Sandstone is an important lithic resource in the South Texas Plains and one that can be found in many different areas of the region. Standing outcrops and surface exposures occur in various of the geological formations that run from northeast to southwest across the central part of the region. At some outcrops, the sandstone is fairly hard (well cemented with silica) and quite suitable for making artifacts. Other outcrops have weakly cemented, friable sandstones (cemented by calcium carbonate) that are not suitable for making tools, but often for cooking stone. Durable pieces of sandstone are also found in cobble form in some upland gravels and in river gravel bars downstream from where the river cuts through sandstone exposures.
Sandstone is a sedimentary rock type that sometimes has pronounced bedding planes – thin layers representing the original layers that were laid down by the sedimentary process by which the sand was deposited (sandbars, sand dunes, and so on). Over geological time, sand deposits are compacted by overlaying layers of rock, and then naturally cemented by crystalline minerals, such as silica and calcite, which impregnate sand deposits turning these into rock.
Hard sandstone works well for ground-stone tools – grinding slabs and manos. The softer the sandstone, the quicker tools made from it would have worn out, but less than ideal materials were often used. Another common tool use is that of an abrader, a piece of sandstone (and other rocks such as quartzite) that has linear grooves formed by back and forth motion. Flintknappers use abraders during manufacture to dull the edges of chipped-stone artifacts, thus creating stronger platforms for flaking. Many other materials were likely abraded – shaped by grinding – including bone and wooden tools.