Role of the Wadena Lobe in the Wisconsin Glaciation of Minnesota

Abstract

The Wadena ice lobe deposited drumlins and other glacial drift in Wadena County and adjacent parts of west-central Minnesota. The area is in the contact zone between drifts of eastern and western provenance; Upham and Leverett had disagreed about the direction of ice flow in this region. The Wadena drumlin field has a fanlike form with its long axis trending southwest and a fan margin of 70 miles along 130° of arc. The fan pattern suggests ice movement from the northeast rather than from the southwest as Leverett suggested. Studies of pebble lithology and carbonate content suggest that initially the Wadena lobe moved southeastward into northern Minnesota from the limestone belt of the Winnipeg lowland, thence was diverted southwestward by the contemporaneous Brainerd lobe advancing from the east. Incorporation of Brainerd ice and debris diluted the carbonate content of the Wadena drift in the northeastern part of the drumlin field, accounting for anomalous leaching depths and pebble counts in this area. This interpretation differs from the writer's earlier assumption that a distinct overburden of noncalcareous till of eastern source (Brainerd till) existed on the northeastern part of the drumlin field. Stone-orientation studies at 22 localities show that elongated stones have a preferential plunge in the up-glacier direction (to the northeast) at moderate angles (15°–25°), and the writer suggests that the drumlin till was deposited by basal ice characterized by shear planes dipping up-glacier. Fabric of stones in the northeastern part of the drumlin area does not support the hypothesis that an overburden of Brainerd till covers the Wadena till. The Wadena drumlin field, formed during the Cary subage of the Wisconsin glacial age, is truncated and buried on the east by the slightly younger St. Croix moraine (Cary?) and its outwash aprons, on the north by the Itasca moraine and its outwash (also Cary?), and on the west and south by overlap of the Des Moines-lobe (Mankato?) till and associated outwash. At its maximum the Wadena lobe may have extended south beyond Minneapolis, east almost to Wisconsin, and west to the Alexandria moraine complex (Altamont-Gary moraine of Leverett). Thus it may have contributed a major share of ice to Minnesota during the Cary phase of glaciation but was crowded and eventually overlapped on the east by the Brainerd and Superior lobes and on the west and south by the Des Moines lobe.