Mortar Holes
These circular depressions on the Roma Sandstone near the city of Escobares are mortar holes made and used by Native Americans for grinding nuts, seeds, and other food products. These are the only known bedrock mortar holes in the Rio Grande Valley.
Although prehistoric mortar holes such as these are common throughout the US, there are large ones visible in publically accessible locations such as Big Bend National Park. The mortar holes pictured above were carved out of an outcrop of the Roma Sandstone.