GEOLOGICAL SURVEY CIRCULAR 31

REPRINTED 1959

March 1949

LANDSLIDE PROBLEMS OF SOUTHWESTERN

COLORADO

By 

Helen D. Varnes
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

REPRINTED 1959

WASHINGTON, D. C.

Free on application to the Director, Geological Survey, Washington 25, D. C.
LANDSLIDE PROBLEMS OF SOUTHWESTERN COLORAI 0

By

Helen D. Varnes

CONTENTS

Page

Introduction............................. 2

Acknowledgments.......................... 2

Regional background...................... 3

Cedar Creek landslide near Montrose,Colo. 3

Location and history................. 3

General description.................. 3

Geology.......................... 4

Topography of the slide.......... 4

Drainage conditions.............. 6

Slide movements and their causes..... Q

Conditions favoring movement..... g

Activating or initiating causes.. 7

Types of movement................ 7

Knife Edge landslide area in Mesa Verde

National Park.......................... Q

General description.................. Q

Landforms and types of movement...... Q

Causes of landslide movements........ Q

Page

Knife Edge landslide area in Hear Verde 

National Park Continued.

Conditions favoring movement,, 8 

Activating causes............. 10

Ames landslide near Telluride, Colo.... n

History............................ H

Geology and topography............. 11

Principal causes of movement....... is

Factors favoring movement..... 12

Influence of vegetation cover. 12 

Activating or initiating causes 

of movement................* 12

Types of movement.................. 12

Remedial measures...................... 12

Cedar Creek landslide.............. 12

Knife Edge landalld e area.......... 13

Ames landslide area................ 13

Conclusions............................ 13

ILLUSTRATIONS

Figure 1. Index map of southwestern Colorado, showing general location of landslides

2. Index map of Cedar Creek landslide area...................................

3. Cedar Creek landslide near Montrose, Colorado.............................

4. Knife Edge landslide area at north end of Mesa Verde National Park........

5. Index map of the Ames landslide area.................................> 

Page

2

4

5

9

10
INTRODUCTION

Landslides of many kinds and of varying 

magnitude are widespread throughout southwestern Colorado. This d.iscussion is limited to 

three typical sites, which illustrate some of 

the more common landslide phenomena and the 

problems that they pose,.

The Cedar Creek, the Ames, and the Knife 

Edge landslide areas (fig. 1) were selected 

partly because the slides are recent and erosion has not yet seriously modified their 

original forms. Equally important reasons for

109°

Denver & Rio Orande Western at Mont rose, 

supplied most of the information on the history of the Cedar Creek landslide. Hr. McDaniells, Surveyor for Montrose Corntty, furnished data on the irrigation system- that 

supplies water to the mesa south of the slide,

The reconnaissance study of the Mesa Verde 

area was made with the permission of the 

Director of the National Park Service, Thanka 

are also due Mr. Peter Nelson, 3uperihtendent 

of the Public Roads Division at Mera Verde

ae"

37'

IOT° 100° 100°

 1

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QGrond Jun

Delta 

Montros

Cortex .

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"KNIFE EDGE 

LANDSLID 

(of Mesa Verde A

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° CEDAR CREE 

LANDSLIDE

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AMES 

LANDSLIDE

Rational Park) \

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Figure 1. Index map of southwestern Colorado, showing 

general location of landslides.

studying these particular s lides are that all 

three have caused serious problems to the 

maintenance of highways or railroads and that 

they were caused, in part, by the activities 

of man. During the 1946 field season, the 

writer, assisted by Leonard Rolnlck, made detailed studies of these slides. In addition 

to routine geologic observations and measurements, a network of points was established by 

triangulation within the Cedar Creek landslide 

and in the stable ground around It; these will 

serve as a basis for measurement and comparison of future movements in that area.

ACKNOWLEDGMENTS

The writer wishes to thank Mr. A.E-Perlman 

and Mr. K. L. Moriarlty of the Denver & Rio 

Grande Western Railroad for permission to 

study the Cedar Creek area and for data on 

recent movement of that slide used in this report. Mr. Fish, section foreman for the

National Park, and to Kr. W. W. 

Assistant Superintendent of the Park for 

their assistance.

Information on the Ames slide wra obtained 

from the section f oreman of the Rio Grande 

Southern Railroad -at the Vance Junction station near Telluride.

The terminology in the field of landslide 

studies is not yet standardized. To simplify 

the following discusaion, C.P.3. SI ftrpe'a I/ 

classification la used throughout the paper.

I/ Sharps, C. F. 3., Landslides and related 

phenomena, % study of mass-movements of soil and roos, 

137 pp.. Mew Toric, Colombia Univ. Press, T938,
REGIONAL BACKGROUND

Throughout the San Juan and La Plata 

Mountains and their extensive borderlands of 

high plateaus and mesas, landslides 2/ are 

spectacular agents of erosion. Regardless of 

the types of rock or the local conditions involved in these landslides, a high percentage 

of them combine the kinds of earth movement 

defined by Sharpe 3/ as slump and earthflow.

3 lump, is the failure of a portion of a 

slope with the resultant downward and outward 

movement of relatively unified blocks of rock 

or earth. This is the kind of movement moat 

generally considered as a typical landslide. 

Both initial failure and subsequent movement 

occur along one or more well-defined surfaces 

commonly known as slip planes or slip surfaces. 

The location and shape of a slip surface depend on several factors: structural and 

stratigraphic conditions, permeability of beds, 

pore-water pressures, number and spacing of 

joints, and the degree to which the slope has 

been disturbed by excavation or undercutting.

Earthflow is the slow, continuous deformatlon of an entire mass. Earthflows that 

are associated with slump masses generally 

extend below the major slide blocks as buckled 

and fissured zones bordered by raised rims of 

debris.

Most of the plateaus of southwestern 

Colorado are composed, entirely or In part, of 

soft clay shale, which is notably prone to 

failure. The Cedar Creek landslide, an almost 

ideal example of mass movement combining slump 

and earthflow, is representative of slope failure in these shales. Initial movement at the 

Cedar Creek site was the downward sliding of 

large blocks of gravel-capped shale. The 

sliding was later accompanied by earthflows 

In the loose weathered and broken shale,which 

was pushed or fell In front of the moving 

slide blocks.

In contrast to the Cedar Creek landslide, 

which affected only weak shale and unconsolidated gravel, some of the landslides involve 

hard beds, usually sandstone or volcanic rock, 

which overlie or are interstratifled with 

shale.

In Mesa Verde 'National Park, landslide 

movements in the Mancos shale affect and are 

affected by a cap of thick, relatively hard 

sandstone of the Mesaverde formation. The 

shale Is weak and easily weathered, but being 

protected by the more durable caprock, it 

maintains abnormally high-angle slopes. This 

oversteepenlng is favorable to slope failure 

in the weak shale and prevents the slopes from 

reaching equilibrium, so that the active life 

of this slide area has been prolonged far 

beyond that indicated for either the Cedar 

Creek or Araes slide areas. Slumping is the 

most common kind of failure in the shale.

2/ Auwood, W. W., and fciataer, K. P., Pnysiograpny and Quarternary sreology of bne Saa Juan Mountains, Colo.i U. S. "-sol. Survey Prof. Paper 166, 

pp. 147-163, 1932.

j/ Sharpe, 0. *. 3.. op. oi-fc., pp. 50-55, 65-74.

Since this process also serves to undermine 

the sandstone, large rock'falls f^om the 

caprock are numerous. Earthflows are rare 

and short-lived because of the s teepness of 

the slopes. Loose material generally moves 

rapidly as debris slides and falls.

The Araes landslide follows a pattern 

similar to the Cedar Creek slide. The upper 

part is composed of several large slump 

blocks; the remainder is a thick train of 

Jumbled earthflow debris. Unlike the Cedar 

Creek and Mesa Verde areas, however, move- * 

ment has been primarily In glacial till 

rather than in shale. Such slides In^unconsolidated till are widespread In the glaciated 

valleys of the San Juan and La Plata Mountains, 

although mass movements have also Involved 

almost every kind o f consolidated and unconsolidated material present in the^e areas.

CEDAR CREEK LANDSLIDE NEAR MONTR7SE, COLO. 

Location and -history

The Cedar Creek landslide crosses the 

Denver & Rio Grande Western Railroad about 

9 miles east of Montrose. (See fig.2.) 

Recently active landslides extend for almost 

a mile along the north-facing aids of the 

valley. Detailed studies were confined -to 

the area where one of the most recent slides 

has disrupted the railroad., This landslide 

(see fig. 3) occupies an area abaa£ 1,000 

feet square with a maximum height of about 

300 feet measured from the railroad grade to 

the top of the cliff.

The first noticeable activity took place 

early In 1941, when a slow slump and earthflow down the hillside pushed som* soil and 

shale debris over the track. l^e largest 

and only sudden movement noted nt any time 

took place early in the spring of 1942, when 

several major slide blocks broke away from 

the cliff and moved down 50 to 75 feet in a 

few days. These movements buried 1,000 feet 

of track so deep that the railroad never recovered it. The original alignment oft he 

track in this section was nearly straight, 

but the large volume of debris that overwhelmed it necessitated relocation of the 

roadbed, and it now curves arcund the toe of 

the slide. The original roadbed is about 

150 feet inside the slide at the point of 

greatest movement. (See fig 3.) Since 

1942 Wiere have been some periods of complete 

quiescence; usually in midwinter, but recurring earthflows and small-scale slumping 

have kept debris moving over the roadbed most 

of the time.

General description

The Cedar Creek landslide (fig.3) was 

formed primarily by the breaking off and 

downward movement of large masses .of Mancoa 

shale along the north end of a mesa. The 

part of the slide above the 1040-foot contour (fig.3) Is composed largely of steplike 

blocks of shale, which broke off and moved
down on a slip plane as unified masses. The 

lower part is made up of unconsolldated earth, 

clay, and broken shale, which moved as earthflows in front of the alide blocks.

probably of middle Pleistocene age.

Topography of the alide. The land forma 

developed by the slide (see fig.3) are of two

Cedar Creek 

landslide area

Figure 2. Index map of the Cedar Creek landslide area.

Qeology. The alide occurred on the 

north edge of a large undulating mesa composed 

of shale capped by 15 to 25 feet of coarse 

boulder gravels.

The Mancoa shale is a soft, almost homogeneous, carbonaceous, clay shale. Individual 

beds range from a fraction of an inch to 

several feet in thickness. Fresh rock is 

daric gray to black, but on exposure the shale 

weathers rapidly to light gray and tan silt 

and clay. It can be found in place only at 

the head of the slide, where it is exposed 

locally along the slide plane. Shale is also 

exposed in the faces of the large slide blocks, 

but there it generally shows some weathering.

The gravels are composed largely of wellrounded cobbles and boulders with interstitial silt and clay. Most of the material 

is more than 4 inches in diameter, and 

boulders 1 to 3 feet across are common. The 

gravels, classed by Atwood and Mather 4/ as 

part of the Florida gravel are widespread 

periglacial stream deposits around the 

margins of the San Juan Mountains. They are

1932.

Atwood, If. f., and Matner, K. f. , op. oit.,

types, large tilted blocks of shale and 

gravel In the upper part and ridged and rolling slopes of loose soil, clay and shale in 

the lower part. The scale and contour interval of figure 3 unfortunately do not permit 

adequate representation of several small but 

significant features, such as the small blocks 

at the head of the slide area, or the ridge 

and trough structure that characterizes the 

lower part.

The part of the slide above 1,040 feet 

is composed almost entirely of lar.Te blocks, 

the width of which increases progressively 

downward from the top of the slide. Just 

below the cliff that rises from the head of 

the landslide is a belt of small blocks. Some 

of these were recently separated f:*om the 

mesa and still retain Intact the complete 

gravel capping and cover of sage and other, 

low brush. On the older blocks, repeated 

movements have shattered the gravel cap sothat only a few elongated pinnacles of weakly 

cemented gravel remain. Rotation of these 

small blocks toward the slide plan* is generally greater than 10°; one has a slope of as 

much as 25°.

Below the belt of small blocks are two 

major blocks that make up most of the upper 

part of the slide. The higher blo^k extends
EXPLANATION

Pond

Railroad alignment

«.* »«-»« * Railroad covered by «arthfiow

Assumed datum

FIGURE 3-CEDAR CREEK LANDSLIDE

NEAR MONTROSE, COLORADO
entirely across the width of the slide and 

probably represents the largest single mass 

movement in the history of the slide. It is 

approximately 850 feet long and has a maximum 

width of 95 feet. Below it and crossing the 

western half of the slide is another block 

about 400 feet long and 140 feet wide. Both 

blocks show a characteristic backward rotation 

of 50 to 10° along a horizontal axis parallel 

to the slide plane. The shale that constitutes 

the v main part of these blocks seems to be only 

slightly shattered .by the downward movement. 

The gravel that formerly capped them has been 

disrupted, partly by the downward movement but 

mostly by rapid weathering of the unconsolidated materials, so that the tops of the blocks 

are covered by a jumble of boulders, soil, and 

disintegrating shale. The outer faces of the 

blocks stand at extremely high angles, but 

their lower slopes are modified by debris falls 

and mudflows. Continuous sloughing of material 

along their outer margins is steadily reducing 

them in width.

The margin of relatively stable ground 

that surrounds these slide blocks is characterized by steep scarps facing the landslide

Prom the base of the large blocks (1,040 

foot-contour on fig. 3) down to the line of 

the railroad, the slide is composed of rolling 

slopes and ridged benches. These show the 

general characteristics of earthflow masses, 

although the portion immediately above the 

roadbed has been somewhat altered by roadbed 

relocations. The outstanding feature of this 

lower section is the development of pressure 

ridges and troughs, which closely parallel the 

toes of the slide blocks above them. As this 

mass of largely unconsolidated material moved 

down in front of the slide blocks, it piled 

up, ridged, and broke because its internal 

friction was greater than that of the better 

lubricated clays along the slip surface. The 

ridges are traversed by two seta of deep 

cracks. One longitudinal set of tension 

cracks follows the crests of the ridges; a 

second set of cracks is almost at right angles 

to the pressure ridges and forms a radial 

pattern from the toe behind them. This lower 

part of the slide is bounded on both sides by 

the prominent raised rims of shale and clay 

characteristic of earthflow masses.

The original toe of the slide has been 

destroyed by reconstruction of the railroad 

alignment around the base of the slide area. 

Probably it was similar to that of the small 

slide to the west, which has a raised and 

overthrust rim of soil and rook debris at the 

toe.

The steep bank below the railroad alignment is the result of efforts by the railroad 

company to maintain the roadbed and is not 

considered a true part of the slide.

.Drainage conditions. In addition to the 

annual precipitation.of 16 inches, Irrigation 

of the grain fields on the mesa above supplies 

abundant water to the slide area. The gravel 

cap is highly porous, and water moves rapidly 

through it to the underlying impervious shale. 

Irregixlarities in the contact of gravel and 

shale direct the flow of ground water into the 

western part of the slide and do not permit 

any significant quantities to reach the eastern part. Thus the slide is effectively 

divided into a "wet" west half and a "dry"

east half.

Because of this abundance of water, the, 

western part shows a greater variety of conditions than the eastern part and ia the site 

of nearly all present slide activity. Eacn 

of the large slide blocks holds r pond on its 

inner margin during the irrigation season. 

Part of the water that moves alorg the gravelshale contact of the mesa seeps down the slip 

plane; the remainder flows down the face of 

the slide to form a pond on the I igher block. 

Enough water escapes from this pond'and aeepa 

through to feed a pond on the block below. At 

times of maximum supply, water ercapes also 

along the outer margin of the lov^r block to 

appear as small springs farther clown its face 

and to form small ephemeral ponds between the 

pressure ridges below. Part of the water 

flows in definite channels as amrll streams, 

and part soaks into the loose disintegrated 

shale that mantles the block.

The finest material from the disintegrated 

shale is carried along by the water moving 

through and over the ali'de until it la deposited as a v ery sticky plastic clay in the 

ponds on the slide blocks. This removal of 

fines leaves a concentration of coarse pebbles 

and boulders between the ponds ar<J the outer 

margins of the blocks.

No appreciable quantities of water penetrate the east half of the slide; there 

lubrication comes only'from rain and snowfall. No ponds or springs are present on 

this side. This, dearth of water Is reflected 

in the absence of any movement except settling 

of small blocks near the top of the slide and 

minor sloughing of dry material from their 

margins.

Slide movements and their causes.

The mass movements of the Oedar Greek 

landslide area can be attributed to welldefined conditions and causes. C"he physical 

setting of very steep slopes and cliffs at 

the edge of a mesa, which is underlain largely 

by weak, easily weathered clay si «le, and only 

indifferently covered by a protective mantle 

of vegetation, offers the essential passive 

conditions favorable to slope failure. To 

touch off actual movement, the excess water 

from the Irrigation of the mes,a is the most 

important factor,, for it is the lubricating 

agent needed to reduce internal friction 

along joint or shear planes and vithin loose 

soil and rock debris, thus permitting sliding 

or flowing of the material.

The resulting slope failures produced two 

primary kinds of movement, namely, true sliding or slump, whereby blocks of shale moved 

suddenly downward as unified masses on a welldefined slip surface, and earthflow, in which 

the material that was pushed or fell in front 

of the sliding blocks continued to move 

throughout the period .during which the whole 

mass was being deformed.

Conditions favoring movement. The underlying factors that make this area susceptible 

to landslide movements are the component 

materials, the vegetation conditions, and the 

topographic and stratigraphic conditions.
The capping boulder gravels are loose and 

porous, thus forming a giant sponge, which 

absorbs an unusually high percentage of any 

water supplied to the mosa top and feeds it 

down to the shales below.

The Mancos shale is a .very fine-grained 

consolidated mud that is almost impervious to 

water. Ground waters percolating through the 

capping-gravels move along»the contact of 

gravel and shale, except where joints or other 

fractures give admittance into the shale. On 

contact with water, -however, the shale breaks 

down readily into fine slippery clay and silt. 

Thus, each fracture or joint that admits water 

is a potential slip plane, along which the 

shale can be converted into an efficient mud 

lubricant. At one point the slip plane of the 

Cedar Greek slide was found to contain about 6 

inches of soft, slick, wet clay.

Exposure of the shale to air results in 

rapid decrepitation, which produces an abundant surface cover of loose material. The 

addition of water readily converts this 

material into moving masses.

The natural vegetation cover is generally 

a sparse growth of scrub oak, sage, and other 

low brush that is not very effective in stopping a sliding or flowing mass.

The youthful profile of Cedar Creek valley 

increases the danger of landsliding. The steep 

valley walls cut in potentially weak gravel 

and shdle will fail much more readily than 

would the moderate slopes of a more mature 

topography.

Other conditions that favor earth movements are present on the south side of Cedar 

Creek Valley, where the slide occurred. Moisture contributed by irrigation or precipitation evaporates less readily than on the north 

side and hence has greater opportunity to penetrate the underlying material. The contact of 

gravel and shale and the bedding of the shale 

have an apparent dip of less than 2° to the 

north. On the south side of the valley, this 

slight dip tends to lead ground waters to the 

cliff face, where the slides develop. On the 

north side of the valley water follows the dip 

away from the valley slopes. Old landslide 

scars and more recently active slides extending for at least a mile on both sides of the 

Cedar Creek alide indicate that such movements have long been a characteristic of the 

slopes on the south side of the valley,whereas 

the north slopes show no good evidence of 

recent landslides.

Activating or initiating causes. Actual 

movement in the Cedar Creek landslide can be 

attributed almost entirely to the relatively 

large amount of water supplied to that area. 

About 300 acre-feet of water is used each 

season in a 12-acre area immediately aouth of 

the active landslide. The porous boulder 

gravels take up all water not held by the top 

soil and transmit it along the gravel-shale 

contact. For a long period some of that surplus water contributed towards the establishment of the slip plane by working into the 

hairline Joints characteristic of the more 

massive phases of the Mancos. The initial, 

quantity able to penetrate such Joints below 

the gravel-shale contact would have been 

almost negligible. Over a sufficient period 

of time, however, continued action of the

water along these minute joints would thoroughly wet the shale and reduce its shearing 

strength. Failure would take place and the 

slip surface would develop wherever the shearing stresses due to gravity had I ecome equal 

to the shearing resistance of the shale. Once 

initial failure had occurred, water moving 

along the slip plane would alter the shale to 

a greasy, slippery clay on which the newly 

developed slide blocks could readily move. »

The remainder of the surplus water flowed 

over the surface of the slide itself/ The 

water thus supplied to the overburden of clay, 

silt, and boulders was often sufficient to 

start this debris in motion by the double process of lubricating the unstable mass and by 

adding weight to it at the same time.

The width of the slide coincides almost 

exactly with the zone that originally received water escaping from the irrigated land 

on the mesa. When re-routing of the irrigation ditches channeled the water into a 

narrower zone, the active part of the landslide decreased in width. The east half, 

which received no irrigation water after 1944, 

has become dormant. Recent activity is confined to the west half of the slide ar^a, 

where water is still abundant.

While the moisture provided by rain and 

snow might have produced the Cedar Creek 

slide, just as this moisture has been the 

activating agent for the older slides in the 

vicinity, it is doubtful whether the slide 

would have attained its present siee so 

quickly or would show such prolonged activity 

without the aid of the large quantities of 

water supplied since irrigation was begun in 

1927.

A secondary factor aiding the activity of 

the landslide has been the unavoidable work 

performed in railroad maintenance. In the 

past few years, clearing away the debris that 

overwhelmed the tracks has removed the toe of 

the slide, which acted as partial support to 

the material above, thus leaving a considerable weight of material resting insecurely 

on its base of wet, slippery clay. Construction during the spring of 1947 avoided this 

difficulty by leaving the toe intact and 

laying the track on a fill built out around 

the landslide area.

Types of movement, Slump ani earthflow 

(see p.3) are the two primary tynea of movement in the Cedar Creek landslide. Slump is 

probably first in importance, especially in 

the early stages of movement. T"T e original 

slip plane probably remains as a kind of sole 

over which the lower part of the slide moves. 

The position of the upper part of the slip 

plane, which forms the head of the slide, has 

continually migrated back into the mesa as 

new blocks break off and slide down. It aeeraa 

probable that the lower end of the slip plane 

came to the surface at or a little above the 

level of the railroad alignment, for there 

is no record that the track was ever buckled 

or broken by the landslide. Rather, it seems 

to have been overwhelmed from above. Earthflow ranks almost equally in importancewith 

slump. The buckled ridges and broad rolling 

benches that make up the lower p<*rt of the 

landslide were shaped almost entirely by this 

type of mass movement.
Although there was no visible movement of 

the large blocks during the time the landslide 

was under'close observation, evidence of some 

movement was supplied by the doming of a large 

mud deposit on the top of the lower block, 

owing to the settling of the upper block. In 

the lower half of the landslide, the ridgeand-trough structure became increasingly prominent, and new cracks appeared and deepened from 

day to day. Maximum activity during August 

1946 occurred on the toe at the northwest corner 

of the landslide, where the first bench above 

the old railroad alignment moved 7 feet in one 

week; a total horizontal movement of 49 feet 

was recorded for this section from June 5 to 

August 20.

Movements other than slump and earthflow 

are of secondary importance in that they 

merely modify and do not essentially change the 

characteristic features of the slide. Of these, 

debris falls are most common. Generally,debris 

falls come from boulder gravels that have been 

undermined by water seeping out at the contact 

of gravel and .shale, or from which the loose 

silt and clay that bind the large rocks have 

been removed.

Along the outer edges of the major blocks, 

narrow wedgelike-blocks break off, producing 

miniature slump movement. After a short 

descent, these small blocks break up into 

debris falls.

True mudflows and sheet flows of fluid 

mud are fairly common in the west half of the 

Qedar Creek landslide.

KNIFE EDGE LANDSLIDE AREA IN 

MESA VERDE NATIONAL PARK

General description

The Mesa Verde is the westernmost part of 

a dissected plateau composed largely of Mancos 

shale and capped by sandstone of the Mesaverde 

formation. Steep escarpments extend almost 

unbroken around the east, north, and west sides 

of the mesa. The southern escarpment has been 

breached by several deep canyons, which extend 

almost to the north end of the mesa.

Along the canyons and marginal escarpments, 

earth mass movements of various types have been 

important destructive processes. Rockfalls 

from the capping sandstone are present along 

all the escarpments. Scars of the slump type 

of landslide movement are especially numerous 

and prominent in the shale on the north and 

east margins of the mesa. Some are evidently 

of very recent origin; others are older and 

their characteristic features are largely 

obscured by erosion. Large-scale slumping in 

the shale is commonly followed by collapse of 

the overlying sandstone.

Southward from the Park entrance the 

road, which leads to the Indian ruins at the 

south end of the mesa, climbs about 6 miles 

along the north and northeast faces of the 

mesa on sidehill cuts and fill in the shale. 

Minor slumping, washing, and sloughing of the 

shale ia prevalent along most of the road. In 

a few places, large-scale slumping, debris 

slides, and subsidence of the grade have caused 

serious trouble ever since the road was built. 

One of the most troublesome parts is a section 

about 1^ miles long generally called "Knife

Edge." (See fig. 4.) This area was the 

focal point of reconnaissance studies in Mesa 

Verde National Park during the 1946 field 

season, because it represents a kind of slope 

failure that is common to the Mancos formation of the southwestern Colorado plateaus 

but is different in several respects from the 

Cedar Creek landslide.

Landforms and types of movement

The Knife Edge area can be divided into 

two parts on the basis of steepness of slope 

and type of movement most prevalent in each.

The eastern part (B in fig. 4} has 

natural slopes of 2:1 to 1:1, although the 

man-made slopes of the road cu>t are locally 

steeper. Movement in this part of the Knife 

Edge area is mostly of the slump ty-e. The 

blocks are composed largely of weathered 

shale and are usually Tess than 15 feet wide 

and from a few feet to 100 or 200 feet in 

length. Each block occupies only a small 

part of the total slide area, which is more 

than 1,400 feet wide. The steepness ofthe 

scopes, together with the highly weathered 

nature of the shale in the slide blocks, 

probably contributes to the tendency of the 

blocks to disintegrate into debris slides, 

and, in wet weather, into small mudflows as , 

they move down the slope.

The location of the slip surfac* appears 

to be determined, at least in part, by the 

boundary between fresh shale and weathered 

shale. Only small amounts of unaltered shale 

are found, even in the -larger blocks.

At its highest point, the upper end of 

the slide area is about 100 feet above the 

road, leaving 50 to 70 feet of undisturbed 

material between the slide and the overlying 

sandstone. Because blocks continue to break 

off and the position of the slip surface 

migrates backward and upward toward the sandstone cap, this zone of undisturbed shale is 

steadily reduced in .width.

In the western part (A in fig.4), the 

plane of movement between the w eath«»red and 

the unaltered shale has retreated $o the base 

of the sandstone caprock. .Old slid* blocks 

below the present road grade indicate that 

slumping was once the primary type of movement. Most of theslopes are now greater 

than i:l, however, and current movement is 

primarily of the rockfall and, to a much 

le&eer extent, the rockslide types.

Causes of landslide movements

Conditions favoring movement. The Manoos 

shale is a dense, impervious mudstone and 

shale when freshly exposed, but it is quickly 

reduced to slippery mud when in contact with 

water. On exposure to air it decrepitates 

rapidly, so that a fairly thick cover of unconsolidated slit and clay will develop even 

under semiarld conditions. Addition 

of water to this loose material can easily 

transform it into a moving mass.

The weak Mancos shale is overlain by a 

thick, relatively competent sandstone sequence, which is,much more resistart to weathering. The sandstone, a part of tbe Mesaverde
EXPLANATION

A. Western part of Knife Edge 

slide area

Movement primarily of rockfall and rockslide types

Slopes generally <

B. Eastern part of Knife Edge 

slide area

Movement primarily of slump 

earthflow, and debris-slide 

types

Slopes > Ml

27° 25' 00"

Base enlarged from portion of U.S.G.S.Topographic mop of 

Mesa Verde National Park, Manlezumo County, Colorado. 

Surveyed 1910-11. Revised 1926.

27° 25'00"

2 Miles 

J

Contour Interval = 100 Feet

FIGURE 4 KNIFE EDGE LANDSLIDE AREA AT NORTH END OF 

MESA VERDE NATIONAL PARK

9
I08°55' J08°50'

Figure 5. Index map of the Ames landslide area,

formation, stands in vertical cliffs in most 

places ajjd partly protects the weaker Mancos 

shale from weathering, so that the shale 

stands at steeper angles than is normal in 

areas where the sandstone is lacking. The 

overateepened shale slopes, capped as they are 

by sandstone cliffs, are more susceptible to 

failure, both topographically and geologically, 

than lower, less precipitous slopes.

The north end of the tableland has the 

greatest precipitation in the Mesa Verde area 

and is thus subjected to more rapid weathering and weakening of the shale. The relative 

abundance of old slides and rockfalls at the 

north end of the Park testifies to the effectiveness of the higher precipitation in this 

area as compared to the drier areas in other 

parts of the mesa. Furthermore, evaporation 

on the north-facing slopes is somewhat slower 

and snow remains longer, thus permitting more 

moisture to be absorbed by the overburden.

Although the mesa top supports relatively 

abundant growth, vegetation generally does 

not prosper on the shale, ao that the cover 

on the slopes below the caprock is relatively 

sparse, leaving the shale largely exposed to 

weathering and wash.

i

Activating pauses. Removal of support 

through natural and artificial means is the 

outstanding activating cause of movement 

along the Knife Edge. Rapid weathering of 

the soft shales has continually undermined 

the resistant sandstone caprock, so that theescarpments are always retreating by means 

of rockfalls and slides. Construction of the 

road has considerably accelerated removal of 

the shale through weathering and washing as 

well as sliding.

Along the west half of the Knife Edge, 

removal of the s hale by sliding has been 

largely a natural process accomplished before

10
the construction of the road. Cutting into 

the shale to build the inner part of the roadbed, however, has undoubtedly increased" the 

tendency to sloughing and sliding. Along the 

eastern half the steep-faced cuts on the inner 

side of the road have left the weathered shale 

and rock debris above the road without adequate support and have facilitated the breaking off and downward movement of several slump 

blocks. 

Another factor producing movement is the 

overloading that resulted from construction 

of the road on unstable ground. Along moat 

parts of the Knife Edge the fill was placed 

on old landslide masses or on slopes where 

the angle was undisturbed but in a precarious 

state of balance that could easily be destroyed by addition of extra weight. The 

settling of most parts of the Knife Edge road, 

which has been almost continuous since construction, can be attributed, at least in part, 

to overloading. Passage of traffic over this 

relatively unstable area may add to the overloading to a minor extent.

The fact that the area of greatest precipitation is also that of the most extensive 

and continuous slide activity indicates the 

importance of water as an activating cause of 

movement. Average annual precipitation is 

about 18 inches, of which 40 to 50 percent 

falls during the winter and early spring. Most 

of the earth movements along the Knife Edge 

occur during March and April, when moisture 

from the melting snow and'thawing ground is 

augmented by the spring rains. Although the 

drainage area that feeds the slide is very 

small, the oversteepened slopes and the nature 

of the materials involved give a maximum 

effectiveness to whatever water reaches the 

unstable area.

AMES LANDSLIDE NEAR TELLURIDE, GOLD.

Small slides in glacial till are fairly 

common in the mountains of southwestern Colorado. Many of them have caused damage to 

* roads and railroads, and their economic significance is often greater than their size might 

indicate. One of the most troublesome is the 

Ames landslide, which is about 7 miles southwest of Telluride in the glaciated valley of 

the South Pork of the San Miguel River. (See 

fig.5.) This slide crosses the Rio Grande 

Southern Railroad about 4.2 miles south of the 

junction of the South Pork with the main 

stream of the San Miguel.

The Ames slide occurs largely in the thick 

glacial till that mantles the San Miguel valley. 

It is a long narrow zone of failure with a 

maximum width of about 400 feet" and a minimum 

length of 1,600 feet. The actual length is 

not known because the toe of the slide, which 

emerged below the railroad grade, has been destroyed by the maintenance work necessary to 

keep the railroad in operation across the 

slide area.

Although the material involved in the 

slide is unconsolidated till rather than shale, 

the kinds of movement in the Ames landslide 

are the same as in the Cedar Creek and Mesa 

Verde landslides. The upper part of the slide 

was a downward movement of unified slide blocks; 

the lower part was a movement of earthflows.

Aa at the Cedar Creek slide, the outstanding cause of movement was an excessive amount 

of water locally in an area where general conditions already favored slope failure.

The railroad has had trouble from slides 

and mudflows along the South Pork of the San 

Miguel since 1890, but no large-scale movement had been recorded for 40 year's until the 

winter cf 1929-30, when the Ames slide Started 

to moveslowly, taking the track with it. Total 

horizontal movement amounted to more than 34 

feet over a period of a month. As movement 

never exceeded one or two feet a day, the railroad was able to continue operation by keeping 

crews constantly at work resetting the track as 

it was displaced by the slide. Originally the 

alignment was straight along this part of the 

valley, but repeated movements since 1929 have 

necessitated relocation of the track around the 

toe of the slide so that it now has a 25° 

curve, the highest along any part cf the line.

geology and topography

The area near Ames is underlain by shale 

and sandstone of the Morrison, Dakota and 

Mancos formations. Exposures are numerous in 

the lower end of the South Pork valley, but 

in the upper part the rock is largely masked 

by glacial till. In the vicinity of the Ames 

slide till blankets most of t he slope with 

layers from 5 to more than 60 feet thick. 

Scarps near the head of the slide show 18 to 

25 feet of till overlying the black Mancos 

shale. The till is composed predominantly of 

rock flour and clay with only 1 or 2 percent 

of rock fragments. Most of the fragments are 

sandstone and monzonite and are less than a 

foot in diameter, although a few boulders of 

monzonite are more than 4 feet across.

The Ames slide has the same general 

topographic expression as the Cedar Creek 

landslide. The upper part is composed of a 

step-like series of blocks formed by downward 

slumping of successive masses of glacial till. 

The higher and younger slide blocks extend 

entirely across the width cf the slide area and 

still retain much of their original form. The 

top surfaces slope gently toward the slip plane 

at angles of 5° to 15°. Three blocks hold 

small reed-grown ponds impounded on their inner 

margins. The lower and older blocks are smaller; none of them is more than about 50 feet 

wide. All are modified, to a greater or lessdegree, by erosion of the soft, unconsolidated 

till, so that they commonly resemble rounded 

hummocky masses rather than individual blocks. 

The oldest ones are difficult to distinguish 

clearly from the irregular earthflow terrain 

that forms the lower part of the landslide.

The earthflow masses are uneven and rolling, with poorly defined ridge-anl-trough 

structure developed at right angles to the 

direction of movement. Slopes are relatively 

steep for this type of mass movement; angles 

of 25° to 30° are fairly common. The slopes 

are scarred by V-shaped ravines carved into 

the soft till by runoff during spring thaws 

and summer rains.

11
Principal causes of movement

Factors favoring movement. Three factors 

are of primary importance in making the valley 

of the South Fork a potential area of landslides. First, the glacial till, the principal material involved in the landslide, is 

unconsolidated and has a very low shearing 

strength, so that It fails under relatively 

light loading. The steep slopes of the 

youthful.glaciated valley of the South Pork 

also favor earth movements, as their angle is 

greater than that required for stability in 

the materials involved. A third factor is 

the seasonal concentration of precipitation. 

The area has a wide annual range in temperature and precipitation. Average annual precipitation Is about 21 inches, of which more 

than 50 percent falls as rain or snow from 

January to the end of May. Because the combined factors of melting snow and thawing 

ground occur together with the spring raina 

during April and May, the lubricating action 

of precipitation and ground water in the till 

is concentrated into a relatively short period. 

Most of the earth movements in the Ames slide 

area occur during this time of maximum lubrication.

Influence, of vegetation cover. Although 

the long cold winters leave only a short growing season, the abundant precipitation helps 

perpetuate a heavy cover of vegetation and 

aids in the reestablishment of cover over 

barren areas created by new road cuts or by 

recent landslides.. The valley is blanketed 

with a thick growth of aspen and scattered 

evergreens. An undergrowth of ferns,grasses, 

and low brush forms a continuous retaining 

mat over all but the cliffed slopes. The 

abundant vegetation cover has apparently been 

effective in preventing earth movements under 

normal conditions, in spite of the adverse 

factors of weak component materials and oversteepened slopes. Earth movements on the 

South Pork of the San Miguel River generally 

occurred where the vegetation cover or the 

slopes have been disturbed by man or where 

there was an abnormal supply of water.

Vegetation conditions vary considerably 

over different parts of the Ames slide. On the, 

blocks near the head, part of the original 

vegetation cover has been retained. Although 

some trees were uprooted by the movement,many 

continue to grow but are usually tilted 10° 

to 300 away from the vertical. The tops of 

the blocks have good grass cover, and the 

ponds are already largely filled with reeds. 

The outer margins are partly covered by vegetation, mostly grass, which is helping to 

stabilize the ground since major movement has 

stopped.

The lower slide blocks and the earthflow 

parts of the slide have relatively sparse 

cover of vegetation. Apparently these parts 

have undergone a more thorough churning movement, which completely destroyed their original vegetation cover. Their present growth 

consists mostly of sparse low brush, grass, 

and some aspen saplings. Slope wash and 

gullying in this much disturbed material 

probably have been effective agents in preventing renewal of a continuous cover of vegetation.

Activating or initiating oausea_of movement. Actual movement at the Ames slide did 

riot take place until enough water penetrated

the .unconsolidated glacial till to overcome 

the internal friction of the mass ard the retaining effects ofthe vegetation cover. In 

addition to precipitation, a considerable 

quantity of water was available from several 

small beaver-dammed ponds on the ed^e of the 

mesa. The localization of the Ames slide to 

the slope immediately below 'the ponds indicated clearly that this abnormally large 

supply of water was the factor that tipped 

the balance and started movement of weak 

material on oversteep slopes, notwithstanding 

the thick cover of vegetation.

Types of movement

In the upper part of the landslide, movement was primarily by slumping. Failure 

occurred as shearing along a definite plane, 

followed by the downward and outward movement 

of blocks of till.

The lower part of the slide has been 

shaped primarily by earthflow in th« till thafc 

was pushed down by the sliding blocl:s. The 

buckled and fissured appearance that Is so 

prominent in the Cedar 'Creek earthf :,ows Is 

scarcely discernible at the Ames landslide. 

 This may be due in part to the steepness of 

of the slopes, which would translate part of 

the earthflow into.mudflow masses and numerous 

miniature slump blocks. Rapid erosion on the 

relatively bare slopes would also twid to modify the typical earthflow structures.

Minor slumping and small mudf lows, common 

as secondary types of movement, are now the 

only obvious indications of reoent novement. 

Tilting of the trees and bushes indicates that 

creep is going on In the area around the landslide. In the zone immediately bordering the 

main slide area, this creep appears to grade 

Into slow slides or earthfIowa.

REMEDIAL MEASURES

The final answers to problems presented 

by landslide areas have yet to be found. Even 

the most successful remedies are usvally 

applicable only to a specific landslide and, 

in most cases, have served only to check movement, rather than to halt it completely and 

thus remove the danger of renewed a otivlty. 

The measures taken on the three landslides 

considered in this paper have met wj th widelyvarying success.

Cedar-Creek landslide. The corrective 

methods employed in the Cedar Creek area have 

proved to be only temporary. Prior to 1947, 

the railroad crews kept the line open by clearIng the debris" from the railroad grade and by 

cutting back the adjacent slopes to low angles. 

These procedures did not stop the earth movements in any way; rather they tended to perpetuate the instability by leaving the unbalanced masses above the grade without support. 

In April 1947, a fill was built to carry thetrack around the landslide without cutting 

into the moving material In the hope that the 

undisturbed slide will stabilize Itself.

Up to December 1946, no major drainage 

measures had been attempted at the Cedar Creek 

landslide, .Probably the only effective one 

would be to cut off the supply of Irrigation 

water to the land behind the slide for at

12
least 2,000 feet south of the active face.

The older landslide areas bordering the 

Cedar Creek landslide appear to be more or 

less stabilized. Excavation into the lower 

slopes of these slides, however, would renew 

tbe danger of large-scale 'earth movements.

Knife Edge landslide area. No satisfactory Solution has yet been found for this 

landslide area. Although the drainage area 

supplying water to the slide faces is very 

limited, the extreme oversteepening in the 

weak shale givea the small quantity of water 

a maximum effectiveness. One section of the 

oversteepened shale face was coated with 

gunite in an attempt to reduce weathering and 

disintegration of the shale. Water working 

down from the capping sandstone and along 

joint and bedding planes and sandy lenses in 

the shale has caused most of the gunite to 

push away from the face and to spall off. 

Steel cribbing was installed along one stretch 

to hold the fill in place and to stop subsidence and sliding of the road. In general,this 

has proved successful, although some displacement of the cribbing has occurred.

Ames landslide area. The methods employed at the Ames slide have demonstrated 

the effectiveness of stabilizing earth mass 

movements by raising internal friction along 

the slide plane and within the mass Itaelf 

through removal or diversion of the water 

lubricant. Large-scale movement has been 

virtually halted since the beaver dams at the 

head of the landslide were destroyed, thus

removing the principal source of excess water. 

However, the high-angle cut 70° to 90° maintained by the railroad across the slide, together with the relatively steep slopes of the 

earthflow masses above it, seem to harbor an 

unstable condition that might result In renewed movement in a season of unusually heavy 

precipitation. Up to the present, the minor 

slumping, sloughing, and small raudflows that 

commonly occur in the spring along this cut 

have presented no new major problems of main* 

tenance. »

CONCISIONS

The corrective measures noted above for 

the three landslide areas illustrate the 

general inadequacy of many common methods of 

landslide control and show the urgent need for 

more knowledge concerning landslides and for 

finding new methods of dealing with them.

In addition to aiding the search for new 

remedial measures, an understanding of the 

geology is needed in planning what can be done 

to atop such landslides before they start, as 

well as in the planning stages cf any major 

construction project, so that ar«ms In which 

conditions favor earth movement can be avoided, 

if at all possible. If construction in such 

areas cannot be avoided, a complete understanding of the geologic conditions may help 

in controlling the activating ceuses before 

slides occur or become serious.

13

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