Jon Bevan

Meghan O’Donnell

December 10, 2003

Introduction and Objectives

• We examined an active landslide along side of the Lee River in Jericho, Vermont.

• The main slide occurred several years ago and recently there have been smaller slides.

• We wanted to investigate:– stratigraphy and cohesion of the sediment.– vegetative cover on and around the slope.

• Infer geologic history of the sediment.• Conclude why the slope continues to fail.

Location Map

37 meters in height50 meters across

See Jon for Scale

Scale = 0.5 km

Scale = 100 km

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Methods

• Dig eight, 1m deep soil pits vertically up the landslide to observe stratigraphy:– Orientation of bedding,– Sediment grain size and graded bedding,– Cross-bedding and Ripples.

• A handheld device to measure cohesion of particular sediment layers.– Cohesion is the soils ability to stick to itself.

The Landslide Data• The slope of the landslide is 28˚. • The sediment continually increases in grain size with

increased elevation, besides Pit 2.• Alternating fine to coarse layers.• All tilted layers, cross bedding, and ripples are oriented

northeast. • Finer grained sediment was more tightly packed and

exhibited greatest cohesion. • Vegetation covered the southern half of the slide was

covered with vegetation.

Pit 2 Data

Unsorted fine to medium grained sediment, 7 meters from river’s edge

Pockets of gravel within unsorted material

Organic debris

See tape measure for scale

See tape measure for scale

See tape measure for scale

Vegetation and Ground Water

Scale = 6 in.

Vegetation covers approximately50% of the slope. The roots stabilize the slope laterally.

See Jon for scale

Outwash of subsurface groundwater at the base of the slope.

Geologic History• 12-13ka the Laurentide Ice mass ablated

from this region of Vermont.– Served as an ice dam for northward flowing

water

• Glacial Lake Vermont

• Creation of deltas into glacial lake

Note: Photo from Benn and Evans, 1998. And please disregard blank spots.

How the Slide has Changed (1962 and 1999)

1962 1999*1962 landslide is almost twice as larger than present scarp. *Vegetation on farmland terrace has increased significantly.

Explanation of why the Slope Fails

• Slope of the slide is quite steep.

• Sub-surface flow of H2O– Normal force decreases as pore pressure stays

the same, decreasing cohesion.

• Undercutting of river

• Insufficient vegetation to completely stabilize the slope.

References• Wright, Steven. 2002 and 2003. Personal Conversations and Class

notes.• Chapman DH. 1937. Late- and Post-Glacial Champlain Valley. In

American Journal of Science. 34:53.• Bierman Paul. 2003. Class notes.• Benn, D, Evans, D. 1998. Glaciers and Glaciation. Arnold

Publishers. London, England.• Bloom, A. 1991. Geomorphology: A Systematic Analysis of Late

Cenozoic Landforms. Prentice Hall. Englewood Cliffs, New Jersey, USA.

• Kochel R, Miller J, Ritter D. 1978. Process Geomorophology. WCB Publishers. Dubuque, IA.