Mass Movement in 

Two Selected Areas of 

Western Washington County,

vania

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1170-B
MASS MOVEMENT IN

TWO SELECTED AREAS OF

WESTERN WASHINGTON COUNTY,

PENNSYLVANIA
Recent earthflows along concave-convex east- to north-facing slopes west of Prosperity, Pa.
Mass Movement in 

Two Selected Areas of 

Western Washington County, 

Pennsylvania

By JOHN S. POMEROY

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1170-B

Landsliding and its relation to geology and an 

analysis of various interpretive elements 

in a region of the Allegheny Plateau 

subject to landslides

UNITED STATES GOVERNMENT PRINTING OFFICE, W AS H INGTON : 1 982
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Pomeroy, John S 1929-

Mass movement in two selected areas of western Washington County, Pennsylvania.

(Shorter contributions to general geology)

(Geological Survey professional paper ; 1170-B) 

Bibliography: p.

Supt. of Docs, no.: I 19.16:1170-B 

1. Mass-wasting-Pennsylvania-Washington Co. I. Title. II. Series: United States.

Geological Survey. Shorter contributions to general geology. III. Series: United States.

Geological Survey. Professional paper ; 1170-B. 

QE599.U5P65 551.3 80-607835

AACR1

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Page

Abstract ______________________________ Bl

Introduction ____________________________ 1

Harmon Creek area ________________________ 2

Geology, soils, and their relation to landsliding ______ 2

Interpretive elements of mass movement between 1939 and

1978 ______________________________ 4

1939 aerial photographs _________________ 4

1952 aerial photographs _________________ 4

1975 aerial photographs _________________ 5

Strip mining and reclamation as related to landsliding __ 6

Page

Short Creek area __________________________ BIO

Geology, soils, and their relation to landsliding ______ 10

Interpretation and analysis of earthflow elements _____ 13

Orientation _________________________ 13

Morphology and topographic form of slope

environment _____________________ 13

Triggering mechanisms __________________ 14

Asymmetrical valleys and their relation to landsliding __ 15

Summary ___________________________ 15

Source and identifying data of the aerial photographs _____ 16

References cited __________________________ 16

ILLUSTRATIONS

Page

FRONTISPIECE. Recent earthflows along slopes west of Prosperity, Pa _______________________________________ n

FIGURE 1. Index maps of Pennsylvania showing location of Washington County and selected areas _____________________ B2

2. Landslide map of part of the Harmon Creek area __________________________________________ 3

3. Photographs of the Harmon Creek area showing old landslide remnant of slumped bedrock, old slump bench adjacent to spoil

flow, and reactivated part of old slump bench ___________________________________________ 5

4. Block diagram of slump-earthflow ____________________________________________________ 5

5. Photograph showing slides from regraded spoil-bank surface _____________________________________ 6 

6-8. Stereoscopic pairs of aerial photographs showing Harmon Creek valley:

6. 1939________________________________________________________________ 7

7. 1952________________________________________________________________ 8

8. 1975________________________________________________________________ 9

9. Landslide map of part of the Short Creek area _____________________________________________ 11

10. Stereoscopic pairs of aerial photographs in the Short Creek area ___________________________________ 12 

11-14. Photographs of mass movement in the Short Creek area showing:

11. Recent earthflows along planar northeast-facing slope ___________________________________ 14

12. Head area of reactivated old slide _______________________________________________ 14

13. View from old slide surface up toward frontal lobe of recent slide ______________________________ 14

14. View down recent slide showing older hummocky surface lower on slope _________________________ 14

v

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

MASS MOVEMENT IN TWO SELECTED AREAS OF WESTERN 

WASHINGTON COUNTY, PENNSYLVANIA

By JOHN S. POMEROY

ABSTRACT

Two areas, each approximately 12 to 15 km2 (Harmon Creek and 

Short Creek), show a great amount of mass-movement phenomena 

typical of western Washington County, Pa. The colluvial slopes are 

underlain by fine-grained clastic rocks of the Conemaugh and Dunkard 

Groups of Pennsylvanian and Permian age. In the Harmon Creek area, 

the Pittsburgh coal bed underlies the hilltops. Strip mining of the coal 

bed has produced extensive spoil bank and regraded spoil-surface 

failures. In many places, slumping followed by debris flowage resulted. 

Movement of rock waste takes place both within the material and at its 

contact with the natural slope. Also, new and old landslides are abundant on colluvial slopes underlain by red mudstone units approximately 

30-80 m below the coal.

The earthflow is the dominant landslide type in the Dunkard terrain of the Short Creek area and is caused largely by natural forces. 

More than 75 percent of the recent slides took place on northwest- to 

east-facing colluvial slopes because of the combination of steeper 

slopes and greater soil moisture. The most conspicuous orientations 

are north and northeast. Recent earthflows, varying widely in their 

morphologic appearance, may be equidimensional, elongated perpendicular to the contour, or elongated parallel to the contour; they commonly take place along concave slopes.

The physical properties of clayey silty landslide material from the 

two terrains are similar. However, soils derived from the Dunkard 

Group tend to have a slightly greater proportion of expandable clays, a 

higher plasticity index, and higher clay content. Clayey silty landslide 

material from both areas is generally high in porosity but low in 

permeability and commonly slakes within a short period.

INTRODUCTION

Reconnaissance studies of the six counties constituting the Greater Pittsburgh area have shown that 

Washington County is the most susceptible to landslides. The region is part of the Allegheny Plateau section of the Appalachian Plateaus province. The 

Washington County study (Pomeroy, 1976a,b; 1977c) 

was an extension of an earlier landslide inventory in 

Allegheny County to the northeast (Pomeroy and 

Davies, 1975; Briggs and others, 1975) and was conducted in late 1975 and early 1976 after an intensive 

study of black and white vertical aerial photographs 

taken in 1975. A map showing landslides and isopleths 

was a result of the investigation (Pomeroy, 1978).

In early 1978, two arbitrarily chosen areas in western 

Washington County (fig. 1), each approximately 12 to 15 

km2 , were examined by means of ground traverses. The 

purpose of the investigation was to determine the causes 

and processes of mass movement and the distribution of 

the landsliding, especially as it related to the geology.

Each area is in a different part of the geologic section. 

The Harmon Creek area, in the northwestern part of the 

county, is mostly within the Conemaugh Group of Pennsylvanian age. Red mudstone sequences in the Conemaugh Group produce colluvial or residual soils highly 

susceptible to sliding in areas to the north and northeast 

in the Greater Pittsburgh region (Pomeroy and Davies, 

1975; Briggs and others, 1975; Pomeroy, 1977a, b) and 

to the south, southwest, and west in West Virginia 

(Lessing and others, 1976) and southeastern Ohio 

(Fisher and others, 1968).

Furthermore, the Pittsburgh coal bed crops out in the 

Harmon Creek area and has been extensively strip 

mined since the 1940's. Many slides originating from 

strip mining have compounded the hazards of naturally 

unstable slopes. Aerial photographs taken over a period 

of years from 1939 to 1975 document the formation of 

landslides resulting from strip-mining activities.

The Short Creek area in the southwestern part of the 

county was selected because it is typical of southern 

Washington County terrain. The combination of steep 

slopes and unstable colluvial or residual soils derived 

from gray to greenish-gray claystone and mudstone of 

the Dunkard Group of Pennsylvanian and Permian age 

is responsible for southern Washington County possessing the highest density of slides in the Greater Pittsburgh region (Pomeroy, 1977a). Most slides are of the 

earthflow type. Many landslides largely in Dunkard 

Group terrain in adjacent areas of Washington County 

were delineated on geologic quadrangle maps and were 

briefly discussed by Kent and others (1969) and Berryhill 

and others (1971). No attempt was made to use older 

photographs to supplement the 1975 aerial photography

Bl
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

0 50 100 150 KILOMETERS

20 MILES

10 20 30 KILOMETERS

EXPLANATION 

W Washington County 

H Harmon Creek area 

S Short Creek area

FIGURE 1.-Index maps of Pennsylvania showing location of Washington County, Harmon Creek area, and Short Creek area. The lower 

map is from U.S. Geological Survey index to 7V2-minute topographic 

quadrangles in Pennsylvania.

because the area has not undergone as much of a change 

as has the Harmon Creek area.

Figures 2 and 9 show recent and old landslides. In addition to currently active slides, the term "recent" includes those slides that are believed to have been active 

within the past century. Recent landslides may be shown 

as discrete individual bodies, but, in some places, a large 

number of closely spaced (and overlapping) slides are 

mapped as one unit.

Although old landslides are commonly represented as 

individual slides, most areas shown as old landslides 

represent a series of coalesced slides that cannot be 

distinguished from one another. Old aggregates of slides 

involve multiple periods of mass movement that 

Carbon-14 dates from old landslides at construction 

sites in the Upper Ohio valley (D'Appolonia and others, 

1967; Philbrick, 1962), indicate probably took place during the Pleistocene and postglacial times.

Because the documentation commonly is lacking, the 

separation of "recent" from "old" slides is difficult. Some 

slides mapped as "old" might actually have moved within 

the past century, but erosion and revegetation have 

masked their features. Old slides geherally lack a clearly 

definable head scarp but can be recognized by the 

characteristic hummocky ground along the lower slope.

The word "landslide" has been widely used as an allinclusive term for almost all types of slope 

measurements and is so used here. The contributors to a 

recent volume on landslides (Coates, 1977) agreed that 

movements involving falling, sliding, and flowing could 

be included as landslides.

The author acknowledges the assistance of K. S. 

Donovan, Monique Moore, and Brad Kauffman, who 

performed physical-properties tests under the supervision of S. F. Obermeier, and of Susan Nabb and Melodie 

Hess, who conducted and interpreted X-ray diffraction 

tests.

HARMON CREEK AREA

GEOLOGY, SOILS, AND THEIR RELATION 

TO LANDSLIDING

The Harmon Creek area as discussed in this report lies 

within parts of the Avella, Steubenville East, Burgettstown, and Weirton 7V2-minute Quadrangles (fig. 1) and 

is approximately 6 km west of the town of Burgettstown, Pa. The average relief is about 130 m. The part of 

the Harmon Creek area in the Avella 7V2-minute 

Quadrangle is shown on figure 2.

The Avella and part of the Steubenville East 

7V2-minute Quadrangles were mapped by Schweinfurth 

(1976) as part of a coal-investigation project in 

Washington County during the 1960's. Stratigraphic sequences in the Harmon Creek area are cyclic and include 

the Glenshaw and Casselman Formations of the Conemaugh Group (Pennsylvanian), the Pittsburgh and 

Uniontown Formations of the Monongahela Group (Pennsylvanian), and the basal part of the Waynesburg Formation of the Dunkard Group (Upper Pennsylvanian 

and Lower Permian). The Pittsburgh coal, at the base of 

the Monongahela Group, occurs near the top of hills in 

the area and has been extensively exploited. The 

bedrock is almost horizontal and dips less than 1° to the
80' 

40°22'30"

MASS MOVEMENT IN TWO SELECTED AREAS OF WESTERN WASHINGTON COUNTY, PA. 

'30' 80° 27'30'

B3

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V- <^t> -.--^m-: .>/ ^o 0& ^%va>-_.,.,.,._l;iirr^~^'^

1 MILE

5 1 KILOMETER 

CONTOUR INTERVAL 20 FEET

EXPLANATION

Area of recent landsliding—Scarp and deposit 

Old landslide—Scarp and deposit

Area completely stripped by surface mining. Includes 

recontoured (regraded) areas

FIGURE 2. -Landslide map of that part of the Harmon Creek area in the Avella 7V2-minute Quadrangle, Washington County, Pa.

southeast. Natural outcrops along the slopes above Harmon Creek are uncommon. Exposures are found along 

access (haul) roads leading to the Pittsburgh coal and in 

areas where railroads cut into the lowermost slopes.

The Conemaugh Group is poorly exposed along Harmon Creek and its tributaries. Rocks of the Conemaugh 

Group include poorly bedded variegated gray, tan, 

green, to maroon mudstone interlayered with claystone, 

shale, siltstone, sandstone, and minor limestone. The 

red mudstone units, commonly containing claystone interbeds, are of varying thickness and highly susceptible 

to landsliding where weathered. Red mudstones correlative with the "Pittsburgh red beds" of the Glenshaw 

Formation (Pomeroy and Davies, 1975; Pomeroy, 

1977a) underlie the lowest slopes of Harmon Creek and 

its tributaries in the western part of the area shown in 

figure 2. Red mudstones identifiable in stratigraphic 

position with the "Clarksburg red beds" of the 

Casselman Formation (Pomeroy and Davies, 1975; 

Pomeroy, 1977a) occupy slopes in the central and 

eastern parts of the area shown in figure 2.

The soils of the Harmon Creek area are the same as 

the unstable Gilpin-Upshur-Vandergrift soils derived 

from the Conemaugh Group of Beaver and Allegheny 

Counties to the north and northeast (Pomeroy and

Davies, 1975; Pomeroy, 1977b; U.S. Soil Conservation 

Service, 1973a, b). The Soil Conservation Service has 

recognized that slopes along Harmon Creek and some 

tributaries are largely slumped, and the word "slumped" 

is included in the description of the mapped unit (U.S. 

Soil Conservation Service, 1974a, b). Samples of red 

clayey to clayey silt soil from 11 landslide localities in 

this and and adjacent areas indicate a moderate to high 

plasticity index ranging from 8 to 31 and averaging 18 

and an average particle-size distribution of 26 percent 

sand, 47 percent silt, and 27 percent clay. X-ray diffraction analyses revealed that the clays consist of a higher 

percentage of illite-kaolinite than of vermiculite and interlayered minerals. The clayey material slakes readily 

and possesses a high porosity but low permeability.

Bedding of outcrops on some slopes (fig. 3A) is tilted 

back toward the slope at an angle much steeper than the 

regional subhorizontal dip. Local exposures that have 

steeply dipping beds are interpreted to be ancient rotational slump blocks. Other indicators of ancient landslides include hummocky ground surfaces and less obvious narrow terraced areas or benches along hillsides 

(fig. 3B). These geomorphic features represent colluvial 

materials from previous sliding episodes that are occasionally subject to reactivation (fig. 3Q; some are not
B4 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

apparent on the topographic map. Benched areas in 

forested land may be only a few meters wide (fig. 35) 

and are commonly discontinuous. Thus, some cannot be 

identified on the aerial photographs.

Characteristic of most large old landslides are their 

origins from concave (laterally and longitudinally) or 

bowl- or amphitheater-shaped slopes, which, owing to 

surface- and ground-water concentrations, are wetter 

than adjacent areas. Old landslides have also been 

detected along planar slopes on both sides of Harmon 

Creek. Distinctive bowl-shaped slopes are present above 

the Pittsburgh coal along colluvial slopes south of Hanlin 

Station (fig. 2). These features are thought to be detachment scars of old landslides.

Small (less than 15 m wide) modern-day landslides, 

mostly slump-earthflows (fig. 4) take place in soils derived from red mudstone and claystone along slopes 

bordering Harmon Creek. These landslides combine the 

process of sliding and flow. The upper part slides 

downward by rotational slip (slump); the lower part 

moves as a viscous flow (earthflow). In the past, most 

slides were probably due to natural causes (precipitation 

and stream-bank undercutting) before the area was strip 

mined. Since the strip mining, most slides have 

originated as spoil-bank failures, slumps along hastily 

constructed access roads, and regraded spoil-bank areas 

(fig. 5) adjacent to steep slopes.

Irrespective of the spoil-bank slides caused by strip 

mining and the probable old landslides above the Pittsburgh coal horizon, most recent and older sliding is concentrated in that part of the geologic section approximately 30-80 m below the Pittsburgh coal horizon 

where red mudstone is common (fig. 2). Landslides 

originating from the 0-30 m stratigraphic interval below 

the Pittsburgh coal horizon are uncommon. For example, this interval along the access road from Hanlin Station to the hilltop immediately to the north is underlain 

mostly by interbedded gray to greenish-gray mudstone, 

shale, siltstone, and minor thin-bedded sandstone. No 

small slides or slips were noticed in colluvium derived 

from this material in this locality.

Mass-movement features shown in selected aerial 

photographs (figs. 6-8) are discussed. Any area identified as an old landslide has been confirmed by field inspection. Not all features discussed are discernible on 

the aerial photographs.

INTERPRETIVE ELEMENTS OF MASS MOVEMENT BETWEEN 

1939 AND 1978

1939 AERIAL PHOTOGRAPHS (FIG. 6)

Areas marked a represent old landslides characterized 

by a hummocky surface and moderate grade on the 

lower slopes. A break in the topography commonly

marks the location of the head of an ancient slumpearthflow as revealed by the benching at an old landslide 

above the north-south road in the eastern part of the 

area shown in the figure. Some bowl-shaped slope surfaces are more diagnostic of old landslides than are 

lower slope surfaces, which may be masked by dense 

vegetation. Planar to convex old landslides (b), where 

small narrow terraces or benches are present on at least 

three distinct levels on each slide, had not been identified before the field examination.

Coal refuse dumps (c) are composed of waste from 

local deep-mining operations located near the hilltop. An 

interesting note is that, earlier in the century, mine 

waste was not dumped at the level of the mining activity 

but instead was transported to small dumps, which were 

generally stable.

The rockfall hazard (d) was present only in outcrop 

areas along man-cut slopes above the railroad. Rockfalls 

are produced by weathering and erosion, which affect 

mudstone and shale more readily than they do sandstone, siltstone, and limestone. Only a few years need 

elapse before unsupported ledges of the more resistant 

rocks break away by falling. Jointing is a significant factor.

Slides (e) were obviously caused by precipitation and 

took place in the 28-36 m interval below the Pittsburgh 

coal.

An earthflow (/) lies within an old landslide and involves approximately the same interval as that shown at 

e. Note the recent slide at g which can be seen on the 

1952 photographs (c on fig. IE).

Summary. Before extensive strip mining, this sparsely populated area was subject to landsliding in colluvium 

derived from red mudstones. Nearly all slides shown in 

this particular stereoscopic pair (fig. 6) are assumed to 

have been caused by precipitation. The lower halves of 

slides designated e and/are inferred to be underlain by 

red mudstone on the basis of a nearby measured section.

1952 AERIAL PHOTOGRAPHS

Strip mining was a radical change in land use. The obvious mine-waste slides shown on figure 7A rarely exceed 30 m in length and stop short of the hummocky less 

steep lower slopes (d) indicative of old landslides (1939 

photographs), except in extensive slide area (b). The 

slides appear to be relatively shallow and reflect a 

failure of zones within the spoil material itself or at the 

spoil material-colluvium interface. The largest individual slide exceeds 6,000 m2 . Harmon Creek has been 

diverted by the emplacement of slag and mine waste (c).

Old landslide surfaces (a) shown on figure IB can be 

seen on 1939 photographs. The surfaces of the probable 

old landslides (b) are underlain by unstable soil (U.S. Soil 

Conservation Service, 1974b) derived from the
MASS MOVEMENT IN TWO SELECTED AREAS OF WESTERN WASHINGTON COUNTY, PA. B5

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Monongahela Group. A recent slide (c) was evident in 

the 1939 photographs (g on fig. 6). Vegetation has not 

covered this slide, probably because of the periodically 

renewed soil movement.

1975 AERIAL PHOTOGRAPHS

In Figure 8A, an access road to the top of the hill 

traverses the slumped area seen on earlier photographs 

and has both directly and indirectly caused many new 

slides (a) in colluvium derived from red mudstones in the 

Casselman Formation. Obvious spoil-bank slides in 1952 

are just faintly discernible 23 years later (6). The top of 

hill has been reclaimed (regraded), and spoil material 

has slumped (c) and flowed downslope. Bedrock ledges 

examined in the field are exposed by subsequent gullying halfway down the slope. Fresh scarps in red soil 

along the hummocky lower slope (d) adjacent to the slide 

have formed because of the additional weight of the 

transported material in addition to drainage changes 

diverting water to the hummocky area. This area is one 

of several old landslide areas where benching is more apparent on the 1939 photographs before surface mining

K_* - V*

FIGURE 3 -A, Old landslide remnant of slumped bedrock. B, Old 

slump bench adjacent to spoil flow; note flattening of slope (a). C, 

Reactivated part of old slump bench; note head scarp (a) and tilting 

of trees (b) produced by slumping.

was begun. Part of an obvious 200-m-wide old landslide 

(e) has been reactivated (fig. 5, foreground) by post-1975 

surface mining. Planar slope (/") shows small recent 

slump-earthflows in red mudstone colluvium above the 

road. The dissected slope is wet where seeps appear at 

the slight break in slope (g).

Obvious flow (h) along a planar slope below a regraded 

area shown in this 1975 photograph was grass covered 

in 1978, and the head scarp was still discernible. Areas 

(i) show well-defined flow movements originating from 

the edge of the regraded surface. An old surface (7), 

though not reactivated in 1975, showed a recent wasterock flow when examined in the field in 1978. Slumping 

and flowage of regraded rock waste have taken place in 

area shown at k.

FIGURE 4.-Block diagram of slump-earthflow (from Varnes, 1978). 

HC, horizontal component; VC, vertical component; L, length. Note: 

A simple earthflow does not show slumping.
B6 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

V ^ «, ^- - v> -y -~""- 7. -.*- i*- T . -, ^ «»i

FIGURE 5.-Slides originating from regraded spoil-bank surface. 

Cracks (a) above head scarp indicate future retrogressive development. Slides in background (b) are similar to slide in foreground.

The major recent landslide at a on figure 85 took place 

after 1969, as determined by an analysis of 1969 

photographs. The slide in colluvium, largely derived 

from red mudstone, was nearly 450 m wide, as seen in 

this photograph, but has since extended several tens of 

meters to the southeast. The terminus of the slide is 

outlined by a frontal margin of partially transported 

trees inclined in several directions; small ponds and cattail marshes are behind them. Sliding of the reclaimed 

(recontoured) surface (6) has taken place above the access road. The diversion of drainage caused by the road 

to the hummocky slope of the old landslide (figs. 6, 1A) 

has contributed to extensive reactivation downslope. 

Area (c) south of the newly constructed dam, was terraced after the landsliding that took place sometime 

after 1969. On earlier photographs (figs. 6, 1A), this convex slope showed natural benches indicative of earlier 

sliding. The hummocky lower slope of largely red 

mudstone colluvium at d was locally reactivated near the 

reservoir, partly because of tree removal. Most of the 

slides sharply defined on the 1952 photographs (fig. 7A)

are no longer discernible at e. New and enlarged failures 

at/took place after regrading.

STRIP MINING AND RECLAMATION AS RELATED TO 

LANDSLIDING

We know that spoil-bank failures in the Harmon Creek 

area took place between 1939 and 1952 because 36 spoilbank slides were identified on the 1952 aerial' 

photographs. Between 1952 and 1969 only two of these 

slides were significantly enlarged, and no new slides 

took place. The 1975 photographs show extensive stripmined areas that have been reclaimed by regrading the 

spoil to the approximate former configuration of the 

land surface. Only one spoil-bank failure had taken place 

during the 1969-1975 period, but 15 slides had formed 

along the edge of the restored surface. During or after 

reclamation, two large areas of sliding (a on fig. SA, E) 

were caused by roadbuilding, which deflected drainage 

onto the sensitive slopes. Field inspection in early 1978 

revealed one new massive slide in a recently stripped 

area (e on fig. SA) and another slide in a regraded section (j on fig. 8A).

In summary, 38 slides resulted from spoil-bank failure, 

16 slides formed at the edge of the regraded spoil surface, and two large areas of sliding resulted after more 

extensive slope-cut and fill operations were made to accommodate the transportation of large earth-moving 

machinery for regrading.

An analysis of the spoil-bank slides shows that most of 

the slides took place along planar slopes (only 14 of the 

36 slides appeared along concave slopes, including heads 

of tributary valleys). All affected slopes were at least 35 

percent grade (20°), and a few were almost 80 percent 

grade (39°), the average was about 60 percent grade 

(31°). Two-thirds of the regraded spoil-surface slides, 

however, took place along concave and generally less 

steep slopes.

The influence of the underlying slope to the spoil-bank 

failures cannot be precisely determined without subsurface instrumentation. However, movement of most 

rock-waste slides is believed to have taken place within 

the waste-bank material itself or at the contact between 

the spoil and the colluvial surface of the slope. Little 

firm evidence was found to indicate that the underlying 

colluvium moved as the result of the overloading of the 

slope by rock waste. However, a few slide areas 1.0-1.3 

km west and west-southwest of Hanlin Station (fig. 2) 

include slightly hollowed-out surfaces, which imply the 

removal of subjacent slope colluvium. In this area, either 

nearby outcrops of red mudstone (less than 2 m thick) 

above manmade benches or small amounts of red soil 

within 30 m of the Pittsburgh coal horizon are indicators 

of conditions that are conducive to soil movement. Sur-
MASS MOVEMENT IN TWO SELECTED AREAS OF WESTERN WASHINGTON COUNTY, PA. B7

1 KILOMETER

FIGURE 6. Stereoscopic pair of Harmon Creek valley, Washington County, Pa., 1939. Explanation of

letters given in the text.
B8 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

O 

o

t 

jf

> 

Jxj

O 

§
MASS MOVEMENT IN TWO SELECTED AREAS OF WESTERN WASHINGTON COUNTY, PA. B9

B (APPROXIMATE SCALE) 1 MILE

KILOMETER

FIGURE 8. Stereoscopic pairs, A and B, of Harmon Creek valley, Washington County, Pa., 1975. 

Explanation of letters given in text.
BIO SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

charging of the slope by rock waste in these areas leads 

to failure.

The presence of tension cracks behind the edge of the 

spoil is the first sign of impeding failure especially where 

the cracks are more than 0.1 m wide and show obvious 

vertical displacement. The cracks serve as incipient 

failure surfaces and act as conduits for surface water 

penetrating the fill.

Increased crack widening and vertical displacement 

along cracks lead to slumping caused by slip-plane formation due to decreased shear resistance. Continued 

rotational movement further breaks up the surface, and 

small blocks are formed. Both increased precipitation 

over a long period of time and sudden intense downpours can lower shear strength of the material. The increased load and the decreased shear strength of slope 

material result not only in slumps but also in debris 

flowage onto lower slopes.

In many areas affected by mine spoil, the initial massmovement process is due to a stacking of too much 

poorly consolidated spoil material along the bench edge. 

The instability of this configuration is compounded by a 

lack of adequate surface-runoff control (Zook and Bednar, 1975; Kimball, 1974). The mining of coal beds near 

the tops of the hills has led to the removal of entire 

hilltops. The overburden has been dumped indiscriminately both into the heads of reentrant valleys 

and onto divergent slopes. Ponds are formed on the flattened spoil surfaces and contribute to the adjacent 

outslope saturation. Detailed engineering investigations, in which appropriate instruments (piezometers 

and slope indicators) and observation wells at specific 

monitoring sites are used, would help in defining future 

problem areas.

SHORT CREEK AREA 

GEOLOGY, SOILS, AND THEIR RELATION TO LANDSLIDING

The Short Creek area lies within the Prosperity 

7V2-minute Quadrangle in southwestern Washington 

County and has an average relief of approximately 120 

m. The village of Prosperity is in the extreme northeast 

corner of the area, nearly 15 km south of the city of 

Washington, Pa. Although the Prosperity 71/2-minute 

Quadrangle has been geologically mapped and mass 

movement was clearly recognized by Kent (1972), landslides were not shown because their representation 

would have obscured the bedrock map units.

The poorly exposed bedrock consists of nearly flat 

lying cyclic sequences of sedimentary rocks belonging to 

part of the Dunkard Group of Permian age. The Greene 

Formation underlies all the area shown in figure 9 except for the drainages northwest of Sparta and in the 

northeast corner of the area shown on the map, where

the uppermost part of the Washington Formation crops 

out. The Greene Formation consists chiefly of mudstone, siltstone, sandstone, and thin lenticular persistent beds of limestone, claystone, shale, and coal. 

Regionally, bedding dips less than 1° toward the 

southeast (Kent, 1972).

Soils having a moderate to high shrink-swell potential 

are present over most of the area and have been included in the Guernsey-Culleoka series (U.S. Soil Conservation Service, 1974b). Although individual soil-slip 

symbols are sparingly used on the Soil Conservation 

Service photo maps, extensive slumped areas as wide as 

3.5 km (the slope south of Short Creek) are denoted. The 

present study generally confirms the interpretation 

shown on the soil maps.

The Washington County isopleth map (Pomeroy, 

1978) shows 40-70 percent of the Short Creek area 

covered by landslide scars and landslide deposits. Landslides take place commonly in colluvial or residual clayey 

to clayey silt soil and highly weathered rock derived 

from mudstone, claystone, limestone, and shale. 

Samples of landslide material from the Short Creek and 

adjacent areas indicate a moderate to high plasticity index, ranging from 10 to 28 and averaging 21; average 

particle-size distribution is 22 percent sand, 46 percent 

silt, and 32 percent clay. X-ray diffraction studies 

revealed that the clay consists of, in decreasing order of 

abundance, illite, vermiculite, kaolinite, and interlayered minerals. The clay slakes rapidly and is 

characterized by relatively high porosity and low 

permeability.

Earthflows and slump-earthflows (fig. 4), the dominant landslide types in the area (fig. 10), consist of soil 

and weathered rock that are rarely more than 3 m thick. 

Evidence of slumping is common in the head area. 

Earthflows are crescent-shaped, rectangular, or oval 

bodies often found along sloping pasture land and less 

often in forested areas. Earthflows range in size from a 

few meters to several tens of acres; most are less than a 

quarter of an acre. Scarps of earthflows may heal in less 

than 10 years, but the hummocky slopes indicate centuries of former slope movements. Sharpe (1938, p. 55) 

observed that many hills in eastern Ohio, West Virginia, 

and western Pennsylvania are "scarred by thousands of 

old earthflows, and almost the entire surface of certain 

slopes show the typical scarps and hummocks of former 

movements."

Unmappable small-scale slumping is typically expressed in the form of "cattle terraces" or "catsteps" produced by animals' hoofs cutting into the hillslope. Scars 

are produced on the uphill side of repeatedly used paths. - 

These features are significant in the distribution of surface runoff to various parts of a slope and influence the 

siting of larger slides.
MASS MOVEMENT IN TWO SELECTED AREAS OF WESTERN WASHINGTON COUNTY, PA.

80°19'30" 18'30" 80°17'30"

Bll

40°02'30" -

Area of recent landsliding—Scarp 

and deposit

40°00'30" -i

Outline of old landslide—Scarp and 

deposit

Area covered by stereoscopic pairs 

with figure numbers

— — Position of Claysville Limestone 

Member of Greene Formation 

(Kent, 1972)

Locality referred to in text

0 1MILE 

J

0 .5 1 KILOMETER 

CONTOUR INTERVAL 20 FEET

FIGURE 9. -Landslide map of part of the Short Creek area, Pa.
B12 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

1 KILOMETER

FIGURE 10. - Stereoscopic pairs of the Short Creek area, Washington County, Pa. A, Terrain 

west of Prosperity, Pa. Note the affinity of recent active earthflows (a) for concave 

northwest- north- northeast- and east-facing slopes. Note abrupt terminal margins (b) of 

old landslides. B, Short Creek Valley. Note the asymmetric valley formation. Many 

episodes of old to recent sliding have occurred along the steep north-facing slope bordering the east-trending Short Creek. Recently active earthflows (a) commonly show a 

strong preference for wetter north- to east-facing slopes. Note the recent slides on all 

configurations of slope surfaces. Note the frontal margins of old slides, especially along 

Short Creek.
MASS MOVEMENT IN TWO SELECTED AREAS OF WESTERN WASHINGTON COUNTY, PA. B13

Most failure surfaces in earthflows appear to originate 

at or close to claystone beds that might underlie a 

limestone or be part of a mudstone sequence along a 

15-40 percent grade slope (9°-22°). Commonly, slopes 

show an overlapping sequence of earthflows largely controlled by a cyclical repetition of claystones. Kent (1972), 

Kent and others (1969), and Berryhill and others (1971) 

recognized the correlation of landslides with largely impervious limestone. Seeps are common at the contact 

between an impermeable unit and overlying permeable 

siltstone or sandstone. Thus, stratigraphy controls the 

wetting of lower slopes and the occurrence of earthflows. Impervious layers are affected by a marked increase in pore-water pressure during saturation. The increased pore-water pressure decreases shear strength, 

and earthflows or slumps are triggered. Flowage commonly ensues as a lobe or sheet that has a bulge at the 

lower end and cracks forming over the surface of the 

mass. An increase in tension upslope from the lower end 

of the hummocky surface might cause slippage at the approximate contact of impermeable clayey material with 

more permeable silty to sandy soil. Rotational slumping 

results at least in part of, but is generally confined to, 

the head area of the newly formed slide. Sharpe and 

Dosch (1942) made comparable observations in describing the mechanics of creep and earthflow on the basis of 

a series of borings near New Philadelphia, Ohio.

The delineation of the Claysville Limestone Member of 

the Greene Formation as mapped by Kent (1972) shows 

in many places a distinct relationship with the upper 

margins of landslides in the northern part of the area 

(fig. 9). Unmapped lower and higher limestone beds 

underlain by claystone elsewhere locally approximate 

the upper part of other slides (fig. 11). However, most 

earthflows take place along concave slopes characteristically mantled by varying thickness of colluvium; 

therefore, only inferences can be made regarding 

lithologic relationships in these areas.

INTERPRETATION AND ANALYSIS OF EARTHFLOW 

ELEMENTS

ORIENTATION

More than 75 percent of the recent slides took place on 

northwest- north- northeast- and east-facing slopes; the 

most conspicuous orientations are north and northeast 

(figs. 9, 10). Areas of old landsliding greater than 1 km 

in length are present on north-facing slopes above Short 

Creek, Hamestring Run, Lick Run, the east-west 

drainage west of the village of Prosperity, Pa., and an 

unnamed drainage 1.5 km southwest of Prosperity.

An examination of the topography for these drainages 

reveals that the steepest slopes are those that face 

north. The slopes are covered mostly by forest growth,

and, because they are steep, only parts of the slopes 

have been farmed as pasture or cropland. Less steep 

south-facing slopes have been extensively farmed and 

show fewer mass-movement features.

Several factors contribute to the high incidence of 

mass-movement features on north-facing slopes. Slopes 

facing north receive less exposure to the sun; after a 

rain, soils there remain wet longer than soils on southfacing slopes. East-facing slopes, of course, receive 

early-morning sunlight, but the drying effect on these 

soils is ineffective because of the low temperatures at 

that time of the day. Snow cover, obviously, lingers 

longest on slopes facing northwest to east. Conversely, 

higher rates of evaporation are found on south-facing 

slopes. The only consistently unstable south-facing slope 

areas are along laterally concave parts, typically at the 

heads of tributary drainages. Although recent slides on 

south-facing slopes are relatively uncommon, hummocky lower slopes (fig. 10) indicate periods of mass 

movement. Hummocky south-facing slopes are much 

more areally restricted than are north-facing slopes.

The combination of steeper slopes and greater soil 

moisture accounts for the higher incidence of mass 

movement on north-facing slopes. Presence or absence 

of trees is less important as a controlling influence on 

slope stability than are either slope steepness or soil 

moisture. Similar landslide and slope-exposure relationships are discussed by Van Buskirk (1977) for northeastern Ohio, Beaty (1956) for California, Harden 

(1976), and Colton and Holligan (1977), both for Colorado. Beaty (1956) observed that 70 percent of the 

earthflows east of Berkeley, Calif., took place on slopes 

facing north and east. Though Hadley (1961) did not 

discuss landsliding, he did mention comparable slope 

characteristics in the intermontane west.

MORPHOLOGY AND TOPOGRAPHIC FORM OF SLOPE 

ENVIRONMENT

In the Short Creek area, nearly 40 percent of the recent earthflows are approximately equidimensional. 

More than one-third are elongated perpendicular to the 

contour, and more than one-fourth are parallel to the 

contour. Most earthflows parallel to the topographic 

contour take place along planar to convex slopes, 

whereas most earthflows perpendicular to the contour 

lie within concave slopes or coves. Equidimensional 

forms are found in a variety of slope settings.

Both recent and old landslides have an affinity for concave slopes (longitudinally as well as laterally) (fig. 9, 

10). Patton (1956) noted that recent earthflows were 

more common in coves or concave slopes than on other 

slope types. Both ground and surface water tend to concentrate along concave slopes. Unstable surface conditions are created where clayey soils that have at least a
B14 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

^^^^vj^y^i^l

14

FIGURES 11-14:

11.-Recent earthflows along planar northeast-facing slope. Thin limestone beds are present at a.

12.-Head area of reactivated old slide. Note head scarp (a) and evidence of slumping.

13.-View from old slide surface up toward frontal lobe of recent slide. Frontal lobe (a) is approximately 1-1.5 m high. Relief of gullies (6) 

increases lower on slope.

14. -View down recent slide showing older hummocky surface lower on slope. *

moderate shrink-swell ratio are present. A lack of outcrops and a relatively thick accumulation of highly 

weathered material are characteristic of these "scoopedout" areas.

Kelsey (1978, p., 364) mentioned large earthflows in 

northern California that have bowl-shaped depressions 

in their upper parts. Amphitheater-shaped basins, 

formed by former episodes of mass movement, were 

noted by Nolan and others (1976) in the same region.

In the Short Creek area, more than 60 percent of the 

recent slides have formed along concave slopes. More 

than 25 percent and 12 percent occur on laterally planar 

and convex slopes, respectively. Slides can form on convex slopes that are better drained than adjacent slopes.

Such occurrences are due to the inherent instability of 

the soil, which, although thinner on noses of slopes, is 

still susceptible to sliding if minimal moisture conditions 

exist. Waltz (1971) discussed the significance of concave 

and convex slopes and their role in reflecting topographic control of surface runoff in two San Francisco 

Bay area counties. In West Virginia, Lessing and others 

(1976, p. 36) determined that 60 percent of their sampled slides took place along concave slopes.

TRIGGERING MECHANISMS

Man's influence in the distribution of present-day landslides in the Short Creek area is relatively minor when 

contrasted with mass movement in the Harmon Creek
MASS MOVEMENT IN TWO SELECTED AREAS OF WESTERN WASHINGTON COUNTY, PA. B15

area or in urbanized Allegheny County (Briggs and 

others, 1975). In the Short Creek area, most of the mass 

movement has resulted from naturally unstable conditions. Prolonged periods of precipitation during late 

winter to early spring while the ground is thawing has 

been conducive to the formation of most earthflows. 

Short periods of intense rain that often take place during the warmer months of the year have had less effect 

on earthflow genesis. Oversteepening of slopes by 

stream erosion takes place at the point of maximum curvature of meander bends. Because the slopes have been 

undercut, slides often result. Overdip slopes l (Briggs, 

1974) do not control the siting of landslides in this area. 

If bedrock structure played a role, then landslides should 

be most pronounced along southeast-facing slopes on 

the basis of the structure contours (Kent, 1972).

A prime example of an area subjected to recent sliding 

due to precipitation alone is shown at locality 1 (fig. 9). 

An arcuate head scarp approximately 50 m in length is 

present along a forested concave slope. Tilted trees imply some rotational movement along an uneven surface 

marked by small ponds (fig. 12). The width of the slide is 

greater than its length. Below the prominent frontal 

lobe is a dissected, less steep forested area (15 percent 

grade) that represents the toe of an older more extensive earthflow (fig. 13). A small outcrop near the base of 

one of the older frontal lobes approximately 4 m in relief 

is almost completely masked by the colluvium of the 

earlier deposit.

Clearing of forested slopes for pasture is a factor in 

the reactivation of parts of steep north-facing slopes 

south of the village of Sparta, west of the village of Prosperity, and, locally, in the southeastern and southwestern parts of the area shown on the map.

At locality 2 (fig. 9), man's influence is apparent, as 

shown by the failure of roadfill in a concave-slope area. 

The area is part of an old landslide deposit, revealed by 

the hummocky and gullied forested surface seen in 

figure 14. Repeated attempts to stabilize the slope 

traversed by the paved road have failed. The author's 

observation indicated that surface water had not been 

diverted from this sensitive area.

ASYMMETRICAL VALLEYS AND THEIR RELATION TO 

LANDSLIDING

North-facing slopes that are steeper than opposing 

south-facing slopes are present in the Short Creek area 

(figs. 9, 10) and adjacent quadrangles in southern 

Washington County. These asymmetrical valleys are 

similar in form to valleys described in southern Indiana 

(Wayne, 1967; Culbertson, 1900; Newson, 1903), south1 An overdip slope is defined as a land surface sloping in approximately the same direction as, 

but more steeply than, the dip of the strata.

western Wisconsin (Judson and Andrews, 1955), 

western Wyoming (Walker, 1948), northwestern Alaska 

(Curry, 1964), eastern Wyoming (Hadley, 1961; Melton, 

1960), southern California (Emery, 1947), New Jersey 

(Bascom, 1909), southern Ohio (Russell, 1924), and 

Virginia (Hack and Goodlett, 1960). Because of significantly greater insolation, the south-facing slopes are affected by more frequent freeze-thaw cycles. The slope 

gradient is reduced through weathering and sheet erosion. If the colluvial debris derived from the south-facing 

slopes and accumulated in an east-west channel cannot 

be effectively removed, the channel will shift its course 

toward the south side of its valley. Such a dynamic 

change leads to the undercutting of the north-facing 

slope (Wayne, 1967; Melton, 1960).

Erosion of the toe of the north-facing slope, commonly 

during flooding, results in its oversteepening and 

enhances the instability of the slope. During the early 

spring months colluvial parts of these slopes are 

extremely sensitive after a prolonged period of precipitation on an already saturated, probably snow-covered 

surface. The added weight of the soil combined with the 

increase in pore pressure and shearing stress is compounded by a decrease in frictional resistance and cohesion of the silty to clayey material. Sliding during or 

after a wet period commonly results.

The steepness and wetness of the north-facing slopes 

thus provide the optimum conditions for creep and 

earthflow activity. The same processes are markedly 

less conspicuous on the less steep and drier south-facing 

slopes where weathering and sheetwash are more characteristic. Hack and Goodlett (1960, p. 41) noted these 

features in their study of asymmetry in the Little River 

area of Virginia.

By contrast, the Harmon Creek valley (fig. 2) to the 

north shows no pronounced asymmetry. Wayne (1967, 

p. 401) reported that valleys occupied by large streams 

in southern Indiana show little or no asymmetry. The 

west-flowing Harmon Creek probably removed debris 

originating from south-facing slopes more effectively, 

and as a result channel shifting and undercutting of the 

north-facing slope were negligible. Then in the waning 

stages of Wisconsin glaciation, swollen waters of Harmon Creek steepened slopes wherever meanders occurred, regardless of the orientation of the slope. As a 

result, no pronounced asymmetry is apparent.

SUMMARY

Colluvium derived from fine-grained clastic rocks of 

the Conemaugh and Dunkard Groups is highly susceptible to landsliding in Washington Country.

In the Short Creek area of the southwestern part of 

the county, earthflows commonly take place in clayey to 

clayey silt soil and highly weathered rock derived from
B16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

mudstone, claystone, limestone, and shale of the 

Dunkard Group. Asymmetrical valleys that have steep 

and moist north-facing colluvial slopes are conducive to 

mass movement.

To the north, red mudstone sequences in the Conemaugh Group correlative with similar strata in the Pittsburgh area underlie colluvial slump-earthflows in a wide 

zone roughly 30-80 m below the Pittsburgh coal bed. 

Strip-mining development has compounded the natural 

landslide environment in the Harmon Creek area. In 

many places, spoil-bank failures, slides along access 

roads, and mass movement of regraded rock waste have 

resulted.

SOURCE AND IDENTIFYING DATA 

OF AERIAL PHOTOGRAPHS

Figure 6.-U.S. Department of Agriculture, Agricultural Adjustment

Administration. APT-92, 92-93. 

Scale 1:20,000. Focal length of camera lens, 21 cm. 

Date of photograph: June 5, 1939

Figure 7. A, B. -U.S. Geological Survey. GS-UG-4, 90-91. 

Scale 1:23,600. Focal length of camera lens, 15 cm. 

Date of photograph: June 6, 1952.

Figure 8 A, B.-A, U.S. Geological Survey. GS-VDWD-3,219-220;

B, U.S. Geological Survey GS-VDWD-3, 266-267. 

Scale 1:24,000. Focal length of camera lens, 15 cm. 

Date of photograph: December 7, 1975.

Figure 10.-U.S. Geological Survey GS-VDWD-3, 52-53. 

Scale 1:24,000. Focal length of camera lens, 15 cm. 

Date of photograph: December 7, 1975.

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