In 1924, E. O. Ulrich described an unconformity at the boundary between his Ozarkian and Canadian systems in the upper Mississippi Valley. This systemic boundary was based on physical and paleontological data, and occurs within the Prairie du Chien Group ("Lower Magnesian" of earlier years) which had been accepted as Ordovician in age.

Prairie du Chien strata have been problematical to stratigraphers for many years, primarily because of a lack of systematic regional study. Most investigators denounced Ulrich's unconformity but a few workers reported local evidences of erosion between the lower Oneota Formation and the upper Shakopee Formation.

Recognition and investigation of rock-stratigraphic boundaries presuppose recognition of the units themselves, which has been the major problem in Prairie du Chien stratigraphy. Once this had been accomplished the presence of an unconformity on the upper surface of the Oneota Formation was obvious.

Physical criteria for erosion at this horizon are (1) local truncation of bedding; (2) irregular upper surface of the Oneota Formation; (3) basal conglomerate of chert pebbles and/or Oneota-type lithology in the lowermost Shakopee Formation; and (4) truncation of grains at contact as seen in thin section.

This unconformity can be recognized throughout the outcrop belt of Prairie du Chien strata. Erosion took place after lithification and seemingly after dolomitization of the Oneota. It may have been subaqueous erosion and probably does not represent the great period of time postulated by Ulrich.

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