FORMATION MECHANISMS OF FAYETTEVILLE GREEN LAKE AND ROUND LAKE: GLACIAL EROSION, WATERFALLS, AND SINKHOLES?

The superficial geology of much of the Northern United States was shaped by the last glaciation. This is the case for Green Lakes State Park, New York. The park contains two small but very deep lakes, Green Lake and Round Lake, within a U-shaped glacial valley. Green Lake has a surface area of 0.26 km² and Round Lake has a surface area of 0.13 km². Both are about 52 m deep. There are two main theories for the formation of these uniquely shaped lakes: 1) vertical erosion by glacial melt water forming a waterfall plunge pool, and 2) dissolution of soluble bedrock including gypsum and halite followed by collapse forming a sinkhole. This study takes a holistic view to address this question by integrating the regional geomorphology and shape of the eroded bedrock surface. Specific focus is given to the thickness of sediment above glacially eroded bedrock and the bedrock surface at the downstream end of each lake.

The top of bedrock at the outflow point of Round Lake constrains the maximum water depth of the hypothetical plunge pool because it would represent the elevation of the downstream spill point. This is a critical variable in testing the plunge pool mechanism as it provides a first order control on the erosive power of falling water at the bottom of the pool. If this depth is found to be too great then this theory can be considered invalid. Plunge pool modeling was conducted using a MATLAB script published by Scheingross and Lamb (2017). The elevation and shape of bedrock at the outflow point of the lake was determined using ground penetrating radar profiles, mapping of sediment fill above bedrock in Green Lake with a submersible ROV, and previous sediment cores that indicated potential bedrock at a depth of about 11 meters.

A bore hole drilled at the downstream end of Green Lake penetrated 42 meters of marl, silt, and peat without reaching bedrock. Other boreholes drilled on either side of the lake combined with ground penetrating radar lines indicate bedrock erosion into a U-shaped valley. The elevation of bedrock at the outflow of Green Lake appears to have been quite deep prior to sediment deposition, and would not allow for the deep lake that exists today. Therefore, the outflow point of Green Lake must have been backfilled after glacial erosion, perhaps with a combination of moraine deposits, glacial outwash, and lake sediment.