SUBGLACIAL DRAINAGE, GLACIAL LAKE HISTORY, AND SUBSEQUENT STREAM INCISION HISTORY, MILLER BROOK VALLEY, NORTHERN VERMONT

Miller Brook drains the eastern slope of the Green Mountains in northern Vermont below Nebraska Notch, the lowest elevation along the Green Mountain ridge between the Winooski River to the south and the Lamoille River to the north. A subglacial drainage system developed across the notch and down the Miller Brook valley while hydraulic gradients in the ice sheet were still sloping to the southeast, across this major north-south topographic divide. Evidence of this flow includes large-scale potholes at the higher elevations and a well-developed segmented esker system at the lower elevations. The esker system has been mapped for ~4 km, sometimes as a single ridge and sometimes as multiple, subparallel, intersecting ridges before it disappears beneath younger lacustrine sediments. Cross-cutting relationships show the relative ages of these intersecting esker ridges. As the ice sheet retreated northwest and west, up the Miller Brook valley, it was bounded by an arm of Glacial Lake Winooski, the glacial lake that occupied much of north-central Vermont when the Winooski and Lamoille River valleys were dammed by the retreating ice sheet. Detailed exposures show a complete transition from coarse esker tunnel facies sediments to sediments deposited in a subaqueous fan at the tunnel mouth to ice-distal lacustrine sediments. Lake water depths over the exposed esker vary from 60 m to 0 m. A delta at the head of the valley is one of the pinning points used to ascertain the isostatic tilt of the former lake surface at 1.25 m/km to 352 (Derr, 2011). A well-exposed section of varved silt and clay in the Waterbury Reservoir ~8 km SE of the Miller Brook valley was used to determine that the ice sheet was retreating up the Miller Brook valley shortly after the Reservoir area was deglaciated ~13,880 years ago. An abrupt change in the lacustrine stratigraphy marks the catastrophic draining of Glacial Lake Winooski ~13,700 years ago and the onset of incision into the recently deposited glacial lake sediments by Miller Brook. A detailed map outlining the incision history in the upper part of the valley is presented in a companion poster (Cronauer, 2012). In the lower part of the Miller Brook valley incision was limited indicating that Miller Brook was graded to another, lower lake level, that of Glacial Lake Mansfield, locally at an elevation of ~270 m.