Jon Bevan Meghan ODonnell December 10 2003 Introduction

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 N 37 meters in height 50 meters across Scale = 100 km See Jon for Scale N N Scale = 0. 5 km

Methods • Dig eight, 1 m 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 See tape measure for scale Pockets of gravel within unsorted material See tape measure for scale Organic debris See tape measure for scale

Vegetation and Ground Water Vegetation covers approximately 50% of the slope. The roots stabilize the slope laterally. See Jon for scale Outwash of subsurface groundwater at the base of the slope. Scale = 6 in.

Geologic History • 12 -13 ka 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 H 2 O – 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.