Late Wisconsinan deglaciation styles of parts of the Contoocook, Souhegan, and Piscataquog drainage basins, New Hampshire

Abstract

This report is the guidebook for the 56th annual meeting and field conference of the Friends of the Pleistocene, held May 22 and 23, 1993. Features were examined at 11 sites in the Contoocook, Souhegan, and Piscataquog Drainage Basins to illustrate the geologic history of this area, about 14,000 years ago, during the time of deglaciation. The Contoocook River Basin is the largest river basin that drains north in New Hampshire and is similar to northwardly draining parts of the Piscataquog and Souhegan River basins. During the retreat of the ice, the drainage divide between adjacent drainage basins acted as a dam and lakes formed behind it. As the ice continued to melt farther north, drainage outlets were uncovered at progressively lower altitudes along the drainage divide. This resulted in catastrophic draining of the lakes. Evidence for the existence of the lakes includes fine-grained lake-bottom deposits and deltas at successively lower elevations. Geomorphic evidence for the catastrophic draining includes Pulpit Rock in Bedford, N.H. and V-shaped notches eroded into till and bedrock. In Henniker, N.H., further evidence of catastrophic draining of a large lake in the Contoocook River Basin is a combination alluvial fan and delta that formed when rapidly draining lake water flowed across a till slope, eroded the till, and redeposited the material where it entered a smaller, much lower lake.