PRELIMINARY GEOMORPHIC MAP OF THE GLACIAL FEATURES OF THE PIERRE SUBLOBE, SD

During the last glacial period, the James Lobe of the southern Laurentide Ice Sheet occupied much of eastern South Dakota, reaching the Missouri River at its western and southern margins, and the Prairie Coteau to the east. Several small sublobes formed along the James Lobe’s western flank, the largest of which was the Pierre Sublobe, which terminated at the Missouri River near Pierre, SD. We recognize three basic drifts in the Pierre Sublobe based on relative dating. The oldest is Tazewell drift, which is exposed along the Missouri River and overall has the least developed moraines. However, several sharp, boulder-rich moraine ridges mark the westernmost Tazewell drift limit in places along the river. Otherwise, this unit is characterized by relatively muted topography, likely because of the loess cover. This is the only place this happens in the James Lobe, where the ground surface slopes gently westward towards Pierre, into the former ice flow direction. The second drift unit, Cary drift, is dominated by low-relief ground moraine with scattered sharp, boulder-rich moraine ridge segments. Also common in Cary drift are stagnation moraines, or extensive, hummocky moraines with low-relief mounds on the surface. Chains of kettles are aligned sub-parallel to the the larger Cary stagnation moraines. Large outwash terraces lie just outboard of the outermost Cary moraines and often have eskers along their edges. The youngest drift unit is Mankato drift, which is characterized by distinct arcuate moraine ridges, which are in many places dissected by glacial outwash channels and terraces. Kettles and push moraines are common along the boundary of Mankato and Cary drift. The Pierre Sublobe is important because it is the only location along the James Lobe margin where the relative sequence of drift units can be observed. This is aided by many of the younger glacial meltwater and post-glacial alluvial channels that cut through the older drift, allowing the development of a relative age.