The Pope Megagroup is a cyclic alternation of limestone and clastic units, whose lower boundary transects Chesterian time and some Valmeyeran. Within the type region these rocks are confined to the Illinois basin, which as it sank was repeatedly filled by sediments carried from eastern Canada by the ancient Michigan River system, and by locally precipitated limestone. A few cycles seem to be correlated with changes in sea level, but direct control of the rhythms was rainfall, not glaciation, corresponding to fluctuations of continental ice in the southern hemisphere.

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