Crowley’s Ridge, Arkansas*
Crowley’s Ridge is an erosional remnant of unconsolidated Tertiary clay, silt, sand, and lignite capped by Pliocene- Pleistocene gravel, sand, and clay, and middle-to-late Pleistocene loess (Haley, 1976). Located in eastern Arkansas and southeast Missouri, Crowley’s Ridge is no more than 11 mi (18 km) wide, but it extends for 186 mi (300 km) from Campbell, Missouri, to Helena, Arkansas (Fig. 1). The ridge stands 100 to 200 ft (30 to 60 m) higher than the adjoining Eastern and Western Lowlands.
Crowley’s Ridge was a divide formed during the Pleistocene, as ancestors of the Mississippi River to the west and the Ohio River to the east of the Ridge eroded Coastal Plain sediments (Call, 189 1; Fisk, 1944) (Fig. 2). At that time the two rivers joined near Simmesport, Louisiana, 404 mi (650 km) south of where they join today. Abundant glacial outwash and a rising sea level contributed to the aggravation of the ancestral Mississippi and Ohio Rivers, causing a decrease in their gradients. This decrease in gradient and tectonic activity along the New Madrid seismic zone may have caused the eastward shift of the Mississippi River, culminating in the cutting of the present channel through the divide at Thebes Gap. Today the Mississippi River joins the Ohio River near Cairo, Illinois, and flows on the east side of the ridge.