Minor end moraines of the Wisconsin Valley Lobe, north-central Wisconsin, USA

Approximately 35 parallel, discontinuous glacial ridges occur in an area of about 100 km(2) in north-central Wisconsin. The ridges are located between about 6 and 15 km north (formerly up-ice) of the maximum extent of the Wis consin Valley Lobe of the Laurentide Ice Sheet. The ridges are between 1 an d 4 m high, up to 1 km long, and spaced between 30 and 80 m apart. They are typically asymmetrical with a steep proximal (ice-contact) slope and gentl e distal slope. The ridges are composed primarily of subglacial till on the ir proximal sides and glacial debris-flow sediment on the distal sides. In some ridges the till and debris-flow sediment are underlain by sorted sedim ent that was deformed in the former direction of ice flow. We interpret the ridges to be recessional moraines that formed as the Wisconsin Valley Lobe wasted back from its maximum extent, with each ridge having formed by a se quence of (1) pushing of sorted ice-marginal sediment, (2) partial overridi ng by the glacier and deposition of subglacial till on the proximal side of the ridge, and (3) deposition of debris-flow sediment on the distal side o f the ridge after the frozen till at the crest of the ridge melted. The mor aines are similar to annual recessional moraines described at several modem glaciers, especially the northern margin of Myrdalsjokull, Iceland. Thus, we believe the ridges probably formed as a result of minor winter advances of the ice margin during deglaciation. Based on this assumption, we calcula te the net rate of ice-surface lowering of the Wisconsin Valley Lobe during the period when the moraines formed. Various estimates of ice-surface slop e and rates of ice-margin retreat yield a wide range of values for ice-surf ace lowering (1.7-14.5 m/yr). Given that ablation rates must exceed those o f ice-surface lowering, this range of values suggests relatively high summe r temperatures along the margin of the Wisconsin Valley Lobe when it began retreating from its maximum extent. In addition, the formation of annual mo raines indicates that the glacier toe was thin, the ice surface was clean, and the ice margin experienced relatively cold winters.