The area's thin layer of topsoil tends to be a light, crumbly caliche—capable of sustaining many agricultural plants, but susceptible to depletion and erosion. Before ranchers and farmers began altering the landscape, the area was once part of a vast grassland. Bison, deer, and pronghorn grazed on tall native grasses. However, because of overgrazing, land clearing, and the suppression of wildfires, these native grasses have been mostly replaced by invasive weeds and tough, woody trees, including Texas live oak, Texas red oak, red juniper (red cedar), and mesquite.