Such native trees as ash, cottonwood, pecan, hackberry, bois d'arc, oak, and elm grow along watercourses. The terrain is level to rolling, with an elevation ranging from 300 to 700 feet above sea level. The area lies in a southeastward-sloping plain and is well drained by many streams that flow into the Trinity River, which forms the eastern boundary of the county. Red Oak, Waxahachie, Mill, and Chambers creeks drain most of the county. The Austin Escarpment, a high chalk ridge, extends in a northeast-southwest direction and cuts off drainage to the southeast.
Mountain Creek flows northeast and drains the northwestern part of the county.
The alluvial soils are mainly clay. Mineral resources include gas, oil, stone, and clays valuable for brickmaking. Chalk and shale provide material for the manufacture of cement. Tonkawa Indians were the earliest inhabitants of the future county, though parties of Wacos, Bidais, Anadarkos, and Kickapoos often hunted in the area. Spanish missionaries worked with the Tonkawas, and as the American settlers began to move into the region in the middle of the nineteenth century, the Indians offered little organized resistance.