GENESIS OF STREAMLINED LANDFORMS AND FLOW HISTORY OF THE GREEN-BAY LOBE, WISCONSIN, USA

The distribution of streamlined landforms in southeastern Wisconsin su ggests that drumlins and flutes formed during several phases of the Gr een Bay Lobe between 18 and 14,000 BP. The largest group of drumlins f ormed during a still-stand of the ice margin during the Johnstown phas e, presumably about 18-16,000 BP. Flutes and smaller drumlins are supe rimposed on larger forms, and larger drumlins are remolded. This indic ates that drumlin modification continued during retreat. Three fields containing smaller drumlins formed after ice re-advanced a short dista nce or stabilized during the Green Lake, Rush Lake, and St. Anna phase s about 16-14,000 BP. The drumlin-forming process included erosion and deformation of pre-existing ice-marginal and proglacial sediments. Dr umlins associated with the Johnstown phase show an increase in length up glacier This is probably the result of variations in flow velocity, length of time of drumlin formation, and sediment availability Flutes and small drumlins are associated with retreat moraines and formed ne ar a thin, retreating ice margin. Ice surface profile reconstructions suggest that margins were relatively steep during the Johnstown, Milto n, Green Lake, and early Rush Lake phases. During the Lake Mills and l ate Rush Lake phases, when flutes were forming, the ice surface slopes were lower and margins were retreating. During retreat minor sublobes developed, some perhaps resulting from surges into small proglacial l ake basins. Thus, steep ice margins and driving stresses on the order of 15-25 kPa were typical during drumlin formation. The formation of f lutes, and the remolding of larger drumlins into smaller forms, took p lace beneath gently sloping ice of retreating margins associated with lower driving stresses (<10 kPa).