The region is bordered on the west by the Caprock Escarpment, on the south by the Edwards Plateau, and on the east by the Western Cross Timbers and Lampasas Cut Plain. Annual rainfall ranges from 22 inches in the west to nearly 30 inches in the eastern portion. May and September usually are high rainfall months. A summer dry period with high temperatures and high evaporation rates is typical. Soils vary from coarse sands along outwash terraces adjacent to streams, to tight clays or red-bed clays and shales.
Soil reaction is neutral to slightly alkaline.
The original prairie vegetation included tall and midgrasses such as bluestems and gramas. Buffalo grass and species of three-awn, among others, tend to increase under grazing. Mesquite is a common invader on all soils. Shinnery Oak and sand sage increase or invade on the sandy soils. Juniper clings to the steep slopes along rivers.