SLUMPING AND EROSION OF THE LAKE ONTARIO SHORE LINE AT STERLING NATURE CENTER, CAYUGA COUNTY, NEW YORK

The region of central NY is underlain glacial till that was deposited during the Pleistocene. The till contains a mixture of clay, silt, sand and gravel. Occasional eratics up to several meters in diameter were observed. At the shore of Lake Ontario, the glacial till forms steep cliffs and bluffs. The Sterling Nature Center resides mostly on ablation till that is about 8 meters thick at the lake shore. The till overlies Ordovician clastic bedrock. Slope failure and wave erosion by Lake Ontario have resulted in steep cliffs in the till at the nature center. The focus of this research was to document the style of slope failure, and examine the factors that contribute to slope instability of the cliffs. A shore line exposure about 450 m long was studies during the summer and fall 2003. Over this distance, 40 slumps were observed, ranging form 5-20 m long, and 10-55 cubic m volume. The slumps occur on steeply dipping failures that dip toward the lake. Debris flows, containing remobilized till, form small (1-3 m) cone-shaped deposits at the based of numerous gullies. A total 35 debris flows were documented in the area. At the base of the cliffs, gravel deposits containing rounded pebbles and cobbles make up the beach. At the top of the cliff, the soil is severely undercut, causing numerous trees to fall. From July to Nov., 2003, there were few changes to the morphology of the cliff face; however, in early December the cliff faces were dramatically modified by wave erosion. The onset of winter storms produces large (1-2 m) waves on Lake Ontario which increased the rate of erosion at the base of the cliffs. In-situ moisture measurements were made in the cliff face during to document the increase with the onset of wet-weather. Over the Fall, moisture content generally increased. By Dec., the moisture content ranged from 2-98 percent, with an average of about 60 percent. Although the till is compacted and strong during the summer months, the addition of moisture in the fall greatly weakens the clay matrix causing debris flows and slope failure. The high moisture percentages observed may account for increased slope instability coupled with the increase in wave action that erodes the base of the cliffs. In conclusion, the slope stability at Sterling Nature Center is influenced by weakening of the till due to increases in moisture. These conditions are compounded by the removal of the external slope support by wave action.