Rio Grande Gravels

The gravel beds along the Rio Grande have particularly diverse materials, because this river has its headwaters far to the northwest in the western Rocky Mountains. Most of the gravels found along the river in south Texas probably don't come from quite that far away. A more likely source area for some of it is the Big Bend area west of the Pecos. And other materials were picked up downstream, including reworked Uvalde gravels, and well as sandstone and other rocks found in bedrock outcrops breached by the Rio Grande. Among the rock types found in the South Texas Plains stretch of the Rio Grande are chert, chalcedony, sandstone, limestone, as well as various metamorphic rocks, such as quartzite, and volcanic rocks, such as rhyolite.

While the materials making up Rio Grande gravels may be particularly diverse, most of the cobbles are relatively small and made out of tough materials that survived long journeys. Large pieces of fine-grained material are rare. It is probably no coincidence that the predominance of the Unstemmed Point Tradition of the South Texas Plains is more or less centered on the Rio Grande, where rocks suitable for making large stemmed dart points are hard to come by.

Rio Grande gravels, some ancient and some more recent, can also be found in the terraces above the river and on the uplands miles away. The upland gravel deposits in the Rio Grande valley are sometimes called "Uvalde gravels" and there is probably little meaningful distinction.

photo of rio grande gravels
Fine-grained Rio Grande gravels collected from surfaces exposed in Falcon Reservoir during prolongued drought conditions. These pieces are probably artifacts ("cores" and "tested cobbles") left behind by prehistoric peoples. The two cobbles on the bottom are of high-quality chert, but note that the cobble size is relatively small. Samples collected by Jame Boyd, TARL Collections.
photo of rio grande gravels
Gravels are trapped by this sandstone oucrop in the Rio Grande River near Rosita under what is today Falcon Reservoir. Gravels can be seen to the right in the background in the enlarged image. TARL Archives.
photo of rio grande gravels
Coarse-grained Rio Grande gravels collected from surfaces exposed in Falcon Reservoir during prolongued drought conditions.These pieces are probably artifacts ("cores" and "tested cobbles") left behind by prehistoric peoples. Note the relatively small cobble size. Not even the most skilled flintknapper could make large thin bifaces from such material. Samples collected by Jame Boyd, TARL Collections.
photo of rio grande gravels
Ridge-top exposure of "Uvalde" or ancient "Rio Grande" gravels near Eagle Pass. The river can be seen in the background. This gravel exposure, like most, is an archeological site as well. The gravels here are most siliceous stones with little or no softer rocks such as sandstone. Photo by Mike Quigg.