FLUCTUATIONS OF GLACIAL LAKE HITCHCOCK IN THE UPPER VALLEY REGION (NEW HAMPSHIRE AND VERMONT)

At the end of the last ice age, the Connecticut River Valley was filled by Glacial Lake Hitchcock (GLH) which was dammed at its southern end near Rocky Hill, Connecticut. GLH received meltwater from the Laurentide Ice Sheet and varved sediments deposited in the lake provide a high-resolution record of ice sheet deglaciation and climate from ~18-12 ka (Antevs, 1922, Ridge, 2004). A breach of the Rocky Hill dam at ~13.5 ka caused drainage of the southern basin of GLH, located to the south of the Holyoke Range in Massachusetts, but the northern basin of GLH (located in the Upper Valley of New Hampshire and Vermont) retained water until ~11.5 ka (Stone et al. 1998). Our study investigates lake level lowering and final drainage of GLH in the Upper Valley region.

Here we present lake sediment cores from two modern lake basins in New Hampshire, Occom Pond (146 m asl) in Hanover and Post Pond (130 m asl) in Lyme. Both of these lakes were located beneath the level of GLH and have varved sediments at their base. We compare the varved sediments from Occom and Post Ponds with the varve record from Newbury, VT (Ridge and Toll, 1999). Based on our initial results, we interpret an increase in varve thickness at Post Pond at ~13.1 ka to register lowering of GLH. A contemporaneous shift from varved sediments to homogenous silty clay in Occom Pond may indicate that GLH lowered below the elevation of Occom.

Our future research will focus on tracking the lowering and eventual drainage of GLH using sediment core records from at least five other modern lakes in the Upper Valley. We have identified nine lakes in New Hampshire and Vermont that are within the GLH basin and therefore should contain varved sediments. These lakes occur over a range of elevations, from near the highest level of GLH to near the elevation of the modern Connecticut River. Varved sediments in the lake cores will be correlated using high-resolution imagery so that thickness patterns and compositional changes can be determined with high accuracy. We will examine these records for sudden shifts from thinner to thicker varves. The observed changes in varve thickness will then be compared to stratigraphic changes in individual lakes indicating lake level lowering and also to paleoclimate records from the region to constrain whether varve thickness changes may be a result of lake level lowering or climatic factors.