Bell County is partly within the Gulf Coastal Plain and is crossed by the Balcones zone of faulting. West of this fault zone, the surface formations are those of the Comanche Cretaceous, while to the east of the fault zone, the formations are largely those of the Gulf series or Upper Cretaceous. In addition to the Cretaceous formations, limited areas of the county are occupied by Pleistocene and Recent stream and terrace deposits, including gravel, sand, and clay. The soils of the county are residual from these various formations. The surface distribution of the Cretaceous formations is indicated in the accompanying map.