THE EFFECT OF GLACIAL RIVER WARREN ON BOULDERS AND BEDROCK IN THE BIG STONE NATIONAL WILDLIFE REFUGE, WESTERN MIINNESOTA: ASSESMENT OF THE EVIDENCE OF CATASTROPHIC FLOW

The granite boulders and bedrock surfaces exposed at the Big Stone National Wildlife Refuge in the former Glacial River Warren spillway of Glacial Lake Agassiz in western Minnesota are studied to determine if Glacial River Warren experienced catastrophic (outburst) flow. Previous studies by Fisher (2004) and Patterson and Boerboom (1999) resulted in very different interpretations of the site. Fisher (2004) suggests that boulder morphology and streamlined bedrock forms oriented parallel to the river valley are evidence of a catastrophic flow regime. On the other hand, Patterson and Boerboom (1999) interpret bedrock morphology and boulder characteristics as the result of chemical weathering and not catastrophic flow.

The goals of the study are to: 1) determine if the study area has a young, unweathered surface, 2) determine if the bedrock has a preferred jointing pattern, and 3) determine if channel orientations are structurally or fluvially controlled. Bedrock outcrops were studied to determine if River Warren had moved overlying boulders. Additionally, boulder, joint, and channel trends were measured to determine if boulders had been oriented by the river and whether fluvially eroded channels were structurally controlled.

The study showed that the boulders are neither fluvially streamlined nor oriented parallel to the flow direction of Glacial River Warren. Rather, boulder morphology is heavily influenced by exfoliation, as stated by Patterson and Boerboom (1999). Bedrock joints and channel trends are of a predominantly easterly orientation. Channels are very minimally developed and appear to reflect erosion along joints by a moderately rapid flowing river. With exfoliation and jointing controlling the surface features, there is insufficient evidence in the study area for a catastrophic flow regime by Glacial River Warren.

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.