Late Valmeyeran and Chesterian sediments that repeatedly filled the subsiding Illinois basin are a quarter autochthonous carbonate, a quarter sand, and half mud brought from remote northeastern sources by a major river system, the Michigan River. These are arranged in cycles implying shoreline fluctuation landward (northeastward) and seaward through a range of a few hundred miles. About 70 minor reversals in shoreline movement are superimposed on about 15 major cycles. Increases in competence, perhaps in phase with modest lowering of sea-level, repeatedly changed the Michigan River into a prograding stream, pushing its delta scores of miles across a marine basin only a few tens of feet deep. Advancing distributaries carved grooves well below sea-level through frontal and ateral mud banks--grooves later filled by elongate sand bodies analogous with modern barfingers and pass fillings. In late stages the delta supported land vegetation. The carbonate phase

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of the cycle was initiated by decrease in stream competence and continued basin subsidence. Changes in sea-level and stream competence depended on climatic variation.

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