DELINEATING GLACIAL FEATURES FROM CHIRP DATA AT HARRINGTON LAKE, MAINE, USA

Glacial features, including eskers, are preserved across Maine. Abundant lakes mask the glacial landscape throughout the state, obscuring spatial continuity of glacial features. Using 2-D CHIRP combined with LiDAR data, glacial features can be mapped below present-day lake surfaces to allow for better understanding of glacial and post-glacial landscape change.

Harrington Lake is a 5.5 km-long lake located in central Maine, situated in a NW-SE trending valley and shallowing and draining to the northwest. The lake was dammed in 1930 with a 4.9 m-high dam. At present, the maximum lake depth is 40.8 m, suggesting the presence of a lake or a series of small ponds prior to damming. CHIRP reflection data were collected in a grid along (NW-SE) and across (NE-SW) the lake. Three NW-SE lines and six NE-SW lines were interpreted in combination with LiDAR data from the adjacent landscape. Points along each line were selected to mark 1) depth to lake bottom, 2) thicknesses of post-glacial sedimentary layers, 3) thickness of the basal layer, and 4) depth to the top of the basal layer. Annotated CHIRP images were georeferenced using ArcMAP and raster contour maps were generated using the natural neighbor technique. These maps were then used to interpret glacial geomorphology and post-glacial evolution of Harrington Lake.

The main esker, which can be seen on the LiDAR, enters the northeastern part of the lake and oriented NW-SE. Another small esker with a similar orientation is west of the main esker. Lake depth is shallowest over the eskers and deepest to the west and south of the eskers. Three post-glacial lacustrine sedimentary layers were measured and mapped. The sedimentary layers are thickest in the deepest parts of the lake, with up to 17 m of sediment just west of the main esker. Post-glacial sedimentary layers pinch out against the eskers and are absent in some areas, suggesting fluvial modification within the basin following glacial retreat. Based on initial core description, the upper unit is post-glacial organic-rich mud and the lower unit has glacial-lacustrine lamination, likely varves. The basal layer is interpreted as glacial till, gravels associated with eskers, and/or bedrock. Basal layer thickness varies, but is thinnest near the esker and along the western shore of the lake.