THE LOWERING AND DRAINAGE OF GLACIAL LAKE HITCHCOCK IN THE UPPER VALLEY OF VERMONT AND NEW HAMPSHIRE AS REGISTERED BY VARVED SEDIMENTS

Following the last glacial maximum, the Laurentide Ice Sheet (LIS) retreated north through New England creating a glacial lake within the Connecticut River Valley known as glacial Lake Hitchcock (GLH). Varved sediments deposited in GLH provide a high-resolution record of LIS retreat and regional climatic conditions. We hypothesize that these sediments also register the lowering and eventual drainage of GLH.

GLH was dammed by glacial sediments near Rocky Hill, CT. When at its largest, GLH may have stretched from Rocky Hill, CT north to the Lyndon, VT area. The Rocky Hill dam was breeched at ~13.5 ka enabling the drainage of the southern basin of GLH, located south of the Holyoke Range in Massachusetts (Stone, 1999). The northern GLH basin (in the Upper Valley region of NH and VT) is believed to have retained water until ~11.5 ka (Stone, 1999). However, no prior studies have focused on lake level fluctuations in the Upper Valley or the exact timing of final drainage of the northern basin of GLH.

We cored several modern lakes to obtain varved sediment in the Upper Valley and are examining the lowering and final drainage of GLH. We demonstrate that modern basins hold varves that can be correlated to the North American Varve Chronology and therefore dated (Ridge et al. 2012). We interpret the transition from varved sediment to non-varved or organic sediments in cores to represent the isolation of the modern basin from GLH, likely resulting from a lowering of GLH. We are using the age of the varves below the transition, combined with radiocarbon dating of organic material above the transition, to determine the timing of GLH lowering. By identifying this transition over a range of paleo-elevations within GLH, we can also understand whether the style of drainage was continuous or sporadic.

We hypothesize that, if sporadic, GLH lowering events are registered by a sudden increase in varve thickness. When GLH lowered, former lake bottom sediments would be exposed at the shoreline and transported into the lower lake level, resulting in an increased sediment flux and thicker varves. To test this hypothesis, we compare the timing of GLH lowering with varved sediments from modern basins that remained submerged below GLH. Other possible explanations for a sudden increase in the thickness of varved sediments are rapid climate changes and flooding.