Uvalde gravel chert

'Uvalde gravels' is a loose name for old river gravels that cap ridges across the northern two-thirds of the South Texas Plains. Ancient rivers laid them down hundreds of thousands to millions of years ago, washing cobbles in from sources hundreds of miles away. The rounded cobbles are mostly under 15 to 20 cm and carry a cortex that is usually much darker than the interior. They include chert, chalcedony, quartzite, agatized wood and jasper. Most pieces are tough, coarse-grained and flawed, but gravel exposures were picked over for 13,500 years and are covered with flaking debris. Eastward, the gravels often hold larger cobbles of finer chert. Do not confuse this gravel with the point type called Uvalde, which is named for the town.

Ancient (Pliocene or Early Pleistocene) stream gravels on upland ridges and in lag deposits, from rivers that crossed the region long ago; the name is applied loosely to old gravels in several geologic units (for example the Catahoula and Willis formations). Cobbles are mostly 15 to 20 cm or less. East of the Nueces River the deposits are often thicker, with larger and finer-grained chert, especially in the Guadalupe basin.