GLACIAL LAKES AND RELATED DEGLACIATION FEATURES IN THE NORTHERN WHITE MOUNTAINS, NEW HAMPSHIRE

The authors have compiled a 1:100,000-scale map showing the extent of glacial lakes, moraines, and related positions of the Laurentide Ice Sheet margin in the northern White Mountains. Most of the lakes resulted from damming of north to west-sloping valleys by the ice sheet as it receded from the Connecticut River basin. From south to north, the ice-dammed lakes were glacial Lakes Franconia, Ammonoosuc, Carroll, Whitefield, and Israel. These lakes typically had stages that changed in size and location as ice retreat opened successively lower spillways. Some of them were identified by Richard Lougee (ca. 1930), and the lake history was defined in greater detail relative to associated ice margins by Thompson et al. (1999). The present compilation is the first to show the glacial lake sequence of the region on a modern topographic base map. It includes Older Dryas moraines in the Littleton-Randolph area, and the Berlin and Androscoggin moraine complexes to the east. The mapped ice margins are largely schematic but locally well constrained where their presence was required to explain the succession of lake spillways and corresponding deltas and fluvial erosion surfaces. Spillways that drained Lake Ammonoosuc SW into the Gale River valley are closely spaced and their elevations correlate with progressively lower delta surfaces near Twin Mountain village. This relationship shows that the receding glacier margin was a tight dam that prevented subglacial escape of lake waters along the terminal zone of the ice sheet. When each of the Gale River and later spillways of Lake Ammonoosuc was uncovered following the Older Dryas Littleton Readvance, the sudden lowering of the lake released a flood into the lower Ammonoosuc and Connecticut valleys. These flood events were detected by Ridge et al. (2012) in a series of thick Lake Hitchcock varves deposited just after the readvance. Other glacial lakes in the study area included parts of Lake Hitchcock in the Ammonoosuc and Connecticut River valleys, and Lake Coos in the upper Connecticut valley. Lake Coos was impounded by till plugs in constricted parts of the valley near the villages of Dalton, NH and Gilman, VT, and extended upvalley to North Stratford. This lake probably had at least two levels as the dams were eroded, with the lower level controlled by a prominent spillway just west of Gilman.