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Topographic and Structural Conditions 

Areas of Gravitational Spreading of 

Ridges in the 

Western United States 

By D. J. VARNES, D. H. RADBRUCH-HALL, and W. Z. SAVAGE 

. In 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1496 

A study relating gravitational spreading of 

ridges to local topography and geologic structure 

UNITED .STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1989 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress Cataloging-in-Publication Data 

Varnes, David J. (David Joseph), 1919-

Topographic and structural conditions in areas of gravitational spreading of ridges in the western 

United States. 

(U.S. Geological Survey professional paper; 1496) 

Bibliography: p. 

Supt. of Docs. no.: I 19.16:1496 

1. Rifts (Geology)-West (U.S.) 2. Geology, Structural. I. Radbruch-Hall, Dorothy H., 

1920- II. Savage, . 

William Z. III. Title. IV. Series. 

QE606.V36 1989 551.8'0978 88-600430 
CONTENTS 

Page 

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Stillwater Complex, Montana ........................ . 

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

Localities in the Sawatch Range, Colorado . . . . . . . . . . . . . . 3 

Bald Eagle Mountain and Mount Massive . . . . . . . . . . . 3 

Joint attitudes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

Joint spacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Joint block sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

Relation of sackungen to topography and rock structure .. 

Topography .................................... . 

Microcracks and joints .......................... . 

Faults and fault zones ........................... . 

Diobsud Ridge, Washington ......................... . 

Mount N ast and Surprise Ridge . . . . . . . . . . . . . . . . . . . 4 Elastic-plastic stress analysis of gravitating ridges ...... . 

Joint attitudes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Conclusions ....................................... . 

Williams Fork Mountains, Colorado . . . . . . . . . . . . . . . . . . . . 10 Acknowledgments .................................. . 

Sangre de Cristo Mountains, New Mexico . . . . . . . . . . . . . . 13 References ........................................ . 

ILLUSTRATIONS 

Page 

18 

18 

18 

18 

21 

22 

22 

27 

27 

28 

Page 

FIGURE 1. Index map showing location of sites in the Rocky Mountains of the Western United States . . . . . . . . . . . . . . . . . 2 

2. Index map showing location of study areas in the Williams Fork Mountains and Sawatch Range, Colorado . . . 2 

3. Sketch of common profiles across spreading ridges in hard, jointed crystalline rocks . . . . . . . . . . . . . . . . . . . . . . . 3 

4. Aerial oblique view looking northeastward down the valley of Busk Creek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 

5. Stereotriplet from vertical aerial photographs of the Busk Creek-Bald Eagle Mountain area . . . . . . . . . . . . . . . . 5 

6. Geologic map of the Mount Nast-Surprise Ridge and Bald Eagle Mountain-Mount Massive study areas . . . . . 6 

7. Prominent joints striking N. 47-67° E. and dipping 73-85° SE. exposed in headwall of cirque . . . . . . . . . . . . . . 8 

8. Aerial oblique view looking southwest at the trench on a northeast-trending ridge northwest of the high point of 

Mount Massive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

9. Poles of joints and foliation observed in the Bald Eagle Mountain-Mount Massive area, plotted on a Wulff net 9 

10. Projections on a Wulff net of the principal joint sets observed at various localities in the Bald Eagle MountainMount Massive study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

11-17. Photographs showing: 

11. Aerial view of Mount Nast, looking northward . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

12. View looking south at closely spaced joints trending N. 10° W. and dipping steeply west at the head of 

Chapman Gulch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

13. Aerial oblique view looking northward along Surprise Ridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 

14. Aerial oblique view looking north along the western side of Surprise Ridge . . . . . . . . . . . . . . . . . . . . . . . 12 

15. Trench on the east side of the crest of Surprise Ridge looking north-northeast . . . . . . . . . . . . . . . . . . . . . 12 

16. Aerial oblique view looking northward of prominent joints Oil the west flank of Surprise Ridge that trend 

N. 5-25° E. and dip steeply westward . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 

17. View looking northwestward along a slot-like trench on the crest of the north end of Surprise Ridge . . 12 

18. Poles of joints and foliation observed in the Mount Nast-Surprise Ridge area, plotted on a Wulff net . . . . . . . . 13 

19. Projections on a Wulff net of the sets of principal joints observed at various localities in the Mount Nast-Surprise 

Ridge study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 

20. Geologic map of the Ute Peak-Old Baldy study area, Williams Fork Mountains, Colorado, showing location of sackungtype features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 

21. Aerial oblique view looking northward toward Ute Peak . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 

22. Stereotriplet of the Ute Peak area showing a prominent southwestward-facing scarp northwest of Ute Peak . . . 15 

23-26. Photographs showing: 

23. View northward toward the trench extending northwestward from Ute Peak . . . . . . . . . . . . . . . . . . . . . . 16 

24. Upward-facing scarps on the convex slope west and northwest of Ute Peak . . . . . . . . . . . . . . . . . . . . . . . 16 

25. Aerial oblique view looking east-southeast over prominent trenches on the western side of the rounded crest 

of the Williams Fork Mountains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

26. Aerial oblique view looking north-northwest toward Old Baldy, showing trenches on the west side of the 

ridge crest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

27. Plot of poles of joints and foliation observed in the Ute Peak-Ptarmigan Peak and Old Baldy areas of the Williams 

Fork Mountains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

III 
IV CONTENTS 

Page 

28-31. Photographs showing: 

28. Southwest-facing slope of Contact Mountain, Stillwater Complex, Montana . . . . . . . . . . . . . . . . . . . . . . . 19 

29. Aerial oblique view of one of the principal uphill-facing scarps on the southwest slope of Contact Mountain, 

Montana . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

30. Joints in the layered intrusive rocks of the Stillwater Complex on Contact Mountain, Montana . . . . . . 19 

31. Large graben on the crest of Contact Mountain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 

32. Photomicrograph of thin section cut from oriented sample from the cirque southeast of Bald Eagle Mountain . 20 

33-36. Photographs showing: 

33. Joint-bounded blocks in the lower part of the cliff below station 3, (fig. 6) . . . . . . . . . . . . . . . . . . . . . . . . 21 

34. Joint-bounded blocks in the quarry above the road in Busk Creek valley northeast of and below the principal area of trenches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

35. View to the southeast over Busk Creek valley showing, on the right, the cliffs and rock-fall deposits below 

the area of sackung trenches .................... _. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 

36. Aerial oblique view of an area southeast of Busk Creek and southwest of the tributary valley that forms 

the southwestern border of the well-developed sackung trenches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 

37. Stereopair of vertical aerial photographs showing area of intricate pattern of shallow trenches above the prominent 

bench and scarp on the southeast side of Busk Creek valley . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 

38. Index map of the State of Washington . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 

39. Conformal transformation for a symmetric ridge in x,y coordinates to a half-plane in u, v coordinates . . . . . . . . . 25 

40. Contour plots of stresses in a symmetric ridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 

41. Diagrams showing predicted zones of potential failure in a symmetric ridge for zero pore-water pressure and angle 

of internal friction <t>=30° . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 

42. Sketch of the potential flow regions and predicted senses of shear on examples of rupture surfaces for a symmetric 

gravitating ridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

TABLE 

Page 

TABLE 1. Joint spacing at several localities in the Bald Eagle Mountain study area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 
TOPOGRAPHIC AND STRUCTURAL CONDITIONS IN 

AREAS OF GRAVITATIONAL SPREADING OF 

RIDGES IN THE WESTERN UNITED STATES 

By D. J. VARNES, D. H. RADBRUCH-HALL, and W. Z. SAVAGE 

ABSTRACT 

Gravitational spreading of steep-sided ridges produces characteristic geomorphic forms including grabens and depressions along ridge 

crests, trenches, and uphill-facing, as well as downhill-facing scarps, 

on the mountain flanks, and outward bulging of the lower slopes. These 

sackung-type features occur in a variety of geologic settings in the 

Western United States. Those discussed here occur principally in high, 

linear ridges separated by glaciated valleys. The ridges are underlain 

by hard, but closely jointed, Precambrian igneous rocks. Topography 

is the primary determinant of the location and direction of the trenches and scarps, but the topographic grain of the terrane is, itseH, 

determined in part by rock structures, such as joints and faults. In 

the Sawatch Range in Colorado, some valleys in the study area follow 

the direction of primary joint systems and, in turn, determine the direction of trenches and scarps parallel to slope contours. The principal 

joint sets are, themselves, parallel to microcracks in the rocks. The 

relation of sackung features to structural elements is close in the 

Sawatch and Williams Fork Mountains in Colorado, not obvious at 

the one site examined in the Sangre de Cristo Mountains of New Mexico, close in the Stillwater Complex in Montana, and apparently close 

in a zone around the Straight Creek fault in the northern Cascade 

Mountains in Washington. Elastic-plastic stress analysis indicates 

that uphill-facing scarps may develop in the upper extending parts 

of a slope preferentially over downhill-facing scarps. 

INTRODUCTION 

Large-scale, deep-seated distortion of steep-sided 

ridges has come under increasing study both abroad and 

in the United States during the past 20 years. One impetus for study of these features comes from the 

resemblance of their surface morphologic features, 

primarily scarps, to those of recent faults, and the need 

to identify and quantify fault movements and recurrence 

intervals for a variety of construction activities. Although the scarps observed are technically fault surfaces, they result from the local adjustments within 

steep-sided ridges rather than far-field tectonic stresses. 

Making the distinction from tectonic faults is not always 

simple, for scarps due to local movement often follow 

pre-existing discontinuities in rock masses, including 

tectonic faults, joints, cleavage, schistosity, or foliation. 

Manuscript approved for publication September 14, 1988. 

The forms of sagging developed in the steep-sided 

slopes depend very much on the lithology of the masses 

involved, the degree of development of existing discontinuities, and the relation of the strike and dip of 

anisotropic elements to the direction and angle of dip 

of the slope. Three general lithologic settings may be 

distinguished: (1) massive, strong (although jointed) 

rocks lying on weak rocks; (2) ridges composed generally 

of metamorphosed rocks with pronounced foliation, 

schistosity, or cleavage; and (3) ridges composed of 

hard, but fractured, crystalline igneous rocks. Slope 

deformations in all these settings have been referred to 

by many authors, including ourselves, variously as 

"sackungen," from the German word for sagging, or by 

the term "deep-seated creep." As the name implies, 

these features can be shown to result from large-scale 

gravitational spreading (Radbruch-Hall and others, 

1976; Savage and Varnes, 1987). The mode of origin is 

illustrated in this report in the section "Elastic-Plastic 

Stress Analysis of Gravitating Ridges." 

Sackungen of the first type, involving the spreading 

of rigid rocks overlying soft rocks, have been described 

previously (Radbruch-Hall, 1978). Gravitational spreading for this case was modeled by finite-element analysis 

(Radbruch-Hall and others, 1976). These kinds of movement are not discussed here. 

The second type shows features as described in detail 

by Zischinsky (1969) from the Austrian Tirol, and was 

referred to by him as "true Sackung." This involves 

much more extensive sagging and bending of foliated 

schists, phyllites, and gneisses than is found in more 

homogeneous and competent rocks. 

Our attention in the present report is directed mainly 

at sackungen of the third type, and we are concerned 

with mostly ridges above glacially oversteepened 

valleys in Precambrian granite and migmatite in central Colorado and to features in some other lqcalities 

in the Western United States. The mechanics of gravitational spreading of such ridges of more or less homogeneous crystalline rocks are not well understood. Hence, 

as part of the study of the origin of sackung features, 

1 
2 TOPOGRAPHIC AND STRUCTURAL CONDITIONS, GRAVITATIONAL SPREADING, WESTERN U.S. 

we have considered their relation to topography and to 

local and regional structure. 

In 1977 and 1978, sites were visited at Bald Eagle 

Mountain, Surprise Ridge, and Mount N ast in the granite and gneiss of the Sawatch Range in Colorado, and 

sites in layered basic igneous rocks of the Stillwater 

Complex in Montana were also visited (figs. 1, 2). Brief 

reconnaissances were made of parts of the Williams Fork 

Mountains of Colorado (fig. 2) and of the Cascade Mountains east of Seattle, Wash. In the Sawatch Range, the 

orientations of jointing and foliation were measured and 

oriented samples were taken, from which thin sections 39°30' 

were made. 

In 1981, sackungen were examined in more detail in 

the Williams Fork Mountains, particularly near Ute 

Peak and Old Baldy, where jointing and foliation were 

measured. In 1982, additional observations were made 

at Mount N ast and Surprise Ridge, a large sackung was 

examined on Mount Massive, and others were investigated in the Sangre de Cristo Mountains of New 

Mexico between Taos and the southern border of Colorado (fig. 1, locality 5). Jointing and foliation were 

measured· at each of these localities. 

, MONTANA 

' x1 I \_-1 ________ 1 

I I 

IDAHO I I 

WYOMING ~-- --r-----1 \ NEBRASKA 

I CHEYENNE L ---- NEVADA' --~------:.::,Q_ - 1 

I 0 SAL T LAKE I 

40° 

1 

CITY r 

4 x2 0 DENVER 

>¢<3 

I UTAH COLORADO 

I 

I 

I 

I 

\ I I I 

~-----

I + -----x5---~L- l1 I I 

"-. ("" ARIZONA I oSANTA FE \ 

350 - 'J I NEW MEXICO 

\ 

300 MILES 

FIGURE 1.-Index map showing location of sites in the Rocky Mountains of the Western United States discussed in the text. (1) Stillwater 

Complex, Montana; (2) Williams Fork Mountains, Colorado; (3) Bald 

Eagle Mountain and Mount Massive, Colorado; (4) Mount NastSurprise Ridge, Colorado; (5) Site in the Sangre de Cristo Mountains, 

New Mexico. 

10 20 MILES 

FIGURE 2.-Index map showing by dashed rectangles the location of 

study areas in the Williams Fork Mountains and Sawatch Range, 

Colorado. 

The sackungen we have studied have the following 

characteristics, not all of which may be developed at any 

one locality: 

1. Uphill-facing scarps on the slope, one to a few meters 

high, trending approximately parallel with topographic contours and commonly somewhat convex 

downslope in plan. The trenches so produced are 

asymmetric in profile: The steep, upward-facing scarp 

is on the downhill side of the trench; the gentler, uphill 

side appears often to be an unmodified hillslope. 

2. A graben or grabens along the ridge crest, commonly 

with closed contours and ephemeral ponds. 

3. Double-crested ridges. 

4. Bulging of the lower parts of the slopes. 

5. Occurrence on the upper flanks of glaciated valleys, 

with the movement apparently being post-glacial. 

6. Local relief more than 1,000 ft. 

7. Occurrence more common on massive and often somewhat rounded ridges, rather than on narrow ridges 

between cirques. Profiles across spreading ridges of 

type 3 are shown in figure 3. 
LOCALITIES IN THE SAWATCH RANGE, COLORADO 3 

A 

:t 

B 

FIGURE 3.-Sketch of common profiles across spreading ridges in 

hard, jointed crystalline rocks (sackung, type 3). A, With ridgetop graben; B, With double-crested ridge. The lower parts of the 

originally glaciated valleys have been modified by bulging and 

the accumulated debris of talus and rock falls. Arrows indicate 

sense of relative displacement along fractures. 

LOCALITIES IN THE 

SAWATCH RANGE, COLORADO 

BALD EAGLE MOUNTAIN AND MOUNT MASSIVE 

A ridge-top graben and hillside trenches mark a major sackung feature on a broad ridge that extends 

southwestward from Bald Eagle Mountain, 9 mi west 

of Leadville, Colo. A mass of rock on the northwest side 

of the ridge has moved northwestward toward the 

valley of Busk Creek (figs. 4, 5). The geologic map of 

the Bald Eagle-Mount Massive area (eastern portion 

of fig. 6), indicates that the rock exposed is granitic (Yg), 

which, here, consists of gneiss cut by stringers and 

masses of granite and pegmatite rock. Figure 6 shows 

the sackung trenches on Bald Eagle Mountain, as well 

as joint and foliation attitudes. The graben and trenches 

trend approximately N. 50° E. Joints and foliation are 

locally well exposed in fresh rock in a cirque on the 

southeast side of the ridge, northeast of Rainbow Lake. 

The major joint set in the cirque trends N. 47-67° E. 

and dips 73-85 o SE. (fig. 7). Foliation, as well as a 

secondary set of joints parallel to the foliation, trends 

approximately N. 45 o W. and dips 30-40 o NE. A prominent joint set at the north end of Bald Eagle Mountain strikes N. 17-20° E. and dips 42-64° NW. (fig. 6). 

Joint spacing is from less than 1 in. up to 6 ft, with most 

being spaced from 6 in. to 2 ft apart. 

Triangulation-trilateration nets established at Bald 

Eagle Mountain in 1975 and 1977 were remeasured in 

1982 and extended in 1984. No movements in excess 

of expected surveying errors have been detected. 

A pronounced trench is visible on a cirque wall on the 

northwest slope of Mount Massive, about 3¥2 mi southwest of the Bald Eagle sackung locality (fig. 6). Here, 

coarse-grained, somewhat foliated granitic rock is cut 

by well-developed joints from a few inches to more than 

6 ft apart. A large and deep, northeast-facing cirque lies 

directly below and north of the highest point on Mount 

Massive. It is separated from a much shallower basin 

to the northwest by a narrow, northeast-trending ridge. 

A block of rock approximately 3,000 ft long and 200 ft 

wide has separated from the ridge and has moved down 

and out toward the large, steep-walled cirque to the 

southeast, forming a long, northeast-trending trench 

and uphill-facing scarp on the gentle, northwest-facing 

slope of the shallower basin northwest of the ridge 

(fig. 8). For much of its length, the face of the uphillfacing scarp consists of plane surfaces that are the walls 

of joints that trend N. 20-35 o E. and dip 62-77 o SE. 

This trend is somewhat more northerly than that of the 

Bald Eagle sackung trenches and that of the major 

joints in the cirque between Bald Eagle Mountain and 

Mount Massive. Other joints in the face of the uphillfacing scarp on Mount Massive trend N. 5-50° W. and 

dip 22-74° NE. In places, joints follow the foliation, 

which is variable. 

JOINT ATTITUDES 

Figure 9 shows the poles, plotted on a Wulff stereographic net, of planes of joints and foliation in the Bald 

· Eagle-Mount Massive area. There is a good deal of 

scatter of the joint directions and dip, particularly of 

the less well-developed joint sets. Foliation is generally 

not well developed here. A concentration of poles of the 

most prominent joint set is visible in the northwest and 

southeast quadrants of the diagram indicating a tendency towards a northeast strike of the main joints. A concentration in north-northeast strike is more easily seen 

in figure 10, in which the arcs of intersection of planes 

on the lower hemisphere are plotted for the most prominent joint at each place of observation. Observations 

of principal joints at three places at the foot of a very 

steep slope below the lowermost trench in areas where 

bulging, dilation, and possible rotation of blocks is to 

be expected are plotted in figure 10 as dashed lines. 
4 TOPOGRAPHIC AND STRUCTURAL CONDITIONS, GRAVITATIONAL SPREADING, WESTERN U.S. 

FIGURE 4.-Aerial oblique view looking northeastward down the valley of Busk Creek. On the broad southwest ridge of Bald Eagle Mountain to the right, a ridge top shallow graben and trenches high on the northwest slope are outlined by snow. Also marked by snow is 

a bench about halfway up the slope, trending parallel to the contours. 

We have not observed sackung-type trenches and 

scarps in essentially unjointed rock. Nor have we observed a shear surface cropping out at the base of a 

slope that has trenches and scarps. Thus, one of the 

requirements for a ridge of granitic rock to undergo 

creep movement appears to be that the rock mass is 

closely divided by joints so that relative displacements 

of a flow-like character can occur without the necessity 

of a through-going basal slip surface. To obtain some 

quantitative measure of the degree to which rock in the 

Bald Eagle Mountain area is divided, we measured the 

joint spacing in bedrock exposures and the size of loose 

blocks on the surface. 

JOINT SPACING 

The spacing of joints of in-place bedrock was measured at several localities in the Bald Eagle area as 

shown in table 1. The spacing of joints having the orientation recorded in column 2 of table 1 was measured on 

intersecting joints having the orientation shown in 

column 3. 

JOINT BLOCK SIZES 

Measurement of the longest, shortest, and intermediate 

dimensions were made of 100 randomly selected blocks 

lying loose along the low scarp on the northwest side of 

the shallow graben on the crest of the southwest ridge of 

Bald Eagle Mountain. The longest long dimension was 

44 in., the median long dimension was about 11.5 in. The 

longest short dimension was 26 in. The median volume, 

assuming a rectangular prismatic shape, was 437 cubic in. 

Measurements of 25 blocks at triangulation station 3, 

just below the lowest trench, yielded roughly comparable 

results. The longest long dimension was 44 in., the longest 

short dimension was 16 in., and the median volume was 

approximately 1,200 cubic in, considerably larger than the 

median volume at the ridge top graben. The reason for 

this difference is unknown. 

MOUNT NAST AND SURPRISE RIDGE 

Sackungen are well developed in the Sawatch Range 

west of Bald Eagle Mountain, on Mount Nast, west of 

(Continued on p. 9) 
FIGURE 5.-Stereotriplet from vertical aerial photographs of the Busk Creek-Bald 

Eagle Mountain area, showing principal area of trenches on the slope southeast of 

Busk Creek and the graben near the ridge crest. A prominent bench with scarp on 

its uphill side extends along the slope parallel to the contours. The scarp is obscured 

by rock-fall deposits in the vicinity of the bulge of the slope below the trenches. The 

Carlton Tunnel diverts water from the west side of the Continental Divide southeastward into the Busk Creek drainage. 

5 

0 

)> 

t"' 

...... 

>-3 

...... 

t.:rj 

r:n 

z 

>-3 

::r: 

t.:rj 

~ 0 

::r: 

::0 

)> 

z 

0 

!"1 

0 

0 

t"' 

0 

~ 0 

01 
Colorado, 1975 ry, 

Base from U s G 1 • 

P

. . · · eo og1cal Survey 1·50 000 

Itkin Counry and Lake Coun , . , 

'h 

SCALE 1 50 000 

2 KILOMETERS 

CONTOUR INTERVAL 80 FEET 

NATIONAL GEODETIC VERTICAL DATUM OF 1929 
.Area of 

ftgure 6 

COLORADO 

LOCATION MAP 
8 TOPOGRAPHIC AND STRUCTURAL CONDITIONS, GRAVITATIONAL SPREADING, WESTERN U.S. 

QUATERNARY 

MIDDLE 

PROTEROZOIC 

EARLY 

PROTEROZOIC 

EXPLANATION 

Oag Alluvium and stream gravels. bog deposits 

Olt Landslide and talus deposits Qu 

Ql Landslide deposits 

Qd Glacial drift 

Yg Granitic rocks of = 1,400 m.y. age group 

Xb Biotite gneiss and schist, migmatite 

---- Contact 

---,:--- Fault-Dashed where approximately located or where 

concealed 

40 

60 

-- Strike and dip of foliation 

Strike and dip of joint set 

Vertical joint 

Triangulation-trilateration station 

----Linear feature of probable gravitational origin-Dashed 

where location uncertain 

~.u....~..u.u Scarp of gravitational origin-Teeth on down thrown side 

Non-glacial surficial 

deposits undivided 

FIGURE 6 (above and 2 previous pages).-Geologic map of the Mount Nast-Surprise Ridge (western part) and Bald Eagle Mountain-Mount 

Massive (eastern part) study areas. Geology in western part by Ludington and Yeoman (1980) and in the eastern part by Tweto (1974) 

and Tweto and Reed (1973a). Linear features and scarps and some joint observations added by Varnes, Radbruch-Hall, and Savage. 

FIGURE 7.-Prominent joints striking N. 47-67° E . and dipping 

73-85° SE. exposed in the headwall of a cirque on the southeast 

side of the Bald Eagle ridge, northeast of Rainbow Lake. 

FIGURE 8.-Aerial oblique view looking southwest at the trench on 

a northeast-trending ridge northwest of the high point of Mount 

Massive. The trench apparently was formed when a block of rock 

approximately 3,000 ft long and 200 ft wide moved down, to the 

left, toward a large, steep-walled cirque. 

rr 
LOCALITIES IN THE SAWATCH RANGE, COLORADO 9 

• 

• 

• 

N 

• 

• • 

• ~ 

X< ·+ X x. 

X • • x 

. . l!ll 

~ 

Poles 

• Most prominent joint set 

o Second joint set 

0 

0 0 

•o 0 

• )( 

X 

• Other sets: or all sets equally developed 

and plotted 

• 

X Foliations: superposed on joint set-symbol 

if joint set follows foliation 

• 

0 

•• 

FIGURE 9.-Poles of joints and foliation observed in the Bald Eagle 

Mountain-Mount Massive area, plotted on a Wulff net. 

South Fork Fryingpan River, and on a north-trending 

ridge east of the river, informally here referred to as Surprise Ridge (figs. 2, 6). Trenches on the top and sides of 

both Mount N ast and Surprise Ridge trend roughly 

northward. 

At Mount Nast, the most prevalent joint set appears 

to trend N. 10-25° W., with dips most commonly 

77-87° SW., although some joints are vertical or dip 

steeply slightly northeast. The northwest trend is 

roughly parallel to most of the sackung trenches near 

the top and along the west side of the north-trending 

ridge south of Mount Nast (fig. 11). Joints and foliation 

were measured near the top of the ridge near the 

sackungen, north of the sackungen, and in a cirque at 

the head of Chapman Gulch near the south end of N ast 

ridge (fig. 12). Joints parallel to foliation are extremely 

variable, ranging from those that strike N. 35 ° W., dipping 28° NE., to others that strike N. 37° E., dipping 

25° SE. 

On Surprise Ridge, joints were measured on the ridge 

top near the center, at the north end, and in a cirque 

N 

/ / / / / / 

/ 

/ 

I 

I 

I 

I 

I 

---

FIGURE 10.-Projections on a Wulff net of the principal joint sets 

observed at various localities in the Bald Eagle Mountain-Mount 

Massive study area. The dashed lines are the plots of observations 

taken within a zone below the sackung trenches believed to be subject to bulging, dilation, and rotation of blocks. 

at the head of Granite Creek, between the two arms of 

Surprise Ridge, which bifurcates at its north end (fig. 

6). Joints in the cirque are pronounced, and foliation is 

well developed. 

The most prominent joint system, measured at 

several places on and near Surprise Ridge, is more 

variable in orientation than the major joint systems at 

Bald Eagle and Mount Nast. The sackungen trenches 

reflect this irregularity, as they are scattered profusely along the top of the ridge, criss-crossing in various 

directions, and give a general impression of shattering 

of the entire ridge top (figs. 13, 14, 15). The most consistent joint set, shown in figure 16, trends N. 5-25 ° 

E. and dips steeply either northwest or southeast. Foliation is variable, but, in general, it trends roughly eastwest, with variations of 5-10° to the north or south. 

At the north end of the ridge, small, but more consistently oriented, trenches trend northwest, in some places 

forming deep slots in the ridge top (fig. 17). The major 

joint trends are also more consistent, striking N. 10-44 o 

W., dipping 64-84° SW.; others are N. 15-40° E., dipping 39-55 ° SE., and lesser sets dip and strike in 

various directions. 

Joint spacing in the Mount N ast-Surprise Ridge area 

generally ranges from 8 in. to 3 ft with an average of 
10 TOPOGRAPHIC AND STRUCTURAL CONDITIONS, GRAVITATIONAL SPREADING, WESTERN U.S. 

TABLE 1.-Joint spacing at several localities in the Bald Eagle Mountain study area. 

[Most prominent joint set is underlined ) 

Location 

(localities 

shown in 

fig. 6) 

On Hagerman Pass 

road, N. of 

Carl ton Tunnel 

Portal, below 

Sta. 5 

Quarry on Busk 

Creek road, 1. 8 mi 

NE. of Carlton 

Tunnel Portal 

Prominent outcrop 

south of farthest 

south switchback 

of trail from Busk 

Creek to top of 

ridge 

At foot of 

cliffs below 

Sta. 3 

Sta. 8 

Measurement of 

Attitude of joint sets spacing of joint Average 

for which spacing was set in column 2 spacing 

N. 

N. 

N. 

N. 

N. 

N. 

N. 

N. 

N. 

N. 

measu~ed was made on joint (inches) 

su~faces with this 

orientation 

5° w. 78° sw. N. 88° w. 75° sw. 16.6 

55° w. 42° NE. N. 5° w. 78° sw. 12.7 

28° E. 81° NW; N. 83° E. 85° NW. 14. 1 

83° E. 85° NW. N. 28° E. 81° NW. 23. 3, 30.7 

65° w. 25° NE. N. 28° E. 81° NW. 16.5 

35° E. 70° NW. N. 60° w. 42° NE. 12.0 

45° w. 58° sw. N. 35° E. 70° NW. 21.1 

60° w. 42° NE. N. 35° E. 70° NW. 31.5 

80-85° E. 75° SE. 18 

62° w. 45° sw. 12 

about 18 in., although, as shown in figure 12, the 

average spacing locally may be much less than 1 ft. 

areas the orientation appears to be consistent, in other 

areas, particularly near ridge crests, the orientations 

have no doubt been disturbed by sackung-type 

movements or possibly by frost action. JOINT ATTITUDES 

Figures 18 and 19 show the poles of joints and foliation, and the projection of the principal joint sets, on 

Wulff nets, for observations in the Mount N astSurprise Ridge area, and illustrate the variability of 

orientation of the discontinuities. Although in limited 

WILLIAMS FORK MOUNTAINS, 

COLORADO 

In the Williams Fork Mountains north of Dillon 

(fig. 2), sackungen are abundant, from the south end of 
WILLIAMS FORK MOUNTAINS, COLORADO 11 

FIGURE ll.-Aerial view of Mount Nast,looking northward. In Chapman Gulch, on the west side of the ridge in the middle distance, 

is the northwest portal of the Mount N ast Tunnel, a part of the large 

Fryingpan·Arkansas transcontinental divide water-diversion 

project. The high western slope of Mount Nast shows numerous 

trenches, here filled with snow; whereas, the eastern slope appears 

to be less disturbed. 

FIGURE 12.-View looking south at closely spaced joints trending 

N. 10° W. and dipping steeply west at the head of Chapman Gulch. 

The direction of Chapman Gulch closely parallels this joint system. 

Length of geologic hammer is 12.8 in. 

the range, northward to Ute Peak (fig. 20). The area 

immediately to the west of Ute Peak has undergone 

major collapse, the movement being down and to the 

west (figs. 21, 22), with the formation of a ridge-top 

graben and many local scarps. A deep trench extending 

northwestward from Ute Peak, on the east side of the 

ridge summit, is shown in figure 23 and upward-facing 

scarps on the western slope are shown in figure 24. 

FIGURE 13.-Aerial oblique view looking northward along 

Surprise Ridge (whose summit at the north end is shown on 

the topographic base, figure 6, as South Fork No.1). The cirque 

between the two forks of Surprise Ridge is on the right. The 

crest of the ridge is greatly disturbed. Trenches occur on 

both sides but are predominantly on the crest and western 

slope. 

The geologists, who previous to this study mapped 

the Williams Fork Range, interpreted the disturbance 

just west of Ute Peak as a fault (fig. 20), and, indeed, 

it may be an old fault zone that was favorably oriented 

to control later sackung movements. There are, however, no marker beds in the Precambrian gneissic 

granite to use in positively identifying tectonic fault 

displacement. 

One set of joints measured in migmatite on the low 

peak north of the deep trench ranges from N. 20 o W., 

65 oNE. toN. 5o E., 46 o SE.; other joints nearly normal 

to the first set trend N. 60-70° E., 50-85° SE., and one 

poorly defined set strikes N. 7 5-80 o W. and dips 54-63 o 

NE. The steep scarp on the west side of Ute Peak shows 

prominent vertical joints trending about N. 15 o W. 

For about 7.5 mi south along the crest of the Williams 

(Continued on p. 13) 
12 TOPOGRAPHIC AND STRUCTURAL CONDITIONS, GRAVITATIONAL SPREADING, WESTERN U.S. 

FIGURE 14.-Aerial oblique view looking north along the 

western side of Surprise Ridge. The eastward-dipping 

surface near the photograph center is a freshappearing, upward-facing sackung scarp. 

FIGURE 15.-Trench on the east side of the crest of Surprise Ridge, 

looking north-northeast. Person on the snowfield in the trench indicates scale. 

FIGURE 16.-Aerial oblique view looking northward of prominent joints on the west flank of Surprise Ridge that trend 

N. 5-25° E. and dip steeply westward. 

FIGURE 17.-View looking northwestward along a slot-like trench 

on the crest of the north end of Surprise Ridge. Orientation is controlled by the major joint set. 
WILLIAMS FORK MOUNTAINS, COLORADO 13 

• 

X 

0 

• 

• 

• Most prominent joint set 

o Second joint set 

II!( 

)( 

)( 

• Other sets, or all sets equally 

developed and plotted 

N 

Ill! 

+ 

X 

X 

X 

Poles 

• 0 

• • 

• eX 

• 

)( 

0 

0 • 

• 

0 • 

X Foliation; superposed on joint 

set-symbol if joint set follows 

foliation 

FIGURE 18.-Poles of joints and foliation observed in the Mount 

N ast-Surprise Ridge area, plotted on a Wulff net. 

N 

Fork Mountains from Ute Peak to Ptarmigan Peak, 

there are many areas of ridge-top grabens and uphillfacing scarps, particularly on the west flank of the crest, 

as shown in figure 25. 

The ridge extending eastward from Ptarmigan Peak 

has uphill-facing scarps on the south side and a welldeveloped ridge-crest graben. In this area, the most 

prominent joints strike N. 10-25 o E., dip 79-83 ° SE., 

and follow the foliation in banded gneiss. Thus, the principal discontinuities cross the ridge at a high angle, yet 

the sackung features are parallel to the ridge, influenced 

not only by the the topography but perhaps, also, by 

secondary sets of joints that trend N. 65 o E. toN. 78 o 

W. and dip to the north. 

East of the main Ute Peak-Ptarmigan Peak ridge and 

separated from it by the South Fork of the Williams 

Fork is another ridge with conspicuous sackung 

features, especially for a mile or so south of Old Baldy 

(figs. 20 and 26). The most prominent joint system at 

Old Baldy trends N. 60-75° W. and dips 60-80° NE.; 

the trend is somewhat more westward than the most 

prominent trenches. Elsewhere on the ridge, the joints 

have various trends, although foliation in the gneiss 

fairly consistently strikes at a high angle across the 

ridge and dips 30-40° to the south. 

The irregularity of joint systems, as observed along 

ridge crests in the Ute Peak-Ptarmigan Peak and Old 

Baldy area, is shown in the plot of poles of joints and 

foliation in figure 27. 

SANGRE DE CRISTO MOUNTAINS, 

NEW MEXICO 

In the Sangre de Cristo Mountains of northern New 

Mexico, sackungen have been observed in granitic rock 

on the peaks east of Taos and east of Costilla, near the 

border between Colorado and New Mexico (fig. 1). No 

obvious correlation could be established between 

jointing and sackungen in this area. Jointing, in general, 

appears to be somewhat irregular, rather than in prominent, well-defined sets. 

(Continued on p. 181 

FIGURE 19.-Projections on a Wulff net of the sets of principal joints 

observed at various localities in the Mount Nast-Surprise Ridge study 

area. 
14 TOPOGRAPHIC AND STRUCTURAL CONDITIONS, GRAVITATIONAL SPREADING, WESTERN U.S. 

39°45' 

Base from US GeologiCal Su<Vey, D•lhon I 62.500, 1929 and 

Ute Peak 1.24,000, uned11ed advance pnm 

T 4 S 

T 3 S 

R 77 W 
SANGRE DE CRISTO MOUNTAINS, NEW MEXICO 15 

QUATERNARY 

CRETACEO US 

EARLY 

PROTEROZOIC 

r"'Area of 

f1gure 20 

COLORADO 

LOCATION MAP 

EXPLANATION 

Oag Alluvium and stream 

gravels, bog deposits 

Landslide deposits Ou 

Glacial drift 

Pierre Shale 

Colorado Group- Niobrara 

Formation and Benton Shale 

Dakota Sandstone 

Granitic rocks of 

= 1700 m.y. age group 

Xb Biotite gneiss and 

schist, migmatite 

----- Contact 

Non-glacial 

surficial 

deposits 

undivided 

---r--- Fault- Dashed wh ere approx imately located, 

dott ed where concea led. Bar and ba ll on 

downthrown side 

_,.,~___..•....._ Thrust fault-Sawteeth o n upper pl ate 

30 

Strike and dip of beds 

Strike and dip of foliation 

Strike and dip of joint set 

--- Vertical joint 

Linear feature of probable gravitational 

origin 

.u..;._:..u...~..w...c Scarp of gravitational origin- Teeth o n 

downthrown side 

SCALE 1:62 500 

'h 

t I I I I I I I I I (II II I I 

.5 1 KILOMETER 

CONTOUR INTERVAL 50 FEET 

NATIONAL GEODETIC VERTICAL DATUM OF 1929 

1 MILE 

I 

+-FIGURE 20 (above and left).-Geologic map of the Ute Peak-Old Baldy 

study area, Williams Fork Mountains, Colorado, showing location 

of sackung-type features. Geology from '!\veto (1973) and '!\veto and 

Reed (1973b) with linear features and scarps of gravitational origin 

and some structural observations added by Varnes and 

Radbruch-Hall. 

FIGURE 21.-Aerial oblique view looking northward toward Ute Peak, 

which is the short ridge that rises above the general summit area 

in the right-central part of the photograph. The west side of the ridge 

summit has moved downward and westward. 

FIGURE 22.-Stereotriplet of the Ute Peak area showing a prominent southwestward-facing scarp northwest of Ute Peak on the east flank 

of the ridge, many scarps along the crest west of Ute Peak, and upward-facing scarps on the western slope. 
16 TOPOGRAPHIC AND STRUCTURAL CONDITIONS, GRAVITATIONAL SPREADING, WESTERN U.S. 

FIGURE 23.-View northward toward the trench extending northwestward from Ute Peak, which is out of view on the right. The 

scarp on the eastern side of the trench is about 30 ft high. 

FIGURE 24.-Upward-facing scarps on the convex slope west and 

northwest of Ute Peak. 

FIGURE 25.-Aerial oblique view looking east-southeast over prominent trenches on the western side of the 

rounded crest of the Williams Fork Mountains, 1.8 mi south of Ute Peak. The animals on the crest of the 

highest scarp are elk. 
SANGRE DE CRISTO MOUNTAINS, NEW MEXICO 17 

FIGURE 26.-0blique aerial view looking north-northwest toward Old Baldy, showing trenches on the west side of the ridge crest; 

the larger trenches are outlined with snow. 

0 

X 

• 

181 

• 

X 

• • 

0 

X 

X 

X X 

xx 

X 

0 

•• 

••• 

N 

0 

0 

X 

0 

• 0 

X 

X 

0 

• 

• 

Poles 

• Most prominent joint set 

o Second joint set 

• Other sets. or all sets equally developed 

and plotted 

X Foliation; superposed on joint symbol 

if joint follows foliation 

FIGURE 27.-Plot of poles of joints and foliation observed in the Ute 

Peak-Ptarmigan Peak and Old Baldy areas of the Williams Fork 

Mountains. 
18 TOPOGRAPHIC AND STRUCTURAL CONDITIONS, GRAVITATIONAL SPREADING, WESTERN U.S. 

STILLWATER COMPLEX, MONTANA 

In the Stillwater Complex, a large, basic, layered 

intrusive of Archean age in the Beartooth Mountains 

of Montana, sackungen have formed along the northeast side of the valley of the Boulder River, on the 

southwest side of a ridge known as Contact Mountain 

(Segerstrom and Carlson, 1982) (fig. 28). The main 

sackungen trenches and uphill-facing scarps strike 

approximately N. 65° W. and are about parallel to the 

slope contours (fig. 29). A major joint set parallels the 

layering, which strikes generally N. 60-65 ° W. and dips 

50-60° NE. into the slope; another near-vertical set 

approximately normal to the layering strikes N. 30-35 o 

E. (fig. 30). A graben lies at the crest of the ridge 

(fig. 31) and movement has taken place along both 

sets of joints to form the graben and trenches. Although the trenches are in bedrock, the ridges that 

bound them on the downhill side are capped in places 

by remnants of glacial till. Therefore, the sackung 

features formed after the glaciers that occupied the 

valley of Boulder River had retreated, which left the 

oversteepened slope unsupported. 

RELATION OF SACKUNGEN TO 

TOPOGRAPHY AND ROCK STRUCTURE 

TOPOGRAPHY 

The primary control on the location of spreadings appears to be topographic; that is, the necessary relief to 

produce stresses sufficient to cause failure must be present, together with a sufficient length of free slope to 

allow their visible expression in shear surfaces. 

However, topography is not the sole determinant, for 

spreadings commonly are not continuous along ridges 

that have almost uniform shape and relief. Local and 

perhaps subtle variations in the spacing and orientation of pre-existing discontinuities, as well as minor 

topographic features, may then fix the limits of the area 

affected. 

Although the relation of spreadings to topography is 

close, the orientation of ridges and the steepness of 

slopes of a mountainous area are themselves largely 

determined by the tectonic fabric of the region, as well 

as by factors that affected the accumulation of ice and 

distribution of glaciers during the Pleistocene. For example, the general northerly orientation of ridges in the 

Mount Nast-Bald Eagle study area (figs. 2, 6) appears 

related to regional joint and fault systems. These valleys were occupied by northward-flowing valley glaciers. 

Owing to predominantly westerly winds and to less 

direct exposure to afternoon sun, the accumulation of 

snow and ice was heaviest on the east or lee sides. Therefore, cirques on the higher slopes developed much more 

commonly on the east- and northeast-facing sides. The 

present ridges are asymmetric-steeper on the east and 

more rounded on the west. Gravitational spreading 

features are more commonly observed high on the west 

sides of these ridges than on the more deeply sculptured 

east sides. This may be due in part to greater remaining 

mass with resulting higher gravitational stresses on the 

west sides or, possibly, in part to removal of the features of gravitational spreading by erosion on the east 

sides. 

MICROCRACKS AND JOINTS 

In relatively homogeneous geologic terranes, 

sackungen appear to be related to joint systems that 

are the macroscopic equivalents of microscopic fractures in the rock. 

A thin section of an oriented sample taken from the 

cirque southeast of Bald Eagle Mountain shows that 

the rock is cut by numerous microscopic open fractures, 

some of which cut across grain boundaries, some of 

which do not (fig. 32). The thin section was cut normal 

to the major joint set, which dips 73-85 o SE., and 

strikes N. 47-67° E. At the point at which the oriented 

sample was taken for the thin section, the major joints 

strike N. 47° E. and dip 85° SE. (fig. 7). This orientation can be compared with the average N. 50 o E. trend 

of the graben and hillside trenches on Bald Eagle Mountain. The thin section also intersects the foliation, which 

here strikes approximately N. 45° W. and dips 30° NE. 

Microscopic fractures in the thin section, parallel to the 

major joint system, cut across the foliation. Some of 

the fractures are partly filled with quartz, others are 

open. 

A thin section, from an oriented sample taken from 

the face of the uphill-facing scarp on Mount Massive, 

was cut parallel to the secondary joint set, and across 

both the irregular foliation and the major joint set, 

which strikes N. 20° E. and dips 62° SE. The section 

was normal to the plane of the major joints. Foliation 

was indistinct in the section. Prominent microscopic 

fractures in the thin section were seen to be parallel to 

the major joints. Some fractures were seen to be wholly 

within individual grains; whereas, others were continuous across several grains. 
RELATION OF SACKUNGEN TO TOPOGRAPHY AND ROCK STRUCTURE 19 

FIGURE 28.-Southwest-facing slope of Contact Mountain, Stillwater Complex, Montana. Layering in the basic igneous rocks dips 

to the left (northeast) at angles of 50-60 ° into the slope. Some of the larger trenches are indicated by arrows. Total height 

of the slope is about 3,850 ft. 

FIGURE 29.-Aerial oblique view of one of the principal uphill-facing 

scarps on the southwest slope of Contact Mountain, Montana, striking about parallel with the slope contour and dipping into the slope 

at the same inclination as layers within the intrusive body. The ridge 

on the downhill side of the trench in the lower center of the 

photograph is capped in places by remnants of glacial till. 

FIGURE 30.-Joints in the layered intrusive rocks of the Stillwater 

Complex on Contact Mountain, Montana. The principal joints, which 

also control the direction of many sackung trenches and scarps, are 

parallel to the layering. Other well-developed joints, nearly vertical 

and nearly normal to the strike of the layering, appear in the 

background. 
20 TOPOGRAPHIC AND STRUCTURAL CONDITIONS, GRAVITATIONAL SPREADING, WESTERN U.S. 

FIGURE 31.-Large graben on the crest of Contact Mountain. The long, high slope to Boulder Creek, shown in figure 

28, is out of view to the left. Joint systems control the zig-zag direction of the principal failure surface on the 

right (northeast) side of the graben. 

Another oriented sample was taken from the cirque 

between the two arms of Surprise Ridge; a thin section 

of the sample was cut normal to the vertical joints at 

the site, which trended N. 20° E. Microscopic fractures 

with the same orientation as the vertical joints (the major set) were clearly visible in the thin section. 

The strike of the major joint system on Bald Eagle 

Mountain and in the cirque to the southwest is similar 

to the strike of the graben and trenches of the sackung 

·mass. The displaced mass, to the southwest of Bald 

Eagle Mountain, consists of blocks of rock ranging from 

a few inches to several feet across. The rock mass that 

has bulged out into the canyon wall of Busk Creek consists of blocks that have moved individually, as indicated in figure 33, as well as in aggregates that form 

larger coherent units. Each larger unit has separated 

from its neighbor along the major joint planes that 

trend northeast about parallel to the contours on the 

slope, and that control the direction of the hillside 

trenches. Individual, smaller blocks apparently also 

separated along this joint set as well as along the other 

FIGURE 32.-Photomicrograph of thin section cut from oriented sample from the cirque southeast of Bald Eagle Mountain. The section 

is normal to the principal joint set, which is paralleled by the 

numerous microscopic fractures. 
RELATION OF SACKUNGEN TO TOPOGRAPHY AND ROCK STRUCTURE 21 

FIGURE 33.-Joint-bounded blocks in the lower part of the cliff 

below station 3 (fig. 6). This is in a zone of some rotation of 

individual blocks and probable dilation, although the mass itself 

has not yet fallen. The observation of principal joint direction, 

shown by dashed lines in figure 10, is different from the usual 

trend of principal joints away from the disturbed zone. 

two sets, one of which is vertical and trends sub-normal 

to the major set (N. 24 ° W.), and one of which trends 

N. 45° W. and dips 30° NE., so that a component of 

the dip is outward toward the valley. Movement of individual blocks also can be seen in a quarry along the 

road below and northeast of the bulge of the sackung 

(fig. 34). 

The direction of principal joints in the Bald Eagle 

Mountain-Mount Nast area has controlled the orientation of ridges and valleys developed during erosion 

by ice and water. Trenches and upward-facing scarps 

developed by gravitational spreading of ridges are 

usually more or less parallel to ridge axes and slope contours, and they also tend to make use of any zones of 

weakness, such as the principal joints. Thus, there is 

commonly a concurrent direction of ridge axes, contours, trenches and scarps, and principal joints. Moreover, other joint sets locally are well developed, so 

that rocks of the ridges are divided into blocks having 

dimensions of one or a few feet. At the scale of the 

mountain masses, the individual rigid units are much 

like grains in a small pile of sand a few feet high. In the 

mountain, the units are better interlocked, and, of 

course, they are subject to much higher gravitational 

forces; but the intimate division permits use of 

mathematical tools for analysis that are appropriate for 

particulate bodies, as is done in a later section. 

FAULTS AND FAULT ZONES 

Fault zones are usually intensely fractured and are 

weak; they are places where sackungen may be expected 

to occur. We have observed uphill-facing hillside scarps 

formed by gravitational movement along faults and 

fault zones in a few places. The principal scarps and 

graben at Ute Peak in the Williams Fork Mountains 

follows the course of a fault previously mapped by 

FIGURE 34.-Joint-bounded blocks in the quarry above the road 

in Busk Creek valley northeast of and below the principal area 

of trenches. Each large block has moved outward 1-4 in. 

relative to the block beneath on joints that dip at a low angle 

obliquely toward Busk Creek. This illustrates bulging 

movements on the lower slopes of the valley that are elsewhere 

obscured by colluvium and talus. 
22 TOPOGRAPHIC AND STRUCTURAL CONDITIONS, GRAVITATIONAL SPREADING, WESTERN U.S. 

Tweto and Reed (1973b). Certainly, there is geomorphic 

evidence for either tectonic faulting or gravitational 

movement since the form of the mountain was established, but it is not known whether there is independent 

evidence for a tectonic fault. The scarp and trench on 

the northwest ridge of Mount Massive, which is 

definitely a gravitational-failure structure, was also 

previously mapped as a fault (Tweto and Reed, 1973a). 

Farther northwest of Mount Massive, on the ridge west 

of Pear Lake, Notch Lake, and Windsor Lake, there are 

also conspicuous uphill-facing scarps and grabens along 

a previously mapped fault (fig. 6). However, this structure may well be a fault that extends to some depth, 

because, beneath its northwest extension, an intensely 

fractured zone was encountered when driving the 

Charles H. Boustead tunnel, about 1,400 ft beneath the 

surface (fig. 6). Remedial work in the fracture zone 

delayed tunneling progress for nearly 11 months. 

The east side of the valley of Busk Creek is marked 

by a prominent linear bench. Above the bench is a scarp 

or steep slope, approximately parallel to the contours 

through much of its course, that extends from well 

southwest of the sackung-affected area, northeastward 

nearly to the mouth of the valley of Busk Creek (figs. 

4, 6, and 35). The trace of the bench on the surface is 

interrupted by rock-fall deposits below the sackung 

area, and the scarp in this area is unusually steep and 

high. This scarp may follow a zone of highly fractured 

rock, which may have had significant influence on the 

erosional development of Busk Creek valley by both 

water and ice, and thus on the topographic setting for 

the sackungen. The influence on the location of the 

sackung may have been more direct. The unusually high 

scarp below station 3 at the lower edge of the zone of 

trenches is on line with the projection of the bench and 

scarp on the slopes to the southwest and northeast, and 

provides a steep, free face toward which gravitational 

movements could occur. 

Farther up Busk Creek valley, southwest of the welldeveloped sackungen, is an area with an unusual 

pattern of intersecting, shallow trenches that is about 

0.6 mi north-northeast of Rainbow Lakes and on the 

slope above the southwest continuation of the bench 

and scarp along the southeast side of Busk Creek valley 

(figs. 36, 37). A few small, uphill-facing scarps do exist; 

this part of the slope may exhibit the initial stages of 

sackung development, before the well-jointed mass has 

formed into defined groups of failure units. 

DIOBSUD RIDGE, WASHINGTON 

A report on the Straight Creek fault zone in the Diobsud Ridge area of northern Washington State 

(McCleary and others, 1978), described uphill-facing 

scarps along a fault-line valley that marks the trend of 

the Straight Creek fault (fig. 38). A trench 5 ft in depth 

that was dug normal to one scarp and the depression 

immediately uphill from it revealed a zone of crushed 

and sheared rock in the depression that was interpreted 

to have formed by tectonic movement, although the 

trench showed very little that would conclusively 

identify the scarp as being of tectonic rather than 

gravitational origin. However, the presence of similar 

scarps on both sides of the fault-line valley strongly suggests that the scarps are gravitational, as the movement that formed them was downhill-side-up on both 

sides of the valley. This means that if the movement 

were tectonic, it would have been in opposite directions 

on opposite sides of the valley, which seems unlikely. 

The pattern of movement is, however, entirely consistent with gravitational movement along pre-eXisting 

weaknesses in the rock-in this locality, along a fault 

zone. 

The study by McCleary and others (1978) included examination of aerial photographs covering 2,100 mi2 of 

the North Cascades. Fifty sackunglike features were 

identified, of which 28 were confidently judged to be 

gravitational-spreading features. Of these, 19 were 

within a restricted zone 10 mi wide centered approximately on the Straight Creek fault zone, and the report 

concluded that the sackung features may have been 

triggered by active seismicity associated with the zone. 

ELASTIC-PLASTIC STRESS ANALYSIS OF 

GRAVITATING RIDGES 

Analytic studies of stresses in ridges and slopes by 

W. Z. Savage and his coworkers (Savage and others, 

1985; Savage and Swolfs, 1986; Savage and Smith, 

1986) have been applied to the problem of gravitational 

spreading of ridges with emphasis on the origin of 

uphill-facing scarps and ridge-crest grabens (Savage and 

Varnes, 1987). The analyses include an exact elastic 

solution for gravity-induced stresses in an isolated ridge 

of generalized parabolic cross section and a plastic solution for gravity-induced deformation of a slope yielding 

under the Coulomb criterion. These are appropriately 

combined to delimit regions of potential failure by 

plastic flow under given conditions of geometry and 

material properties. 

The elastic solution by Savage and others (1985) for 

gravitational stresses in symmetric ridges is based on 

the Kolosov-Muskhelishvili (Muskhelishvili, 1953) 

method of complex potentials for plane elasticity. A conformal mapping function is used to transform an 

isolated symmetric ridge into a half-plane in which expressions are derived for gravity-induced stresses in the 
ELASTIC-PLASTIC STRESS ANALYSIS OF GRAVITATING RIDGES 

FIGURE 35--View to the southeast over Busk Creek valley showing, on the right, the cliffs and rock-fall deposits below the area 

of sackung trenches. On the left, the prominent bench and low scarp continue to the northeast about halfway up the slope 

from the road. 

FIGURE 36.-Aerial oblique view of an area southeast of Busk Creek and southwest of the tributary valley that forms the 

southwestern border of the well-developed sackung trenches. This slope is in a similar topographic setting, just above the bench 

and scarp along the southeast side of Busk Creek valley. It may be in the initial stages of sackung development. 

23 
24 TOPOGRAPHIC AND STRUCTURAL CONDITIONS, GRAVITATIONAL SPREADING, WESTERN U.S. 

FIGURE 37.-Stereopair of vertical aerial photographs showing area of intricate pattern of shallow trenches (at arrow) above 

the prominent bench and scarp on the southeast side of Busk Creek valley. This area is in a topographic setting similar 

to that of the well-developed sackung across the tributary valley to the north, and may be in an earlier stage of development. 

ridge (fig. 39). The mapping used to transform a symmetric ridge in x,y coordinates into a half-plane in u,v 

coordinates and the definition of the parameters a and 

b, which describe the shape of the ridge, are illustrated 

in figure 39. Specifically, a and bare parameters used 

in the conformal mapping where b is the ridge height 

and a + b/2 represents the x coordinate of the inflection point on the ridge flank. 

Savage and others (1985) derived expressions for 

horizontal, ax, and vertical, ay, total normal stresses 

and shear stress, axy• in and away from the ridge of the 

form 

- llpgy () ax- pgbFH(u, v, a, b, 11)+ - - 1 

1-11 

ay= pgbFy(u, v, a, b, 11)+pgy (2) 

axy= pgbFs(u, v, a, b, 11) (3) 

where pis the density, g is the gravitational acceleration, 11 is Poisson's ratio, and FH, Fy, and Fs are com: 

plex functions of u, v, a, b, and 11. Effective stresses ax 

and a~ are defined as a~=ax-P and a~=ay-P, where Pis 

pore pressure. Stresses given by equations 1, 2, and 3 

satisfy the conditions that shear and normal stresses 

are zero on the ridge surface and satisfy the condition 

of plane strain parallel to the ridge. The stress state 

away from the ridge is given by assuming vanishing farfield horizontal displacements, which requires that the • II 

stresses far from the ndge are ax= 1_11 pgy, ay=pgy, 

and axy=O. 

Figure 40 shows contours of normalized effective 

stresses aJpgb, aJpgb, and a~ylpgb by the exact solution for alb = 1, P = 0.0, and 11 = 0.25. Note that by 

decreasing a for a given b, narrower and steeper ridges 

can be created. 

The predicted magnitudes of gravity-induced stresses 

from point to point in idealized ridges of the type shown 

in figure 39 are then compared to stress levels necessary 
ELASTIC-PLASTIC STRESS ANALYSIS OF GRAVITATING RIDGES 25 

WASHINGTON 

.,--.. 921-~~- 0REGON 

,--

1 

I 

I 

I 

I I IDAHO 

I N 

----U 50 100 MILES 

FIGURE 38.-Index map of the State of Washington showing location 

of the Skagit River, Diobsud Ridge, and, by dotted line, a part of 

the trend of the Straight Creek fault zone. 

for yielding. The yield condition is that of Coulomb in 

the form given by Drucker and Prager (1952); 

or as 

(4b) 

where cJ> is the angle of internal friction and c is the cohesion (resistance to shear) across a plane having zero 

normal stress. 

In equation 4b, F represents the ratio of resisting 

shearing stresses (term in numerator) to maximum 

shearing stresses (term in denominator) at a point in the 

0 0 

- 1 -1 

.!J .!J 

~ '-> 

-2 0.9 -2 

-3 1.2 - 3 

-4 -4 

0 2 3 4 0 

x/b 

y 

v 

u 

FIGURE 39.-Conformal transformation for a symmetric ridge in x,y 

coordinates to a half-plane in u,v coordinates and the definition of 

the parameters a and b which describe the shape of the ridge. Here, 

b represents the ridge height. When u=a and v=O, then x=a+(b/2) 

and y= b/2, which are the coordinates of the inflection point on the 

ridge flank. 

0 

- 1 

.!J 

~ 

-2 

-3 

2 3 4 2 3 4 

x/b x/b 

FIGURE 40.-Contour plots of u'.jpgb, u/pgb, and u~Jpgb from left to right, respectively, for a symmetric ridge where alb = 1.0 

and Poisson's ratio, P = 1/4. Compressive stresses are taken to be positive and pore pressure is zero. 
26 TOPOGRAPHIC AND STRUCTURAL CONDITIONS, GRAVITATIONAL SPREADING, WESTERN U.S. 

gravitating elastic body shown in figure 39. When F=1, 

the maximum shearing stresses on suitably oriented 

planes at this point is equal to the resisting shearing 

stress, and failure is imminent. When F> 1, the resisting 

shear stress exceeds the maximum shear stress, and the 

material at the considered point is stable. 

Some predicted failure zones, calculated by equations 

1 through 4, for zero pore pressure and a few values of 

the parameters a, b, v, cp, and c, are shown in figure 41. 

For simplicity, zero pore pressures are assumed, and 

because of symmetry, only the right half of the ridge 

is shown. 

The depth to which the zone of potential instability 

of the slope extends is seen to depend not only on the 

steepness of the slope but also on the value assumed 

for cohesion (under conditions of zero pore-water 

pressure and angle of internal friction, ¢=30°) (fig. 41). 

The depth of the failure zone increases with steepness 

of the slope, markedly decreases with increasing cohesion, and can be shown (Savage and Smith, 1986) to increase with increasing pore-water pressure. 

Plastic flow occurs in the failed region in response to 

gravity loading. Stresses in the failed region must 

satisfy the yield condition (equation 4), and the conditions of static force equilibrium. Also, velocities in the 

failed region must satisfy the continuity condition. Expressions for stresses and velocities that satisfy these 

conditions for a two-dimensional uniformly sloping halfspace of Coulomb plastic material under elevated pore 

pressure and under gravity loading are given by Savage 

and Smith (1986). 

Savage and Smith also give expressions for a system 

of coincident stress and velocity characteristics along 

which discontinuities in velocity are propagated 

through the plastically flowing mass. These characteristics are physically manifested as rupture surfaces. 

The expressions for stress, velocity, and associated 

rupture surfaces given by Savage and Smith (1986) are 

exact only for uniformly sloping failed regions of constant thickness. However, these expressions apply approximately for failed regions of the type shown in 

figure 41, if the variation in depth of failure along the 

ridge flank is small and if the radius of curvature of the 

surface slope is large, compared with the failed 

thickness. Clearly, the approximations are poorest for 

steep slopes and best for gentle slopes. 

A sketch of the predicted slip-line field in potential 

flow regions is shown in figure 42, which shows results 

of an analysis of a complete ridge with convex-upward 

slope at the upper part, a straight part or an inflection, 

and concave-upward lower valley wall. These three 

regions have different responses. The upper subregion 

is in extending flow, and the two complementary sets 

of potential rupture surfaces have opposite sense of 

0 

-1~----~------~----~------~----~ 

1~~--~------~----~------~-----. 

~ 0 

0 

c 

2 3 4 5 

xlb 

FIGURE 41.-Diagrams showing predicted zones of potential 

failure in a symmetric ridge for zero pore-water pressure and 

angle of internal friction </>=30°. A, Shows the effect of cohesion, c, on depth of potential failure: cohesion is 0.01 in the 

diagonal-ruled case, 0.06 in the cross-hatched case. Cohesion 

is in units normalized with respect to pgb (density X gravita· 

tional constant X slope height). B, Shows effect of Poisson's 

ratio: P=0.35 in diagonal-ruled area, "=0.45 in cross-hatched 

area. Cohesion is 0.01 in both cases. C, Shows effect of steeper 

slope; c=0.01, "=0.35. The x andy coordinates are normal· 

ized with respect to slope height, b (Savage and V ames, 

1987). 

shear. Uphill-facing surface scarps will develop along 

the steeply dipping set. More gently dipping downhillfacing scarps of the other set are possible but are commonly absent if the steep set is developed. Very often, 

the region of extension is preferentially developed or, 

at least, is more commonly preserved and easily observed than the lower region of compression in the lower 

valley wall. A subregion of plug flow may be present 

if the slope is long and planar. The subregion of compression in the concave part of the slope may have been 

removed by erosion in precipitous valley walls or may 

be obscured by colluvium, landslide debris, or talus. One 

should regard the basal surface forming an envelope 

to potential shear· surfaces as a mathematical 

construction-the boundary between an elastic region 

below and a potentially failing region above. 

The plastic-flow solution requires that in the subregion in extension an initial discontinuity on the steep 

set will propagate upward from the plastic-elastic 
ACKNOWLEDGMENTS 27 

-1~----~------~----~------~----~ 

0 2 3 4 5 

x/b 

FIGURE 42.-Sketch of the potential flow regions and predicted 

senses of shear on examples of rupture surfaces for a symmetric 

gravitating ridge. Inactive rupture surfaces are dashed. The x 

and y coordinates are normalized with respect to slope height, 

b (Savage and Varnes, 1987). 

boundary with exponentially increasing relative 

displacement toward the ground surface; the converse 

is true of the downhill-facing discontinuities. In the 

compressive-flow region, the opposite holds. 

In summary, the principal contributions of the 

analysis toward an understanding of sackungen 

mechanics in homogeneous terranes are as follows: 

1. The steeper the slope, the deeper the zone of potential. instability. 

2. Any significant cohesion drastically decreases the 

thickness of the failure zone. 

3. Development of the set of uphill-facing scarps, complementary to the more usual downhill-facing 

scarps of conventional landslide, is not only possible but may be favored under the proper conditions 

of topography and rock structure. Preference for 

the uphill-facing scarp set suggests that the mountain has "solved" a least-work problem and has 

chosen to activate slip surfaces of the shorter set 

that extend to the free surface rather than longer 

surfaces that penetrate to increasing qepth. 

4. Increase in pore-water pressure increases the depth 

of the unstable zone. 

CONCLUSIONS 

Field observations in the Sawatch Range and the 

Williams Fork Mountains of Colorado, where the rocks 

are relatively homogeneous migmatites and granitic 

rocks or gneiss, show that the rocks involved in 

sackungen commonly move valleyward normal to major joint sets that strike approximately parallel to valley 

walls. Dips of the joints are variable and joints may dip 

steeply either into or out of the slope. The rocks are 

generally broken by minor joint sets and(or) planes of 

foliation that are more or less normal to the valley sides, 

as well as by joints or planes that are nearly flat or dip 

slightly valleyward, so that at least three sets of discontinuities commonly are involved. 

Joint sets and other planes generally cannot be 

measured reliably within the sackungen, because the 

blocks composing the moving masses have rotated so 

that the original orientation of joints and foliation cannot be determined. Exposures are generally good and 

measurements reliable, however, in nearby glaciated 

areas above timberline in these high mountain regions, 

particularly in glacial cirques. Thin sections of samples 

of fresh rock taken where jointing and foliation can be 

measured show that microscopic fractures seen in thin 

section have an orientation similar to that of the open 

principal joints visible in the field and similar to that 

of the trenches characteristic of the sackungen in two 

areas. 

Where gravitational sackung-type movement has 

taken place along tectonic shear zones, it may be difficult to ascertain whether the scarps are the result of 

tectonic activity or the result of gravitational movement that has used a previously existing zone of 

weakness, or both. Uphill-facing scarps in any seismically active area should be examined carefully, to attempt 

a determination of whether they are of gravitational or 

tectonic origin. A possible criterion for making this 

distinction is that scarps of gravitational origin are 

more likely to have their location, orientation, continuity, and sense of displacement determined by local 

topography than are fault scarps resulting from 

regional tectonic stresses. 

Grabens and hillside trenches that have formed where 

bedding planes or other discontinuities dip into a slope 

may have formed by toppling, bending of beds, or 

spreading of a ridge. All these mechanisms have been 

suggested by various authors. An elastic-plastic 

analysis indicates that ridge-crest grabens and uphillfacing scarps may result from activation of the set of 

potential shear surfaces that are complementary to the 

usual surfaces of slump-type landslides. 

ACKNOWLEDGMENTS 

We owe much to Czechoslovakian geologists, who, for 

many years, have made excellent studies of gravitational spreading of ridges in their country. We are indebted, particularly, to Arnold Nemcok and Tibor 

Mahr, whose untimely deaths dealt a serious blow to 

sackung research, and to their colleagues Josef Malgot, 

J aroslav Pasek, and Milan Matula. In our fieldwork in 

the Western United States, we have been helped at 
28 TOPOGRAPHIC AND STRUCTURAL CONDITIONS, GRAVITATIONAL SPREADING, WESTERN U.S. 

various times and for various periods by our coworkers 

Sigrid Asher-Bolinder, Robert Fleming, Kenneth 

Segerstrom, William K. Smith, and Katharine Varnes, 

and by field assistants Greg Twombly and Erik Shaw. 

Lastly, the special skills of helicopter pilots Burt 

Metcalf, Paul Kinsey, Marc Pasewalk, and Jim Innes 

enabled us to go wherever required and brought us 

back, often under difficult conditions. 

REFERENCES 

Drucker, D.C., and Prager, W., 1952, Soil mechanics and plastic 

analysis or limit design: Quarterly of Applied Mathematics, v. 10, 

no. 2, p. 157-165. 

Ludington, Steve, and Yeoman, R.A., 1980, Geologic map of the 

Hunter-Fryingpan Wilderness Area and the Porphyry Mountain 

Wilderness Study Area, Pitkin County, Colorado: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1236-A, scale 

1:50,000. 

McCleary, Jeff, Dohrenwend, John, Cluff, Lloyd, and Hanson, 

Kathryn, 1978, 1872 earthquake studies, Washington Public 

Power Supply System, Nuclear Project Nos. 1 and 4, Straight 

Creek Fault Zone Study: Woodward-Clyde Consultants, San Francisco, Calif., prepared for United Engineers and Constructors, Inc., 

Contract No. H.O. 52028, 72 p. 

Muskhelishvili, N.l., 1953, Some basic problems of the mathematical 

theory of elasticity: Groningen, The Netherlands, N oordhoof, 

718 p. 

Radbruch-Hall, D.H., 1978, Gravitational creep of rock masses on 

slopes, in Voight, Barry, ed., Rockslides and avB.lanches, Vol. 1, 

Natural Phenomena: Developments in Geotechnical Engineering 14A, Amsterdam, Elsevier Scientific Publishing Co., 

p. 607-657. 

Radbruch-Hall, D.H., Varnes, D.J., and Savage, W.Z., 1976, Gravitational 

spreading of steep-sided ridges ("sackung") in western United States: 

International Association of Engineering Geology Bulletin 14, p. 

23-35. 

Savage, W.Z., and Smith, W.K., 1986, A model for the plastic flow of 

landslides: U.S. Geological Survey Professional Paper 1385, 32 p. 

Savage, W.Z., and Swolfs, H.S., 1986, 'Thctonic and gravitational stress 

in long symmetric ridges and valleys: Journal of Geophysical 

Research, v. 91, no. B3, p. 3677-3685. 

Savage, W.Z., Swolfs, H.S., and Powers, P.S., 1985, Gravitational stresses 

in long symmetric ridges and valleys: International J oumal of Rock 

Mechanics and Mining Sciences & Geomechanics Abstracts, v. 22, 

no. 5, p. 291-302. 

Savage, W.Z., and Varnes, D.J., 1987, Mechanics of gravitational 

spreading of steep-sided ridges ("sackung"): International Association of Engineering Geology Bulletin, no. 35, p. 31-36. 

Segerstrom, Kenneth, and Carlson, R.R., 1982, Geologic map of the banded upper zone of the Stillwater Complex and adjacent rocks, 

Stillwater, Sweet Grass, and Park Counties, Montana: U.S. 

Geological Survey Miscellaneous Investigations Series Map 1-1383, 

scale 1:24,000. 

1\veto, Ogden, 1973, Reconnaissance geologic map of the Dillon 

15-minute quadrangle, Summit, Eagle, and Grand Counties, Colorado: U.S. Geological Survey Open-File Report 1776, scale 1:62,500. 

1\veto, Ogden, 197 4, Geologic map and sections of the Holy Cross 

quadrangle, Eagle, Lake, Pitkin, and Summit Counties, Colorado: 

U.S. Geological Survey Miscellaneous Geological Investigations Map 

I-830, scale 1:24,000. 

1\veto, Ogden, and Reed, J.C., Jr., 1973a, Reconnaissance geologic map 

of the Mount Elbert 15-minute quadrangle, Lake, Chaffee, and Pitkin 

Counties, Colorado: U.S. Geological Survey Open-File Report 1777, 

scale 1:62,500. 

1\veto, Ogden, and Reed, J.C., Jr., 1973b, Reconnaissance geologic map 

of the Ute Peak 15-minute quadrangle, Grand and Summit Counties, Colorado: U.S. Geological Survey Open-File Report 1779, scale 

1:62,500. 

Zischinsky, Ulf, 1969, Uber sackungen: Rock Mechanics, v. 1, no. 1, 

p. 30-52. 

U.S.G.S. PROFESSIONAL PAPER 1496 C2. 

TOPOGRAPHIC AND STRUCTURAL CONDITIONS 

IN AREAS OF GRAVITATIONAL SPREADING 

OF RIDGES IN THE WESTERN UNITED STATES 

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CO 80225. (See latest Price and Availability List) 99 lPunll»RAcSI~onns crrftllne GecR~AcSIR Sllllll"vey, Jl.379- JI.96JI." may be purchased by mail and over the counter in paperbaek book form and as a 

set ofmicrofiche. 

. 99 1PIIllll»RAcSI~Ol!D.S orr ilie G~on~lcBIR §u!l"vey, R962-ll~»70" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche. 

"lPunbUca~Aons cUilne lU.§. Geoioglcmn §uney, 1197ll- JI.93JI." may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) ,and as a set of microfiche. 

SllllJlliJ!llllementtsfor 1982,1983, 1984,1985,1986, and for subsequent 

years since the last permanent catalog may be purchased by mail and 

over the counter in paperback book form. 

§~SI~e CBliS!Ro~s, "List of U.S. Geological Survey Geologic and 

Water-SupplyReportsandMapsFor(State),"maybepurchasedbymail 

and over the counter in pape~ack booklet form only 

"lPirfice B~nn(ll A"VBIAnB~bllUy JLAsa clr lU.S. Geo!~AcSIR Sllllll"vey lPunbRicaalcns,99 issued annually, is available free of charge in paperback booklet form only. 

s~nedeltll CC]p)Ae§ olr Sl moniliny CSiftB!Rog "New Publications of the u.s. 

Geological Survey" available free of charge by mail or may be obtained 

over the counter in paperback booklet form only. Those wishing a free 

subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 

Nation81 Center, Reston, VA 22092. 

No~.--Prices of Government publications listed in older catalogs, 

announcements, and publicationS may be incorrect.. Therefore, the 

prices charged may differ from the prices in catalogs, announcements, 

and publications·.