The Dubuque Formation of Upper Ordovician age crops out in the Upper Mississippi Valley. It comprises interbedded carbonate and argillaceous rocks that are approximately 35 feet thick in Iowa and Illinois, but thicken to a maximum of approximately 45 feet in southern Minnesota. Three proposed informal subdivisions: Frankville, Luana, and Littleport beds, are differentiated on the basis of bed surface topography ranging upward from nearly planar beds in the Frankville to prominently undulose surfaces in the Littleport beds. The Frankville beds represent a transition from the massive dolomite of the underlying Stewartville Member of the Wise Lake Formation to the overlying interbedded carbonate rocks and shale of the upper Dubuque. The base of the Dubuque Formation in Iowa and Minnesota is placed at a prominent, approximately 8 inch thick, carbonate bed at the base of the Frankville beds. This \u27\u27marker bed\u27\u27 provides a more precise datum for lithostratigraphic correlation than the lowest prominent shale parting employed by previous workers to identify the base of the Dubuque
