DATING THE VARVES OF GLACIAL LAKE HITCHCOCK

Laura B. Levy
Department of Geology
Mt. Holyoke College

Glacial Lake Hitchcock, a large proglacial lake in the Connecticut Valley existed during late Wisconsinan deglaciation. Originating when glacial debris formed a dam at Rocky Hill, Connecticut, the lake eventually extended into northern Vermont1. Although the lake sediment in the valley is varved (representing annual layers of sedimentation), the duration of the lake is only generally known to be ca 12-16 ka BP. Organic matter is notably absent in the varves making the resulting high resolution record a "floating chronology."

The two study sites, River Road east and west, are both located in south Deerfield, Massachusetts, west of the Connecticut River. These varve exposures are exposed due to erosion from a tributary of the Connecticut River that runs between them. During the summer of 1997, these exposures were studied. Vertical faces that overlapped stratigraphically were dug into the varves and then the varves

Ridge and Larsen (1990) have received a radiocarbon date from Vermont on organic material from varve #6150 of 12,095-12,815 yrs. BP. This dates the River Road sites between 13,242 and 13,962 yrs. BP.

Carbonate concretions which are small nodules, usually a few centimeters in diameter, were also found at the River Road sites. If these concretions originated from carbon from organic matter, then they could be potentially useful in dating the varves. If the source of their carbonate is from detrital carbon, they would yield an infinite date and if the source of their carbonate is from modern carbon they would date recent. On a whim, a carbonate concretion was AMS C-14 radiocarbon dated. The results dated varve # 5003 at 13,0401 160 yrs. BP, which is close to the range of Ridge and Larsen's date and within the age of the lake (1). Either the date received on the concretion yields and accurate date of the varves or it is a fortuitous mixture of modern and detrital carbonate.

__________________
1Ridge, J. C., Larsen, F. D., 1990, Re-evaluation of Antevs' New England varve chronology and new radiocarbon dates of sediments from glacial Lake Hitchcock, Geographical Society of America Bulletin, 102, 889-899. 2Antevs, Ernst, 1922, The Recession of the Last Ice Sheet in New England, American Geographical Society: New York, 120 p.