LAURENTIDE ICE SHEET RECESSION RECORDED IN A LATE-WISCONSIN PROGLACIAL LAKE IN THE CONTOOCOOK RIVER BASIN, SOUTHERN NEW HAMPSHIRE

The retreat history of the Laurentide Ice Sheet in New Hampshire during the last deglaciation is poorly constrained due to the relative scarcity of well-preserved recessional moraines with age control. As a result, correlation with ice-recessional histories from elsewhere along the southern margin of the ice sheet and with regional paleoclimate oscillations is limited. The distribution and stratigraphic relationships of glaciofluvial and glaciolacustrine deposits are among the best available lines of evidence for establishing relative age chronologies of ice margin position during retreat. In this study, we mapped and described paleo-shorelines, deltas, ice-contact deposits, and spillways associated with Lake Contoocook, a 215 km² ice-dammed proglacial lake that occupied the Contoocook River valley in southwestern New Hampshire during recession of the Laurentide Ice Sheet. The timing of ice retreat from the study area has previously been estimated at 15.6 cal ka BP by ¹⁴C dating of bulk organic material. Modern-day elevations and locations of glaciolacustrine deposits indicate a systematic northward retreat of a well-defined ice sheet terminus, as opposed to recession through large-scale stagnation and regional downwasting. The calculated gradient of delta topset-foreset contacts along an azimuth of N20ºW is 1.12 m/km, rising toward the north. The trend in elevations of topset-foreset contacts indicates that postglacial isostatic rebound has been greatest in the north-northwest portion of the former lake basin. The extent of glacial Lake Contoocook was reconstructed on a topographic base map using this inclined plane as a guide to infer paleo-shoreline position where deltaic deposits were absent. Our new reconstruction is consistent with and adds critical details to previously published maps indicating the distribution of glaciolacustrine deposits. Results of this study contribute to an understanding of the deglacial history of New Hampshire, and provide a foundation for developing a refined chronology of ice margin retreat through application of numerical dating methods to surficial deposits in the region.