WATER PLANE HISTORY DERIVED FROM LAKE AGASSIZ SHORELINES, CENTRAL POLK COUNTY, MINNESOTA

Recent high-quality LiDAR has made mapping complex sequences of Lake Agassiz strandlines possible. Proglacial Lake Agassiz formed between the retreating Laurentide Ice Sheet (LIS) and the Big Stone Moraine between 14-8 ka. The opening of successively lower outlets caused overall water level decline across the basin which was complicated by readvances of the LIS and glacio-isostatic adjustment.

The field site in central Polk County, Minnesota contains over seventy strandlines consisting of beach ridges, spits, and scarps. Above the Campbell scarp and Tintah gap, ten strandline complexes are marked A (highest) through J (lowest). The northern end of each complex consists of a few beach ridges and the number of ridges and spits in each complex increases to the south across 8 km. Water plane elevation is estimated from the lower break in slope of the basinward (west) side of the strandlines. In each complex the water plane elevation decreases westward and the elevation of the youngest ridge in each complex about 4 meters lower than the highest. In general, there is a 5-meter difference in elevation between complexes. These data suggest many more episodic drops in water level than the number of named water planes at Lake Agassiz’s southern outlet. Within the study area (Polk Co.) what is inferred to be the Campbell strandline is recorded by an 8-meter-high scarp, a few ridges at the base of the scarp, and a large spit complex built into a bay. Further west at lower elevations there are well-developed but small beaches recording lower lake levels. Optical ages are pending from across the entire strandline complex. A detailed water-plane history is expected from this site based on the high number of strandlines paired with optical ages.