DECIPHERING PRE-ILLINOIAN FLOW PATHS IN THE WESTERN CENTRAL LOWLANDS

Substrate lithology and slope have a large influence on flow paths of continental scale glaciation- potentially over multiple iterations of the North American Ice Sheet. To illustrate, the advance of the Late Wisconsinan (Marine Isotope Stage [MIS] 2) Keewatin-sourced Des Moines Lobe followed a trough formed between the boundary of the Phanerozoic-aged and Proterozoic-aged bedrock through Canada, Minnesota, and the Dakotas. This bedrock trench is over two hundred kilometers wide and hundreds of meters deep; in places it is partially filled with older Quaternary sediments, thus indicating that it was established before the last glacial stage. In Canada, the trough is bounded to the west by the Manitoba escarpment formed in Cretaceous sedimentary rocks. These Mesozoic deposits rise in elevation toward the Rocky Mountains. Further south, the Prairie Coteau -formed from thick glacial diamicton and Cretaceous-aged sedimentary bedrock- continues to serve as the margin in the Dakotas. The trough’s eastern boundary is formed by Archean bedrock which shallowly rises to the east. The Des Moines Lobe streamed within this boundary and may have encountered Labrador sourced ice or till to form its eastern margin. Additionally, this flow path appears to have been established before the Late Wisconsinan due to the recognition of Middle Wisconsinan (MIS 3) tills that shared a similar provenance and flow path. Furthermore, river orientations and subtle curvilinear landscape features that crosscut basins beyond the Wisconsinan margins in Iowa suggest that ice flowed down this route during the last Pre-Illinoian (>MIS 6) advances of the North American glaciers. Pre-Illinoian Ice sheet configurations and flow paths are poorly understood but are required to reconstruct continental scale landscape evolution.