SURFICIAL MAPPING OF THE MAD RIVER WATERSHED, CENTRAL VERMONT, AND PRELIMINARY RESULTS CONCERNING ITS DEGLACIATION HISTORY

Quaternary deposits in the drainage basin of the Mad River record Late Wisconsinan ice advance, development of a series of small proglacial lakes during initial ice retreat, and establishment of three large proglacial lakes at successively lower levels with continued retreat. Glacial Lake Granville drained south through a 1410 ft. threshold, Lake Winooski and Lake Mansfield drained north through 915 ft. and 750 ft. thresholds, respectively. Today the Mad River flows north through rugged uplands with relief of 3620 ft.

Ice retreat was complex and included small-scale readvances that apparently were isolated in sub-basins. Multiple deltas of pre-Granville proglacial lakes occur in the Clay Brook basin. In the southern part of the basin a large morphosequence of Lake Granville age helps establish the ice margin for that time. Deltas of Granville age are restricted to the southern two-thirds of the watershed; however, varves in tributary valleys to the north suggest the ice margin was complex. Very little sand was deposited in Lake Granville, but thick clay varves with silt partings are common. We suggest that Granville ice in the Mad River basin was cold-based and the lake was frozen much of the time, and in a regional sense, glacial plumbing carried coarse sediments east to the Dog River basin.

Lake Winooski was established when ice retreat exposed a lower threshold to the north. Lake Winooski bottom sediments consist of thick pebbly sand with ripple flow to the north. Near Warren, several valleys supplied sediment to a local depocenter and pebbly sand nearly filled the lake. A coarsening upward of these sediments is a product of decreasing lake depth and increasing bottom current velocities. Lake Winooski sediments are often collapsed, indicating that deglaciation between Granville and Winooski times was rapid, leaving abundant dead ice in the valley bottom.

Lake Mansfield also drained north and was restricted to the northern third of the watershed. The present valley is dominated by fan-terrace and stream-terrace deposits. Timing of channel incision and drainage development is not fully understood, but wood in an old landslide, recently exposed by a recent landslide, gave an age of 5430 +/- 50 14C ybp, revealing that lateral migration has been ongoing in the valley bottom since at least the middle Holocene.