Rock Springs uplift by Gilbert Peak erosion surface.

Regional dissection; lowering of base levels in basins; climatic cooling and drying.

Eastward drainage of Green River Basin toward North Platte River.

Climate turns cooler and drier.

Continued deposition of Duchesne River Formation (Andersen and Picard, 1972; Emry, 1981).

Continued rise of Uinta Mountains.

Rise of Axial Basin anticline; resurgence of Rock Springs

uplift and Douglas Creek arch (possibly Oligocene);

renewed compressive movement on Uinta fault.

Continued subsidence of Tertiary basins.

Deposition of Bridger, Uinta, Washakie, and Duchesne River Formations.

Blockage of north-south drainage across Axial Basin by rise of Axial Basin anticline.

Extinction of Lake Uinta 41 -40 m.y. ago (Mauger, 1977); opening of permanent drainage south

out of Uinta Basin (Hunt, 1969).

Extinction of Lake Gosiute 45-44 m.y. ago (Mauger, 1977). 

Interconnection of Lake Gosiute and Lake Uinta about 45 m.y. ago. 

Burial of Rock Springs uplift by Green River Formation (Roehler, 1965). 

Burial and inundation of Douglas Creek arch, but thinning of Green River Formation and Wasatch

Formation across arch (Dyni, 1981, p. 100).

Expansion of Eocene Lakes Uinta and Gosiute, Green River Formation. 

Continued alluviation, Wasatch Formation. 

Uinta Mountains eroded to Precambrian core; development of Wild Mountain upland erosion

surface across Eastern Uinta Mountains.

Compressive movements along Uinta, Yampa, and Island 

Park faults and others.

2,400-3,050 m of dissection into crest of Uinta anticline.

Unconformable overlap by Fort Union Formation across Cretaceous rocks (Erickson, Rock

Springs,and Hilliard Formations) of Eastern Uinta Mountains; thinning of Fort Union over Rock

Springs uplift (Roehler, 1961); truncation of Cretaceous on Douglas Creek arch and Rock

Springs uplift. 

Consequent drainage off Uinta anticline and beginning of subaerial erosion; alluviation begins

in newly formed basins; inception of Lake Uinta (Ryder and others, 1976).

Initial rise of Uinta Mountains, Rock Springs uplift and 

Douglas Creek arch and subsidence of adjacent basins 

(Bradley, 1964; Gow, 1950; Hansen, 1965; Hunt, 

1956, 1969; Ritzma, 1965a; Roehler, 1961).

Withdrawal of Cretaceous Interior seaway.
DEFORMATION 41

to the south and the Wyoming Basin to the north. The 

Uinta fault has a pre-Laramide history, probably as a 

normal fault, and the Laramide arching of the Uinta 

anticline would have effectively reduced the dip of the 

fault to an attitude conducive to thrusting along the 

preexisting fracture (Hansen, 1984, p. 9). The asymmetry of the fold suggests that the range has translated 

northward relative to the Green River Basin. It verges 

northward, and the north limb is steeper than the 

south. Perhaps the Uinta Basin boundary fault is a 

back thrust on which the south limb has ridden passively over an underthrust Uinta Basin margin.

The long period of crustal stability needed to form 

the Gilbert Peak erosion surface and to deposit the 

overlying Bishop Conglomerate across thousands of 

square kilometers perhaps as much as 26,000 km2 

ended with renewed deformation in an extensional 

mode that brought about notable changes in the pattern 

and regimen of drainage. This deformation is well 

documented, but in some places it is masked by movements that followed the deposition of the Browns Park 

Formation. These movements may, in fact, have been 

part of a continuum of alternate activity and quiet, but 

the greater part of the deformation seems to have preceded the deposition of the Browns Park Formation.

After the Bishop was deposited the whole eastern 

part of the range began to founder, partly by regional 

warping and partly by localized displacements along 

faults. Figure 26 explains the observed deformation as 

a consequence of north-to-south rotation of the range. 

Rotation is expressed in tilted erosion surfaces. I 

suggest that the Eastern Uinta Mountains have rotated 

over a root severed by Laramide or earlier faulting 

that a faulted, semirigid upper crust has rotated over 

a ductile lower crust at a depth of perhaps 20 km in 

response to regional north-south extension. Such rotation requires a curved fault surface and a mass imbalance. Imbalance is indicated by a relative positive 

Bouguer gravity anomaly over the north flank of the 

range (Behrendt and Thiel, 1963) and by the fact that 

the axis and heights of the range are closer to the north 

flank than the south. Rotation has not taken place 

under the western part of the range, where flanking 

subthrusts appear to be planar (Clement, 1977; Gries, 

1981). The rotation and subsidence as thus visualized 

would be taken up partly by fault slippage and partly 

by monoclinal flexing under the south flank over the 

concealed Uinta Basin boundary fault.

Middle to late Tertiary extensional deformation is 

common in other ranges of the Rocky Mountain foreland. (See Ingersoll, 1982, for example.) Among other 

things, it is responsible for the subsidence of the south 

part of the Wind River Range, of the south part of 

the Owl Creek Mountains, and of the Granite Mountains, all in central Wyoming (Reefer, 1970; Love, 1960, 

1970, 1971). In general, post-Laramide extensional deformation was more complicated along the north flank 

of the Eastern Uinta Mountains than along the south 

(not surprisingly, as Laramide uplift and faulting were 

greater on the north flank), but the overall effect was 

a lowering of the crest of the range, relative to both 

flanks, and inward tilting of both flanks. Maximum subsidence was along the zone of faults that trends eastward to southeastward from Bear Mountain to Browns 

Park, thence along the northeast border of Browns 

Park to and beyond the east end of the range (fig. 27).

NORTH FLANK

Most of the post-Bishop deformation on the north 

flank was concentrated between the above-noted zone 

of faults and the Uinta fault a few miles north. Some 

faults that had Laramide or earlier movements were 

reactivated. A reversal of movement on the south-dipping Uinta fault has been recognized since the time of 

Powell. Its large compressional Laramide displacement up on the south, with a probable component of 

left-lateral strike slip (Hansen and Bonilla, 1954, p. 15; 

Hansen, 1965, p. 158) was countered by a much smaller, post-Bishop gravitative reversal. This movement, 

down to the south, amounted to about 520 m between 

Cold Spring Mountain and Diamond Peak (Bradley, 

1936, fig. 18) and about 790 m at Goslin Mountain. It 

was accompanied by marked warping between the 

Uinta zone and the Bear Mountain-Browns Park zone. 

Bear Mountain has been lowered about 975 m since 

Bishop time by gravitative movement on the Uinta 

fault. (See p. 57.) North of the Uinta fault, the Miller 

Mountain area (Bradley, 1936, p. 185) and the lookout 

Mountain area were tilted northward, increasing the 

northward gradient of the Gilbert Peak erosion surface.

South of the Uinta fault, 0-Wi-Yu-Kuts Mountain 

was bowed upward relative to adjacent Cold Spring and 

Bender Mountains its crestline now forms a broad 

east-west arch but the whole area of these mountains 

dropped with respect to the north side of the Uinta 

fault. As one crustal block, the area of Goslin, Bender, 

0-Wi-Yu-Kuts, and Cold Spring Mountains also was 

tilted southward, flattening and reversing the slope of 

the Gilbert Peak erosion surface and thereby redirecting the runoff into the newly formed drainage line of 

Browns Park.

The southerly component of tilt gradually increased 

westward from Cold Spring Mountain to a maximum 

at Goslin Mountain, where the present southward slope 

of the Gilbert Peak surface is about 83 m/km. Southerly 

tilt extended west from Goslin Mountain at a diminishing rate at least as far as Bear Mountain and probably
-190 i-

-230

BOUGUER GRAVITY ANOMALY PROFILE

NORTH

GREEN RIVER BASIN

Flattop Mtn Little Mountain 

(projected)

SOUTH

Goslin Little 

Mountain Hole

UINTA ^ MOUNTAINS UINTA BASIN

£ $8 Split Yampa Cliff Ridge

; Cj Diamond Mountain Mountain Plateau (projected)

Plateau i i VT.~ /T

LATE PALEOCENE

LATE OLIGOCENE 

Gz

25 MILES

in boundary fault 

I^f*T'\* * \" * %* - ' (subsurface only)

' '-"'-^rrf^^.0 " "' ' 30 KILOMETERS

I

0 

EXPLANATION

SURFACE COVERED BY 

BISHOP CONGLOMERATE 

(OLIGOCENE) 

RESTORED GILBERT PEAK

EROSION SURFACE 

.......... PRE-ROTATION PROFILE

Gz CENOZOIC ROCKS

MESOZOIC AND PALEOZOIC 

ROCKS

n

20 MILES

PROTEROZOIC ROCKS 

ARCHEAN ROCKS

CONTACT

FAULT Queried where inferred

FIGURE 26. Conceptual cross section drawn to explain the middle to late Tertiary deformation of the 

Gilbert Peak erosion surface in the Eastern Uinta Mountains after the compressive mode of the 

Laramide orogeny had given way to Tertiary extension. The offsets shown on the faults are mostly 

Laramide, but the arrows indicate later Tertiary slippage. The center of gravity of the range in 

this reconstruction is beneath the Diamond Mountain Plateau, owing to the asymmetry of the fold, 

and the resultant imbalance has rotated the entire mountain mass along the old Laramide fault 

lines, lowering the north flank and raising the south. Gravity and aeromagnetic data, moreover, 

suggest rock of greater density (amphibolite?) under the crest and north flank than under the south flank (Behrendt and Thiel, 1963). The rotation of Goslin Mountain is detailed in figure 28. Gravity profile from Sears and others (1982). Inset diagrams, left, show steps in the hypothetical severing 

of the anticline. Top, before Laramide uplift; middle drawings, beginning and climax of Laramide 

thrusting; bottom, maximum extent of Gilbert Peak erosion surface. No attempt is made to depict 

the complex Precambrian structural relationships beneath the north flank.

52! M525 

H 

O

2i

525 

G

g O

525

CO
] 5 10 

i i i 1 1

. i > . 1 

5

15 KILO 

1

1 

10 MILES

EXPLANATION

MAIN BODY OF BROWNS PARK FORMATION 

PHANEROZOIC ROCKS, UNDIVIDED

p PRECAMBRIAN ROCKS, UNDIVIDED

FAULT Bar and ball on downthrown side 

of most recent movement

THRUST FAULT-Sawteeth on upper plate: 

u indicates upthrown side of post-Bishop 

movement; ruled pattern indicates wide 

fault zone

FIGURE 27. General extent and attitudes of faults on the north flank of the Uinta Mountains between Flaming Gorge and Browns Park. Downthrows to the south and 

west predominate. Note two opposed directions of movement shown on the Uinta fault, the first up on the south, the second up to the north; several other faults have moved 

more than once, but not necessarily in opposed directions. Generalized from Hansen (1965, pi. 1). CO
44 NEOCENE OF EASTERN UINTA MOUNTAINS

to Phil Pico, west of Manila. The southerly slope of 

this mountain's flattish summit is most distinct as seen 

from an aircraft. Inasmuch as the initial slope of the 

Gilbert Peak erosion surface on Goslin Mountain was 

northward probably about 38 m/km, judging from 

Bradley's reconstruction of the undisturbed Hickey 

Mountain profile farther west (1936, table, p. 174) the 

total amount of southward tilting on Goslin Mountain 

probably was about 120 m/km.

North to south, Goslin Mountain is about 5.6 km 

across. If tilted southward at 120 m/km, its south end 

at Little Hole has been lowered about 700 m relative 

to its north end at the Uinta fault since Gilbert Peak 

time. Using those figures and relating the tilting of 

Goslin Mountain to the area immediately to the south, 

and to 0-Wi-Yu-Kuts Flats and Pine Mountain to the 

east, I estimated (Hansen, 1965) that the total collapse 

of the crestline of the Uinta Mountains south of Goslin 

Mountain was about 1,370 m. If that figure is added 

to the present mean altitude of the crestline south of 

Goslin Mountain now about 2,740 m the restored altitude before collapse is 4,115 m. This figure almost 

exactly equals the maximum present height of the 

western part of the range that was unaffected by postGilbert Peak subsidence (Hansen, 1965, p. 172).

Comparable but more refined results can be obtained 

graphically by projecting the gradient of the Gilbert 

Peak erosion surface southward from Little Mountain 

in Wyoming across the Uinta fault to Goslin Mountain 

in Utah and assuming that the surface steepened from 

a measured 24 m/km at Little Mountain to an assumed 

38 m/km at Goslin Mountain, as explained in the preceding paragraphs. This reconstruction (fig. 28) assumes that the Uinta fault is a downward-steepening 

fracture that dips about 60° south a dip indicated by

the attitude of sheeting in the fault gouge just east 

of Goslin Mountain at Red Creek and by the attitude 

of large tabular slices of resistant rock caught in the 

fault zone. The dip may be a bit lower at Cold Spring 

Mountain, perhaps about 45° (Hansen, 1984, p. 17). 

Downward steepening, or upward flattening, is 

suggested by overturning in the foot wall far back from 

the fault trace. With that geometry, any renewed 

down-to-the-south movement of the Uinta fault would 

rotate the Gilbert Peak erosion surface southward. The 

observed rotation geometry of the Gilbert Peak erosion 

surface, in fact, demands a curved fault surface. The 

present southward slope of Goslin Mountain would result if the rotation were about 5° of arc, the radius 

of curvature were about 12 km, and the downthrow 

were about 790 m. This downthrow is appreciably more 

than what Bradley (1936) calculated (520 m) for the 

subsidence of Cold Spring Mountain 32 km or so to the 

east, but the southward rotation of Cold Spring Mountain is much less than that of Goslin Mountain, and the 

downfaulting, accordingly, should be less also. Cold 

Spring Mountain's altitude about 150 m higher than 

Goslin Mountain also indicates less displacement. Furthermore, the Gilbert Peak erosion surface has a strong 

westerly tilt component between Cold Spring Mountain 

and Goslin Mountain (Hansen, 1965, p. 163, fig. 63), 

which would increase the displacement at Goslin Mountain also. Projecting the Gilbert Peak erosion surface 

south to Little Hole suggests that the south end of Goslin Mountain has been lowered about 1,280 m.

South of Little Hole, across the Dutch John fault, 

zone at the foot of Goslin Mountain, the crest of the 

Uinta Mountains has been lowered about 300 m more, 

relative to the south end of Goslin Mountain, judging 

from the profile, figure 28. Added to the 1,280 m of

METERS 

4000-

3000 -

2000 -

1000 -

1590 m

^JL^sjonjLur^^

Little Hole 1280 m Goslin Mountain 60°v / Surface eroded from

*790 m this area

i i

**^Heave:6f 

500 m extension

FEET

- 10,000

 5000

- 0

4 KILOMETERS 

J

I ' I T

0 1 2 MILES

FIGURE 28. Cross section through Goslin Mountain area showing post-Gilbert Peak gravitative movement on the Uinta fault, rotational 

displacement of the Gilbert Peak erosion surface, and extent of displacement (in meters).
DEFORMATION 45

deduced subsidence at the south end of Goslin Mountain, this figure gives a total collapse of the crestline 

of about 1,590 m. This figure can be duplicated by simply projecting the restored Gilbert Peak erosion surface 

south from Goslin Mountain and measuring its height 

above remnants preserved south of Little Hole.

Tilting and displacement similar to that described in 

the preceding paragraphs can be obtained if a dip lower 

than that postulated here is assumed for the Uinta 

fault, so long as the fault surface remains convex upward. Some geologists have suggested dips as low as 

10°. However, the amount of extension required to 

achieve the observed tilting increases rapidly as the assumed dip angle is reduced.

SOUTH FLANK

A northerly tilt along the south flank of the Uinta 

Mountains, as expressed in the attitude of the Bishop 

Conglomerate, is visible from many vantage points on 

or above the Bishop outcrop all the way from Diamond 

Mountain northeast of Vernal to the extreme east end 

of the range. This tilting reversed the drainage of several small streams along the crest of the range, redirecting them from their old south-flowing courses into 

the newly formed Browns Park valley. Even though 

the slope to the north is low, mostly only 1 or 2 degrees, the pattern of barbed drainages is obvious and 

remarkable. Inasmuch as the original slope of the pediment and the depositional slope of the Bishop were 

southward, the present slope is a minimal measure of 

the actual amount of tilt.

Tilt is well documented in the Stuntz Reservoir quadrangle, where the Gilbert Peak erosion surface is well 

preserved and is little deformed except for tilting (fig. 

29). There it is partly mantled with patchy Bishop ConNORTH

glomerate and hence is readily depicted in profile (Hansen and Rowley, 1980b, map and section C-C'). In that 

area the northward slope of the surface is about 16-17 

m/km, some parts being a bit steeper than others. The 

original slope of the erosion surface in the opposite direction, however, may have been as much as 13 m/km, 

perhaps appreciably more, judging again from the undeformed Hickey Mountain profile on the north slope 

of the range. An original slope of 13 m/km toward the 

south added to the present northward slope would yield 

a minimal northerly tilt component of 28 m/km since 

the Bishop Conglomerate was deposited. That order of 

tilt, if uniform over a distance of 56 km the distance 

from the south boundary of the Stuntz Reservoir quadrangle to the crest of the Uinta anticline would 

amount to differential subsidence of about 1,600 m at 

the crestline. Without a known fixed datum the absolute subsidence cannot be determined, but this figure 

agrees closely with the estimated subsidence farther 

west at Goslin Mountain.

NONUNIFORM TILTING ON THE SOUTH FLANK

Farther west on the south flank of the range, nonuniform tilting has been noted northwest of Vernal, where 

remnants of the Bishop Conglomerate straddle the 

South Flank fault zone (Kinney, 1955, p. 127). There 

the Gilbert Peak erosion surface has been warped into 

a shallow syncline whose axis is parallel to the fault 

zone and is about 3 miles south of it. The limbs of the 

syncline slope 3&-57 m/km (D. M. Kinney, written commun., 1959). A synclinal structure there could result 

from a north-side-down movement on the subsurface 

Uinta Basin boundary fault of Ritzma (1969). Kinney 

also noted that the Gilbert Peak surface gradually decreases in altitude eastward from the Uinta River toward Diamond Mountain.

SOUTH

BLUE MOUNTAIN

r

2 MILES

FIGURE 29. Geologic section across Blue Mountain near Colorado-Utah state line showing northerly tilt of Gilbert Peak erosion surface, 

partly mantled by Bishop Conglomerate (heavy line). Surface here truncates folded Paleozoic rocks and originally sloped south Weber 

Sandstone (shaded) is outlined to show truncation. Vertical scale is twice horizontal. From Hansen (1984), based on Hansen and Rowley 

(1980b, section C-C')-
46 NEOCENE OF EASTERN UINTA MOUNTAINS

In the Diamond Mountain area, generalized structure 

contouring on the base of the Bishop Conglomerate indicates nonuniform warping centered on Island Park 

(fig. 30). This warping probably is a result of postBishop activity along the Island Park fault zone at the 

downwarped northwest border of the Split MountainRuple Ridge-Harpers Corner crustal block what Powell (1876, p. 177) called the Island Park sag. The

Diamond Mountain side has been lowered 60-120 m relative to Harpers Corner, judging from the structure 

contours. This displacement is plainly discernible on the 

ground from distant vantage points. Quaternary faulting also cuts the Bishop on Diamond Mountain along 

a west-northwest fault zone just south of Diamond 

Gulch. Several kilometers northeast of Diamond Gulch 

along Pot Creek, a synclinal closure straddles a zone

109W

7200 -. __

R24E R25E R 104 W R103W

10 KILOMETERS

I

5 MILES

FIGURE 30. Structure contours drawn on the Gilbert Peak erosion surface (base of Bishop Conglomerate) in the area of Diamond Mountain, 

Island Park, and Blue Mountain, showing post-Bishop faulting and warping. Contour interval is 200 feet (about 61 m). Dashed contours 

project above eroded ground level. Hachures indicate negative structural closure. Bar-and-ball symbol shows downthrown side of fault.
DEFORMATION 47

of very old faults that have had Quaternary movement 

in the Grouse Reservoir area (Hansen and others,

1981). One of these faults in Lodore Canyon contains 

a dike of Late Cambrian or Early Ordovician age (Hansen and others, 1982, 1983). These faults also trend 

west-northwest.

WEST-TO-EAST TILTING ON THE SOUTH FLANK

Although west-to-east tilt is clearly visible on the 

south flank of the Eastern Uintas, it is difficult to quantify because the original surface sloped away from the 

crestline and because the surface has also been modified 

by the northerly tilt component. As a rough measure 

of the easterly tilt north of Vernal, however, the present surface descends about 900 m west to east, from 

about 3,050 m just west of the Uinta River to about 

2,135 m at Diamond Gulch a rate of about 12 m/km. 

This tilt is shown graphically by figure 31.

CLIFF RIDGE AREA

In the Dinosaur area, easterly tilt is also evident 

from Cliff Ridge, one of the most southerly parts of 

the Gilbert Peak erosion surface and the highest point 

on that surface south of the line of the Yampa River 

(summit elevation about 2,524 m). From Cliff Ridge the 

Gilbert Peak surface and the Bishop Conglomerate 

slope northeast and east toward the Stuntz Reservoir

area and Mud Springs Draw (a tributary of Hells Canyon). Stuntz Ridge, which is a topographic continuation 

of Cliff Ridge, was a monadnock that stood above the 

Gilbert Peak surface and received no gravel. Its 

smoothly graded crestline suggests that it is a remnant 

of the Wild Mountain upland surface. The gravel that 

spread out into Mud Springs Draw must have come 

from the north via Cliff Ridge, because it contains 

abundant red quartzite clasts from the core of the 

range to the north, but it detoured around the west 

side of Stuntz Ridge. Bishop Conglomerate preserved 

on the south side of Stuntz Ridge caps a narrow berm 

that slopes east toward Mud Springs Draw (fig. 32).

The northerly component of tilt from Cliff Ridge is 

manifest in the slope to the Yampa Plateau, whose conglomerate-capped summit is about 2,350 m in altitude, 

and to Split Mountain, a bit farther northwest at 2,315 

m. Split Mountain's summit is a point on the Gilbert 

Peak erosion surface, but it lacks a gravel cap. Thirteen 

kilometers due north of Split Mountain, the rim of 

Diamond Mountain also reaches slightly above 2,315 m, 

but the conglomerate is at least 90 m thick there, so 

the elevation of the underlying Gilbert Peak surface is, 

accordingly, about 2,225 m.

MUD SPRINGS DRAW AND WOLF CREEK

East along Mud Springs Draw and Wolf Creek the 

general easterly tilt component is expressed in a rather

SOUTH FLANK

40 80 KILOMETERS

\

20 

VERTICAL SCALE x 10

40 MILES

FIGURE 31. Profiles along the flanks of the Uinta Mountains, showing the west-to-east decline in the altitude of the Gilbert Peak erosion 

surface. Vertical scale is 10 times the horizontal scale. Profile segment from Heller Lake to Island Park fault is about 24 km farther 

north than the segment from Cliff Ridge to Elk Springs. Part of the altitude difference between Cliff Ridge and Diamond Mountain 

is due to offset fault blocks, but part is due also to a northerly component of tilt. Points on the profiles are taken from maps by 

Kinney (1955, plate 1), Rowley and others (1979), Hansen and others (1983), and the Vernal I°x2° topographic sheet (1:250,000).
48 NEOCENE OF EASTERN UINTA MOUNTAINS

^f"'^

FIGURE 32. A berm on the south flank of Stuntz Ridge, upper right, capped by Bishop Conglomerate, in Moffat County, Colo., just 

east of the Colorado-Utah State line. Dark, rounded cobbles are from the Uinta Mountain Group and light ones are mostly from 

Paleozoic limestones. Cliff Ridge, in the distance to the west, is topped by a remnant of the Gilbert Peak erosion surface and by 

patchy Bishop Conglomerate. The berm rises west toward Cliff Ridge.

narrow belt of Bishop Conglomerate about 32 km long. 

In that distance the altitude of the Bishop descends 

about 300 m, a rate of 10 m/km. Some of this slope 

may be initial dip and may represent the depositional 

gradient of the Bishop Conglomerate before tilting, inasmuch as the proportion and coarseness of clasts derived from the Uinta Mountain Group diminish eastward. Figure 33 shows that a shallow synclinal warp 

at the base of the Bishop south of Hells Canyon probably is a response to post-Bishop movement along the 

Mud Springs monocline. This downwarp is centered 

along the length of Blue Mountain, and because the 

Bishop Conglomerate coincides with the downwarp, the 

monocline appears to have helped localize the deposition of Bishop in that area. The downwarp thus had 

negative relief before the gravel was deposited, as well 

as after.

Fingers of gravel reached north from Wolf Creek up 

narrow tributaries that drained the heights of Round 

Top Mountain, Marthas Peak, Tanks Peak, and Bear 

Valley Ridge. Many patches of this gravel still remain, 

now loosely cemented into conglomerate or coarse sandstone, but the tilt component cannot be measured, 

owing to the scatter of the remnants and uncertainty 

as to their original gradients. Northerly tilt is indicated, however, by a reversal of streams that now

drain north to the Yampa River but formerly flowed 

south into Wolf Creek. Johnson Draw (figs. 33 and 39) 

is a good example (Hansen and Carrara, 1980). Turner 

Creek and Mud Springs Draw are others captured by 

north-draining Hells Canyon (Hansen and Rowley, 

1980a).

SOUTHEAST END OF THE RANGE

At the southeast end of the range, the subsidence 

of the Uinta arch and its southeastward extension, the 

Axial Basin anticline (west part) lowered the Bishop 

Conglomerate below present drainage. Much of the deformation there is younger than the Browns Park Formation, but some of it is older, and it produced a sag 

into which the Browns Park Formation was deposited 

on or partly on the Bishop. In nearly continuous outcrop the Bishop extends from Bear Valley east to Elk 

Springs, declining in altitude from 2,440 m at the head 

of Bear Valley to 1,950 m at Elk Springs, where it 

passes beneath the white sands of the Browns Park 

Formation. East of Cross Mountain, where the Bishop 

is deeply buried by the Browns Park Formation, its 

altitude is less than 1,460 m (Dyni, 1980).

At Elk Springs, what I identify as Bishop Conglomerate is well exposed at the base of the Browns Park 

Formation in a large gravel pit. It contains the usual
DEFORMATION

1D8°45'

49

MUD SPRINGS 

MONOCLINE

R 103 W R102 W R101W

5 KILOMETERS

I

R100W R99W

1 

5 MILES

EXPLANATION

 7200 STRUCTURE CONTOUR-On base of the Bishop 

Conglomerate, showing elevation in feet

NORMAL FAULT Bar and ball on downthrown side 

THRUST FAULT Sawteeth on upper plate

ANTICLINE

SYNCLINE

MONOCLINE

FIGURE 33. Structure map of the Bishop Conglomerate in the Woll Creek area, showing contours drawn at the base of the conglomerate; 

contour interval is 200 feet (61 m). Contouring indicates that post-Bishop warping resulted from renewed movements along preexisting 

structural features.

assemblage of Uinta Mountains rock types red quartzite, gray limestone, and gray chert in cobbles 10-15 

cm across and occasional boulders as large as 75 cm 

across (Dyni, 1980). The bedding attitude changes from 

a very low northeast dip almost flat to about 20° to 

25° north in response to post-Browns Park deformation. 

Dyni (1980) has suggested that the Bishop (which he 

called the lower conglomerate unit of the Browns Park 

Formation) may have been deformed before or during 

deposition of the upper sandstone unit (of the Browns 

Park Formation). Dyni's suggestion corroborates my 

view about the disparate ages of the two formations.

East from Elk Springs the conglomerate continues 

along strike as a narrow line of outcrop at the base 

of the Browns Park Formation along Elk Springs 

Ridge, dipping about 25° north and finally disappearing 

under Browns Park overlap east of Cedar Springs 

Draw. A deep downwarp between Elk Springs Ridge 

and Cross Mountain, broadening east toward Maybell, 

contains as much as 520 m of Browns Park rocks (Dyni, 

1980). According to Dyni this trough may have formed

over growth faults that were active during Browns 

Park deposition.

Due north from Elk Springs, the conglomerate is 

offset by several small faults, but it slopes gently north 

toward Lily Park at the confluence of the Little Snake 

and Yampa Rivers, forming the cap rock on the high 

mesas there (such as Twelvemile Mesa) 300 m above 

river level (Dyni, 1968). Eastward on Twelvemile Mesa 

it passes beneath the Browns Park Formation near 

Cross Mountain, but to the north, correlative deposits 

on Klauson Pasture stand about 200 m above the 

Browns Park Formation in the canyon of the Little 

Snake River. (See p. 32.) Though it is a loosely 

cemented, bouldery gravel and is nearly bare of the 

white sandstone cover west and north of Elk Springs, 

geologists back to the time of A. R. Schultz have included it in the Browns Park Formation. According to 

Dyni (oral commun., 1981), it is discontinuous beneath 

the Browns Park Formation and, hence, is hardly 

mappable. I suspect that it was partly removed by preBrowns Park erosion.
50 NEOCENE OF EASTERN UINTA MOUNTAINS

NORTHEAST END OF THE RANGE

At the northeast end of the Uinta Mountains, between Vermillion Creek and Sand Wash, the whole 

flank of the Uinta anticline has been obscured by 

downwarping, faulting, erosion, and sedimentary overlap by the Browns Park Formation. Just west of Dry 

Mountain, the Browns Park Formation is as much as 

600 m thick (Ritzma, 1965b, p. 131). A zone of mostly 

normal faults trends southeast from the Vermillion 

Creek area to and beyond the Little Snake River near 

Sunbeam. Most of these faults are downthrown toward 

the south. This zone is a structural continuation of the 

faulting along the northeast side of Browns Park, and 

it also merges with the Uinta-Sparks zone north of Cold 

Spring Mountain. It is partly overlapped by an unconformity within the Browns Park Formation (Ritzma, 

1965b, p. 131), which suggests intra-Browns Park deformation. Sears (1924a, p. 296) noted a similar local 

unconformity northwest of the mouth of Vermillion 

Creek. I interpret the faulting as a gravitative response 

to post-Laramide extension of the northeast flank of 

the range.

The Browns Park Formation itself, as well as the 

underlying Bishop or basal Browns Park of earlier reports is sharply tilted along the northern margin of 

its outcrop belt between Vermillion Creek and the Little Snake River. Dips commonly reach 20° south and, 

locally, are as much as 40° to 65° south (McKay, 1974). 

The subjacent Eocene rocks, chiefly the Bridger Formation, dip northward beneath a marked unconformity 

into the Sand Wash Basin. This unconformity is presumably the Gilbert Peak erosion surface. The Bishop 

there is discontinuous. In part it has been deleted by 

faulting, and its locally steep dip is surely caused by 

drag, but in part the Bishop either was not deposited 

or was removed by pre-Browns Park erosion. Abrupt 

changes in thickness and the absence of Bishop in some 

sections point toward pre-Browns Park erosion rather 

than nondeposition. For example, between the Little 

Snake River and Simsberry Draw, a distance of only 

about 2.4 km, the thickness diminishes abruptly from 

about 24 m (Izett and others, 1970) to less than 1 m 

(McKay, 1974), and the provenance changes from the 

Uinta Mountains to the Park Range. A few miles to 

the east, north of Sunbeam, S. J. Luft and I found 

the Browns Park Formation resting directly on the 

Wasatch, with no intervening conglomerate at all.

The main body of the Browns Park Formation 

through most of its exposure has the structure of a 

shallow syncline turned up sharply along its north and 

northeast margin by warping and discontinuous faulting. Mapping its western part, I called it the Browns

Park syncline (Hansen, 1965, pi. 1); McKay and Bergin 

(1974), mapping its eastern part, called it the Lay 

syncline, but it is a continuous structure and its axis 

is one and the same. Its axis trends east-southeast 

through most of its length, from the head of Browns 

Park to Sunbeam, then veers gradually eastward (Rowley and others, 1979). S. J. Luft (written commun., 

1983) has traced it nearly to Craig. Luft has also traced 

a secondary axis southeast from Sunbeam toward Axial 

Basin.

This syncline coincides with the old erosional valley 

of Browns Park along the breached crest of the Uinta 

anticline and its extension, the Axial Basin anticline 

(Sears, 1924a, fig. 1; Hancock, 1925, pi. 19). I visualize 

the valley as developing along the depressed sag in the 

Gilbert Peak erosion surface. At Axial Basin the 

Browns Park Formation fills the valley floor below 

cliffs and highlands eroded from the Mesaverde Group 

on both flanks; hence the topographic configuration of 

Axial Basin at the time of Browns Park deposition was 

not greatly different from what it is now.

Mapping in Moffat County, Sears (1924a, p. 288) attributed the form of the syncline primarily to deformation, but he recognized the effects of initial dip into 

the old trough. Mapping farther west, I attributed the 

structure to a combination of (1) initial dip on the sloping floor of the old valley, (2) valleyward thinning of 

coarse-grained beds, and (3) differential compaction of 

fine-grained sediments, the total compaction being 

greatest in the thickest part of the deposit over the 

deepest part of the buried valley (Hansen, 1965, p. 

154). Steep dips along the north and northeast margins 

of the old valley, however, are due to drag and flexing 

along and near faults. Along the Mountain Home fault 

at Jesse Ewing Canyon (figs. 27 and 40) near the head 

of Browns Park, for example, the dip approaches vertical (Hansen, 1965, pi. 1 and fig. 59). Less steep inclination along the south margin of the old fill along Elk 

Springs Ridge east of Elk Springs may be due to resurgence of the Elk Springs anticline, whose axis is 

just south of the outcrop (Dyni, 1968). There, beds that 

originally sloped southward from the Uinta Mountains 

now slope north.

At the confluence of the Little Snake and Yampa 

Rivers, the short, south-trending Lily Park syncline 

(Dyni, 1968; McKay, 1974) separates the Cross Mountain anticline to the east from the east plunge of the 

Uinta anticline to the west. This syncline is expressed 

by sharp folding in the pre-Tertiary rocks and faulting 

on the east margin. Overlying these rocks, the Bishop 

(or basal Browns Park of earlier reports) forms a 60-mthick caprock on high terrace remnants on both sides 

of the valley. These terraces project south across the
DEFORMATION 51

Yampa valley to Twelvemile Mesa. If the slopes of the 

terraces were projected to the center of the valley, the 

base of the Bishop would form a gentle swale superimposed on the Lily Park syncline. This swale represents 

initial dip on the floor of a consequent valley eroded 

into the syncline, because the source of Bishop debris 

for the west limb was the Uinta Mountains and the 

source for the east limb was Cross Mountain, but the 

swale might have been enhanced by subsequent warping. A post-Bishop tilt to the north, in any event, is 

unmistakable.

The pre-Bishop Lily Park syncline plunges southward about 120 m/km (Dyni, 1968), whereas the 

superimposed swale structure plunges north; the north 

plunge is an expression of the widespread post-Bishop 

tilting of the region, which amounted to about 9 m/km 

in that area and was not sufficient, therefore, to reverse the southerly plunge in the older rocks. One may 

recall that the crestline of Cross Mountain, which is 

a relic of the pre-Gilbert Peak, Wild Mountain upland 

surface, also slopes southward despite the general 

northerly post-Gilbert Peak (post-Bishop) tilt of the region. This southward slope, like that of the Lily Park 

syncline, predates the Bishop Conglomerate.

SUMMARY OF POST-BROWNS PARK 

DEFORMATION

Tectonic movements in the eastern Uinta Mountains 

involving the Browns Park Formation have already 

been mentioned, particularly movements in the Elk 

Springs area. These movements appear to have been 

continuations of earlier deformation; rather large-scale 

deformation preceded, accompanied, and followed deposition of the Browns Park Formation. Some of this 

deformation has been discussed by Sears (1924a, p. 291) 

and by Bradley (1936, p. 185), both of whom believed 

that the major collapse of the Uinta arch was after the 

deposition of the Browns Park. I believe it was before, 

inasmuch as it produced the broad, erosionally modified 

tectonic downwarp into which the Browns Park Formation was deposited the Uinta arch was depressed hundreds of meters before the Browns Park Formation was 

deposited. Further deformation then followed.

DIAMOND PEAK AND BROWNS PARK

Post-Browns Park deformation involved widespread 

tilting and the reactivation of previously formed faults. 

Between Diamond Peak and Cold Spring Mountain, for 

example, the Browns Park Formation extends northwest up the valley of Talamantes Creek along the 

Uinta-Sparks fault zone, probably as an old valley fill, 

resting on the older rocks with a sharp angular unconformity. South-dipping drag in the Browns Park indicates renewed movement along the fault, down on the 

south toward the collapsed Uinta arch (Weber, 1971, 

fig. 4), a continuation of movements that earlier had 

lowered Cold Spring Mountain relative to Diamond 

Peak.

Similar faulting in Browns Park is better exposed, 

better documented, and even more pronounced. Near 

the head of Browns Park, movement on the Mountain 

Home fault, the Beaver Creek fault, and other faults 

reversed the dip in the Uinta Mountain Group, forming 

drag anticlines and sharply tilting the Browns Park 

Formation (Hansen, 1965, plate 1, fig. 59; 1957 a,b). 

Just east of Jesse Ewing Canyon, however, the Mountain Home fault passes beneath the Browns Park Formation without cutting it, showing that the fault antedates the Browns Park Formation. Renewed movement 

on parts of the Mountain Home fault and the Beaver 

Creek fault after deposition of the Browns Park Formation, south sides down, indicates further subsidence of 

the Uinta arch.

The Browns Park Formation is broken by displacements along these and other faults all the way from 

the head of Browns Park to points north and west of 

Craig, Colo. (Hansen, 1957a,b; Ritzma, 1959; Tweto, 

1976; Rowley and others, 1979). In the Dry MountainDouglas Draw-Vermillion Creek area, in an added complication, the axis of the Browns Park syncline plunges 

gently northwestward at a rate of about 9.5 m/km. This 

plunge also reflects subsidence of the Uinta arch.

YAMPA FAULT

One of the major structural features on the south 

flank of the Eastern Uinta Mountains is the faulted 

Yampa fold bordering the north side of Blue Mountain 

(figs. 25 and 33). This fault is a low-angle thrust between Hells Canyon and Johnson Draw. At Hells Canyon its dip is about 22° south and its upper plate is 

dragged into a sharp bend, the Yampa monocline. 

Stated differently, the fault is the ruptured synclinal 

bend of the monocline. Although most of the fault's 

1,500-m displacement is Laramide, Sears (1924a, p. 

291) ascribed large-scale post-Browns Park movement 

to it and invoked this movement to explain the course 

of the Yampa River. I know of no clear evidence of 

any post-Browns Park offset; evidence to the contrary 

near the western end of the fault indicates little or no 

movement since the Bishop Conglomerate was deposited.

Dyni (1980), however, found evidence of inter- and 

post-Browns Park deformation east of the Yampa 

monocline in the Elk Springs area, some of which he 

attributed to recurrent movements on growth faults. 

Near the eastern terminus of the Yampa fault and
52 NEOCENE OF EASTERN UINTA MOUNTAINS

monocline the Bishop Conglomerate has been displaced 

by renewed warping on the monocline (fig. 33), but 

whether or not this movement was post-Browns Park 

in age is uncertain. Structure contouring based on 

abundant remnants of Bishop Conglomerate indicates 

that post-Bishop warping is superimposed on preexisting structural features. In T. 5 N., R. 99 W. and R. 

100 W., a noselike swing of the contours (fig. 33) coincides with the plunging south branch of the Yampa 

monocline. Just to the north, in T. 6 N., a synclinal 

reentrant in the contours coincides with the north 

branch of the Yampa fault and its synclinal extension. 

These features suggest resurgence of older structures. 

Here the Bishop and the subjacent Gilbert Peak erosion 

surface have been warped about 120 m. Inferred postBrowns Park warping on the nearby Deerlodge monocline (see next section) suggests possible warping of 

that age at the eastern end of the Yampa structure 

also.

West of Hells Canyon, however, the Yampa fault 

dies out, and its monoclinal extension is truncated by 

the Gilbert Peak erosion surface and the Bishop Conglomerate. (See fig. 29.) Some minimal post-Bishop distortion of the surface across the monocline is possible 

in this area, but it isn't obvious. At any rate, because 

the fault does not displace the Bishop Conglomerate, 

the trough-like or bench-like outer canyon of the 

Yampa River between Douglas Mountain on the north 

and Blue Mountain on the south is a product of erosion, 

not faulting (fig. 34). The trough resulted from the differential erosion of soft Triassic and younger rocks from 

the downthrown side of the Yampa fault or monocline 

in post-Bishop time.

Sears (1924a, p. 303; 1962, p. 23-24) visualized a new 

Yampa River flowing west into a drag syncline just 

north of the Yampa fault, but inasmuch as the Gilbert 

Peak surface and the Bishop Conglomerate extend unbroken across the Yampa monocline west of Hells Canyon near Harpers Corner, the fault zone and downwarp 

could not have been the avenue for the Yampa River. 

The present river is far north of the fault zone and, 

because of the low southerly dip of the Yampa fault 

along most of its length, moreover, any gravitative 

"collapse" of the downthrown (north) side as visualized 

by Sears is unlikely. Unaware of its low southerly dip,

FIGURE 34. View from the rim of Johnson Draw (foreground) west-northwest along the escarpment of the Yampa fault, Moffat County, 

Colorado. In the distance the fault becomes a monocline. On the distant ridge, center and upper right, the Gilbert Peak erosion surface 

capped by Bishop Conglomerate extends unbroken across the fault/moncoline trace. The broad, troughlike valley in the middle distance 

to the right of center has been differentially eroded from the soft Triassic and younger rocks that once filled the area. Yampa Canyon 

is at the extreme right.
DEFORMATION 53

Sears assumed, as Powell had before him, that the 

Yampa fault was a high-angle normal fault. Following 

Sears' lead, other authors have similarly concluded that 

the Yampa fault is the southern boundary of the socalled Browns Park or Uinta Mountain graben, in the 

mistaken belief that the Yampa fault dips north. This 

graben simply does not exist.

DEERLODGE MONOCLINE

Geomorphic relationships along the Little Snake and 

Yampa Rivers suggest warping on the Deerlodge 

monocline during or after Browns Park deposition. This 

monocline is the short, abrupt fold that terminates the 

Uinta uplift on the east (Hansen and others, 1984) and 

provides the dramatic entry into Yampa Canyon from 

Deerlodge Park (fig. 13). Just upstream from Deerlodge Park along the Little Snake River, a basal conglomerate beneath the Browns Park Formation between Cross Mountain and the eastern Uintas (p. 32) 

indicates through drainage by a vigorous stream in that 

area in a canyon-like setting at the onset of Browns 

Park deposition. Where this drainage went is problematical, but it almost certainly turned west along the 

present Yampa Canyon, because egress in other directions is blocked by higher ground, even though there 

is little likelihood that a canyon as deep and narrow 

as the present one could have persisted since Miocene 

time. A south-flowing course toward the White River 

looks plausible on a planimetric map, but geomorphic 

evidence is against it. More likely, the drainage turned 

west into a then-shallower, more juvenile Yampa Canyon, which deepened as the resurgent Deerlodge 

monocline rose beneath it (or as the connecting Lily 

Park syncline sank).

The rise of the monocline would have inhibited drainage upstream and would have abetted the aggradation 

that attended the deposition of the succeeding Browns 

Park Formation. This reconstruction of events accords 

well with Hunt's concept of anteposition (1967, 1969), 

whereby a drainage temporarily blocked by a rising uplift overtops the obstruction by aggrading its bed, then 

resumes its previously established course. The presence of loose Browns Park sandstone at rim level on 

Bishop Conglomerate upstream from Yampa Canyon 

(Dyni, 1980, map) adds credence to this hypothesis.

East Cactus Flat, which truncates the Deerlodge 

monocline, slopes gently southward its rim is highest 

next to Yampa Canyon. The slope probably postdates 

the entrenchment of the river; otherwise the river 

should have migrated downslope to the south, away 

from its present position. The southerly slope probably 

is a manifestation of resurgence of the Deerlodge 

monocline during or after the deposition of the Browns 

Park Formation.

DRAINAGE DEVELOPMENT

Drainage off the eastern Uinta Mountains formed 

concomitantly with Laramide uplift, and the gradual 

degradation of the mountains in Paleocene and Eocene 

time produced immensely thick deposits of clastic sediment and carbonates in the adjoining basins. Drainage 

at that time was centrifugal, away from the mountains 

in all directions, unlike the modern drainage, and it remained largely so until after the deposition of the 

Bishop Conglomerate (fig. 35). One exception was the 

structurally controlled drainage of Blue Mountain, 

where the Wolf Creek-Mud Springs Draw drainage 

was controlled by the east-west folds and faults on Blue 

Mountain. Even that drainage was consequent to the 

structural trough between the subsidiary folds of the 

Blue Mountain highland. The distribution of Bishop 

Conglomerate shows that Wolf Creek was constrained, 

then as now, by the bounding upwarps of Blue Mountain. Modern Wolf Creek and Mud Springs Draw, flowing across Bishop Conglomerate, are re-incising themselves into the sub-Bishop bedrock.

As drainage from the mountains flowed out toward 

the basins, it deposited the Bishop Conglomerate in 

broad, sheetlike fans on the subjacent Gilbert Peak erosion surface. The present major streams of the area 

obviously did not yet exist; their courses are younger 

then the pedimentation and the alluviation that produced the Bishop Conglomerate.

Deposition of the Bishop Conglomerate ended when 

renewed deformation terminated the long period of 

crustal stability that followed Laramide uplift. One consequence was a drastic overhaul of the drainage regime. Gravitative movements on the zone of faults 

along Red Canyon and Browns Park tilted the whole 

crestal region and south flank of the range northward 

and eastward, deforming the Gilbert Peak erosion surface and lowering the crestline as much as one and a 

half kilometers. These movements began in late Oligocene time and lasted intermittently through the Miocene and, perhaps, the Pliocene. Some faults predated 

the Browns Park Formation; others displaced and 

dragged it to steep angles. Some faults have had 

Quaternary movements. At the east end of the range 

the lowering of the Bishop below present drainage 

formed a broad downwarp that became a catchment 

basin for the Browns Park Formation. Between 

Browns Park and the Uinta fault, the north flank of 

the range was tilted unevenly southward.

All these crustal movements helped set the stage for 

the drainage changes that followed. Much of the runoff 

that had flowed from both flanks of the range toward 

the Tertiary basins was reversed and became tributary 

to the newly forming valley of Browns Park. The new
54 NEOCENE OF EASTERN UINTA MOUNTAINS

GREEN RIVER, BASIN

WYOMING 

COLORADO

111° 110° 109°

42°

50 MILES

40°

drainage lines in Browns Park then carried the combined runoff eastward to the downwarp in the LayMaybell-Little Snake area. The former crest of the 

range was now a deepening valley that ultimately 

would carry the full flow of the Green River. As discussed later, the Green was eventually diverted south 

from Browns Park into Lodore Canyon.

ADJUSTMENTS ON THE SOUTH FLANK IN 

THE POT CREEK AREA

Former southward drainage south of Browns Park 

and Red Canyon is indicated by the barbed tributary 

patterns of all the major valleys between Cart Creek 

(fig. 36) and Lodore Canyon (Kinney and others, 1959; 

Ritzma, 1959, p. 87; Hansen and others, 1960, p. B258; 

Hansen, 1965, p. 174; 1969a, p. 34; Hunt, 1969, p. 97). 

During Bishop time all these valleys drained into the 

Uinta Basin and contributed detritus to the Bishop 

Conglomerate (fig. 37). All still contain Bishop Conglomerate, consisting mostly of coarse, nearly white, 

friable pebbly sandstone and conglomerate. As these 

fills merged southward they formed an extensive 

bajada on the south flank of the range (Hansen and 

others, 1981).

With northerly and easterly tilting, however, all 

these streams stagnated and partly reversed their flow 

directions, leaving barbed tributaries. In part these reversals were caused simply by capture due to tilting. 

For example, Cart Creek, Gorge Creek, and Jackson 

Creek were captured by vigorous north-flowing 

streams emptying into Red Canyon through deep, narrow tributary canyons. All the reversed streams, in

FIGURE 35. Steps in the development of the Green River drainage 

system, telescoped in time and generalized. Details in text.

Top. Late Oligocene time. Diagrammatic drainage off the Gilbert 

Peak erosion surface and across the Bishop Conglomerate into 

the adjacent basins. At this stage the Bishop formed a nearly 

continuous bajada around the flanks of the Uinta Mountains.

Center. Latest Oligocene to early Pleistocene time. Subsidence 

and northeasterly tilting have redirected the drainage in the 

eastern part of the range. Note new drainage through Browns 

Park (B) along the former crestline, leading to the downwarped 

area (D) at the east end of the range. Note reversals of drainage 

direction on the northeast flank and the new southeasterly drainage at Diamond Mountain (DM). Vigorous headward erosion into 

the Green River Basin from the south is threatening the eastflowing ancestral Green and its northeast-flowing tributaries, 

Blacks Fork (BF) and Henrys Fork (HF). Hells Canyon (H) has 

captured the headwaters of Wolf Creek (W). Yampa Canyon (Y) 

is eroding headward toward D.

Bottom. Present drainage. The southeasterly drainage along the 

Uinta axis spilled across the range, in late Miocene or early Pliocene time, at Lodore Canyon (L). The Green River was captured 

at Green River, Wyo. (GR), in the early Pleistocene. Black Fork 

and Henrys Fork have been redirected southward. Note the 

barbed confluence of Bitter Creek with the Green River.
DRAINAGE DEVELOPMENT 55

109°30' 109"22'30"

FIGURE 36. Drainage map of the Cart Creek area, showing barbed 

headwater tributaries that formerly flowed south to Reader Creek. 

These tributaries meander sluggishly through McKee Draw and 

East McKee Draw before plunging into the Cart Creek canyon, 

shown by hachures (hachures coincide with canyon rims).

fact, now pass through such canyons before joining the 

Green. Most of the valleys at some midpoint contain 

low divides in Bishop Conglomerate, the northerly 

reaches flowing to Red Canyon or Browns Park and 

the southerly reaches to Pot Creek. A person can cross 

such a divide by automobile without realizing that the 

drainage direction has changed 180 degrees. Similar

low divides separate the drainages of Pot Creek and 

Diamond Gulch.

Pot Creek and Diamond Gulch are post-Bishop drainages that formed by the diversion of southward runoff 

into east-southeasterly courses after regional tilting. 

Because of the concavity of the original Bishop profile, 

tilting created new low points along the formerly southflowing streams, as shown diagrammatically by figure 

38. Pot Creek and Diamond Gulch sought out these low 

points and consequently funneled the runoff toward the 

east-southeast. The near coincidence of Pot Creek and 

Diamond Gulch with fault lines, however, suggests that 

their drainage direction was influenced partly by faulting. I visualize south-building alluvial fans gradually 

rising toward and locally overtopping low places at fault 

lines in the interfluves. With tilting, drainage found its 

way across these partly buried interfluves.

Drainage adjustments are still taking place and, on 

a geomorphic time scale, Pot Creek itself is threatened 

with imminent capture by Grouse Creek (Hansen and 

others, 1981). Grouse Creek heads within a few 

hundred feet of Pot Creek, has the advantage of a 

steeper gradient than Pot Creek, and is cutting rapidly 

downward into the intervening divide of soft Bishop 

Conglomerate (mostly friable sandstone at that locality). Pot Creek, on the other hand, flows alternately 

across the Bishop Conglomerate and the Uinta Mountain Group and is held at grade by numerous thresholds 

of hard red quartzite. If left to itself, the entire discharge of Pot Creek upstream from the divide will 

eventually flow north into Grouse Creek. The capture 

of Pot Creek, in fact, has been partly fulfilled artificially by a diversion ditch that directs water from Pot 

Creek across the low divide into Grouse Creek to irrigate hay fields in Browns Park. A narrow trench 10-15 

m deep has already been eroded into the divide.

ADJUSTMENTS ON BLUE MOUNTAIN

Middle Tertiary drainage adjustments on Blue Mountain parallel those of the Pot Creek area in time and 

style. The pre-tilt drainage flowed east down Wolf 

Creek in a valley floored with Bishop Conglomerate. 

Hells Canyon, which drains north to the Yampa River, 

has captured most of the headwaters of Wolf Creek 

and in so doing has greatly expanded its watershed at 

the expense of Wolf Creek (fig. 39). Hells Canyon is 

a steep, narrow gorge that drops nearly 600 m in about 

11 km at an overall rate of 38-57 m/km; the gradient 

steepens locally to as much as 75 m/km. Wolf Creek 

has a gradient of only about 15-19 m/km. Mud Springs 

Draw, Spike Hollow Draw, and Bobcat Draw are 

sharply barbed former tributaries of Wolf Creek.
56 NEOCENE OF EASTERN UINTA MOUNTAINS

109=15'

Matt Warner 

Reservoir v^. (

"

10 KILOMETERS

5 MILES

FIGURE 37. Distribution of Bishop Conglomerate (shaded), anomalous present drainage pattern, and barbed tributaries of north-flowing 

streams in Pot Creek area. Hachures indicate canyons. Note how close heads of Jackson Creek and Crouse Creek are to Pot Creek. 

Pot Creek is held at grade by the numerous crossings of the Uinta Moutain Group (unshaded), and its capture by Crouse Creek 

is imminent at point shown by arrow.

Turner Creek, a former reach of Wolf Creek, has been 

reversed and is now separated from Wolf Creek by a 

low divide. Northerly tilt probably shifted the hydraulic 

advantage to Hells Canyon. Farther upstream, northflowing Meadow Creek captured the headwaters of K 

Creek, probably at about the same time.

Johnson Draw, which once drained south independently into Wolf Creek, now drains north to the Yampa

River via Johnson Canyon, a gorge that resembles 

Hells Canyon but is smaller. Johnson Draw heads only 

1 Vs km from Wolf Creek at a wide low pass in Bishop 

Conglomerate, then flows 11 km north to the Yampa. 

A few kilometers to the east, Bear Valley Draw, shown 

on figure 26, still drains into Wolf Creek, but part of 

its headwaters has been diverted to the Yampa via 

Bear Draw through a deep gorge cut in the Madison
DRAINAGE DEVELOPMENT 57

FIGURE 38. Effect of tilt on the low point of a concave-upward profile. New low points are indicated by arrows on tilted profile. The 

local flattening of the original profile at the future site of Pot Creek 

is attributed to a constriction in the valley upstream from a belt 

of resistant Precambrian rock across the drainage line.

Limestone. Bear Valley is being threatened by 

Thanksgiving Gorge at East Bear Valley Draw, but 

capture is not yet imminent.

ADJUSTMENTS ON THE NORTH FLANK

Drainage adjustments on the north flank of the Eastern Uinta Mountains, north of the anticlinal axis (fig. 

40), have had far-reaching geomorphic consequences. 

Many adjustments were imposed by the deformation 

that ended the Bishop depositional episode. Erosion in 

the southern Green River Basin since the end of the 

Bishop episode in late Oligocene time has been extensive, and evidence for ensuing drainage changes has 

been partly lost to erosion. In brief, a reversal of the 

regional gradient, accompanied by a large-scale redirection of the drainage, was caused by the foundering of 

the Uinta arch in late Oligocene and early Miocene 

time.

From the time Lake Gosiute disappered (middle Eocene) until the foundering of the Uinta arch (late Oligocene), the drainage of the southern Green River Basin 

was northward, away from the Uinta Mountains. The 

northerly gradients on Bishop-capped remnants of the 

Gilbert Peak erosion surface and the northward fining 

of the gravels leave no doubt about the direction. The 

northward drainage must have arisen south of the exposure area of the Red Creek Quartzite, as cobbles and 

boulders from the Red Creek and from the Uinta 

Mountain Group are abundant in the Bishop Conglomerate on Little, Miller, and Pine Mountains and as far 

north as Aspen Mountain. The drainage divide was well 

south of the Uinta fault, for the Red Creek Quartzite 

is truncated south of the fault by the Gilbert Peak erosion surface. The crestline must have been as far south

as Browns Park, along the axis of the Uinta anticline, 

because the reconstructed profile of the Gilbert Peak 

surface projects into the air above Browns Park. The 

coarseness of the Bishop, furthermore, indicates that 

its source to the south had strong relief. Bradley (1936, 

p. 177) suggested that the master drainage was eastward or northeastward out of the Green River Basin, 

possibly toward the ancestral Platte River, a view endorsed by Ritzma (1959, p. 87) and elaborated by Hansen (1969b). Sears (1924a) had hinted at it decades earlier.

As pointed out previously (p. 32), the shift in drainage direction along the mountains followed the deposition of the Bishop Conglomerate and preceded the deposition of the Browns Park Formation, because the 

Browns Park Formation contains abundant clasts derived from sources to the north. New south-flowing 

drainage lines must have begun to form all along the 

north flank of the range concomitantly with tilting. 

With modifications, these lines persist in the modern 

drainage. The master drainage was diverted much later 

(p. 31).

FLAMING GORGE AREA

The foundering of the Uinta arch almost surely 

played a critical part in redirecting the drainage of the 

Green River Basin toward the south and in establishing 

the present Green River as the master stream of the 

basin. Some idea of the extent of the subsidence at 

Flaming Gorge, where the Green enters the mountains, 

can be had by comparing the present height of Bear 

Mountain with Cedar and Little Mountains in Wyoming 

(fig. 41); all three of these are remnants of the Gilbert 

Peak erosion surface. (Cedar and Little Mountains are 

also capped by Bishop Conglomerate; Bear Mountain 

is not). In longitude, Bear Mountain is about half way 

between Cedar and Little Mountains, and it stands 

about 18 km to the south. The elevation of Cedar Mountain approaches 2,620 m, and Little Mountain exceeds 

2,774 m. A point on the restored Gilbert Peak surface 

halfway between at their latitude would be about 2,700 

m, or about 2,670 m if 30 m is subtracted for an assumed capping of Bishop Conglomerate, which is about 

30 m thick on Twin Buttes, 4.8 km to the north. Bear 

Mountain, however, reaches only 2,356 m, about 300 

m lower than the projected surface midway between 

Cedar and Little Mountains. Originally, though, Bear 

Mountain was higher than the restored midpoint. If the 

restored pre-tilt gradient of the Gilbert Peak erosion 

surface (there assumed to be about 38 m/km) were projected south to Bear Mountain, it would reach about 

3,350 m. Thus, the Gilbert Peak surface at Bear Mountain probably has been lowered at least 975 m. That
NEOCENE OF EASTERN UINTA MOUNTAINS

10 KILOMETERS

5 MILES

FIGURE 39. Drainage map of the Blue Mountain area. Stippling indicates Bishop Conglomerate. Note how former drainage of Wolf Creek 

has been diverted into Hells Canyon, reversing the direction of Turner Creek. Meadow Creek formerly flowed into K Creek. Note 

barbed tributaries of Johnson Draw.

amount of lowering requires a back tilting toward the 

mountains of about 17 m/km, and with that amount of 

tilt, a reversal of the drainage should not be surprising.

In fact, the new southward and eastward drainage 

should have been vigorous and should have agressively 

enlarged its watershed at the expense of disadvantaged
DRAINAGE DEVELOPMENT 59

109°30' 109W 108°30'

30 KILOMETERS

I 

I

20 MILES

FIGURE 40. Principal hills and drainages of northeast flank of Uinta Mountains and vicinity and their relationship to the Uinta anticline 

and the Uinta-Sparks fault zone. Bar-and-ball symbol indicates Laramide downthrow; (D) indicates post-Bishop downthrow; hachures 

outline mountains and escarpments. Notice at upper center of the map how Salt Wells Creek has captured the headwaters of Vermillion 

Creek.

north-flowing streams nearby. The flow through Red 

Canyon toward Browns Park should have rapidly extended its watershed north into the Green River Basin, 

capturing such streams as Henrys Fork and Sage 

Creek in the process. Just when it captured the master 

drainage of the basin, thereby greatly shifting the position of the Continental Divide, is uncertain, but it probably did so in Pleistocene time (Hansen, 1969b; Hansen 

and others, 1983). Such a momentous capture, diverting 

and acquiring the headwaters of the Platte River, 

would have vastly enlarged the competence and erosive 

capacity of the system. The effect on the Platte should 

have been the opposite aggradation and reduced competence.

Drainage changes in the various tributaries of the 

Green River on the north slope of the Eastern Uinta 

Mountains since Bishop time have been outlined by several investigators (Sears, 1924a; Bradley, 1936; Ritzma,

1959; Hansen and others, 1960; Hansen, 1965, 1969a, 

1969b, 1984). In brief, drainage quickly adjusted to the 

topographic changes brought on by the foundering of 

the Uinta arch, and the earliest new stream courses 

were largely consequent down the new slopes.

Subsidence and topographic distortion caused by the 

foundering diminished gradually westward to a minimum. Drainage west of Flaming Gorge continued to 

flow northward, but all of it as far west as Henrys 

Fork was eventually captured by structurally controlled subsequent drainage that flowed directly to the 

Green River in response to the new gradients (Hansen, 

1965, p. 175-180).

East of Flaming Gorge the topographic dislocations 

were severe, and the resultant drainage changes were 

correspondingly drastic. New south-flowing consequent 

streams included Spring, Goslin, Red, Willow, Beaver, 

and Vermillion Creeks, the Little Snake River, and
60 NEOCENE OF EASTERN UINTA MOUNTAINS

110°

40° 

45'

NORTH

Flaming

Gorge j^* Little

Reservoir!.,' Mountain

WYO

Sage y

Creek ' T>A/i /Twin Buttes 

Butte A *«f cedar 

i<c-^ ^ ~ Mountain

1 Point halfway between 

Cedar Mountain and 

Little Mountain, 

elevation 2670 m

>-t\\t 915'

3D KILOMETERS

~\I

20 MILES

SOUTH

ELEVATION

Former elevation of , IMETERSI 

Bear Mountain

PeaVc 

-

eteyafen:

-ppst-t ||f d if fere ncfs ih 

"elevation

Present elevation of 

Bear Mountain

-18 KILOMETERS-

- 2670

- 2500 

2356

- 2000

FIGURE 41. Diagram showing subsidence of Bear Mountain and reversal in gradient of Gilbert Peak erosion surface north from Bear 

Mountain. The restored gradient is assumed to have been a uniform 

38 m/km, although it probably was actually concave upward. In 

any event, subsidence of the magnitude indicated would have drastically altered the drainage directions across the area. Vertical 

scale is 10 times horizontal scale.

many lesser streams. (Incidentally, remnants of the 

Browns Park Formation deposited within the valleys 

of Vermillion Creek and the Little Snake River are confirmatory evidence that southerly directed drainage 

and hence southerly tilting predated the Browns Park 

Formation). Further realinements took place along 

some of these streams as they adjusted to the structure 

of the underlying bedrock and as subsequent drainage 

developed along belts of nonresistant rock. Spring 

Creek, for example, was later diverted west into a 

strike valley in the Hilliard Shale (Cretaceous), leaving 

its initial course high and dry at Dutch John Gap 

(Ritzma, 1959, fig. 8C; Hansen, 1965, fig. 67). (Dutch 

John Gap is a conspicuous wind gap just northwest of 

Dutch John, Utah).

VERMILLION CREEK AND IRISH CANYON

Changes in the course of Vermillion Creek at the 

southeast end of Cold Spring Mountain have been described briefly by Sears (1924a, p. 298). With a drainage area of more than 2,600 km2, Vermillion Creek has

by far the largest basin of the south-flowing tributaries 

of the Green between Flaming Gorge and the Little 

Snake River. Vermillion Creek crosses the flank of the 

Uinta arch in a narrow canyon about 215 m deep. Sears 

attributed this canyon, correctly I think, to superimposition down through a former cover of the Browns 

Park Formation. However, the broad basin of Vermillion Creek upstream from the canyon plainly was excavated before the Browns Park Formation was deposited in it. The present valley may have evolved through 

a process of cutting, back filling, and renewed cutting, 

perhaps by the mechanism of antepositoin, as postulated by Hunt (1956, 1967, 1969) for many stream 

valleys of the West. Sears also pointed out the probability that Vermillion Creek formerly flowed through 

nearby Irish Canyon (fig. 42) but was captured by a 

tributary of the "East Fork of Vermillion Creek." (The 

East Fork is now called Dry Creek.) With that capture, 

Irish Canyon was abandoned by perennial drainage and 

is now a dry but impressive beheaded valley, 6.5 km 

long and nearly 300 m deep.

Sears did not elaborate on the origin of Irish Canyon, 

but he noted that Irish Canyon's downstream extension, Bull Canyon, though incised into the Uinta Mountain Group, has undoubtedly been superimposed. The 

Browns Park Formation still exists at the head and 

mouth of Bull Canyon and on both rims. Whether or 

not the Browns Park Formation also once covered the 

rims of Irish Canyon, however, is doubtful. The head 

of the canyon truncates a bermlike remnant of a pediment that flanks Cold Spring Mountain on the northeast. Just 2.4 km farther north the pediment passes 

beneath a basal conglomerate of the Browns Park Formation that, in turn, underlies typical Browns Park 

sandstones. Soon after the pediment formed, the 

Browns Park Formation probably was deposited on it. 

Irish Canyon, which cuts the pediment, probably is 

younger, therefore, than the nearby Browns Park Formation. The slope of the pediment, moreover, is wrong 

for drainage into Irish Canyon. A hypothesis that 

would not require superposition of Irish Canyon down 

through the Browns Park Formation is as follows:

Irish Canyon, like other north-flank drainageways, 

began to form with the onset of tilting and collapse 

of the north flank. Eroding headward across the Uinta 

Mountain Group and the lower Paleozoic rocks, Irish 

Canyon then uncovered the soft Morgan Formation and 

started to excavate a strike valley on the northeast side 

of Cold Spring Mountain over the present upper reach 

of its canyon. Meanwhile, another south-flowing drainage was excavating Vermillion Creek basin. On the 

southwest side of the basin on the northeast flank of 

Cold Spring Mountain the pediment that is preserved 

today as the aforementioned berm was forming at that
DRAINAGE DEVELOPMENT

108°45'

61

5v^f'^'7?^<X?v'« «V» - y^X^f'^iSiFi £*'*?' $ '

R 102 W R 101 W R 100 W

10 KILOMETERS

Q

Tbp,

0

QUATERNARY DEPOSITS

BROWNS PARK FORMATION (TERTIARY)

BISHOP CONGLOMERATE (TERTIARY)

PALEOGENE ROCKS

CRETACEOUS ROCKS

JURASSIC ROCKS

EXPLANATION

1

5 MILES

TRIASSIC AND PERMIAN ROCKS 

;S:'| WEBER SANDSTONE (PENNSYLVANIAN)

MORGAN AND ROUND VALLEY 

FORMATIONS (PENNSYLVANIAN)

MISSISSIPPIAN AND CAMBRIAN ROCKS 

UINTA MOUNTAIN GROUP

CONTACT

FAULT Bar and ball on downthrown side

SYNCLINE

FIGURE 42. Geologic map of Vermillion Creek area affected by the beheading of Irish Canyon. Heavy dashes mark wind gap at head

of Irish Canyon. Geology modified from Rowley and others (1979).
62 NEOCENE OF EASTERN UINTA MOUNTAINS

time also, graded to the floor of Vermillion Creek 

basin. The Browns Park Formation soon began to accumulate on the pediment, on the floor of Vermillion 

Creek basin, and in the valley of Browns Park below 

Irish Canyon. Irish Canyon continued to erode headward, finally truncating the pediment along the strike 

of the Morgan Formation, leaving the stranded berm. 

Perhaps it then extended its headwaters north to become the upper reach of ancestral Vermillion Creek.

At that time, the level of Irish Canyon was about 

425 m above modern drainage in the Vermillion Creek 

basin, as indicated by the altitude of its rim. Ancestral 

Vermillion Creek would have continued to flow through 

Irish Canyon until a tributary of Dry Creek beheaded 

the canyon and set the present drainage pattern. Dry 

Creek, which probably was flowing on the Browns Park 

Formation at that time, would have had an erosive advantage over Irish Canyon, which had to cross the 

quartzites of the Uinta Mountain Group and the resistant Paleozoic limestones.

Downstream from the point of capture, the southeast-trending reach of Vermillion Creek meanders in 

a trench that parallels, crosses, and recrosses the 

Sparks fault in a way that suggests a causative relationship between faulting and drainage. The fault is 

downthrown to the northeast, and the Browns Park 

Formation is tilted northeastward by drag in the upthrown block. Late movement on the fault thus postdates the Browns Park Formation. This movement 

moreover was directly athwart ancestral Vermillion 

Creek and was parallel to the capturing tributary of 

Dry Creek. It therefore would have inhibited the flow 

of ancestral Vermillion Creek while encouraging headward erosion by the capturing tributary. Capture, 

therefore, probably shortly followed movement.

The capture of Irish Canyon must postdate the diversion of the Green River into Lodore Canyon, because 

the Green River is the base level of Vermillion Creek 

and because the wind gap at the head of Irish Canyon, 

at an altitude of about 2,035 m, is nearly 300 m lower 

than the rims of Lodore Canyon at the Gates of Lodore. 

The diversion of the Green River into Lodore Canyon 

should have rejuvenated all upstream drainage. The 

capture of Irish Canyon probably was an early Pleistocene event, inasmuch as it significantly postdated the 

deposition of the Browns Park Formation and inasmuch 

as the wind gap at its head is only about 180 m higher 

than nearby modern Vermillion Creek. The latest 

ground-breaking movement on the Sparks fault, therefore, may be as recent as early Pleistocene. An earthquake on September 24, 1983, strengthens the case for 

Quaternary movement. This earthquake, of magnitude 

4.2, was centered beneath Cold Spring Mountain at lat

40.8° N., long 108.8° W. (John Minsch, U.S. Geological 

Survey, oral commun., October 3, 1983), and it may 

have been caused by a movement on the Sparks fault, 

which dips southwestward beneath Cold Spring Mountain.

Bull Canyon, like Irish Canyon, also could not have 

been eroded until after the diversion of the Green 

River into Lodore Canyon, inasmuch as the rims and 

floor of Bull Canyon are much lower than the rims of 

Lodore. Bull Canyon has an average depth of about 

75 m, and at its mouth its floor is only about 75 m 

above its base level downstream at nearby Vermillion 

Creek. Bull Canyon, therefore, must have been incised 

largely in later Pleistocene (early Wisconsin?) time, 

well after the beheading of Irish Canyon. Despite the 

loss of its Irish Canyon headwaters, Bull Canyon still 

drains about 30 km2 of steep terrain on Cold Spring 

Mountain, much of it possibly gained at the expense 

of the drainage of Little Joe Basin.

PHYSIOGRAPHIC SETTING AND 

DEVELOPMENT OF LODORE CANYON

For more than 100 years geologists have pondered 

the course of the Green River through the Uinta Mountains. Most rivers flow away from mountains, not toward them. But the Green, draining the mountains and 

plains of southern Wyoming, cuts sharply into the 

Uinta Mountains at the Wyoming-Utah State line, then 

flows 175 km east and south across the range through 

Utah and Colorado without regard for topographic relief or geologic structure. Why, at the Gates of Lodore, 

does the river turn south from the wide valley of 

Browns Park, where logical egress is eastward, reenter 

the mountains to the south, and for the next 80 km 

drain one canyon after another before reaching the 

Uinta Basin? The hypothesis I've advanced in earlier 

publications (Hansen, 1969a, p. 42-44, 58; 1969b; Hansen and others, 1982) is briefly recapitulated here:

The Green River was flowing southeast through the 

valley of Browns Park in middle Miocene time on a 

thickening fill (the Browns Park Formation) toward a 

possible junction with the Yampa River and ultimately, 

perhaps, the White River. The fill eventually overtopped the valley rim to the south, and the river, turning southward at the site of the present Lodore Canyon, found a new course across the Uinta Mountains 

toward the Uinta Basin. From the Gates of Lodore 

southward, the river established itself on the old upland 

surface of the Bishop Conglomerate, ultimately eroding 

its way down through the underlying rocks to the present canyon bottom. Then, as now, the Bishop Conglom-
DRAINAGE DEVELOPMENT 63

erate extended deep into the mountains as a network 

of coalesced alluvial fills (figs. 5, 8, 13, and 37). On 

the south slope of the range the Bishop is still so widely 

disposed along and near both rims of the canyon that 

there can be no doubt about the superposition of the 

river through it. Quite likely, a shallow south-flowing 

drainage had already been established over the site of 

Lodore Canyon on or through the Bishop Conglomerate 

at the time of spillover. If so, the Green simply took 

advantage of the already available course.

This concept calls for a much-thickened valley fill in 

Browns Park and for the subsequent removal of much 

of it by the Green River, but the evidence seem irrefutable. East of Lodore Canyon the Browns Park Formation still reaches altitudes comparable to the rims of 

the canyon, which are about 2,285-2,315 m. The highest 

little-deformed remnant is at John Weller Mesa (shown 

on fig. 13), on the drainage divide between the Green 

River and the Little Snake River, 30 km southeast of 

the Gates of Lodore. There, the eroded top of the 

Browns Park Formation is about 2,195 m above sea 

level. John Weller Mesa slopes gently northeastward 

and probably has been gently tilted in that direction. 

Beds stratigraphically higher in the Browns Park Formation crop out to the northeast but, because of dip, 

at lower present altitudes. Twenty kilometers north of 

John Weller Mesa at Dry Mountain, remnants of the 

Browns Park Formation stand even higher 2,340 m 

in an area that has been involved in strong late Tertiary warping. Their altitude may formerly have been 

higher than now.

In any event, a vast amount of material has been 

removed from the valley since the onset of canyon cutting. The highest part of John Weller Mesa is 540 m 

above the level of the Green River at the Gates of Lodore, a height only about 122 m lower than the canyon 

rim, or about 82 percent of the height of the canyon 

wall. A rough guess would place the volume of material 

removed from the old valley of Browns Park below the 

level of John Weller Mesa at 210 km3 . (Here, the "old 

valley of Browns Park" is considered to be the area 

between the head of modern Browns Park, to the 

northwest in Utah, and Lone Mountain, to the southeast in Colorado a total length of about 72 km.) If 

the fill reached to the rim of Lodore Canyon, as postulated here, about 80 km3 of additional fill must have 

been removed. An even larger volume of Browns Park 

material has been eroded from areas to the east in Moffat County, and much more still remains.

The above hypothesis varies from views of earlier 

workers. Major Powell (1876), using the Uinta canyons 

as his type example, advanced the then-new concept

of antecedence, which assumed erroneously that the 

Green River predated Laramide uplift of the Uinta 

Mountains and that as uplift progressed the river kept 

pace in eroding its canyons. Powell alluded to a stationary saw cutting a moving log: "the river was running 

ere the mountains were formed * * * before the rocks 

were folded" (Powell, 1876, p. 152). Antecedence is an 

elegant concept, but cogent evidence rules it out for 

the Eastern Uinta Mountains, as S. F. Emmons (in 

Hague and Emmons, 1877, p. 194) was the first to point 

out.

Noting the abundance of high-level Tertiary deposits 

(Bishop) in the mountains, Emmons advocated superposition, a concept that nearly coincides with the view 

expressed here. Bradley (1936, p. 189) agreed that the 

Green was superimposed upstream and down from Lodore Canyon, but he doubted the supposed depth and 

extent of the old Tertiary fill and, elaborating on a suggestion of Sears (1924a), proposed instead that an ancestral east-flowing Green River was captured by a 

small but vigorous stream in Lodore Canyon, "Lodore 

Branch of Cascade Creek," and was thus diverted 

southward. (Cascade Creek is now Pot Creek.) This 

hypothesis disavows any extraordinary thickness of 

Browns Park fill and calls for headward erosion by "Lodore Branch," rather than a spillover by the Green, 

as postulated here. Bradley did not address the difficult 

problem as to how a small stream with a very small 

watershed in hard rock would erode across a divide to 

capture a much larger stream a problem obviated by 

spillover. Hunt (1969, p. 90-99), in his brilliant analysis 

of the history of the Colorado River and its tributaries, 

proposed a compromise position between Powell and 

Emmons, a combination of antecedence and superposition, which he called "anteposition." Also questioning 

the extent of the fill, Hunt suggested that much of the 

depth of Lodore Canyon is an erosive response to renewed uplift of the range since the drainage was established. Hunt's surmise probably is true, but there is 

no known demonstrable evidence that the canyon area 

has been uplifted differentially relative to the valley 

of Browns Park in a way that would account for the 

depth of Lodore Canyon. The evidence suggests otherwise: The Green has eroded hundreds of meters downward in Browns Park also. The small difference in altitude owing to post-Bishop tilting between the head 

of Lodore Canyon and Browns Park is just a few 

meters per kilometer and, in that short a distance, is 

negligible compared to the depth of canyon erosion. 

Browns Park and Lodore Canyon are both parts of the 

same crustal block, and if renewed uplift has occurred, 

it probably was regionwide epeirogenic and included 

Browns Park as well as Lodore Canyon.
64 NEOCENE OF EASTERN UINTA MOUNTAINS

YAMPA RIVER

The physiographic evolution of the Yampa River in 

the Uinta region is more obscure than that of the 

Green, and its complete history may never be fully resolved. Some episodes in its development, however, are 

reasonably clear. Like the course of the Green River, 

the course of the Yampa through the Uinta Mountains 

is younger than the Bishop Conglomerate; the conglomerate is well preserved south of the Yampa River, and 

its transport direction was plainly from the north, from 

the crestal part of the range toward the flanks at right 

angles to the present course of the river. Neither the 

river nor the canyon could have existed at the time 

the Bishop was deposited. Upstream from the Uinta 

Mountains, however, before the main body of the 

Browns Park Formation was deposited, an ancestral 

stream perhaps the Yampa was delivering pebbly 

gravel from the Park Range to the basin in the LayMaybell area. This basin was formed after the deposition of the Bishop Conglomerate; tilted Bishop crops 

out on the distal side of the basin, dipping toward its 

source.

With the tectonic subsidence of the Eastern Uinta 

Mountains, the broad downwarp in the Lay-May bell 

area became an accumulation basin for the dominantly 

lacustrine and eolian deposits of the Browns Park Formation. Fluvial deposits coarser than medium-grained 

sand are rare in the Browns Park of this area (Luft 

and Thoen, 1981), and currents were seldom strong 

enough to transport pebble-size debris (except at the 

onset of Browns Park deposition, when the basal conglomerate derived from the Park Range, described on 

previous pages, was laid down).

Through drainage, therefore, was weak during deposition of the main body of the Browns Park Formation. No outlet from the basin at that time has yet been 

identified, but trunk drainage probably skirted the 

north end of Cross Mountain, essentially along the 

present course of the Little Snake River between Cross 

Mountain and the easternmost Uintas. It then probably 

turned west along the present Yampa Canyon. (See discussion of Deerlodge monlocline, p. 53.) The Yampa 

River as such did not yet exist through Cross Mountain 

Canyon or Juniper Canyon, because both are superimposed canyons that postdate the deposition of the 

Browns Park Formation. In any event, drainage probably was overwhelmed by sediment. The abundance of 

eolian bedding in the Browns Park Formation in the 

Lay-Maybell area suggests aridity, and the meager 

runoff into the rather capacious basin may have been 

incapable of overtopping the basin rim. Basically, the 

Lay-Maybell area was an aggrading sink throughout 

Browns Park time. Dyni (1980) noted sandy calcitic concretions within the eolian sequence and thin horizontal

beds of sandy limestone that he regarded as deposits 

of interdunal ponds. Continuing subsidence of the basin 

floor, moreover, may have prolonged its filling.

Downwarping of the magnitude indicated by the displacement of the Gilbert Peak erosion surface and the 

Bishop Conglomerate must have required a long period 

of time. Guided in part by faulting, the ensuing basin 

over the subsided Uinta arch was partly structural and 

partly erosional, and concomitant runoff from the 

higher mountains to the west must have followed available low ground and weakened rock. Erosional breaching of anticlines is common in a first cycle of erosion 

following uplift. A prominent set of joints parallel in 

trend with the axis of the Uinta anticline, moreover, 

suggests lateral extension normal to the axis. Such 

fracturing along the crest of the fold would abet breaching. Thus, the depositional basin in the Browns ParkLay syncline nearly coincides with the subsided Uinta 

arch and Axial Basin anticline. Inasmuch as the subsidence and valley cutting postdated the deposition of the 

Bishop Conglomerate and predated the deposition of 

the Browns Park Formation, a significant time break 

must have intervened between the deposition of the 

two formations. Radiometric dating indicates a break 

of perhaps 4 m.y. (See paragraph on age of the Browns 

Park Formation, p. 31.)

Meanwhile, drainage must have begun to develop 

along the present course of the Yampa River in Dinosaur National Monument soon after the Gilbert Peak 

surface was formed and the Bishop Conglomerate was 

deposited. That area is an asymmetrical, partly faulted 

structural trough between Douglas Mountain to the 

north and Blue Mountain to the south (Hansen and 

others, 1980, cross sections), and the early drainage 

should have been guided by the trough into a roughly 

east-west trend. The exact course is unclear; I view 

it as flowing west but extending itself east by headward erosion, possibly as far as the east boundary of 

the monument. Structurally, the trough is not nearly 

as deep as the Browns Park-Lay syncline, but it is 

deepest along its southern border next to the Yampa 

fault. Bishop Conglomerate is preserved at the east and 

west ends but not in the center; the Gilbert Peak erosion surface is sparingly preserved as accordant 

ridgelines and summits and, as noted previously, it 

slopes gently eastward and northward in response to 

post-Bishop tilting.

Thus, the westerly flow of the Yampa River in Dinosaur National Monument is against the direction of the 

present slope of the Gilbert Peak erosion surface. The 

rim at Harpers Corner, near the confluence with the 

Green, is more than 300 m higher than East Cactus 

Flat; westward drainage, therefore, must predate eastward tilting. Otherwise, the Yampa would have flowed 

east and would still do so.
DRAINAGE DEVELOPMENT 65

Before tilting, drainage wandered rather widely between Blue Mountain and Douglas Mountain, and in so 

doing, it planed off the smooth platform still well preserved 300 m or so above present drainage on East 

and West Cactus Flats (and less well preserved at 

Schoonover Pasture). This platform truncates bedding 

at an angle of 8°-10°. The northerly component of tilt, 

then, would have encouraged the river to shift gradually northward, away from Blue Mountain and the 

Yampa fault and toward the flank of Douglas Mountain, 

where it is today.

The incised gooseneck meanders between Harding 

Hole and the Yampa's confluence with the Green at 

Echo Park may have formed as a hydraulic response 

to a rising base level induced by tilting up to the west 

and south against the direction of drainage. Canyon 

cutting followed, and it faithfully preserved the meander pattern. As entrenchment progressed, however, 

the meanders gradually shifted, undercutting the outsides of bends, slipping off the insides, and forming the 

dramatic overhangs for which the canyon is famous. 

The largest of these is more than a thousand feet high. 

Meander shifting was influenced by dip, inasmuch as 

most of the overhangs are on the downdip sides of

meanders. The time of entrenchment can only be surmised, but downcutting would have been greatly 

enhanced by the spillover of the Green into Lodore 

Canyon and the resultant rejuvenation of the drainage 

system. Further entrenchment surely would have been 

enhanced by the postulated diversion of the Upper 

Green River near Green River, Wyo. (See next section.)

When drainage was flowing across East and West 

Cactus Flats, it also was flowing between Cross Mountain and the east end of the Uintas, now the valley 

of the Little Snake River. At that time it was cutting 

a broad terrace into the Bishop Conglomerate at 

Klauson Pasture (fig. 43) present altitude about 1,980 

m. This truncation predated the northerly tilt of the 

Eastern Uinta Mountains. Whether or not that stream 

should properly be called the Little Snake River is a 

moot question: It was the outlet for the growing playa 

or sedimentary basin in the Lay-Maybell area, it probably carried the overflow from the Browns Park area 

as well as the Little Snake and Yampa headwaters, 

and the Little Snake as now constituted did not yet 

exist. Drainage downstream along the present Yampa 

Canyon appears to be anteposed established before

FIGURE 43. View north toward Klauson Pasture (flat surface in distance at foot of easternmost Uinta Mountains), Moffat County, Colo. 

Klauson Pasture is capped by Bishop Conglomerate and has been terraced here by a second, lower erosion surface. Such incision 

of the Bishop Conglomerate is fairly common along the Uinta Mountains. Bradley (1936, p. 198) noted a similar incision of the Bishop 

Conglomerate on Cedar Mountain in Wyoming.
66 NEOCENE OF EASTERN UINTA MOUNTAINS

tilting but maintained by entrenchment as tilting progressed and as aggradation upstream raised its bed on 

the thickening fill of the Lay-Maybell area.

The present course of the Yampa through Juniper 

Canyon and Cross Mountain Canyon did not exist until 

the end of Browns Park deposition (Hancock, 1915). 

Both canyons are accidents of superposition that 

formed after the level of fill had risen high enough to 

superimpose the drainage. Farther upstream, what is 

now the Yampa merely emptied into the playa and was 

lost in a sink of shifting sand. When the drainage 

changed from a depositional to an erosional mode, the 

Yampa became organized as a through-flowing river 

across the buried spurs of Cross and Juniper Mountains. This change may have been triggered by the diversion of the Green into Lodore Canyon.

Hunt (1969, p. 89) has suggested that Cross Mountain has risen 150 m since the superposition of the 

Yampa and, hence, that the lowest 150 m of canyon 

is due to antecedence after superposition (fig. 44). Such 

a rise should be expressed by upturning or faulting in 

the flanking Browns Park Formation. Exposures are 

inconclusive on the east side of Cross Mountain upturning, if present, is minimal and the Browns Park 

Formation is not preserved on the west, but just south 

of the Yampa River on the west side, the Bishop Conglomerate (Dyni's lower conglomerate unit of the 

Browns Park Formation) has been lowered as much as 

150 m in a narrow graben along the Cross Mountain 

fault, well shown on Dyni's map (1968). The time of 

faulting is not precisely known, but Dyni (1980) noted 

post-Browns Park monoclinal folding and inferred

FIGURE 44. Yampa River emerging from Cross Mountain Canyon, incised into the Madison Limestone. The course of the river here is 

superimposed, but Hunt (1969) has suggested that the full depth of the canyon in this view, about 150 m, is antecedent to Pliocene-Pleistocene renewed uplift of Cross Mountain. The Cross Mountain fault at the base of the slope has had post-Bishop movement (Dyni, 

1968).
DRAINAGE DEVELOPMENT 67

growth faulting in adjacent areas. Farther upstream, 

the rather tortuous meandering of the Yampa River 

above Juniper Canyon suggests that the river there has 

been base leveled by late resurgence of the Axial Basin 

anticline. These meanders are incised to a depth of 

about 150 m.

Large horseshoe-shaped or amphitheatrical basins in 

the north wall of Yampa Canyon at various places are 

not "meander-migration scars" cut by the Yampa River 

at an early time, despite the belief of Sears (1962, p. 

9), and they thus have no bearing on the development 

of canyon drainage. Rather, they are products of retrogressive landsliding down a dip slope (figs. 45 and 

46). The hummocky but essentially translatory slides 

are restricted to places where the downcutting river 

has uncovered the incompetent lower member of the 

Morgan Formation dipping into the canyon. Failure 

within the Morgan removed support from the canyon 

walls above, and failure then retrogressed updip and 

radially outward from the points of origin. Subsequent 

gullying through some slides has exposed their rubbly 

interiors as well as the underlying Round Valley Limestone, which provides the resistant base of failure. The 

most accessible slide of this kind is not in Yampa Canyon but in Diamond Gulch along the road to Jones Hole, 

where ongoing failure is indicated by road damage, 

open ground cracks, and toppled blocks of sandstone 

tens of meters on a side.

POSTULATED CAPTURE OF THE MASTER 

DRAINAGE OF THE GREEN RIVER BASIN

The most significant major drainage change, in terms 

of the total budget of the streams involved, and probably also the most recent, was the postulated diversion 

of the Upper Green River3 from its ancestral easterly 

course. As a first step, by late Eocene or early Oligocene time, the Rock Springs uplift had been truncated 

by the Gilbert Peak erosion surface. Aspen Mountain 

remained as a monadnock, with a mantle of Bishop 

Conglomerate at its flanks. Drainage out of the Green 

River Basin was presumably eastward, across the present Continental Divide, for reasons discussed below.

As postulated previously (Hansen, 1969b, p. 99), the 

slow rise of the incipient Continental Divide in postBishop time stagnated easterly drainage. The Great Divide Basin rose so much in late Pliocene time that it 

was largely stripped of post-Eocene rocks (Love, 1971,

8 As the following discussion postulates, the part of the Green River above the mouth 

of Bitter Creek, near the town of Green River, Wyo., was once a separate stream and 

has a significantly different history from the downstream reaches of the Green. For simplicity and clarity, this part of the stream is here referred to as the "Upper Green River," 

even though, in more common usage, the Upper Green is regarded as the reach above 

the confluence of the Yampa River, the Green's chief tributary.

p. 79). As the eastern Uinta Mountains subsided, 

moreover, the new drainage along and across the range 

gradually extended its watershed, and the Upper 

Green River finally was captured near the present 

mouth of Bitter Creek at the town of Green River, 

Wyo. At that instant in geologic time the entire Green 

River system was turned south toward the Uinta 

Basin. Ritzma (1959, p. 88) has suggested that a rise 

of the Rock Springs uplift in Pliocene time and the extrusion of lavas over the north part of the uplift may 

have blocked easterly drainage. These lavas crop out 

in the Leucite Hills 30-45 km northeast of Rock 

Springs. They are now deeply dissected but were once 

more extensive, and they surely would have deflected 

any drainage in their path. Pilot Butte, 13 km northwest of Rock Springs, is an outlier, base altitude about 

2,380 m, resting on soft gray sandstone that resembles 

a fine facies of the Bishop Conglomerate. The base of 

the sandstone, altitude about 2,315 m, rests on the 

Laney Member of the Green River Formation. Pilot 

Butte itself is about 520 m above the nearby Green 

River. The Leucite Hills lavas have been dated at 

about 1.25 m.y. (Bradley, 1964, p. A58) and more recently at 1.1 m.y. (McDowell, 1971). Their age, therefore, is compatable with Pleistocene diversion of the 

Upper Green River, but they seem too high and too 

old, as explained later, to have played a direct part. 

They could, however, have shifted drainage southward 

away from an earlier hypothetical course toward the 

Sweetwater River.

As a topographic feature, the present Continental Divide is very subdued all the way from the Wind River 

Range to the Sierra Madre. The divide splits and encircles the Great Divide Basin, which has interior drainage and contains many ill-defined watersheds and 

closed depressions, some of which contain playas. If the 

Great Divide Basin were filled to overflowing, it would 

spill to the North Platte River across a broad threshold 

about 2,000 m above sea level. The low point in the 

west rim of Great Divide Basin is just west of Tipton, 

Wyo., at an altitude of about 2,070 m. This is the true 

position of the Continental Divide. It is about 245 m 

lower than the floor of the Gilbert Peak erosion surface 

near the center of the Green River Basin and about 

the same amount lower than the rims of Lodore Canyon 

at the Gates of Lodore. Just north of the Great Divide 

Basin the Sweetwater River, arising on the southwest 

side of the Wind River Range in an area that is structurally part of the Green River Basin, turns south, then 

east, and flows to the North Platte. Thus, even now, 

part of the drainage from the Green River Basin flows 

to the Gulf of Mexico, and has, according to Denson 

and Chisholm (1971, p. C125), since the end of the Eocene.
68 NEOCENE OF EASTERN UINTA MOUNTAINS

^^;:ir»IW^,:,:,^^^

Older Paleozoic and 

Proterozoic rocks

1500

6000

-500G

MILE

CONTOUR INTERVAL 40 FEET 

NATIONAL GEODETIC VERTICAL DATUM OF 1929
DRAINAGE DEVELOPMENT 69

Once drainage turned across the Uinta Mountains at 

Lodore Canyon, and was thus flowing to the Uinta 

Basin, it was rapidly rejuvenated. The Uinta Basin is 

not only the deepest basin of the Colorado Plateau 

structurally (Hunt, 1956, p. 2), it is also some 300 m 

lower topographically than the Green River Basin. In 

late Pliocene time, as thus visualized, the main drainage of the Green River Basin (the Upper Green River) 

was still flowing eastward at a hydraulic disadvantage 

across the rising Rock Springs uplift and the rising incipient Continental Divide. The invigorated trans-Uinta 

drainage, however, with its new, lower base level, was 

incising itself into the Uinta Mountains, carving out the 

spectacular meander loops of Flaming Gorge, Horseshoe Canyon, and Red Canyon. Near Flaming Gorge 

it captured drainage that once flowed north into the 

Green River Basin, including Henrys Fork, Burnt 

Fork, and Birch Creek, as noted previously on page 

59. These streams flow north off the Uinta Mountains, 

then turn southeast to join the Green. Other, lesser 

streams do the same (Hansen, 1965, p. 176). Some of 

the details can be gleaned from topographic maps of 

the area. Henrys Fork probably once flowed north 

through a gap between Hickey Mountain and Sage 

Creek Mountain ("Sage Creek Butte" in old reports;

EXPLANATION

Qa

Qtc

Qoa

Pw

FLOODPLAIN AND CHANNEL DEPOSITS OF YAMPA

RIVER (HOLOCENE) 

EOLIAN SAND (HOLOCENE)

DEBRIS FAN (HOLOCENE)

ALLUVIUM IN VALLEY BOTTOMS (HOLOCENE)

LANDSLIDE DEPOSITS (HOLOCENE AND PLEISTOCENE)

TALUS AND OTHER COLLUVIUM (HOLOCENE AND

PLEISTOCENE) 

OLDER ALLUVIUM (PLEISTOCENE)

WEBER SANDSTONE (MIDDLE PENNSYLVANIAN)

MORGAN FORMATION (MIDDLE PENNSYLVANIAN): 

Upper member

Lower member

ROUND VALLEY LIMESTONE (LOWER PENNSYLVANIAN)

CONTACT Dashed where approximately located 

STRIKE AND DIP OF BEDS 

TRACE OF MASTER JOINTS

FIGURE 45 (on facing page). Geologic map and section of part of 

the Haystack Rock quadrangle (Hansen and others, 1980) showing 

relationship of large, retrogressive landslides (Ql) in Yampa Canyon to amphitheatrical headwalls and to the incompetent lower 

member of the Morgan Formation.

Bradley, 1936, p. 199). Burnt Fork and Birch Creek, 

now flowing independently into Henrys Fork, may have 

merged and flowed north through a gravel-covered gap 

between Cedar Mountain and Black Mountain. Working 

headward against the prevailing regional slope, the expanding new river system then captured the Upper 

Green.

This capture may have been a middle Pleistocene 

event, although modern indigenous fish species in the 

Upper Green River drainage differ markedly from 

those in the North Platte and the Sweetwater. Only 

the mountain sucker and two closely related daces are 

common to the two drainage systems, according to R. 

S. Behnke (Colorado State Univ., Dept. of Fisheries 

and Wildlife Biology, written commun., 1983). Any 

postulated connection, therefore, must take into account these differences.

The present Green River at Green River, Wyo., is 

flowing at an altitude of about 1,850 m, only about 215 

m lower than the Continental Divide at Tipton, 90 km 

to the east. Extending up the Green from a point a 

few kilometers west of the town of Green River is a 

series of high gravel-capped terrace remnants. These 

remnants are about 120-180 m above modern drainage. 

Richmond (1948) correlated them with the Buffalo glaciation of the Wind River Range, though he later abandoned the name "Buffalo glaciation" because it represents three separate glaciations (Richmond, 1965, p. 

218). The highest terrace, which will be mentioned 

further, is referred to as the "Peru bench" for its occurrence north of Peru station on the Union Pacific 

Railroad 10 km west of Green River (Hansen, 1969b, 

p. 99), where it has an altitude of about 2,040 m and 

is about 195 m above the river.

The gravel on the Peru bench contains an assortment 

of resistant rock types particularly light-colored to

Upper member of 

Morgan Formation

Headwall

Lower member of 

Morgan Formation

Round Valley Limestone

FIGURE 46. Block diagram showing relationship of a retrogressive 

landslide to Yampa Canyon and the amphitheatrical form taken 

by the headwall scarp. Sliding begins when the lower member of 

the Morgan Formation is undercut by the river. Where such slides 

encroach on the river, they cause rapids.
70 NEOCENE OF EASTERN UINTA MOUNTAINS

nearly black quartzite and pink granite derived 

mainly from the Wind River and Wyoming Ranges, including many pebbles that may have been reworked 

out of the Pinyon Conglomerate and (or) Pass Peak 

Formation. It contains only subordinate amounts of red 

quartzite from the Uinta Mountains, because little 

Uinta Mountain drainage ever joined the Green River 

above that point. (Blacks Fork once joined the Green 

just above or about at the Peru bench and shared a 

common flood plain with the Green at that level.)

Significantly, the Peru bench does not extend 

downstream from Green River. However, a high 

gravel-capped bench 50 km east at the Rock Springs 

airport, referred to here as the "Rock Springs bench," 

may be its easterly correlative and may thus mark the 

former course of the Upper Green River, roughly along 

the present course of Bitter Creek, which now drains 

that area in the opposite direction. Other gravel 

benches stand at intermediate points between the Peru 

bench and the Rock Springs bench. The gravel on the 

Rock Springs bench is very similar to that on the Peru 

bench, although it is not as coarse and contains fewer 

granite pebbles and more red quartzite. Perhaps the 

granite was worn away by attrition in its transit 

downstream from the Peru bench area; the red quartzite may have been augmented by material carried out 

of the Uinta Mountains by Blacks Fork and by material 

reworked out of the Bishop Conglomerate south of 

Rock Springs.

The Rock Springs bench is about 130 m above the 

level of Bitter Creek. There is little likelihood, however, that its gravel was deposited by a diminutive 

stream like the present Bitter Creek, which is manifestly underfit (Drury, 1964) and carries no gravel. A 

local source for the gravel, moreover, is improbable, 

because the predominant rock types are derived from 

Precambrian sources and are much the same as the 

rocks in the Peru bench. No source for such rock types 

exists within the drainage basin of Bitter Creek. A 

more likely source, therefore, was the Upper Green 

River, before its diversion to the south.

The most easterly deposit carrying these clasts 

known to me is at Creston Junction, east of the Continental Divide on U.S. Interstate 80, about 130 km east 

of Rock Springs, Wyoming. This deposit overlies a bed 

of volcanic ash (Sanders, 1975) sampled by J. D. Love 

and identified by Izett and Wilcox (1982) as Lava Creek 

B ash from the Yellowstone caldera complex. The 0.62- 

m.y. age of the Lava Creek B ash (Izett and Wilcox, 

1982) indicates that the Upper Green River was still 

in its east-flowing course 600,000 years ago.

A continued late Tertiary and Quaternary uplift of 

the Continental Divide across southern Wyoming 

(Love, 1971, p. 79) probably was partly responsible for 

the Upper Green's capture and diversion. The gravel

deposits rise toward the east, and their rise is a measure of the uplift of the Continental Divide. Thus, from 

a point near the town of Green River east to the Continental Divide near Creston, a distance of about 145 km, 

the gravel deposit rises about 155 m, at an average 

rate of about 0.94 m/km. Assuming that the gradient 

of the river was about the same before capture as now, 

about 0.9 m/km, the rise has been about 285 m in the 

600,000 years since capture, or about 475 mm per 

thousand years. The tilting up toward the east has been 

about 1.84 m/km, or roughly 3.1 mm/km per thousand 

years.

RELATIONSHIP OF UPPER GREEN RIVER CAPTURE TO 

MODERN FISH FAUNAS

Any hypothesis proposing that the Upper Green 

River was captured as recently as middle Pleistocene 

time must account for marked differences between the 

indigenous fish fauna of the present upper ColoradoGreen River drainage and that of the North Platte and 

its tributaries. A mixing of these faunas might be expected, but they are distinctly different. The Platte 

River is tributary to the large Mississippi-Missouri system and shares its faunal diversity, which includes 

many different families and species adapted to seasonally warm waters. It also includes a few cold-water 

taxa such as salmonids (trouts and whitefish) that are 

restricted to mountain headwaters in areas of geologically recent transfers across the Continental Divide 

from the Columbia River basin (Miller, 1958, 1965; 

Uyeno and Miller, 1963; Behnke and Benson, 1983). 

The North Platte, however, contains no native salmonids, for reasons mentioned below.

In contrast with the Missouri, the upper ColoradoGreen drainage has a narrowly limited fauna of only 

four families and 14 species adapted to two separate 

habitats: a lower altitude, seasonally warm-water 

habitat and a higher altitude, cold-water habitat. The 

warm-water species are endemic: that is, they are restricted to the Colorado drainage system and have 

evolved adaptations unique to that drainage, owing to 

long isolation from other river faunas (Behnke and Benson, 1983). These species are now greatly depleted in 

range and numbers, owing to habitat changes caused 

by river impoundments and irrigation, artificial population controls, and the introduction of highly competitive 

exotic species. Some of the endemic, warm-water 

species are endangered, such as the Colorado River 

squawfish (Ptychocheilus lucius), the humpback chub 

(Gila cypha), the bonytail chub (Gila elegans), and the 

razorback sucker (Xyrauchen texanus), which were 

abundant in the Green as far north as Green River, 

Wyo., before the construction of Flaming Gorge Dam 

(Behnke and Benson, 1983). They are adapted to tur-
DRAINAGE DEVELOPMENT 71

bid, seasonally warm water, and their former ranges 

were mostly restricted to the main stem of the Colorado River and the downstream reaches of its larger 

tributaries, such as the Green and the Yampa, where 

the water is suitably warm in summertime to sustain 

the habitat (Behnke and Benson, 1983).

Nonendemic fish native to the upper Colorado-Green 

drainage include cold-water, clear-water species transferred from the Columbia River basin in geologically 

recent time, such as the cutthroat trout (Salmo clarki), 

mountain whitefish (Prosopium williamsoni), speckled 

dace (Rhinichthys osculus yarrowi), and sculpins of the 

genus Coitus. None of these fish is indigenous to the 

North Platte drainage, although they thrive elsewhere 

in the headwaters of the Missouri River basin. All have 

close ties to the Columbia River basin.

A mountain sucker, Catostomus platyrhynchus, is 

native to both the Green and the North Platte drainage 

(R. J. Behnke, Colorado State University, Department 

of Fisheries and Wildlife Biology, written commun., 

1983; Behnke and Benson, 1983) and poses a perplexing 

question, therefore, of interbasin fish dispersal. This 

fish also formerly lived in Picket Lake a small lake 

without outlet in the Great Divide Basin along with 

the lake chub, Couesius plumbia, which was native to 

the Sweetwater River but not the Green. Both species 

have been extirpated from Picket Lake by introduction 

of exotic species (G. T. Baxter, University of Wyoming, 

written commun., 1983).

Because of the marked differences in the native fish 

faunas of the Green and the North Platte, the Upper 

Green is postulated to have been captured at a time 

and under circumstances unfavorable to the interbasin 

transfer of fish populations. The least favorable time 

would have been at the height of a glacial stage, when 

the riverine environment was cold and hostile and the 

headwaters were locked in ice. At such a time the 

Green River Basin was a frigid semidesert, cold in summer and bitterly cold in winter. Widespread relict frost 

wedges, frost polygons, and remains of cold-climate terrestrial fauna suggest the existence of permafrost and 

a periglacial tundra throughout the intermontane basins 

of Wyoming during late Pleistocene time (Mears, 1981). 

Malde (1961) noted similar evidence in western Idaho 

and adjacent areas. Mears calculated that the average 

Wyoming temperature in late Wisconsin time was 

10°-13°C colder than now. Similar conditions probably 

prevailed during pre-Wisconsin glaciations.

The Green River Basin today remains one of the coldest areas of Wyoming, winter and summer (Lowers, 

1978). Hardly in jest, the local people proclaim Big 

Piney, Wyo., the nation's icebox. The prediversion fish 

of the Green River Basin linked to the North Platte 

and adapted to more moderate climates, therefore 

would have been forced far downstream to warmer

waters during the climatic deterioration of a glacial 

stage, leaving the basin void of fish life. East of Wyoming, the midcontinent was ice covered as far south as 

northeastern Nebraska during the Wisconsin stage, and 

even farther south during earlier glaciations (Flint, 

1955, fig. 27; Mickelson and others, 1983).

Eroding northward from Flaming Gorge toward 

Green River, Wyo., the stream that captured the 

Upper Green River probably was ephemeral and contained no viable fish population. Inasmuch as its 

watershed was the dry, central part of the Green River 

Basin even drier in late Wisconsin time than now 

(Gates, 1976) it probably held little if any water except during prolonged wet periods or after heavy showers, when its flow would have been large and energetic. 

Moreover, because it was working headward through 

the saline Wilkins Peak Member of the Green River 

Formation, it would have contained high concentrations 

of alkaline salts. I, therefore, postulate a vigorous but 

ephemeral stream, devoid of fish life, capturing a much 

larger but also fish-free Upper Green River whose indigenous North Platte fish population had been forced 

far to the east, out of the Green River Basin, by the 

harsh Pleistocene climate and, perhaps, by the turbidity of the glacial meltwater. At that instant in geologic 

time, the entire drainage of the Upper Green was 

diverted south through the Uinta Mountains, and the 

diversion was without any transfer of fish populations. 

Then, with the warming of the interglacial climate, the 

endemic warm-water species of the Lower Green began 

to colonize the Green River Basin.

The native but nonendemic cold-water fishes of the 

headwaters and mountain tributaries of the ColoradoGreen River system the cutthroat trout, mountain 

whitefish, speckled dace, mountain suckers, and sculpins are all regarded as geologically recent entrants 

into the drainage system, inasmuch as they have 

evolved but little since their interbasin transfer 

(Behnke and Benson, 1983, p. 9). Because they are absent from the North Platte drainage (except the mountain sucker, Catostomus platyrhynchus), they must 

have migrated into the Green from the Columbia River 

basin by way of the Snake River, rather than by way 

of the Missouri, after the Upper Green was captured. 

Though absent from the North Platte, all these fish 

are native to the Upper Missouri; Marias Pass in northwestern Montana was an obvious transfer point across 

the Continental Divide, according to R. S. Behnke 

(written commun., 1983), until it was blocked by railroad construction early in the 20th century. The cutthroat trout, the species most adapted to cold water 

and least tolerant of warmth, never became established 

much below Great Falls, Mont. The whitefish reached 

the mouth of the Yellowstone but no farther down the 

Missouri, and the mountain sucker, the most warm-
72 NEOCENE OF EASTERN UINTA MOUNTAINS

tolerant species, became widespread throughout the 

upper Missouri, including the Sweetwater (R. S. 

Behnke, written commun., 1983).

The mountain sucker may have entered the Platte 

drainage (the Sweetwater River) from the Wind River, 

according to G. T. Baxter of the University of Wyoming, Department of Zoology (written commun., 1983). 

If so, it should have transferred from the Columbia by 

way of the Flathead, the Missouri, and the Yellowstone. This fish tolerates the warm water of the 

Missouri and the lower Yellowstone, but the salmonids 

and sculpins do not, hence they were unable to reach 

the Sweetwater. The mountain sucker might also have 

traveled all the way down the Missouri to the mouth 

of the Platte, though the turbidity of the lower Platte 

would have been a deterrent to its further migration.

In any event, the cold-water fauna of the Green 

River must be high-altitude transfers from the Snake, 

since they are not natives of the Platte. The mountain 

sucker probably entered the Green along with the rest 

of the cold-water fauna, but being native to the Missouri and the Snake, as well as the Green, it theoretically could have made the transfer in either direction. 

The sucker family Catostomidae has a nearly continentwide Nearctic distribution, from Arctic Canada to 

Guatemala (Miller, 1958, fig. 10; Patterson, 1981, p. 

274).

The time (or times) of fish transfer to the Green 

River is uncertain. At the time of any pre-late Wisconsin fish transfer, the topography of the interbasin divide would have been somewhat different from now, 

and possible transfer points accordingly would have 

been different also. A likely Holocene or very late Wisconsin transfer point is a broad, marshy morainal area 

straddling the Snake-Green divide in the extreme 

northwest corner of Sublette County, Wyo., drained by 

Raspberry Creek on the Snake River side and Wagon 

Creek on the Green River side (Mosquito Lake 71/2-minute quadrangle). The headwaters of these streams intermingle in a maze of ponds and bogs left behind by 

the withdrawal of the large late-Wisconsin (Pinedale) 

Green River glacier (G. M. Richmond, oral commun., 

1984). Raspberry Creek is a first-order tributary of the 

Gros Ventre River, which in turn joins the Snake in 

Jackson Hole, Wyo. Wagon Creek flows directly to the 

Green. A few kilometers to the southwest, at another 

possible transfer point, Tepee Creek (Green River 

drainage) and Kinky Creek (Gros Ventre drainage) are 

separated by a chain of lakelets along another obscure 

divide in the same Pinedale moraine complex.

In one late Wisconsin transfer scenario, partial withdrawal of the Green River glacier from its terminus 

formed small proglacial lakes on the Wagon Creek side 

of the divide. These lakes then spilled north across the 

low divide into Raspberry Creek, allowing fish to migrate across to the Wagon Creek (Green River) side. 

When the ice melted back far enough to drain the lakes, 

the divide closed to further transmigration, and the fish 

moved into the Green River drainage. Whether or not 

a viable fish population could survive that close to the 

glacier terminus, however, is uncertain. The Colorado 

River subspecies of the cutthroat trout (Salmo clarki 

pleuriticus) requires a water temperature of about 

7.2°C for spawning (Behnke and Benson, 1983, p. 29), 

and the meltwater temperature would have been close 

to freezing. Moreover, the turbidity of the meltwater 

would tend to bury and suffocate the eggs and suppress 

the growth of aquatic plants and small invertebrates 

needed to sustain a food chain. More likely, therefore, 

well after deglaciation, a physical connection through 

the bogs and ponds allowed fish to move freely from 

one drainage basin to the other. Such dual drainage 

is not uncommon in glaciated areas. Sedimentation in 

the ponds and gradual expansion of the bogs at the 

expense of the ponds eventually closed off the transfer 

routes.

Various low passes along the present Snake-Green 

River divide, including points along the headwaters of 

the Hoback River south of Jackson Hole, Wyo., suggest 

other possible earlier Pleistocene transfer points, some 

probably involving minor headwater stream captures. 

Transfers might have happened at more than one place 

and time. Twin Creek, near Kemmerer, Wyoming, is 

another early possibility, which could have linked the 

Bear River and Hams Fork of the Green River, at a 

time when Hams Fork was flowing at a terrace level 

about 100 m above present grade (Rubey and others, 

1975). At that time the Bear must still have flowed 

into the Snake. Two Ocean Pass, on the Continental 

Divide just south of Yellowstone National Park, is a 

modern transfer point from the Snake to the upper 

Yellowstone River and perhaps also to the lower Yellowstone and the Missouri (Evermann, 1892, p. 28; Jordan and others, 1930), but the Upper and Lower Falls 

of the Yellowstone prevent counter migration from the 

Missouri to the Snake. Few fish, moreover, probably 

survive the descent of the Lower Falls, which plunge 

94 m to a rocky bed.

Fish transfers from the Snake River to the Green 

most likely took place during one or more interglacial 

stages, inasmuch as the Snake-Green divide was partly 

ice clad during glacial maximums, as already noted, and 

the streams that remained open probably were heavily 

laden with sediment downstream from the glaciers. 

These fish prefer clear water and have low tolerance 

of turbidity. The subsequent dispersal of the cold-water 

fauna throughout the upper Colorado River basin, however, probably followed during a later glacial stage 

(Pinedale?), when the downstream water temperature 

throughout the basin was lowered enough to be toler-
DRAINAGE DEVELOPMENT 73

* - * 

i.aSsr'tsK.fcspfeMk-.. ' " '< ^ »' "',- ,

* ^^**- * m..*? »j*-:* *

FIGURE 47. Quaternary terraces in the Eastern Uinta Mountain area.

Top. Quaternary terraces along Brush Creek northeast of Vernal, Utah. (See Kinney, 1955, p. 128). Remnant at upper right is about 

120 m above Brush Creek. Diamond Mountain Plateau, on the skyline, is capped with Bishop Conglomerate more than 760 m 

above Brush Creek; its top appears uneven here only because the irregular rim of the plateau is viewed from below.

Bottom. Gravel-capped Quaternary pediment remnants rock fans along northeast side of Browns Park at Utah-Colorado State line. 

Canyon of Beaver Creek, upper left. Cold Spring Mountain on the skyline. The highest Quaternary pediment remnant in the 

background is about 90 m above the level of Beaver Creek and about 150 m above the Green River, out of view at right (Hansen, 

1965, p. 132, plate 1).

ated. Meanwhile, the warm-water fauna would have 

been forced farther downstream. Then, as the water 

temperature slowly rose again during a succeeding interglacial, or the postglacial, the cold-water fauna

would have abandoned the trunk drainages and moved 

gradually into the high-altitude, cold-water tributaries 

throughout the upper Colorado River basin. This event 

may have been Holocene.
74 NEOCENE OF EASTERN UINTA MOUNTAINS

QUATERNARY REENTRENCHMENT 

OF THE GREEN RIVER

A postcript to the diversion of the Green River out 

of the Green River Basin and across the Uinta Mountains is the relatively brief history of the canyons since 

then. Field relations suggest reentrenchment of the 

Green River since the initial cutting of the canyons, 

especially of Lodore. Despite the steepness of the walls 

of Lodore Canyon, cross-canyon topographic profiles 

show a distinct slope break, steepening downward, 

about 245-300 m above the canyon floor (frontispiece). 

Profiles in Red Canyon are comparable to those in Lodore. The compound nature of the profile is most pronounced down spurs. Unrelated to lithology or stratigraphic shelving, the break in the profile is regarded 

as a result of accelerated downcutting and reentrenchment by the river, probably in early or middle Pleistocene time, after the canyon had been partly excavated 

(Hansen, 1969a, p. 48; Hansen and others, 1982). Thus, 

after Lodore Canyon had been eroded to a depth of 

about 460 m, downcutting slowed and the river began 

to widen the valley bottom, but the river was then rejuvenated, and it cut down an additional 245 m or so 

to its present level. This figure is comparable to the 

amount of canyon deepening along the Colorado River 

in the Rocky Mountains in Quaternary time 230 m, 

according to Hunt (1969, p. 72).

Tributaries of Lodore Canyon responded to rejuvenation by renewed downcutting also, which led to breaks 

in their gradients at heights above the Green River 

in accordance with the breaks in the cross-canyon profiles (Hansen and others, 1982).

The cause and timing of renewed downcutting are 

uncertain. Regional uplift in response to unloading or 

some other factor would have rejuvenated the entire 

drainage system through the mountains. The capture 

of the Upper Green River could also have caused reentrenchment by greatly increasing the discharge and 

competence of the system. Capture and uplift may both 

have been involved. If initial cutting began in latest 

Miocene time after the Uinta Mountains were overtopped at Lodore Canyon, most of Pliocene time may 

have elapsed during the erosion of the upper 460 m 

of canyon. Reentrenchment, then, may have begun in 

early Pleistocene time.

The rate of canyon cutting can be inferred. If 760 

m is taken as the mean depth of Lodore Canyon and 

if cutting took all of Pliocene and Quaternary time 

about 5 million years then the average rate was about 

15 cm per thousand years. This rate compares favorably with that of the Black Canyon of the Gunnison 

River in central western Colorado, where the estimated 

rate was about 30 cm per thousand years (Hansen,

1965) but where the river, though smaller, has a much 

steeper gradient.

The actual cutting surely involved times of faster and 

slower erosion. Differential rates of Quaternary cutting 

are thus expressed by terracing upstream and 

downstream from the canyons. Terrace remnants are 

uncommon in the canyons but are abundant and well 

preserved in the softer rocks outside the mountains and 

in Browns Park (fig. 47). Climatic fluctuations contributed to Quaternary terracing, inasmuch as the terrace 

deposits correlate with episodes of mountain glaciation, 

but regional uplift must also have contributed to 

downcutting and terracing. The highest gravel-capped 

surfaces stand more than 150 m above river level, and 

downcutting of that magnitude can hardly be ascribed 

solely to climatic change. I favor the view that the terraces represent episodic aggradation in an otherwise 

degradational regime, when downcutting was temporarily overwhelmed by increased sedimentation.

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