Between the area five miles south of Georgetown and Tullos, the road rolls up and down across the valleys of Bear Creek, Dugdemona River, and Castor Creek, which slice through sediments of the Prairie Complex exposed on the hilltops. Georgetown stands on the higher elevations of the Prairie Complex. Tullos is at the northern margin of a band of rocks that trend from northeast to southwest, part of the Jackson group of late Eocene formations. All you see of them are patches of pale tan sand next to the road in Tullos. The formation typically includes clays interlayered with thin beds of lignite coal, sands stained red and brown by iron oxides, and fossil beds.
--Darwin Spearing, 2007
Roadside Geology of Louisiana, Second Edition