GEOMORPHIC ASSESSMENT AND SURFICIAL MAPPING OF GLACIAL LAKE AGASSIZ STRANDLINES IN POLK COUNTY, MINNESOTA

The purpose of this study is to quantify the shoreline history in the southern basin of glacial Lake Agassiz by assessing the geomorphology of its strandlines in northwest Minnesota. Study components include: 1) the identification of strandlines, including beach ridges, escarpments, and spits, and comparing their elevation changes determined from LiDAR-based DEMs, 2) surficial geologic mapping and ground penetrating radar (GPR) data collection to determine shoreline sediment thickness over till, and 3) comparative analysis of the morphodynamic processes and the effects of glacioisostatic rebound (GIA) on strandline geomorphology and elevation. Sediment samples were collected for a surficial geologic map project for the USGS EDMAP Program. The littoral silt, sand, and gravel collected along the slopes of the strandlines serve as indicators of the lake’s various waterplane levels and lake phases. Shoreline thickness from GPR data overlying till ranges from ~1–3 m beneath ridge crests and ~2–6 m near the base of their slopes. Additionally, shoreline tilting from north to south is greatest along the highest elevation strandlines (Herman?), compared to the lowest elevation sub-Campbell strandlines. Based on DEM investigations, field data collection, and using comparative analysis of similar Pleistocene lakes in the North American Midwest, there is reason to conclude that these geomorphological and elevation trends are a result of GIA in the southern basin of glacial Lake Agassiz. Identifying the geomorphic processes and shoreline deposits during the lake’s various waterplane levels provide important data that can be used for reconstructing the shoreline history of glacial Lake Agassiz.