Variations in the relative abundance of kaolinite and illite in the clay-size fraction of the Ordovician Glenwood Formation of southeastern Minnesota, western Wisconsin, and northeastern Iowa suggest that the source area for this shaly unit was to the southwest. Kaolinite is most abundant in the southwesternmost samples; it decreases in amount to the northeast and is absent in the most northeasterly samples. Illite increases in abundance as kaolinite decreases. Limestone and dolomite beds in the shale are more abundant in the illite-rich region to the northeast.

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