GLACIAL DEPOSITIONAL FEATURES REVEALED ON LIDAR IMAGERY: IMPLICATIONS FOR THE BEHAVIOR AND MARGIN POSITION OF THE LAURENTIDE ICE SHEET DURING RETREAT

It has been suggested that a calving bay existed in the Penobscot lowlands during the retreat of the Laurentide ice sheet from the Maine coast about 15 ka to 12 ka (Syverson and Thompson, 2008; Lowell, 1994). However, the distinct path of the Katahdin esker suggests the presence of a lobe here. We used LiDAR imagery to map DeGeer moraines, presumed to be largely annual and riddling the Maine coast below the marine limit, and found that margin positions so defined fail to support the presence of either a lobe or a calving bay. Rather, they suggest a relatively straight northeast-southwest margin at the time of moraine formation. The lobate geometry may have developed later as the ice thinned over some low mountains.

Flutes are common west of Penobscot Bay but rare east of it; this may reflect the roughness of the bedrock topography to west, promoting till deposition and flute formation. Locally, moraines are not normal to flutes, suggesting possible differences in ice flow direction during advance and retreat. Large moraines suggest a stillstand where the margin remained in the same location for two or more years, building the morainic deposit with each winter advance. One large moraine, likely contemporaneous with the Pineo Ridge moraine, crosscuts a series of smaller moraines. This supports (Borns,1973) hypothesis that Pineo Ridge was deposited during a re-advance. Variations in moraine spacing reflect variations in retreat rate. However, at least four deltas, which typically form during stillstands (Sirkin, 1968; Dietrich et.al., 2017), appear in a line parallel to ice flow in a sequence of widely-spaced moraines. While moraines below the marine limit are relatively continuous and evenly spaced, hills rising above the marine limit display discontinuous and haphazard moraines. The influence of local topography on ice behavior and subsequent moraine formation and erosion begs further investigation.