This paper is concerned with the origin and geologic history of the boundary that separates the widespread St. Peter sandstone from the underlying Shakopee dolomite. That international surface is highly undulatory, and most contemporary geologists who have examined it believe that the contact irregularities result from pre-St. Peter subaerial erosion of the upper Shakopee surface. Some earlier investigators do not entirely concur.
This report is a by-product of work done for the U. S. Geological Survey during an investigation of the lead-zinc resources of Wisconsin by the U. S. Geological Survey in cooperation with the Wisconsin Geological and Natural History Survey. Attention was focused on the problem by stratigraphic studies of the Prairie du Chien formations, particularly of the Shakopee dolomite, in which structural and lithologic characters observed in the carbonate sediments seemed to be incompatible with the widely accepted erosional origin of the upper Shakopee surface. It was decided, in 1950, to explore the whole problem of the Shakopee-St. Peter relations by detailed outcrop examinations, supplemented by subsurface and other studies that might contribute to a better understanding of the genetic relationships involved. The area of the investigation was restricted to three counties in southwestern Wisconsin and closely adjacent areas in Iowa(fig. 1), differing in this respect from the broader and more general studies that previously have tended to be the rule.
