The eastern part of Williamson County is in the black land belt of Texas where roads must be surfaced to be passable in wet weather. Road materials are scarce in this area, so much of the time spent on the mineral resource survey was concentrated on finding gravel.

Two main streams traverse Williamson County-San Gabriel River, composed of two forks which join at Georgetown, and Brushy Creek. These streams head to the west in areas of Lower Cretaceous rocks from which they obtain hard limestone and flint, which is carried to the lower reaches of these streams and deposited as gravel.

The streams, during the past, have been excavating their valleys and thus lowering their beds. During this process, gravel deposits were isolated along the sides of these valleys and are now scattered throughout the area as terrace remnants. These terraces range from the present stream bed well up the sides of the stream valleys.