The Eastern stable interior is characterized by arches, domes, and synclinal basins usually covered by sedimentary layers dating back to the Cambrian. On the basis of data from outcrops and deep tests drilled into or through the sedimentary cover, activity in the underlying crustal (basement) rocks can be interpreted from many of the sedimentary tectonic features created by this movement. Geophysical studies of crustal rocks indicate that as much as 12 km (7.5 mi) of relief exists at their base and that there is layering within the crust. Since Precambrian time, new crust, probably supplied by a stationary hot spot in the asthenosphere as continental drift shifted the overlying plate, created uplift of features such as the La Salle anticlinal belt, while dense crustal lay rs subsided to create riftlike grabens. During periods of plate collision or very rapid drift, the mantle and crust were decoupled; this action produced an upward vector of thrust against downward crustal bulges, and arches and domes responded by uplift. Platewide unconformities developed during the decoupled movement of crust and mantle. The sedimentary rocks record the time and nature of the activity.

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