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Northeastern Section (45th Annual) and Southeastern Section (59th Annual) Joint Meeting (13-16 March 2010)

Paper No. 5
Presentation Time: 8:00 AM-12:05 PM

THE MULTI-STAGE FORMATION OF THE BIG STONE MORAINE, WEST-CENTRAL MINNESOTA: GEOMORPHOLOGIC ANALYSIS OF THE ICE-MARGINAL, GLACIAL CHIPPEWA RIVER


CONDIT, Cailey, Department of Geology, Middlebury College, Middlebury, VT 05457, EVELETH, Rachel, Department of Geology, Bowdoin College, Brunswick, ME 04011 and COTTER, James F.P., Geology Discipline, University of Minnesota, Morris, Morris, MN 56267, ccondit@middlebury.edu

The Big Stone Moraine is the last major constructional feature of the Des Moines lobe prior to the formation of Lake Agassiz. Although critical to the understanding of the deglacial history of the Des Moines lobe, the formation and climatic significance of the Big Stone Moraine is poorly understood. The goal of this study is to analyze the terraces and sediments of the glacial Chippewa River to determine if the Big Stone Moraine was deposited over a significant interval of time in multiple phased events.

New geomorphic analysis indicates that both the Chippewa and the Pomme de Terre rivers formed as ice marginal streams during the retreat of the Des Moines lobe in west-central Minnesota. The Chippewa River is on the lateral (eastern) side of the Big Stone Moraine while the Pomme de Terre River (which is parallel to the Chippewa) is within the moraine complex. Calculations of glacio-fluvial paleoflow rates based on terrace cross-sections were nearly identical for the two drainages. The single terrace within the Chippewa Valley grades to a deltaic structure within Glacial Lake Benson while the terraces of the Pomme de Terre River grade to a post-drainage plain in the Glacial Lake Benson basin.

Paleoflow calculations and geomorphic observations support the conclusion that the Chippewa and the Pomme de Terre were ice marginal streams during the deposition of the Big Stone Moraine. This suggests that the Chippewa represents an early stage of the construction of the Big Stone Moraine during the existence of Glacial Lake Benson and that subsequent retreat of the Des Moines lobe led to the drainage of Glacial Lake Benson followed by the formation of the Pomme de Terre as an ice marginal stream. Therefore, both rivers represent distinct stages of the formation of the Big Stone Moraine over a significant period of time.

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

Session No. 10--Booth# 22

Quaternary Geology, Geomorphology, and Archaeological Geology (Posters)
Sunday, 14 March 2010: 8:00 AM-12:05 PM
International ABCDF (Sheraton Baltimore City Center)
Geological Society of America Abstracts with Programs. Vol. 42, No. 1, p.67
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