ORIGIN OF LOW RELIEF RIDGES NEAR PADUA, STEARNS CO., MINNESOTA

The goal of this study was to determine the origin of multiple, northeast – southwest trending, low relief  (7 to 10m high and 2 to 2.5km long) ridges, located near Padua, Stearns Co., Minnesota.  These ridges lie north east of the Alexandria moraine complex in an area that has been mapped as Des Moines lobe ground moraine.  Three hypotheses have been suggested for the origin of these landforms: 1) the ridges are ice marginal (e.g. lateral moraines), created during the advance of the northeastward flowing Granstburg sublobe of the Des Moines Lobe; 2) the relief is palimpsest - pre-existing ridges of undetermined origin covered by Des Moines till during the (post-Grantsburg) over-riding of the Alexandria Moraine; and 3) these are end moraines of a southeasterly flowing sublobe (named the Bonanza Valley sublobe) of the Des Moines lobe.  

Research focused on the stratigraphy of ridges and the texture and provenance of the till units. If the ridges are composed of units of multiple sources (e.g. Wadena lobe drift overlain by Des Moines lobe drift) then the relief of the ridges is palimpsest.  On the other hand, if the ridges were composed entirely of Des Moines lobe drift then they were formed by the northeast flowing Grantsburg sublobe or the southwest flowing Bonanza Valley sublobe.  Since the model for the Bonanza Valley sublobe suggests it actively eroded older units (e.g. Wadena lobe till), provenance analysis should differentiate Grantsburg sublobe deposits from Bonanza Valley sublobe deposits.

Sediment from four outcrop locations and 2 drill holes were described and collected. Samples were prepared, dry sieved, and point counts were preformed on the coarse sand size fraction. Lithologies were identified and percentages were calculated for each sample. Initial results indicate that the ridges are not homogeneous.  However the provenance of the multiple till and sand units is difficult to determine.  The units may represent overriding or they may represent late stage stagnation.  Additional analysis is underway.

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