GIS ANALYSIS OF EVIDENCE FOR A LATE WISCONSINAN CALVING EMBAYMENT IN THE PENOBSCOT RIVER VALLEY REGION NEAR BANGOR, MAINE

The Penobscot River Valley region near Bangor, Maine, has land-surface elevations ranging from sea level to 350 m asl. During the Late Wisconsinan ice retreat, the marine limit was 97 m above current sea level based on a marine delta topset/foreset contact (Thompson et al., 1989). The glacier terminated in much deeper water in the Penobscot Valley. Because calving rates are controlled by depth, a calving embayment may have developed at this time. Glacial striation and crag and tail data have been analyzed in this study to evaluate evidence for a possible calving embayment.

Ice-flow data collected over 15 years by Maine Geological Survey researchers were plotted as point features using ArcGIS within the 1,185 km² study area. The relative-size criterion was used to determine age relationships between different flow-indicator sets. The study area was divided into upland/lowland regions (above or below 67 m asl) and areas west/east of the Penobscot River. A geospatial and statistical analysis was conducted for striations (n=257 non-unique flow indicators) and crag and tail features (n=108 unique flow indicators) in each defined region.

Ice flowed south (170^o-180^o azi) during the Late Wisconsinan maximum. As deglaciation began, ice flow converged toward the Penobscot Valley (Syverson and Thompson, 2008). In the western lowland, secondary flow became more easterly (90^o-110^o azi). In the eastern lowland, secondary flow became westerly (270^o-290^o azi). Transitional flow indicators were not observed east of the Penobscot Valley.

Convergent secondary flow patterns in the lowlands are evidence for a calving embayment in the region. The lack of transitional flow indicators suggest that the flow direction changed abruptly east of the modern river. The lack of secondary flow indicators in the highland region is likely because the uplands were above the marine limit during glacial retreat, so these areas were pinning points for the glacier. Evidence for major changes in ice-flow directions is observed 18 to 5 km to the west and east of the Penobscot River, respectively. The results of this study suggest a narrow calving embayment within the Penobscot Valley during the Late Wisconsinan retreat. LiDAR imagery has not revealed morainal banks to delimit the width of the calving embayment.