North-Central Section - 42nd Annual Meeting (24–25 April 2008)

Paper No. 1
Presentation Time: 8:00 AM-12:00 PM


THOMPSON, Andrew H. and SYVERSON, Kent M., Geology, University of Wisconsin - Eau Claire, 105 Garfield Avenue, Eau Claire, WI 54702-4004,

The Penobscot River valley is in a moderate-relief portion of coastal Maine deglaciated approx. 13,000 yrs ago. Glaciomarine deltaic sediments indicate sea-water depths ranging from 0 to 100 m in the valley during deglaciation (W.B. Thompson et al., 1989). Because one of the main controls on the rate of calving is water depth, some have informally suggested that a calving bay must have been present in the Penobscot River valley, but this has been disputed. According to Lowell (1994), a calving embayment did not develop in the Penobscot valley because the deep-water area was too narrow. The purpose of this study was to map ice-flow indicators and determine if major changes in ice-flow direction suggest the existence of a calving embayment in the Penobscot River valley near Bangor, Maine.

We mapped the Bangor quadrangle as part of a STATEMAP project to find smooth bedrock surfaces preserving ice-flow indicators. We measured the orientations of 116 striations (showing non-unique flow direction) and crag-and-tail features (showing unique flow direction). The relative size criterion was used to evaluate ages of flow indicators. The data was analyzed using RockWorks99 to discern ice-flow patterns and calculate vector means. Only unique ice-flow direction data are reported here.

Ice flowed to the south (175° azimuth vector mean, n=18) during the oldest event (flow maximum) throughout the map area. West of the Penobscot River, a continuous range of younger flow indicators becomes more easterly (100° azimuth flow toward the river). East of the Penobscot River, a younger, robust westerly flow is indicated toward the river lowland (280° vector mean, n=14). Flow indicators between 174° and 280° are lacking, suggesting a rapid change in flow direction. These flow direction changes are in an area with gentle surface slopes, so changes were not caused by ice sliding down the bedrock surface. Westerly flow away from the coast has not been observed previously in this part of Maine and is evidence for a calving embayment.

An area within the embayment should only record flow indicators from the ice-flow maximum. Changes in flow direction can be seen as close as 1 km east and west of the river. Thus, a relatively narrow calving embayment (<2 km wide) must have existed in the Penobscot River valley near Bangor.