Many north-trending thrust faults have been observed in coal mines of southern Illinois and adjacent western Kentucky. Displacement ranges from less than an inch to about 3 m (10 ft). No known ancient tectonic stress field or nontectonic process has been found to have produced these faults; they correspond to the contemporary tectonic stress field in the region—maximum horizontal compression along an axis trending slightly north of east. This compression is thought to account for the preferred orientation of roof failure in underground coal mines, jointing in roof shales, and minor earthquakes in southern Illinois.

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