Landslide Hazards in Vermont 

U.S. GEOLOGICAL SURVEY BULLETIN 2043 

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Landslide Hazards in Vermont 

By CHARLES A. BA~KERVILLE, FITZHUGH T. LEE, and 

CHARLES A. RATTE 

Prepared in cooperation with the 

Vermont Geological Survey 

A discussion of recent and historic landslides in Vermont, 

detailing hazards, processes, and slope movement in numerous 

soil and rock types 

U.S. GEOLOGICAL SURVEY BULLETIN 2043 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names 

in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Government. 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1993 

For sale by 

U.S. Geological Survey, Map Distribution 

Box 25286, Bldg. 810, Federal Center 

Denver, CO 80225 

Library of Congress Cataloging in Publication Data 

Baskerville, Charles A. 

Landslide hazards in Vermont I Charles A. Baskerville, Fitzhugh T. Lee, Charles 

A. Ratte 

p. cm.-(U.S. Geological Survey bulletin ; 2043) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:2043 

1. Landslides-Vermont. I. Lee, Fitzhugh T. II. Ratte, Charles A. Ill. Title. 

IV. Series. 

QE75.B9 no. 2043 

[QE599.U5] 

557.3 s-dc20 

[551.3'07] 92-23611 

CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Natural Slope Processes in Soil and Bedrock 1 

Mass Movement of Soils and Debris 1 

Mass Movement of Bedrock 1 

Human-Induced Slope Instability 2 

Active Geomorphic Processes Contributing to Instability 3 

Acknowledgments 3 

Historical Record of Slope Movement in Vermont 3 

Research Investigations 5 

Movements of Soil Materials on Slopes 6 

Soil Types in Vermont 6 

Types of Soils in which Slope Movements Occur in Vermont 6 

Natural and Human-Induced Slope Movements in Vermont Soils 8 

Differing Rates of Movement in Soils in Vermont 9 

Debris Avalanches 10 

Slope Remediation 11 

Slope Movements in Bedrock 12 

Conditions that Promote Rock-Slope Movement 12 

Types of Structural Discontinuities in Rock 13 

Rock-Slope Movement Types 13 

Rockfall 13 

Block Slides 14 

Topples 14 

Slope Movements on Vermont's Interstate Highways 16 

History and Construction of the System 16 

General Geology 18 

Design, Design Changes, and Construction of Rockcuts 18 

Analysis of Rockcuts 20 

Rockcut Slope Failures 20 

Results of the Phase I Project 22 

Conclusions 22 

References Cited 22 

FIGURES 

1, 2. Photographs showing: 

1. The intensity of jointing in the Cheshire Quartzite 2 

2. An example of a vertical joint set with a second set intersecting at a large 

angle 2 

3. Map showing locations of soil landslide sites described in the text 4 

4, 5. Photographs showing: 

4. A, The 1984 debris flow on East Mountain at Mendon, looking downslope; 

B, The same slide area, looking upslope, from U.S. Route 4 a year later 6 

5. A rhythmite sequence at North Springfield flood-control reservoir 7 

Contents Ill 
IV Contents 

6. Diagram illustrating a typical rhythmite sequence 7 

7-12. Photographs showing: 

7. Shoreline slope failure on Lake Champlain near Vergennes 8 

8. A slide being repaired on East Mountain, Mendon 9 

9. A debris avalanche scar from failure on August 10, 1976, on Dorset 

Mountain 10 

10. The riprap berm used to add weight to the toe of a slope on a road 

shoulder after a slide west of Bridgewater Corners on U.S. Route 4 12 

11. The Andover Brook where it has undercut its bank in front of this 

house 12 

12. The corrugated vertical drain used at Killington Mountain 12 

13. Map showing locations of some rock-slope movements in western 

Vermont 14 

14. Generalized section through Smugglers Notch showing rockfalls and debris 

accumulation at the base of the slope 14 

15-17. Photographs showing: 

15. The 6,000-ton King Rock on Vermont Route 108 in Smugglers 

Notch 15 

16. Block slide at White Rocks, near Wallingford, south of Rutland 15 

17. Damage to trees caused by blocks of Cheshire Quartzite tumbling down 

the dip slope 15 

18. Diagram showing slope deformation in Cheshire Quartzite at Mad Tom Brook, 

looking west 16 

19. Map showing the location of Interstates 89 and 91 and the periods of their 

incremental construction and locations of rockslides and rockfalls 17 

20. Generalized geologic map of Vermont 19 

21-25. Photographs showing: 

TABLES 

21. The rockslide of May 4, 1985, along the southbound roadway of 

Interstate 89 at South Barre 20 

22. The Palisades rockcut at Fairlee 21 

23. A vehicle damaged by the March 9, 1987, rockfall at location 5 shown in 

figure 19 21 

24. The scar of the October 10, 1968, rockslide near Interchange 6 

(Rockingham) on Interstate 91 21 

25. The scar and resulting debris of the April 30, 1977, rockslide along the 

southbound cut face of Interstate 91 at Fairlee 22 

1. Types of discontinuities in rock causing failure of slopes 13 

2. Criteria used in the 1977-78 analysis of rockslide-rockfall potential along 

Vermont's Interstate Highway System 20 
Landslide Hazards in Vermont 

By Charles A. Baskerville,1 Fitzhugh T. Lee, 2 and Charles A. Ratte3 

Abstract 

Landsl ides are one of the major geologic processes 

that cause change on the Earth's surface. Many landslides 

in Vermont involve unconsolidated materials such as 

colluvium, various glacial materials, and glacial lakebed 

deposits. Naturally occurring slides are common along 

riverbanks as part of the process of stream erosion. 

Landslides, including debris slides, debris flows and 

avalanches, earth slumps, and a variety of wet soil flows, 

develop in many areas because of the presence of unconsolidated materials. Slides can also result from human 

activity. For example, improper design of cut slopes for 

highways can give rise to slope failures. Two hundred 

years ago (1783), Vermont landslides were made famous in 

newspaper accounts of devastating spring flood events. 

Such slope movement can cause economic losses as well 

as loss of life. 

The mountainous areas of Vermont are underlain by 

fractured bedrock that typically has a very thin soil cover. 

The nature and orientation of joints, faults, foliation, and 

bedding, which are called geologic discontinuities, are the 

main factors in rock-slope stability. In addition, several 

natural factors such as precipitation and freeze-thaw 

action play an important part in rock-slope stability, as 

well as does topography. The predominant types of rockslope failures in Vermont are rockfalls, block slides, and 

topples . Although the majority of falls involve a small 

quantity of rock, they typically occur without warning. 

Debris avalanches triggered by rockfalls have occurred in 

western and central Vermont, where they typically take 

place on south-facing slopes in foliated metamorphic 

rocks. 

Design and construction considerations for rockcuts 

in Vermont's Interstate Highway System (1-89 and 1-91) 

were constrained by economics. Construction began in 

1956 and was completed in 1982. Consideration of rock 

structure was not taken into account in the design of 

slopes that had cut rocks. 

Manuscript approved for publication, July 16, 1992. 1

U.S. Geological Survey. Present address: Department of Physics 

and Earth Sciences, Central Connecticut State University, 1615 Stanley 

St. , New Britain, CT 06050--4010. 2

U.S. Geological Survey. Present address: 211-2 Bluffs Dr. , Kala 

Point, Port Townsend, W A 98368. 3

State Geologist, State of Vermont, Agency of Natural Resources, 

103 South Main St., Center Building, Waterbury, VT 05676. Present 

address: Department of Geological Sciences, Bowman Hall, University of 

Kentucky, Lexington , KY 40506-0059. 

INTRODUCTION 

The primary agents required to trigger slope 

movement processes are gravity and precipitation (Hamblin, 1989). In Vermont's climate and with its glaciated 

mountainous terrain, these ingredients are frequently 

present. Steep slopes promote gravitational acceleration. 

Heavy winter snows are accompanied by spring melting; 

heavy rains occur in the spring and fall. 

As the pressure for development of potentially 

unstable areas increases, it becomes critical that slope 

movements and their causes are analyzed and explored. 

When slope stability problems in Vermont are being 

addressed, determination of the engineering characteristics 

of the various soil and rock types present in the differing 

geomorphic settings is paramount. 

Natural Slope Processes in Soil and Bedrock 

Soils. -The use of the term "soil" in this report is in the 

engineering sense; that is, soil consists of unconsolidated 

material such as clastic sediment, weathered bedrock, rock 

fragments, and organic matter (Varnes, 1978). This term is 

synonymous with earth. 

Debris. -This term refers to material containing a relatively 

high percentage of coarse fragments (Varnes, 1978). 

Mass Movement of Soils and Debris 

Many types of mass movement on slopes involve soil 

and debris, including debris slides, debris flows, earth 

flows, and debris avalanches. Debris avalanches at times 

may be carried on a cushion of air and (or) water. Except for 

dry flows, the rapidity of flow depends on water content and 

slope angle (Varnes, 1958). 

Mass Movement of Bedrock 

Bedrock.- Discontinuities control the stability and the 

failure mode of bedrock slopes. "Discontinuity" in this 

report refers to bedding in sedimentary rocks, foliation in 

metamorphic rocks, and simple to complex joint sets in 

various rock masses. 

Introduction 
Figure 1. The intensity of jointing in the Cheshire Quartzite 

facilitates failure by allowing gravity as well as normal 

freeze-thaw action to open the joints more rapidly. 

Mass movements in bedrock include (1) rock topples, 

which occur when tall, slender blocks of rock fall forward 

because their center of gravity is high- a block of this type 

will rotate about its lowest contact edge (forward edge) 

(Hoek and Bray, 1977); (2) rotational rock slumps, which 

occur when forces cause a backward rotation about a point 

above the center of gravity; this movement is extremely 

slow to moderate; (3) block slides, which occur when there 

is translational movement of a coherent mass of rock along 

a relatively planar surface; (4) lateral spreads, which are 

movements in a fractured mass distributed as a lateral 

extension; and (5) rockfalls and rockslides, which may be 

moderate to rapid in movement (Varnes, 1978). A rockfall 

is a fall of a newly detached mass from an area of bedrock, 

and a rockslide is movement consisting of shear strain and 

displacement along a surface or surfaces that may be visible 

or inferred (Varnes, 1978). Rockfalls can trigger debris 

avalanches (Baskerville and others, 1988; Jibson, 1989). 

Failure of a jointed bedrock slope commonly occurs 

where two intersecting joint surfaces emerge on a slope face 

to form a V -shaped notch (fig. 1). Hydrostatic pressure 

along joint surfaces can reduce the effective stress and cause 

sliding in the rock mass bordered by these joints. 

Toppling failures develop when planar discontinuities 

are about vertical and subparallel. The rock column thus 

created is prone to fail, especially where another set of 

planar surfaces intersects the vertical surfaces at some large 

angle (fig. 2). Hydrostatic pressure and frost wedging will 

help push away the failing column or block from the main 

rock mass. 

2 Landslide Hazards in Vermont 

Figure 2. An example of a vertical joint set (upper right) 

with a second set intersecting at a large angle (subhorizontal). Toppling failure would occur in the direction of 

dip of the subhorizontal joint set. This site is on U.S. 

Route 7 near East Dorset. 

Human-Induced Slope Instability 

Although many landslides are due to natural geomorphic processes, slope instability can also be aided by the 

actions of humans. Causes of human-induced failures 

recognized in Vermont include insufficient design of stormwater drainage systems, improper construction and 

maintenance of logging roads, and inadequate design of 

slope cuts made for highways. 

About 520 km of four-lane Interstate Highways (1-91 

and I-89 plus short links) were built in Vermont between 

1956 and 1982. During the early years of Interstate Highway construction, the general emphasis was on keeping 

excavation and right-of-way costs at a minimum (El Ashmawi and Greenfield, 1972). There was little experience in 

slope-cut design for highways, particularly in strongly 

foliated, heavily fractured, steeply dipping metamorphic 

rocks. Slope stability considerations in Vermont during the 

1950's were in their infancy (Sharpe, 1989). By the early 

1970's, Federal highway safety specifications were enforced 
in Vermont when the government required widening of all 

pre-1971 rockcuts to a 9-m clear zone from the edge of the 

pavement (Vermont Department of Highways, 1971). 

Active Geomorphic Processes Contributing to 

Instability 

Whether the failing slopes are rock or soil, the active 

processes include factors that generate high shear stresses 

and low shear strength (Varnes, 1958). One factor 

generating high shear stress is removal of lateral support. 

This loss of support can come about by stream erosion or, 

as along the shores of Lake Champlain, erosion by waves 

and associated longshore currents. High shear stresses can 

also come about by construction of highway rockcuts, by 

adding unevenly distributed weight to subsoil as a result of 

building highway embankment fills, and by earthquakes. In 

contrast, low shear strength in soil results from such factors 

as increased moisture content. In rock, low shear strength is 

attributed to weathering and discontinuities. Along discontinuities such as joints, faults, and cleavage, high porewater pressure due to rainfall or snowmelt can also develop, 

and this pressure decreases strength of rocks. 

Acknowledgments 

We thank the Vermont Agency of Transportation for 

their cooperation in supplying data on slides affecting 

various highways. Special appreciation is extended to Roy 

Nickelson, former Director of the Materials Division, and 

Frank Lanza, former Chief Geologist of the Vermont 

Agency of Transportation, for their suggestions. We are 

also grateful for the constructive peer reviews given this 

manuscript by Gerald Wieczorek and Eleanora Robbins of 

the U.S. Geological Survey. 

HISTORICAL RECORD OF SLOPE MOVEMENT 

IN VERMONT 

While researching historical data as far back as 1783 

to learn about slope failures in Vermont, we discovered that 

the term "landslide" or any related terms were not used in 

accounts earlier than 1796, although widespread damage 

from floods is mentioned. For example, in Spooner's 

Vermont Journal, May 27, 1796, the following account 

appeared: "In the afternoon of Sunday, the 15th inst. [sic] 

was experienced at the upper part of this state, the most 

uncommon shower of rain ever known in this country . . . 

many mill dams and bridges were carried away, and roads 

injured materially." 

Additional historical examples describe a significant 

amount of damage due to storms. (See fig. 3 for site 

locations.) It was noted that the words "landslide'' and 

"slide" began to appear after the first quarter of the 19th 

century: 

July 22, 1811 (Spooner's Vermont Journal, August 12, 

1811): 

In Ira [site 1, fig. 3], the inhabitants were not exempt 

from the general calamity. The waters in the east part 

of the town carried the most awful destruction over a 

beautiful tract of land of many hundred acres, and in 

many instances totally destroyed the soi~ .... A 

number of once beautiful farms now exhibit a dreary 

waste of rocks and flood wood .... Where once 

were meadows and fields of grain, nothing but huge 

rocks and trees, torn from hills and mountains, are 

now to be seen .... The mass of rocks and trees 

which were brought from the mountains, is 

wonderful to behold. Trees which measured one, two 

and three feet through, are strewed over hundreds 

and hundreds of acres of the once fertile plains ... 

and in some instances piled up 20 and 30 feet high. 

Also, rocks which probably weighed several tons, 

covered the rich and fertile vale of several hundred 

acres. 

June 3, 1827 (Monthly Weather Review, July 1897, U.S. 

Department of Agriculture, Weather Bureau): 

Warren, Vt. [site 2, fig. 3] ... about 100 acres slid 

from Fayston Mountain down to the valley beneath. 

Circa 1870 (no day or month given) (Rutland Herald, 

March 24, 1920): 

Near the northern opening of the [Smugglers] Notch, 

and about four miles from Carlton's hotel in 

Cambridge [site 3, fig. 3] ... is a large rock about 60 

feet in circuit, which became detached from its parent 

bed far up the mountain side about 50 years ago, and 

came thundering down the valley, laying waste the 

giant trees and whatever chanced to be in its 

downward course, producing a rumbling not unlike 

an earthquake, which heard far off, and shook the 

hills for miles around. 

July 1897 (Monthly Weather Review, July 1897, U.S. 

Department of Agriculture, Weather Bureau): 

The landslide of the present month [Wednesday, july 

14, 1897] had been preceded by heavy rains, but it 

seems to have been started by the fall of an immense 

boulder near the top of the mountain [Mt. Ellen] in 

the northwest of Warren township at an altitude of 

about 3890 feet [site 8, fig. 3]. Simultaneously, another 

slide began at a point a little south of this boulder; the 

two slides joined together after a path of about 100 

rods, and the combined avalanche continued not only 

Historical Record of Slope Movement in Vermont 3 
I 

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Figure 3. The locations of soil landslide sites described in 

the text. A few cities are shown directly on the map; other 

locations are shown by number and listed below. 

1 Ira 

2 Warren 

3 Cambridge 

4 Brownsville 

5 Vergennes 

6 Hardwick 

7 Topsham 

8 Clay Brook 

9 East Dummerston 

10 East Middlebury 

11 Fayston 

12 Barre 

13 White River Junction 

14 Ludlow 

15 Mendon 

4 Landslide Hazards in Vermont 

16 West Bridgewater 

17 Northfield 

18 Rochester 

19 Bolton 

20 Waterbury 

21 Springfield 

22 St. Johnsbury 

23 Woodstock 

24 Hartland Four Corners 

25 North Hartland 

26 Marlboro 

27 lrasville 

28 Castleton 

29 Chester 

30 Killington Mountain 

31 Waitsfield 

three miles eastward to the foot of the mountain, but 

a mile farther over comparatively level ground 

.... The foot of the slide now covers an area of 

nearly a quarter of a mile square ... about 1300 to 

1478 feet above sea level, making an actual fall for the 

whole mass of about 2400 feet. The slide apparently 

ran down the ravine occupied by Clay Brook and 

landed at its mouth in the valley of the Mad River. 

April 1, 1920 (Rutland Herald, April 1, 1920): 

... moving out of the ice jam opposite this village, 

which caused two slides on the Boston and Maine 

railroad ... the roadbed north of the junction of the 

Connecticut and West Rivers slid out for a distance of 

150 feet making a gulch 30 feet deep and leaving the 

track suspended in mid-air. A similar slide occurred 

north of the Dummerston station [site 9, fig. 3], being 

100 feet long and 20 feet deep. 

March 8, 1922 (Rutland Herald, March 8, 1922): 

Heavy flooding as temperatures go to 20 degrees 

above freezing for two days ... rain and melting 

snow send all streams over their banks in torrents ... 

railroads and trolley tracks were inundated along with 

roads and low lying fields in East Middlebury [site 10, 

fig. 3] and Rutland and Fayston [site 11, fig. 3]. A 

landslide from a bank near Fort Ethan Allen covered 

the tracks of the Central Vermont railroad on March 7 

(at night). 

April 13, 1922 (Rutland Herald, April 13, 1922): 

... after two days of rain over all of Vermont ... 

landslides hinder railroad traffic ... bridges out, 

trains derailed ... mountain streams overflowing 

their banks ... many miles of highways damaged 

... damage in hundreds of thousands of dollars. 

November 3-7, 1927 (Rutland Herald, excerpts cover 

November 3-7, 1927): 

... seven inches of rain in less than 18 hours .... 

Property damage in Rutland was estimated at 

$500,000 .... In Bennington [fig. 3] water reached 

the second floor of some homes .... Nine lost their 

lives in Barre [site 12, fig. 3]. At least 8 fatalities were 

reported by November 5 .... The Lt. Governor 

perished in flood in Montpelier [fig. 3] where waters 

were 20 feet deep .... Damage in the Connecticut 

River Valley between St. Johnsbury [site 22, fig. 3] and 

Bellows Falls was estimated at $1,000,000 .... A 

reservoir collapse in Montpelier was said to have 

killed 137 to 212 people .... Damage at White River 

junction [site 13, fig. 3] estimated at $1,500,000. Water 

rose 30 feet in the river in 12 hours killing 3 .... A 

landslide on Mt. Tabor [site 5, fig. 13] carried rocks 

and timber down slope .... Montpelier and Barre 
flood losses $4,000,000 .... Many landslides 

occurred near Ludlow [site 14, fig. 3] and in the 

Connecticut River Valley .... Landslides occurred in 

Mendon Woods between West Bridgewater [site 16, 

fig. 3] and Rutland. From Cary's Mills down Rochester 

Mountain to Rochester [site 18, fig. 3], landslides 

covered the roads .... Northfield [site 17, fig. 3] on 

the Dog River, Moretown and other towns in the Mad 

River Valley along with Bolton and Waterbury in the 

Winooski Valley suffered heavily from the disastrous 

flood .... 3000 cattle drown in Winooski Valley ... 

over 50 persons killed .... Entire hills from Bolton 

[site 19, fig. 3] to Waterbury slid into the river .... A 

landslide occurred on Lincoln Mountain about two 

miles from Warren [site 2, fig. 3]. Landslides also 

occurred between Northfield and Roxbury along with 

washouts. 

April 11, 1934 (Rutland Daily Herald, April 11, 1934): 

Montpelier-Winooski River rose to the high water 

mark by 10 P.M .... Gale driven rainstorms caused 

thousands of dollars worth of damage throughout 

Vermont .... Landslides and washouts blocked 

main highways .... Ground was still frozen and 

unable to absorb the water. Heavy snow fell in the 

mountains .... A landslide cut off the highway 

between Springfield [site 21, fig. 3], Vermont and 

Charlestown, New Hampshire .... 

In 1934, construction began on the Winooski Valley 

flood control project by the Civilian Conservation Corps 

(CCC) and the Army Corps of Engineers to ease damage 

from floods of the 1927 proportions (Rutland Daily Herald, 

October 18, 1934). The 1927 floods and slides took 

numerous lives and did $13,500,000 in property damage. 

This project included the Wrightsville Dam (completed in 

1935) on the North Branch, a tributary to the Winooski 

River, at the northern outskirts of Montpelier (fig. 3). The 

North Branch flows through the heart of Montpelier joining 

the Winooski within a few blocks of the State Capitol. The 

flood control dams in the Winooski basin "tamed the 

roaring waters" (Rutland Daily Herald, March 13, 1936). 

But on March 13, 1936, torrential rains, landslides, 

and avalanches killed three people, and two others drowned 

in other wild streams. In the same month, a large snowslide 

blocked the road between St. Johnsbury and Newport (fig. 

3). By March 19, flood deaths in Vermont had increased by 

another five. There were washouts and frequent landslides 

between Bridgewater and Woodstock (site 23, fig. 3) in the 

Ottauquechee Valley. The flood waters were higher during 

these 1936 events than in 1927. Food shortages and 

spoilage were big factors in 1936 due to electric power 

failure that shut down the use of refrigerators. Spoilage of 

food in 1927 was not a factor because refrigeration was with 

ice (Rutland Daily Herald, March 19, 1936). According 

to then State Works Progress Administration (WPA) 

administrator, Harry W. Witters, overall damage was less 

in 1936 than 1927 because of construction of hard surface 

roads and new bridges since 1927 (Rutland Daily Herald, 

March 19, 1936). In the spring of 1936, Congress 

appropriated $13,373,000 to construct a system of 10 

reservoirs on the Upper Connecticut River in New England. 

These funds came out of a Senate Commerce Committee 

bill for $305,000,000 for flood control projects in the 

recently flooded Eastern States. 

This short digest of landslides and floods in Vermont 

covers 150 years from the late 18th into the early 20th 

century. With the amounts of precipitation and flooding 

described, some slope failures also undoubtedly occurred. 

The slope movements probably did not affect the valley 

dwellers to the same extent as did the floods. The floods 

that devastated Connecticut Valley communities during the 

1920's and 1930's led to construction of numerous flood 

control projects by the Army Corps of Engineers on many 

Connecticut River tributaries. While these structures have 

solved the major flooding conditions, potential for landsliding remains. For example, in 1976, and again in 1984, 

slides occurred on East Mountain in Mendon, (site 15, fig. 

3) and blocked U.S. Route 4 (fig. 4). 

Research Investigations 

We conducted research on slope-movement problems 

in the State of Vermont from 1983 through 1989. One of the 

goals of this work has been to determine the dominant 

processes that activate these slope failures. We took 

measurements of some of the slope failures so we could 

begin to understand the magnitude of these processes. 

These measurements included length and width of various 

categories of landslides that included soil. Auger holes were 

also drilled in landslides and logged. These data were used 

to determine the volume of material in each slide. 

Additionally, we performed field tests, such as the vane 

shear and penetrometer, to determine engineering 

characteristics of the materials. Both disturbed (shovel) 

samples and undisturbed (pressed in Shelby tubes) samples 

of landslide materials were taken. These samples were used 

for such tests as grain-size distribution and unconfined 

compressive strength. X-ray analyses were performed on 

many of these samples to determine the amount and types of 

clay and other minerals in the slide masses. This 

information assisted in the determination of whether certain 

clays may have contributed to slide movement and whether 

the clay minerals are prone to absorbing large volumes of 

water (Baskerville and Ohlmacher, 1988; Ohlmacher and 

Baskerville, 1991). In the study of bedrock failures, we 

applied dial-gage and tape extensometer instrumentation to 

some of the rock slopes in areas where potential for failures 

was suspected, including Smugglers Notch, White Rocks 

Recreation Area, and Mad Tom Brook (Lee, 1989). 

Historical Record of Slope Movement in Vermont 5 
Figure 4. A, The 1984 debris flow on East Mountain at 

Mendon, looking downslope; clear area in background is 

pavement of U.S. Route 4. This slide developed when a 

breach occurred across a logging road upslope. Water 

courses higher on the mountain were diverted to this 

breach by continued erosion. B, A view of the same slide 

area, looking upslope, from U.S. Route 4 a year later. The 

artificial drainage structures blend in well with the natural 

drainage. 

The instrumentation was used to determine the rates 

of movement on discontinuities, such as joints, and the 

mechanisms that control these movements (Baskerville and 

others, 1988; Lee, 1989). We also sampled many of the 

rocks for chemical analysis to determine which major 

oxides were in the various rocks and in what percentages. 

These data, along with petrographic study, helped in the 

determination of the susceptibility of a particular rock unit 

to decomposition, especially along discontinuities. 

MOVEMENTS OF SOIL MATERIALS ON SLOPES 

Soil Types in Vermont 

Residual soils, which commonly form over bedrock 

in situ by a long period of chemical decomposition, are 

uncommon in Vermont. Instead, many soils in Vermont are 

the result of glacial transport and deposition and are 

generally unrelated to the underlying bedrock (Doll and 

others, 1961; 1970). Glacial tills that are quite compact 

underlie many stream valleys throughout the State. Some 

areas such as in the granitic terrains of northeastern 

Vermont contain clean ablation tills. These are loosely 

6 landslide Hazards in Vermont 

combined glacial rock material that has been winnowed of 

fines as the ice disappeared by ablation and evaporation in 

place. Other glacial deposits are kames and thick till 

deposits, both of which form terraces in many river valleys. 

All of these sediments are stratified, being crossbedded if 

formed in deltas or rhythmitic units if formed in glacial 

lakes (fig. 5). Rhythmites are cyclical deposits composed of 

fine sands, silts, and clays (Bates and Jackson, 1980). 

Types of Soils in which Slope Movements Occur 

in Vermont 

As the continental glaciers retreated during the late 

Pleistocene, the meltwaters formed ice-marginal lakes 

(Hamblin, 1989). Various sedimentary deposits are 

associated with these lakes, such as varves, rhythmites, and 

deltas. Some glacial lake deposits are prime examples of 

potentially unstable materials. Typically, the stratigraphic 

section where rhythmitic sediments occur contains thick 

deposits of crossbedded deltaic sands and fine gravel 

overlying the rhythmite unit of alternating beds of silt and 

clayey silt. The rhythmite sequence generally overlies a 

coarse gravel and boulder ablation till or kame deposit. The 
Figure 5. Rhythmite sequence at North Springfield floodcontrol reservoir. Slickensides (arrow) from where the 

material slid on the scarp can be seen in the extreme 

upper left. 

top of the entire sequence may or may not have a cap of 

Holocene colluvium (fig. 6). 

Such soil profiles as described above are present 

along terraces in many Connecticut River basin stream 

valleys in Vermont, such as the Black River valley along 

Vermont Route 11 in Springfield and the White River valley 

along Vermont Route 14 near Sharon (Baskerville and 

Ohlmacher, 1988). Springs fed by base flow from streams 

on terrace surfaces can lead to failures by increasing the 

pore-water pressure in the less clayey layers of rhythmites. 

Increased pore-water pressure reduces effective normal 

stress and triggers slope failures. This was the failure 

mechanism on Vermont Route 11 in Springfield (Baskerville and Ohlmacher, 1988). Where these terraces occur 

within reservoirs, failure can develop during rapid drawdown of reservoir water. In this process, the rapid drawdown suddenly lowers the water to normal pool level and 

leaves the water table perched high in the reservoir slopes. 

The saturated slopes begin to fail initially in the rhythmites, 

aided by the differential pore-water pressures in the silt and 

clayey silt layers. These conditions developed in 1984 over 

extensive areas of the perimeters of the reservoirs of the 

North Springfield and the North Hartland flood-control 

dams (Baskerville and Ohlmacher, 1988). 

High pore-water pressure caused by prolonged 

precipitation helps to generate sliding in glacial lakebed 

terrace deposits. These deposits almost always have a 

rhythmite unit 2.4 or more meters thick in the upper 

one-third of the terrace . Clayey layers that alternate with 

coarser silt layers in these rhythmites prevent rapid 

infiltration of heavy precipitation. Increases in pore-water 

EXPLANATION 

Arrows follow groundwater f low path and 

pressure direction 

Draped 

col luvium 

Rhythmites 

Roc k 

Figure 6. A typical rhythmite sequence. Streams often cut 

channels parallel to the rock wall. The water feeds into the 

more permeable layers, approximately following the baseflow vector arrows shown, and thus increases pore-water 

pressure. Slope failures are triggered because effective 

normal stress is reduced by the increased pore water. 

pressure occur in two places: at the contact of the rhythmite 

with overlying crossbedded deltaic or colluvial deposits and 

in the silt and fine sand layers within the rhythmite. 

Deposits of basal till along streambanks are also 

subject to slope movement by both bank undercutting and 

saturation of the clay-rich components. When these clays 

become saturated, the basal portion of the mass becomes 

fluidized and sets the stage for sliding (Ohlmacher and 

Baskerville, 1991). Till is a heterogenous mixture of 

boulders, gravel, and sand-sized material in a matrix of 

clayey silt. It generally is very dense and has a relatively 

high shear strength. The types of failures in till range from 

block slides to earth flows. These types of slope failures are 

frequent in the Clay Brook and Mill Brook Valleys near 

Warren (site 2, fig. 3). Similar failures occur along Brook 

and Holbrook Hill Roads between Hartland Four Corners 

and Brownsville in the town of West Windsor (sites 4 and 

24, fig. 3). 

An example of a naturally occurring slide in such tills 

is in the town of West Windsor (site 4, fig. 3). These 

compact tills have very high shear strength. Natural 

moisture content, 5 percent or less in these deposits, is so 

low as to make them almost dry. The clayey silt matrix has 

its shear strength reduced drastically when saturated (Baskerville and Ohlmacher, 1988; Ohlmacher and Baskerville, 

1991). 

Some soil deposits are on steeply dipping bedrock 

slopes. Heavy prolonged rainfall causes high pore-water 

Movements of Soil Materials on Slopes 7 
pressures that trigger sliding on bedrock surfaces (see table 

1 in Baskerville and Ohlmacher, 1988). The deposits on 

Brook Road are an example of this. Ohlmacher and Baskerville ( 1991) have determined that high clay content of 

these soil masses is not a prerequisite for movement; water 

pressure buildup in the pores of fine sand and silt layers 

restricted by less permeable layers appears to be the primary 

triggering mechanism. 

Natural and Human-Induced Slope Movements 

in Vermont Soils 

Natural slope failures in Vermont are often triggered 

by abnormally intense and prolonged rainfall. In the spring, 

snowmelt adds to the problem by presaturating the soil 

(Baskerville and Ohlmacher, 1988; Baskerville, 1990). 

These conditions also aid in the creation of torrential 

streamflows that undercut banks and cause instability 

(Baskerville, 1990). Storms and high water levels have 

combined to cause coastal erosion and some slope failures 

on the Vermont shore of Lake Champlain. Along the lakeshore, Vermont Route 125 near Chimney Point State Park 

and the Blake Lawrence estate near Vergennes (fig. 7), are 

examples of this. 

Some of the human-induced slope instabilities are 

related to logging practices, such as clearcutting on some 

slopes, or to poor maintenance and abandonment of logging 

roads. Examples of these failures are near Hardwick and in 

Mendon (fig. 4A). In the case of the Hardwick slide, at least 

150 ha (hectares) were clearcut. This site is in the Lamoille 

River valley. It appears that without the forest cover for 

protection, heavy precipitation was able to infiltrate and 

saturate a rhythmic substrate that is composed of clayey silt 

alternating with more clay-rich layers. 

Ritter ( 1986) indicated that as much as 50 percent of 

the precipitation is intercepted by a forest canopy. Without 

a canopy, the added water at Hardwick infiltrated into the 

ground and created a saturated fluidlike zone upon which a 

massive movement of the entire tract occurred, similar to 

slides at North Springfield (Ohlmacher and Baskerville, 

1991). 

Following the 1984 failure, the Hardwick site had an 

extremely heavy rate of runoff that was concentrated into 

channels that were about 1 m wide and deep. Additionally, 

tension fractures both surrounded and crosscut the slide 

mass. On August 2, 1984, vane shear tests were taken on 

the scarp faces of some of the fracture zones. Torvane tests 

gave maximum shear strength values of 49 kN/m2 (kilonewtons per square meter), indicating a low-strength 

material. On July 31, 1985, Shelby tube samples (VT -10 

and VT-11) were obtained from substrate material in a 

tension fracture. The material at the very base was wet but 

too thin to sample; the estimated unconfined compressive 

strength of this material is approximately 20 kN/m2 (see 

8 Landslide Hazards in Vermont 

Figure 7. Shoreline slope failure on Lake Champlain near 

Vergennes. Both soil and bedrock failed. The timber 

cribbing has been placed in an attempt toward stemming 

further failures. 

table 2:6 in Sowers, 1979). Torvane and penetrometer 

(unconfined shear strength) tests gave results of 354 to 462 

kN/m2 and 216 to 236 kN/m2

, respectively. These results 

show that these slide materials have relatively high strength 

when drained. 

Examination of the undisturbed samples showed that 

this substrate material contained no coarse particles such as 

pebbles. The undisturbed samples also showed many large, 

jagged, primary void spaces; most of these spaces were 

oriented normal to the axis of the cylindrical soil samples. 

These void spaces appear to be pull-apart tension fractures 

spaced a few centimeters apart, possibly a result of ground 

movement. 

Results from unconfined uniaxial compression tests 

on samples VT-10 and VT-11 indicate that, without 

saturation, this section has a reasonably high strength. If the 

total precipitation falls directly on the soil without the 

intervention of vegetation, surface runoff and infiltration 
will become the only methods of water dissipation and will 

help to decrease soil strength. 

The 1984 Mendon slide was generated following 

erosion of a logging road high upslope on East Mountain. 

This road was built to haul logs and carry drainage east or 

west to existing stream channels and off the mountain. 

Through time, without maintenance, this road developed a 

transverse breach that allowed the drainage to develop a 

new channel. This new channel ran directly downslope, and 

subsequently a debris flow developed along this new track 

(Baskerville and Ohlmacher, 1988; fig. 4). 

Another example of human-induced sliding 

developed in Topsham at the upper end of Powder Spring 

Brook valley (site 7, fig. 3). The slide mass measured 40 m 

in length and 23 m in width at about midlength. The slope 

angle was 3r. At one time, the toe area of the slide was a 

source of soil borrow. The excavation of the pit destabilized 

the slope above. During a series of heavy rains, which 

supplied excess pore pressure, the slope above the pit face 

failed. 

Clearcutting and poor maintenance are not the only 

causes of human-induced slope failures. Other causes are 

overly steep cut slopes and slopes cut where the bedding or 

foliation of rock dips toward the roadway. 

Differing Rates of Movement in Soils in Vermont 

Free-draining coarse materials may present slope 

stability problems as exemplified on the south side of 

Vermont Route 9 near Woodford, just east of Bennington 

(fig. 3). At the Woodford site, the slope contains glacial 

deposits composed mostly of coarse quartzite. These 

deposits range from gravels to boulders on a slope that 

stands about 60 to 90 m higher than the road. This slope 

continually shows signs of creep or landslide movement, 

particularly in wet weather. 

An initially slow-moving slide occurred near 

Vermont Route 9 on Whetstone Brook near Marlboro (site 

26, fig. 3). Heaving played a part in the slow slope motion 

of the Whetstone Brook slide. Heaving is usually an integral 

part of creep motion on slopes in that the forces disrupting 

the mass operate by expansion normal to the surface (Ritter, 

1986). This greatly facilitates gravitational motion downslope. The headscarp at the top of the Whetstone Brook 

slide measured 12 m in height, and the slide surface 

contained disrupted vegetation. Anecdotal evidence from 

the auto repair garage owners across the brook from the 

slide suggests that small annual movement of the mass has 

been occurring since 1971. Examples of slides that slowed 

to creeping movement occurred in Mill Brook valley west 

of Irasville (site 27, fig. 3). Trees on some slopes of the 

brook exhibit a pipestem structure, which is a curvature at 

the base of the tree trunks. These pipestem structures 

suggest that the slope had initially undergone a rapid 

Figure 8. Slide being repaired on East Mountain, Mendon. 

The slide chute has carried debris onto U.S. Route 4 

several times. Regrading and the placement of drains to 

move excess water flow have mitigated the danger 

associated with this slide. 

translational slide, which tilted the trees downslope. Later, 

the movement slowed enough for tree trunks to realign 

vertically toward sunlight. 

Examples of rapidly moving flow slides are those that 

have occurred in glacial till on the north slope of East 

Mountain on U.S. Route 4 east of Rutland (fig. 3). At one 

of the East Mountain sites, a large slide recurred in the 

spring of 1983 (fig. 4A). This debris flow developed after 

heavy spring rains and completely blocked U.S. Route 4. 

Fortunately, no vehicles were passing the site at the moment 

of this event. Records show that damage was restricted to 

the highway and adjacent property. The Vermont Agency of 

Transportation has alleviated this problem by installing 

drains to reduce the water pressure in the slope. The slide 

chute was also lined with large riprap to prevent further 

erosive degradation of the channel (figs. 4B and 8). 

Extremely rapid slide and flow movements can 

exceed 3 m/s (Varnes, 1978). The Dorset Mountain debris 

avalanches in north-central Bennington County (see site 6, 

Movements of Soil Materials on Slopes 9 
Figure 9. Debris avalanche scar from failure on August 10, 

1976, on Dorset Mountain. This was one of at least four 

separate landslides that occurred on the south-facing 

slopes of this mountain (site 6, fig. 13). Several thousand 

cubic meters of soil, detached joint blocks of rock, and at 

least 40 ha of trees rushed downslope along runout paths 

up to 4.2 km in length. These slope failures were preceded 

by several days of rain that culminated in a heavy 

cloudburst (Ratte and Rhodes, 1981). 

fig. 13) of 1976 illustrates high-velocity flows. This 

avalanche covered a distance of 3 km in less than 5 min 

traveling at an average rate of 10 m/s. Four almost 

simultaneous debris avalanches from Dorset Mountain 

destroyed 40 ha of trees (Ratte and Rhodes, 1981). 

Debris Avalanches 

Debris avalanches are long, narrow slope failures 

made up of masses of bedrock, soil, and, in some instances, 

trees that flow downslope (fig. 9). Failure is sudden, and 

the debris avalanche rapidly gains momentum, adding mass 

during its descent. This type of mass movement is relatively 

common in western Vermont, where many scars attest to 

their occurrence. Of the various types of rock-slope failures 

recognized in Vermont, debris avalanches are by far the 

most destructive to forests, roads, and streams. Debris 

avalanches obviously have a potential for causing 

considerable damage to mountainside developments. Field 

studies have identified several conditions that favor the 

initiation of a debris avalanche. Foliated metamorphic 

rocks, mainly chlorite-muscovite phyllites and fine-grained 

schists, are the predominant rock types associated with 

these failures. The foliation of these rocks when related to 

failures is typically horizontal or has gentle dips down 

toward the roadway . Well-developed, open, vertical joint 

10 Landslide Hazards in Vermont 

sets strike about parallel and perpendicular to the axes of 

debris chutes in and next to the headwalls. These joints 

truncate foliation surfaces, and the result is a zone of 

loosened blocks that average 1 m in thickness. Soil cover is 

thin to nonexistent on upper slopes next to slide chutes and 

increases to 1 m in thickness midway down the slope. 

Bedrock, where exposed in the avalanche scars, is fresh, 

and joints are closed. Headwalls of debris avalanches are 

formed as steep ( 40° to 90°), south-facing cliffs or benches 

located below flatter, more stable mountaintops. The 

middle and lower portions of debris avalanches have 

concave-outward topography suggesting that they may 

occupy old drainage paths that are perhaps related to the 

foliation. Vertical drops of over 600 m and slide chute 

lengths of more than 3 km are on record (Ratte and Rhodes, 

1981). Avalanche debris contains abundant angular blocks 

of fresh bedrock having a wide range of sizes. Historical 

records give a sense of the weather conditions before a 

failure. Typically, several days of rain culminate in an 

intense local downpour of 6 to 8 em of rain in 3 to 4 hours. 

This type of storm occurs most frequently in the late spring 

and summer months. This timing coincides with many 

documented debris avalanches such as at Mt. Greylock in 

western Massachusetts, in the eastern Adirondacks of New 

York, and at other places in eastern North America 

(Eschner and Patrie, 1982). 

A vivid account of a debris avalanche that occurred 

on June 27, 1827, near Waitsfield in Washington County 

(fig. 3), was given by Theron Baldwin (Baldwin, 1829, p. 

228-232): 

On the 30th of june, I went, in company with sixteen 

of my neighbors, to visit the spot so singularly marked 

by Providence, which I am now about to describe. I 

found the slide to commence near the top of the 

mountain, between two large rocks which were 

stripped of earth, opening a passage of four rods 

wide, from it proceeded in a south easterly direction, 

gradually widening for a distance of two hundred 

rods, to the south branch of Mill Brook in Fayston. In 

its course it swept every thing in its way; overturning 

trees by their roots, divesting them of roots, branches 

and bark, and often breaking them in short pieces. A 

number of rocks, judged to weigh from fifteen to 

twenty tons were moved some distance. From where 

it entered Mill Brook, its course was in a north 

easterly direction, two hundred and eighty rods, the 

natural course of the brook being very small; but the 

channel cut by this torrent is now from two to ten 

rods in width; and on either side are large quantities 

of flood wood piled up very high; and from fifteen to 

twenty rods of the lower part it is blocked up across 

the channel in every direction; some of the trees are 

standing on their tops, and generally stripped of 

roots, branches and bark, and broken into many 
pieces. The pile in some places is ten feet high. Much 

of the timber is apparently buried several feet in sand 

and mud. One large tree was broken off square, 

measuring three feet nine inches where it was 

broken. One black ash was literally pounded into a 

broom, whose brush is seven feet long. The whole 

distance of these ravages, is a mile and a half, and the 

quantity of land thus suddenly metamorphosed into a 

barren waste, is twenty five acres. The force of water 

must have been very great, at which, we cannot 

wonder, when we consider its probable depth. In 

some places, from appearances, it must have been 

thirty or forty feet high, and there was mud on them 

at that height. Heavy rains for several days preceded 

this debris avalanche, which produced a roar that 

could be heard for miles, and was thought by some to 

be an earthquake or a clap of thunder. 

The preceding description is probably accurate 

because the effects of similar destructive slope movements 

were seen by the authors on Dorset Mountain (see fig. 13) 

where a debris avalanche occurred after several days of 

rainfall on August 10, 1976. These rains culminated in a 

cloudburst that produced 8 to 10 em of rainfall in a 24-hour 

period (Ratte and Rhodes, 1981). Similar conditions have 

quite likely occurred at many times and places in the past. 

Slope Remediation 

When development on relatively steep slopes cannot 

be avoided, priority should be set on a determination of the 

potential for movement by analysis of the types of materials 

composing the slopes. Materials encountered at construction sites should be sampled, and their laboratory and 

engineering characteristics should be determined. 

Studies of natural drainage systems are needed at new 

construction sites. These studies will determine the slide 

susceptibility of materials; susceptible materials can then be 

prevented from becoming saturated and losing shear 

strength. Artificial drainages associated with construction 

should complement the natural drainage so that conditions 

for a potential slope failure do not develop. 

Some slopes may have a history of intermittent creep, 

such as those along streambanks where the trees show 

pipestem structures. These streambank slopes may be in 

equilibrium because of their vegetative cover. If a 

determination is made to develop such slopes, it is 

imperative that the project not disturb the existing 

vegetation. Artificial drainage installed during construction 

must be properly maintained afterward. As much 

vegetation as possible should be replaced after construction 

so a change in surface-water runoff and infiltration regimes 

can be prevented (Greenway and others, 1984). Changes in 

the surface-water regime also affect the subsurface water 

table. When both the surface and subsurface water regimes 

are altered, slope failure can result. 

Several slope failures have occurred in the Castleton 

River valley between Castleton and Rutland as a result of 

construction in the 1980's. Two shallow slides occurred on 

new U.S. Route 4 while construction was in progress. A 

complex slide developed on old Route 4A to the west of 

those cited on new Route 4 (site 28, fig. 3). One of the U.S. 

Route 4 slides occurred in 1985 during construction of a cut 

slope on the section between West Rutland and U.S. Route 

7 south of Rutland. In these cases, overconsolidated slope 

materials (basal tills) failed. These tills became unstable 

when saturated. The slopes of 26° or more failed in both 

rotation and translation. 

In applying corrective procedures to the landslide at 

Mendon, the placement of artificial drainage structures 

complemented the natural drainage. These corrective 

procedures on East Mountain have worked very well (fig. 

4B). 

Regrading to a flatter slope has been a remediation 

strategy used in Vermont on failed slopes. Other measures 

include installation of vertical and horizontal drains in 

slopes to relieve the buildup of pore-water pressures, and 

the placing of a heavy stone or massive concrete block berm 

on the toe of slopes to prevent further movement. Many 

failed slopes are beyond rehabilitation without very high 

costs. Sometimes, even after the remedial efforts, the 

results are not permanent. 

The U.S. Army Corps of Engineers regraded and 

revegetated as remedial steps for failed slopes in the North 

Springfield flood control reservoir. Subsequently, high 

water levels in the reservoir generated further failures on 

some of these repaired slopes (Baskerville and Ohlmacher, 

1988). 

Adding riprap to the toes of failed slopes can stabilize 

highway cut slopes in soils (fig. 10). Riprap was added as 

bank protection for a house on Andover Brook west of 

Chester (fig. 11). Such steps aid in preventing headward 

continuation of failure in stream channels, although there is 

no guarantee that failure will not occur on adjacent channel 

ground. 

A debris flow occurred just south of Windsor during 

heavy rains in May 1984 on U.S. Route 5 and blocked the 

highway (Baskerville and Ohlmacher, 1988). The Agency 

of Transportation placed a heavy stone counterweight berm 

along the toe of the slope to try to prevent a future landslide. 

The intent of this berm is to stabilize the toe of the slope and 

prevent further slope movement. 

Perforated vertical corrugated drain is also used to 

lower the soil pore pressure (fig. 12). This procedure 

reduces the potential for sliding on a slope composed of 

glacial till on Killington Mountain (site 30, fig. 3). Slope 

failures occurred in tills along the East Creek and Ottauquechee River drainage basins, on East (site 15, fig. 3) and 

Killington Mountains, respectively. Systems of horizontal 

Movements of Soil Materials on Slopes 11 
Figure 10. Rip rap berm used to add weight to the toe of 

slope on a road shoulder after a slide west of Bridgewater 

Corners on U.S. Route 4. This procedure is used as an aid 

toward preventing further movement. 

Figure 11. The Andover Brook has undercut its bank in 

front of this house. Rip rap has been placed on the slope in 

the hope that future undercutting and loss of the house 

will be prevented. 

and vertical drains placed in these slopes are effective 

methods for reducing further movement. This procedure 

was developed in Japan (Hoek and Bray, 1977; Yamaguchi, 

1980). 

12 Landslide Hazards in Vermont 

Figure 12. Corrugated vertical drain used at Killington 

Mountain. This device helps to lower water pressure in 

the slope so as to reduce sliding potential. Horizontal 

drains farther downslope, connected to the bottom of the 

vertical drains, carry the excess water out of the slope. 

SLOPE MOVEMENTS IN BEDROCK 

In this report, bedrock denotes consolidated earth 

material that underlies unconsolidated or so ill ike materials. 

Bedrock is not necessarily strong or durable. On the other 

hand, the strength and deformation properties of bedrock 

commonly far exceed those of soils or glacial deposits. 

Soil cover in the mountainous parts of Vermont is 

typically very thin, particularly at higher elevations. 

Bedrock, or ledge, in various parts of the State differs 

considerably in hardness, color, grain size, and mineral 

composition. However, these properties are not the most 

significant in an analysis of the stability of rock slopes in 

Vermont. 

Conditions that Promote Rock-Slope Movement 

As noted by Piteau ( 1970), the basic parameters of 

interest in understanding rock-slope behavior are (1) the 

systems of joints and other discontinuities, (2) the 

relationship of discontinuities to potential failure surfaces, 

and (3) the strength parameters of discontinuities, which 

include properties of both the joint surface and any infilling 

material. Besides the intrinsic features of the rock mass, 

several external factors are important for analyzing rockslope stability. The most important of these are 

precipitation, freeze-thaw cycling, and earthquakes. 

Precipitation is critical because additional water may 
Table 1. Types of discontinuities in rock causing failure of 

slopes 

[Definitions are from the American Geological Institute's Glossary of 

Geology (Gary and others, 1972)] 

Joint .................. A surface of actual or potential fracture or 

parting in a rock, without displacement; the surface is usually planar and 

often occurs with parallel joints to 

form part of a joint set. 

Tension joint ... ..... A joint that is a tension fracture. 

Shear joint .......... A joint that is a shear fracture; a potential 

plane of shear. 

Sheeting structure .... The type of fracture or jointing formed by 

pressure-release jointing or exfoliation. 

Fault ............... . .. A surface or zone of rock fracture along 

which there has been displacement, 

from a few centimeters to a few 

kilometers in scale. 

Shear zone ........... . A tabular zone of rock that has been 

crushed and brecciated by many parallel 

fractures due to shear strain. 

Bedding surface ....... A surface, usually conspicuous, within a 

mass of stratified rock, representing an 

original surface of deposition; the surface of separation or interface between 

two adjacent beds of sedimentary rock. 

If the surface is more or less regular or 

nearly planar, it is called a bedding 

plane. 

Foliation .............. A general term for a planar arrangement 

of textural or structural features in any 

type of rock. Cleavage in slate or 

schistosity in a metamorphic rock, for 

example. 

increase hydrostatic pressures on the joints and thus 

decrease stability. Freeze-thaw is known to have displaced 

joint blocks in the Green Mountains and as a result to have 

caused rockfalls (Lee, 1989). Earthquakes have triggered 

landslides in many parts of the world and are responsible for 

much damage annually; however, in Vermont earthquakes 

are infrequent and mild, so this cause of slope failure is 

rare. 

Types of Structural Discontinuities in Rock 

The term "discontinuity" is applied to those geologic 

structural features, such as joints, faults, and bedding 

surfaces, that divide the rock mass into individual blocks. 

Researchers in many countries agree that most failures of 

rock slopes are controlled by such geologic structures 

(Schuster and others, 197 5; Goodman, 197 6; Tharp, 1984). 

A list of types of discontinuities in rock-slope studies and 

their definitions is given in table 1. Terzaghi (1962) 

demonstrated the function and importance of discontinuities 

to the maintenance of stable slopes. Terzaghi calculated the 

critical height, He, of a vertical cliff of relatively weak, 

intact (free of discontinuities) rock as being approximately 

1,290 m: 

H =qu=1 290 c 'Y ' 

where the unconfined compressive strength, qu, = 34.5 

MPa and the unit weight, -y, is 2,720 kg/m3 x 9.8 m/s2 . 

Even for stronger, intact hard rocks such as granite, no 

vertical faces as high as 1,290 m have been found. 

Countless numbers of nonvertical slopes of much lesser 

height have failed. Therefore, the critical height of rock 

slopes is largely related to the structural discontinuities of 

the rock and not the strength of the rock itself. In contrast, 

the slope behavior of soils, and especially of relatively soft 

soils, depends upon the properties of the intact soil almost 

independent of discontinuities in the soil. In hard rock, the 

path of failure follows preexisting surfaces or zones of 

weakness, and such is the case for rock-slope failure in 

Vermont. 

ROCK-SLOPE MOVEMENT TYPES 

Rock-slope movements in Vermont are mainly rockfalls, rock avalanches, block slides, and topples. 

Combinations of these failure modes occur at some 

locations. Representative of all the types of movement 

mentioned above, figure 13 shows the locations of several 

rock-slope movements in western Vermont. 

Rockfall 

In rockfall, a detached rock mass falls from a steep 

slope or cliff. Blocks of rock can fall, bounce, and roll 

down the slope, as shown in figure 14. Falls are the most 

rapid type of rock-slope movement, although they are 

usually preceded by minor movements leading to 

progressive separation of blocks from the main mass. 

Rockfalls account for the smallest volumes of rock of any of 

the major kinds of rock-slope movements in Vermont. 

While they typically occur in the upper, steeper slopes of 

mountain ranges, they also occur on steep slopes at lower 

elevations (sites 1, 3, and 5, fig. 13). On July 13, 1983, in 

a rockfall at Smugglers Notch, one or more blocks of schist 

fell that are estimated to have weighed more than 10,000 

tons. The joint surfaces that bordered these blocks may have 

acted as conduits for water. The cumulative effects of 

repeated freezing and hydrostatic pressure caused block 

displacements leading to the instability of this cantilevered 

overhanging mass (Baskerville and others, 1988). This rock 

bounded upon loose rock, soil, and vegetation and triggered 

a debris slide. There are many such unstable blocks in this 

part of the Green Mountains. In 1910, a block called King 

Rock dislodged from a high valley wall in Smugglers Notch 

Rock-Slope Movement Types 13 
Figure 13. Locations of some rock-slope movements in 

western Vermont. Numbers indicate sites referred to in 

the text: 1, Smugglers Notch; 2, Mill Brook; 3, Mount 

Horrid; 4, White Rocks Recreation Area; 5, Mount Tabor; 

6, Dorset Mountain; 7, Mad Tom Brook. 

and tumbled intact to the valley floor. King Rock has an 

estimated weight of 6,000 tons (fig. 15). Some rockfalls 

that occur during periods of heavy rain may fall or tumble 

onto saturated soil and unconsolidated glacial deposits and 

trigger a debris avalanche. Curiously, the 1983 Smugglers 

Notch event occurred during a relatively dry period. 

Block Slides 

Block slides are commonly controlled by defined 

joints that are usually at some large angle to bedding or 

foliation surfaces. These slides occur on slopes that are 

close to the angle of stability for the particular block of rock 

(for quartzite, this is 45° to 60°). If blocks slide over a step 

14 Landslide Hazards in Vermont 

West 

(/) 

0: 

UJ 

1-

UJ 

2: 

740 

East 

640 rz~~~&a~~~t212~~~~ 

Figure 14. Generalized section through Smugglers Notch 

(site 1, fig. 13) showing rockfalls and debris accumulation 

at the base of the slope. The open joints observed at 

several locations at the top of many slopes indicate that 

significant horizontal displacement, probably caused by 

frost action, occurs at early stages in the rockfall process. 

The collision of a large block with a relatively thick 

vegetated soil mat downslope may trigger a debris 

avalanche. 

or bench, they may tumble and develop rotational 

momentum. An excellent example of block slides, which 

exhibited both translational and rotational movement, is at 

White Rocks Recreation Area, 29 km south of Rutland (site 

4, fig. 13; fig. 16). The rock here is moderately massive 

Cambrian Cheshire Quartzite. The westerly facing mountainside at this location is parallel to the dip of the bedding. 

Initial block sliding was probably in response to freeze-thaw 

cycling. Under such conditions, large blocks that have 

become unstable more than likely slid down the dip slope of 

a joint, destroying trees (fig. 17). 

Topples 

Topples are not a common type of rock-slope failure 

in Vermont, mainly because the geologic and topographic 

conditions that promote their formation are not common. In 

this failure mode, major movement occurs where foliation 

and (or) bedding have high dip angles and low base slope 

angles. Closely spaced bedding or foliation favors topples, 

as opposed to larger, more massive rock, which is prone to 

rockfalls. 

Possibly, a topple failure is developing on Mad Tom 

Brook near East Dorset (site 7, fig. 13). The rock, 
Figure 15. The 6,000-ton King Rock on Vermont Route 108 

in Smugglers Notch. Note the person in front of the rock 

for scale. 

Figure 16. Block slide at White Rocks, near Wallingford, 

south of Rutland (site 4, fig. 13). Bedding dips down to the 

right (west). 

Figure 17. Damage to trees caused by blocks of Cheshire 

Quartzite tumbling down the dip slope. The block 

movements occurred in the early spring, the characteristic 

time for freeze-thaw-induced displacements. 

moderately to severely fractured Cheshire Quartzite, 

contains thin beds of graphitic schist. Figure 18 illustrates 

how slope deformation may enhance topples. Slope 

movements have rotated the beds so their strike is about 

parallel to the brook. The slide axis, oriented N. 50° W., 

has an average slope of 60°. At the headwall and near the 

base, the beds are vertical to overturned. The slope distance 

is 37 m. Although streamflow is rapid and water levels 

change dramatically in response to precipitation, our 

observations show that little undercutting of the toe is now 

occurring. Displacements are primarily at the headwall 

scarp and midway down the slide axis, where a prominent 

bulge has developed. While sliding-friction angles of blocks 

are as high as 55° on slopes underlain by massive, unaltered 

quartzite (as at the White Rocks Recreation Area mentioned 

in the previous section), the sliding-friction angles 

determined at the Mad Tom site are in the range of 30° to 

40° (Lee, 1989). The overall slope angle is greater than this 

range because at many locations on this slope ( 1) no 

well-developed, throughgoing weakness surfaces are 

parallel to the strike of the bedding, (2) individual blocks 

are commonly keyed or interlocked with adjacent blocks, 

and (3) beds are dipping into the slope. These structural 

conditions make overturning and toppling mechanically 

more likely than sliding. The overturning and toppling beds 

shown in figure 18 are moving at an average rate of 44 

Rock-Slope Movement Types 15 
deposits 

A 

Figure 18. Slope deformation in Cheshire Quartzite at Mad 

Tom Brook (site 7, fig. 13), looking west. A, Initial postglacial condition showing steeply dipping beds of quartzite and graphitic schist. Although fracturing is severe, 

infiltration of water is impeded by schistose layers and the 

steepness of the slope. The stream is not actively downcutting its bedrock channel. 8, Present condition of the 

mm/yr (F.T. Lee, unpub. data, 1989); these data are based 

on 3 years worth of slope instrumentation. Overturning 

produces unstable overhangs; individual blocks tumble 

downslope, and many reach the brook below. 

Much of the smaller debris falling from the slope 

washes away in the spring floods of Mad Tom Brook. The 

streambed contains large, well-rounded glacial boulders and 

large pieces of slide debris that the stream has been unable 

to transport. A sudden failure of a large part of the slope 

could likely dam the entire stream width. A partial 

damming occurred when a section on the east end of the 

slope failed in the early 1980's. 

16 Landslide Hazards in Vermont 

slope. A topple has formed because of overturning of 

beds, particularly at the top and near the base of the 

slope. Bending has opened joints at several locations, and 

most deformation is by discrete displacement of blocks 

rather than by changes in block geometry. Access of 

rainfall and snowmelt to the rock mass has now been 

greatly increased. 

SLOPE MOVEMENTS ON VERMONT'S 

INTERSTATE HIGHWAYS 

History and Construction of the System 

Construction of Vermont's Interstate Highway 

System began in 1956 and took 26 years to complete (fig. 

19). In southeastern Vermont, work on I-91 began in 1956 

Figure 19. The location of Interstates 89 and 91 and ..,. 

the periods of their incremental construction. The 

locations of rockslides and rockfalls and their dates of 

occurrence are also shown. 
~ 

' 

~ 

'..J 

.X 

0: 

<:...;) 

0 

).. 

_______ £_6_N_AD~-- __ 

22 Derby Line 29 

I 

) 

~----------- ,' MASSACHUSETTS I 

28 

s 

LL.J 

< 

0 

~ 

-?-' ~ 

~ 

~~ 

N 

EXPLANATION 

EARTH AND ROCKSLIDE LOCATIONS 

AI GEORGIA 10/11/78 

£2 BOLTON 4/81 

£3 SOUTH BARRE 5/4/85 

£4 SOUTH BARRE 6/7/73 

£5 FAIRLEE 4/5/85 

3/9/87 

£6 FAIRLEE 3/12/73 

£7 THETFORD 4/30/77 

£8 ROCKINGHAM 10/10/68 

CONSTRUCTION PERIOD 

~ 1956-59 

D 2 

1960-69 

1970- 78 

1980-82 

DIRECTION OF PROGRESS 

INTERCHANGE 

16 32 48 KILOMETERS 

Slope Movements on Vermont's Interstate Highways 17 
and progressed northward from the Vermont-Massachusetts 

border. In 1970, construction started at the Canadian border 

on 1-91 in Derby Line and progressed southward. A link 

with 1-91 to 1-93 in New Hampshire, east of St. Johnsbury, 

was completed in 1982. Construction of 1-89 started in 

1960 in Montpelier and headed north. In 1963 construction 

began on 1-89 from Highgate at the Canadian border and 

progressed southward. The south end of 1-89 went to 

contract at White River Junction in 1966 and progressed 

northwestward. 

General Geology 

Highway contractors encountered many kinds of 

rocks during construction (fig. 20). The dominant strike 

orientation of both bedding and foliation of Vermont 

bedrock is north or northeasterly. East of the Green 

Mountain front, the foliation of the folded and metamorphosed bedrock units dips vertically or is near vertical 

(steep east and (or) west dips). These attitudes are typical 

along major portions of both 1-91 and 1-89 (fig. 21). 

However, north of Interchange 18 on 1-91, in the Barnet-St. 

Johnsbury area, the highway right-of-way is almost 

exclusively within two bedrock formations, the Silurian(?) 

Waits River Formation and Lower Devonian Gile Mountain 

Formation (Doll and others, 1961). These rock units consist 

mainly of schist, phyllite, and crystalline limestone (fig. 

20). The bedding and (or) foliation of these formations at 

this location dip more gently (20° to 30°) to the east and 

west. Mostly, the strikes of bedding, foliation, and 

formational contacts are parallel. 

West of the Green Mountain front, at Interchange 13 

in South Burlington and north to the Canadian border, the 

1-89 right-of-way is in folded and faulted sedimentary 

bedrock of Cambrian and Ordovician age (fig. 20). 

Limestones, dolomites, and shales define the sedimentary 

units in northwestern Vermont, and their sedimentary 

characteristics are maintained. However, a secondary metamorphic cleavage foliation also is present in these rocks. 

The highway crosses the contacts of north-striking 

formations at an acute angle in this area. These rock units 

vary in dip from very gentle (3° to 5°) to vertical. Bedding, 

the dominant planar feature in these formations, dips both 

easterly and westerly. 

Design, Design Changes, and Construction of 

Rock cuts 

Design of the Interstate Highway System, including 

rockcut slope design, was done by both State and consulting 

engineers. Consulting firms designed 177 km, and State 

engineers designed 346 km of the system. 

Before 1972, specifications for rock excavation 

blasting did not include presplitting, which helps maintain 

18 Landslide Hazards in Vermont 

the integrity and stability of the finished rock face by 

reducing undercutting of the slope toes. As early as 1966, 

some contractors, for convenience, practiced presplitting 

blasting procedures to better control the volume of rock 

eligible for payment. Early design, therefore, was based on 

economy of construction, cost of right-of-way that the State 

would have to buy, and whether right-of-way was available. 

By 1972, 1-89 was finished, and 1-91 was completed 

between the Massachusetts line and Fairlee on the south, 

and between the Canadian border and Barton in the north 

(fig. 19). 

Rockcut slope design called for 4 (vertical) on 1 

(horizontal) slopes throughout the system. Rock type and 

structure were ignored in slope design. Precedence for the 

value of such consideration was evident in mountainous 

terrains in other States. Similar construction had occurred or 

was in progress in Pennsylvania at the same time Vermont's 

interstate system was under construction (Philbrick, 1953; 

Flint and Hamel, 1971). 

In 1972, Section 207 of the Standard Specifications 

for Highway and Bridge Construction (Vermont 

Department of Highways, 1972) and later revisions 

contained procedures for rock blasting including presplitting. Additionally, under Section 203.04 of the manual, in 

describing rock excavation, wording included details on 

controlled blasting procedures for rockcut slopes. However, 

presplitting techniques did not include specifications for 

cutting slopes parallel to dips on existing foliation or other 

planar surfaces in metamorphic rock. As a result, toes of 

slopes were often undercut. Many segments of the interstate 

have rockcut slope faces constructed at steeper angles than 

the dip angle of the foliation or bedding planes. Thus, 

potentially unstable and hazardous conditions were created 

in places where undercut and unsupported slabs sloped 

toward the highway. These conditions are similar to the 

"overdip" situations described by Pomeroy (1982) in 

Pennsylvania where the natural slope is steeper than the dip 

angle. 

In 1971, design specifications for newly constructed 

slopes extended the rockfall area at the base of slopes 

(called the clear zone) from 7 m to 9 m. The use of the 9-m 

width for clear zones was the result of tests that showed this 

width is a safer "recovery" distance for drivers who have 

lost vehicular control and strayed from the paved highway. 

A secondary benefit was the creation of a wider fall zone 

that could prevent some rockfalls from entering the traveled 

portion of the highway. In 1972, a change was made in the 

clear zone by adding a mound of rounded earth at the toe of 

each rockcut slope. This was also a safety measure to 

redirect errant drivers away from the rock slope face and 

secondarily to cushion falling rock. 

Figure 20. Generalized geologic map of Vermont ~ 

(modified from Doll and others, 1961). 
N 

EXPLANATION 

- Igneous rocks 

Q Devonian and Silurian rocks 

Ordovician rocks 

Cambrian rocks 

Precambrian rocks 

Green Mountain Front 

12 18 24 KILOMETERS 

I I I 

Slope Movements on Vermont's Interstate Highways 19 
Figure 21. Rocks I ide of May 4, 1985, along the southbound 

roadway of Interstate 89 at South Barre (location 3, fig. 19). 

Foliation dips approximately 60° east (toward the 

highway). Such dip surfaces serve as failure surfaces. 

In some areas, rock structure conditions presented 

problems. The use of geological examination and analysis 

became important in trying to change the standard design. 

Often, recommendations by highway geologists during 

construction helped solve problems not considered during 

the design stage. 

Analysis of Rockcuts 

A reconnaissance survey began in the fall of 1977 and 

spring of 1978 to rate the rockslide-rockfall potential of all 

rockcuts along the Vermont Interstate Highway System. 

The State Geologist and the Chief Geologist of the Agency 

of Transportation collaborated in this survey. This 

reconnaissance was the first of an intended three-phase 

program (Ratte, 1978). In Phase I, these geologists made an 

assessment of all bedrock cuts over 6 m in height. These 

cuts were checked and given a hazard rating based on 7 

criteria (table 2). A rating of 1 showed a low hazard; a 

rating of 10 was an extreme hazard. Phase II called for 

detailed geological and engineering analysis of those cuts 

thought to present the most hazard to public health and 

safety. Phase III would provide recommendations for and 

implementation of remedial work, and a plan for continued 

monitoring. It was expected that new design criteria for 

rockcuts, which considered geologic conditions unique to 

Vermont, would evolve from this program. 

20 Landslide Hazards in Vermont 

Table 2. Criteria used in the 1977-78 analysis of rocksliderockfall potential along Vermont's Interstate Highway 

System 

[An evaluation number was selected from each of the seven categories, and 

these numbers totaled for a final score] 

Criteria Evaluation 

Number 

1. Height of rockcut 

<6m ......................................... . 

6 m to 9 m .................................. . 

9 m to 12m ................................. . 

>40 m ........................................ . 

2. Distance from toe to pavement if <6 m .......... . 

3. Planar geologic structures dipping toward the 

highway 

<30° .......................................... . 

30° to 70° ................................... . 

70° to 90° ................................... . 

4. Topographic slope toward highway .............. . 

5. Presence of ground water and (or) surface water .. . 

6. Past history of rockslide-rockfall activity ......... . 

7. Other recognizably adverse conditions (such as 

being strongly jointed) .......................... . 

Rockcut Slope Failures 

0 

1 

2 

3 

1 

3 

Not all problems were solved or recognized. Several 

large rockslides and rockfalls occurred along Vermont's 

Interstate Highway System. The large failures that entered 

or crossed traffic lanes, fortunately, occurred at times of 

sparse traffic. This latter factor has averted injury to the 

traveling public. Sites of recorded rockslides and rockfalls 

are shown with dates of occurrence on figure 19. 

Some of the rockcut slope failures are instructive. In 

the early morning hours of April 5, 1985, a large block of 

rock fell from near the crest of the Palisades cliff between 

posted mile 99.75 and 99.85 in Fairlee on southbound I-91 

(location 5, fig. 19; fig. 22). The block landed in the fall 

area and shattered, spraying 11- to 21-cm-diameter chunks 

across the southbound lanes. A pickup truck passing the 

area sustained damage to all four tires. On March 9, 1987, 

at this same location, a rockfall caused extensive damage to 

two automobiles (fig. 23); fortunately, the occupants of 

these vehicles escaped injury. 

The bedrock at the Fairlee site is the massive Middle 

Ordovician(?) Post Pond Volcanic Member of the Orfordville Formation (Doll and others, 1961). It contains multiple 

intersecting joint sets related to the Ammonoosuc fault, 

which traverses this area. This rockcut scored 7 in the Phase 

I study. 

Another rockfall of about 3,500 m3 occurred about 

7:00 p.m. on October 10, 1968, along southbound I-91 

between posted mile 34.70 and 34.90 in Rockingham 

(location 8, fig. 19). This failure took place along the steep 

east-dipping foliation in the phyllite-slate unit of the Littleton Formation (Doll and others, 1961) in a very deep, 
Figure 22. View of the Palisades rockcut at Fairlee (location 

5, fig. 19). The southbound lane is hidden behind the 

guardrail in the foreground. Loose blocks originated near 

the crest of this rockcut at a height of several meters above 

the pre-split area (indicated by the closely spaced drill 

marks seen on the rock face). 

unbenched cut (fig. 24). The slide scar was about 12m high 

and 46 m wide. Rock debris covered the two southbound 

lanes. 

This type of failure can occur where foliation strikes 

parallel to the highway and dips steeply toward the 

highway. Fall areas (clear zones) through these deep cuts 

are narrow as a result of economic considerations made 

during original design and construction. This cut scored 9 in 

the Phase I study. 

The Georgia rockfall occurred on the night of October 

11, 1978, on the northbound side of I-89 at posted mile 107 

between 107.50 and 107.75 (location 1, fig. 19). A massive 

block of dolomite, weighing several hundred tons, dropped 

6 m to the clear zone next to the traveled lanes of I-89. 

Several smaller blocks and fragments slid onto the edge of 

the pavement. This type of rockfall can occur in massive, 

thick-bedded rock units having penetrating, intersecting 

joint sets, particularly where bedding planes dip toward the 

highway. The failure occurred in the Dunham Dolomite of 

Figure 23. A vehicle damaged by the March 9,1987, rockfall 

at Fairlee (location 5, fig. 19). 

Figure 24. Scar of the October 10, 1968, rockslide near 

Interchange 6 (Rockingham) on Interstate 91 (location 8, 

fig. 19). The view is looking south along a steep, eastdipping foliation surface upon which sliding took place. 

Note the hanging undercut slab at the south end of the 

cut. 

Cambrian age (Doll and others, 1961). Many more bedded 

and jointed Cambrian sequences dominate the bedrock 

along I-89 from Burlington to the Canadian border. This 

site scored a 10 in the Phase I survey. 

The last example of rockfalls from Interstate Highway 

rockcuts is a topple that occurred on southbound I-91 

between Thetford and Fairlee on the morning of April 30, 

1977 (location 7, fig. 19; fig. 25). The rock is the 

Meetinghouse Slate Member of the Lower Devonian Gile 

Mountain Formation (Doll and others, 1961). The foliation 

Slope Movements on Vermont's Interstate Highways 21 
Figure 25. Scar and resulting debris of the April 30, 1977, 

rockslide along the southbound cut face of Interstate 91 at 

Fairlee (location 5, fig. 19). 

at this site strikes parallel to the highway (northerly) and 

dips 70° to 80° westerly, away from the highway. 

Results of the Phase I Project 

The phase I reconnaissance survey rated the interstate 

highway cuts on the basis of the criteria shown in table 2. 

Forty sites received a score of 7 or greater and were given 

recommendations for detailed geologic and engineering 

analysis (Phase II). A highway safety reconstruction project 

began in 1989-90. The intent of this project is to widen all 

rockcuts that predate 1971 to a 9-m clear zone width 

between rockcut faces and the road pavement. 

CONCLUSIONS 

The cases of landslide events discussed in this paper 

are presented to create a public awareness of natural hazards 

and hazards accentuated by humans. When valuable 

property is exposed to potential landsliding, methods for 

mitigation need to be weighed. Some of the mitigation 

procedures may be as simple as the development of a 

drainage system to lower the water level of the slope. The 

use of counterweight berms can be applied to prevent 

further movement on the toes of slopes. When planning the 

creation of rock slopes, engineers should consider cutting 

the rock back until as flat a slope as possible is made for 

stabilization purposes. 

Sloping land that has been set aside for tree harvest 

may be susceptible to landsliding and consequently should 

22 Landslide Hazards in Vermont 

be examined carefully for slope stability before clearcutting 

takes place. Controlled selective cutting and immediate 

reforestation may be desirable to prevent destruction of 

these valuable properties. 

The design of new highways must be made with a 

clear understanding of the effects of geologic structure, soil 

conditions, and drainage on the behavior of constructed 

slopes. Proper design and construction practice will help 

assure stable cut slopes and avoid costly maintenance work. 

Before commencing commercial or residential 

development, qualified professionals, such as engineering 

geologists or geotechnical engineers, should perform a 

detailed study on land that appears susceptible to landsliding. The State Geologist or individual city or county 

geologists can be consulted on such subjects. If the situation 

seems potentially threatening, development should be 

reconsidered. 

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and debris slide at Smugglers Notch, Mount Mansfield,Cambridge, Vermont: Vermont Agency of Natural Resources, 

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751 p. 

Doll, C.G., Cady, W.M., Thompson, J.B., Jr., and Billings, 

M.P., 1961, Centennial geologic map of Vermont: Vermont 

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1:250,000. 

El Ashmawi, N., and Greenfield, B.J., 1972, A review of vertical 

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Association of Engineering Geologists Bulletin, v. 9, no. 4, 

p. 395-398. 

Eschner, A.R., and Patrie, J.H., 1982, Debris avalanches in 

eastern upland forests: Journal of Forestry, v. 80, no. 6, p. 

343-347. 

Flint, N. K. , and Hamel, J. V. , 1971 , Engineering geology at two 

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Pittsburgh, Pennsylvania, in Thompson, R.D., ed., 

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Goodman, R.E., 1976, Methods of geological engineering in 

discontinuous rocks: St. Paul, Minn., West Publishing 

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Greenway, D.R., Anderson, M.G., and Brian-Boys, K.C., 1984, 

Influence of vegetation on slope stability in Hong Kong: IV 

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Hamblin, W .K., 1989, The earth's dynamic systems: A textbook 

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Hoek, Evert, and Bray, J. W., 1977, Rock slope engineering: 

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---1972, Standard specifications for highway and bridge 

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References Cited 23 

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interest. 

Water-Resources Investigations Reports are papers of an 

interpretive nature made available to the public outside the formal 

USGS publications series. Copies are reproduced on request unlike 

formal USGS publications, and they are also available for public 

inspection at depositories indicated in USGS catalogs. 

Open-File Reports include unpublished manuscript reports, 

maps, and other material that are made available for public consultation 

at depositories. They are a nonpermanent form of publication that may 

be cited in other publications as sources of information. 

Maps 

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 7.5- or 15-minute quadrangle formats (scales 

mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or 

engineering geology. Maps generally include brief texts; some maps 

include structure and columnar sections only. 

Geophysical Investigations Maps are on topographic or planimetric bases at various scales; they show results of surveys using 

geophysical techniques, such as gravity, magnetic, seismic, or 

radioactivity, which reflect subsurface structures that are of economic 

or geologic significance. Many maps include correlations with the 

geology. 

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also 

includes 7 .5-minute quadrangle photogeologic maps on planimetric 

bases that show geology as interpreted from aerial photographs. Series 

also includes maps of Mars and the Moon. 

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial 

geology, stratigraphy, and structural relations in certain coal-resource 

areas. 

Oil and Gas Investigations Charts show stratigraphic 

information for certain oil and gas fields and other areas having 

petroleum potential. 

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases for quadrangle or 

irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post -1971 

maps are primarily black-and-white maps on various subjects such as 

environmental studies or wilderness mineral investigations. 

Hydrologic Investigations Atlases are multicolored or blackand-white maps on topographic or planimetric bases presenting a wide 

range of geohydrologic data of both regular and irregular areas; 

principal scale is 1:24,000, and regional studies are at 1:250,000 scale 

or smaller. 

Catalogs 

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available 

under the conditions indicated below from the U.S. Geological 

Survey, Map Distribution, Box 25286, Bldg. 810, Federal Center, 

Denver, CO 80225. (See latest Price and Availability List.) 

"Publications of the Geological Survey, 1879-1961" may be 

purchased by mail and over the counter in paperback book form and as 

a set of microfiche. 

"Publications of the Geological Survey, 1962-1970" may be 

purchased by mail and over the counter in paperback book form and as 

a set of microfiche. 

"Publications of the U.S. Geological Survey, 1971-1981" may 

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

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

Supplements for 1982, 1983, 1984, 1985, 1986, and for 

subsequent years since the last permanent catalog may be purchased by 

mail and over the counter in paperback book form. 

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by 

mail and over the counter in paperback booklet form only. 

"Price and Availability List of U.S. Geological Survey 

Publications," issued annually, is available free of charge in paperback 

booklet form only. 

Selected copies of a monthly catalog "New Publications of the 

U.S. Geological Survey" are available free of charge by mail or may 

be obtained over the counter in paperback booklet form only. Those 

wishing a free subscription to the monthly catalog "New Publications of 

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

Survey, 582 National Center, Reston, VA 22092. 

Note.-Prices of Government publications listed in older 

catalogs, announcements, and publications may be incorrect. 

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

announcements, and publications. 
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