Depiction of the configuration of the top of Precambrian rocks in the New Madrid region has long been tentative because of the relatively small number of datum points available and the difficulties in projecting known structural trends downward through the complex faulting that exists at the base of the Phanerozoic sedimentary section. Geological, geophysical, and seismological studies during the past 15 years have contributed new data that suggest considerable geodynamic activity and the presence of an ancient rift complex in this area.

A map showing the postulated configuration of the top of Precambrian crystalline rocks constructed on the basis of a new understanding of tectonic movements since the close of Precambrian time differs markedly from those prepared previously. The Rough Creek Graben and Reelfoot Rift are fault-bounded, linear depressions with several thousands of feet of vertical offset. These down-dropped blocks, in addition to being filled with thick layers of Cambrian sediments, also served as depocenters during later periods of Phanerozoic deposition. A marked, northwest-trending uplift, the Pascola Arch, is shown as a 10,000-foot uplift intersecting the Reelfoot Rift. The Pascola Arch was a later Paleozoic to middle Mesozoic feature that effectively formed the southern boundary of the Illinois Basin.

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