THE RELATIONSHIP OF THE BLUE MOUNDS TO THE SWIFT FALLS DELTA COMPLEX, POPE AND SWIFT COUNTIES, MINNESOTA

The Swift Falls Delta Complex is located two miles south of Swift Falls, Swift County, west-central Minnesota. The Delta was formed in Glacial Lake Benson, a moraine-dammed lake that formed during the retreat of the Des Moines lobe. The source of the deltaic sediments may be the Blue Mounds, a unique glacial landform located ten miles north of the delta. The Blue Mounds are made up of (roughly) parallel ridges, two to fifteen meters tall, that trend northwest – southeast for approximately 6 miles. The goal of this study was to determine the origin of the Blue Mounds and to reconstruct the sequence of events that formed both the Blue Mounds and the Swift Falls delta complex.

Sedimentary features in the delta were observed and described in the field (Maus, and Reiss, 2000). These features provide evidence for an ice-proximal setting in which ice blocks, flow till and ice-rafted boulders were regularly added to the delta. The lithologic character of the deltaic sediment indicates that meltwater of the Des Moines lobe created this feature. The implied position of the ice margin during formation of the delta complex is to the north, perhaps within or near the Blue Mounds.

In the field, sedimentary structures and stratigraphy within the Blue Mounds were observed and compared. Provenance studies were done on the Blue Mounds sediments and compared to that of the deltaic sediments. It appears that the Blue Mounds were formed as Des Moines lobe sediments filled in supraglacial topographic low spots. These low spots were probably crevasses that formed as the Des Moines lobe overrode the Alexandria moraine. It is also possible that the Blue Mounds formed as an interlobate moraine, between the Wadena and Des Moines lobes while the delta was deposited.

Research for this study was funded by grants from the N.S.F.-R.E.U. Program (NSF-EAR 9820249) and the University of Minnesota - Undergraduate Research Opportunities Program (UROP).