The upper part of the Mississippian column in the type region is the Pope Megagroup, a cyclic alternation of limestone-dominated and clastic-dominated units. Its lower boundary transects time, so that in different districts the Pope equals part of the Chesterian, all of the Chesterian, or includes the Chesterian plus some late Valmeyeran. Within the type region these rocks are essentially confined to the Illinois basin, which as it gradually sank was repeatedly filled by sediments carried from eastern Canada by the Michigan River system, and by locally precipitated limestone. The fill averaged half mud, a quarter sand, and a quarter limestone.
The shoreline fluctuated several hundred miles landward (northeastward) and seaward, with 70 or more minor reversals in direction superimposed on about 15 major cycles of advance and retreat. Sea-level is inferred to have been relatively static, and variation in rainfall was the primary factor controlling sedimentary cycles. A clastic unit was deposited when the Michigan River delta prograded scores or hundreds of miles across the subsiding basin in response to increased sediment yield that was triggered by a drier climate in the source area. Lunate sand bars at the mouths of advancing distributaries were lengthened into bar fingers. Unweathered grooves, later filled with more sand, were gouged far below sea-level behind the advancing mouth bars. The delta surface supported vegetation, and in some cycles became a great sea-level swamp when increased humidity and more effective plant cover in the source region so reduced sediment yield that clastic sedimentation could no longer keep pace with basin sinking. When plant growth could not keep up with continued subsidence, the sea again covered the area and limestone was deposited until return to a drier climate at the sediment source brought the cycle full turn.
A few cycles seem correlated with changes in sea-level, as were many Pennsylvanian cycles. The alternation between tropical pluvial and subtropical interpluvial stages that caused the cyclic sedimentation in what is now the northern hemisphere corresponded in some manner to fluctuations of the boundary of continental ice in the southern hemisphere. However, the direct control of the sedimentary rhythms was rainfall, not glaciation.