SURFICIAL GEOLOGY OF THE THIRD BRANCH OF THE WHITE RIVER, CENTRAL VERMONT

Surficial deposits of the Third Branch watershed record the formation and drainage of glacial Lake Hitchcock, which locally was fed by glacial Lake Roxbury via the Roxbury threshold at 1010 ft asl. Today, the Third Branch flows south from Roxbury, joins a major tributary, Ayers Brook, in the town of Randolph, and then continues south to its confluence with the White River. Uplands are rugged with relief of 2700 ft.

Late Wisconsinan ice advanced south-southeast based on an indicator fan of erratics from the Braintree pluton and striae. Ice-contact deposits are relatively rare. From Roxbury threshold to Randolph, glaciolacustrine deposits are limited and ice-contact deposits are found in the valley bottom. Apparently, high flow through the spillway produced a sediment bypass zone in the upper Third Branch valley.

Complete deltas were not found for the Lake Hitchcock level, but trimmed foresets are found in the Third Branch valley north of Randolph. As Lake Hitchcock drained, sheets of sandy gravel were deposited out of tributary valleys and across the former lakebed. These are mapped as fan-terrace deposits. In the Randolph area, at the confluence of Ayers Brook and the Third Branch valleys, a thick section of Lake Hitchcock thick, sandy varves, with paleo-flow indicators of north, east and south, documents sediment focusing and development of a large subaqueous fan. This fan dammed flow from Ayers Brook valley after the drainage of Lake Hitchcock, resulting in a 1.3 mi long lake (“Ayers Pond”) containing thick, organic-rich ponded sediments dating from 9980 +/- 130 to 8700 +/- 150 14C ybp. Liriodendron tulipifera wood from Ayers Pond corroborates a suggestion that the early Holocene was warmer than present in central Vermont.

Stream terraces comprising sandy gravel over lake bottom are common in upper reaches of the watershed. These grade down valley to terraces of fining-upward alluvial sequences, occasionally over lake bottom. Meander scars on low terraces suggest slow lateral migration followed by relatively recent incision of the Third Branch. A greater than 100-yr flood in 1998 produced several mass failures when channels rapidly migrated laterally. Randolph alone experienced damage in excess of $1.4 million.