POST-LAURENTIDE DEGLACIATION OF THE MIDDLE SWIFT RIVER BASIN, BARTLETT AND CHOCORUA 7.5-MIN. QUADRANGLES, NEW HAMPSHIRE

The Middle Swift River Basin lies on flat terrain that stretches west to east in Albany, New Hampshire. Mountain ridges flanking the basin to the south are formed by Mts. Passaconaway, Paugus, and Chocorua and on its north by Owl Cliff, Bear Mt. and Table Mt. These ridges direct the Swift River to the east into the Saco Valley at Conway. During deglaciation, the basin initially contained a +/-12 sq. mi. +/- 2,000-ft. thick mass of wasting glacial ice that had been isolated from the rest of the downwasting ice sheet as its thinned and separated around the region’s higher terrain. The initial stage of its disintegration within the basin involved extensive meltwater flowing to the south off its high surface through the 2,200-ft. cols at the heads of the Kelly and Paugus Brook valleys. After the ice surface lowered below these cols, deposits on the basin’s north-facing slopes show small lakes locally ponded into progressively lower elevation pockets variously trapped between raggedly disintegrating ice and the north facing slopes. As the ice surface continued to lower and the basin became ice-free, these small lakes progressively drained and then coalesced into Glacial Lake Passaconaway, which was dammed by a cross-valley morainal “plug” at the present location of the Rocky Gorge Upper Falls on the Swift River. Several abandoned progressively lower and heavily eroded meltwater channels there show the lake drained in several stages until the plug was breached and the bedrock threshold exposed.