ANALYSIS OF THE SUBSURFACE STRATIGRAPHY OF THE EAST BRANCH OF THE CHIPPEWA RIVER, POPE CO., MINNESOTA: ARE THE BLUE MOUNDS CONTEMPORANEOUS WITH GLACIAL LAKE BENSON?

The Blue Mounds are a series of sub parallel to parallel ridges between two and fifteen meters tall located in Pope County, Minnesota. Consisting mainly of sand and gravel distinctly different from the underlying till of the Alexandria Moraine, the mounds extend ten kilometers southeast and are one kilometer wide. The mounds are thought to have been deposited by glacial activity although it remains unclear as to exactly how and when. The goal of this study was to determine the origin of the Blue Mounds and if they formed while Glacial Lake Benson, a moraine-dammed lake formed during the retreat of the Des Moines lobe, existed.

Core samples were retrieved from four sites in and along the flood plain of the East Branch of the Chippewa River an apparent paleo-drainage system from the Blue Mounds to the Swift Falls Delta, which formed in Glacial Lake Benson. The stratigraphies of the five cores were described and the sediments underwent sieve analysis, point counts, thin-section analysis, and scanning electron microscope (SEM) analysis. Sieve analysis show high percentages of sand with textures ranging from a loam to loamy sand, and the point count analysis results suggest a Des Moines lobe provenance. Both the thin-section and scanning electron microscope analyses confirmed these results.

Although the relationship between the Blue Mounds and Glacial Lake Benson remains unclear, the methods used in this study have the potential to solve the problem. Additional coring and higher resolution of the late glacial stratigraphy may enable the correlation of the Blue Mounds and Glacial Lake Benson.

Research for this study was funded by a grant from the N.S.F.-R.E.U. Program (NSF-EAR 0640575) and the University of Minnesota, Morris.