PALEOGLACIOLOGICAL CONTEXT OF ROGEN MORAINE, NORTHEASTERN MINNESOTA

Rogen, or ribbed, moraines are enigmatic ridges characteristic of continental glaciations. Transverse to ice flow and of uncertain origin, they are generally assumed to be subglacial landforms. Recent work (Hättestrand and Kleman, 1999) suggests that the Rogen landscape is essentially a “boudinaged” till sheet that has fractured due to extensional forces at the base of the ice sheet. During retreat, these forces are generated at the transition from cold-based (ice frozen to bed) to warm-based (ice sliding over bed) basal thermal regime.

Rogen moraine in northeastern Minnesota offers a unique perspective on potential modes of formation and paleoglaciology of the Late Wisconsin Rainy Lobe. Eskers and striated bedrock provide little evidence that cold-based conditions existed in this area; however, an abrupt change in basal conditions along this flowline may have resulted in initiation of extensional flow and Rogen formation. The Rogen landscape is developed in an area of thin till, down-glacier from ice-scoured Canadian Shield bedrock and up-glacier from a thicker drumlinized till sheet. This transition poses intriguing questions as to the behavior of the Laurentide Ice Sheet along this flowline and suggests that in this setting, a transition in the glacial substrate rather than basal temperature may be responsible for Rogen moraine formation.

Our results suggest that Rogen formation can occur anywhere ice flow accelerates over unconsolidated substrate. Both a transition from a frozen to thawed bed and from crystalline bedrock to unconsolidated bed result in ice acceleration and thinning, producing extensional forces at the base of the ice sheet.