The deglaciation of the northern White Mountains of New Hampshire

Abstract

The Laurentide Ice Sheet flowed southeast across the northern White Mountains during the Wisconsinan Stage. In late Wisconsinan time, the regional snowline was probably elevated above the highest peaks. Downwasting ice masses progressively ceased to flow (stagnated) as they became cut off from regional sources to the northwest. Indirect lines of geomorphic evidence and a short radiocarbon chronology indicate that deglaciation proceeded very rapidly once major elements of the regional terrain were exhumed from beneath the ice; there is a distinct downward and locally north-westward progression of deglacial events. North of the city of Berlin, regional ice flow was sustained in the Androscoggin and Dead River valleys. Ice eventually retreated in these valleys and left sequences of stratified drift graded to bedrock at the head of the 70-m knickpoint in the Androscoggin Valley at Berlin Falls. Stratified moraines record temporary positions of these retreating ice fronts. Outwash began to accumulate in the deepest part of the Androscoggin Valley downstream from Berlin after the ice in this area was beheaded at the falls. The local deglaciation was probably complete between 12,100 and 12,600 yr B.P. There is evidence for two separate phases of activity in the relict rock glacier in King Ravine in the Presidential Range. The earliest rock glacier may have formed from a stranded remnant of Laurentide ice. Later activity may have occurred during a period of significant climatic cooling.