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The oldest Paleozoic rocks exposed in Iowa are found in the northeastern corner of the state in Allamakee County. Cambrian exposures in the county form part of the classic reference area in the Upper Mississippi Valley for the Upper Cambrian Croixan Series. Overlying dolomites of the Prairie du Chien Group are exposed as bold cliffs in the bluffs along the Mississippi River and its tributaries. Cambrian and Lower Ordovician strata collectively encompass the Sauk Sequence, the first large-scale transgressive-regressive cratonic marine cycle of the Phanerozoic (Sloss, 1963). A major episode of erosion followed deposition of the Sauk Sequence. A widespread unconformity separates Lower and Middle Ordovician strata in the region, marking the boundary between the Sauk Sequence and the succeeding Tippecanoe Sequence.

The most complete sequence of Cambrian and Ordovician strata in northeast Iowa is exposed between Lansing and Church as one ascends from the Mississippi River up the valley of Clear Creek to the crest of Lansing Ridge along Iowa 9 (Fig. 1) 1). Vertical relief along this profile is 630 ft (192 m), and the composite stratigraphic section measured totals 619 ft (189 m). The base of the section begins along Iowa 26 north of the Mississippi River bridge (loc. 1, Fig. 1), and continues through a series of roadcuts and natural exposures in Mount Hosmer Park (loc. 4, Fig. 1). Strata equivalent to the lower half of the section at locality 1 can be seen behind Knopf’s Standard Station

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