Discover Texas and Explore the Geodiversity and Fossils of the Amistad National Recreation Area
- Wayne Munday
- Apr 9
- 4 min read
Sip back and discover south west Texas and explore the geodiversity and fossils of the Amistad National Recreation Area located within the Great Plains Physiographic Province and whose name translates in Spanish to friendship. A 3-hour drive west from San Antonio on the US-90 this natural oasis is the 265 Km2 Amistad Reservoir and 10 Km embankment Amistad Dam located at the confluence of the Pecos River, Rio Grande, Devils River near Del Rio in Val Verde County. This arid landscape has a rich cultural, historical and fossil record as well as being a haven for water sports, bass fishing, camping and hiking the trails. The bedrock of the Amistad National Recreation Area is made up of multiple formations of 100 million year old Cretaceous Period limestone rich in marine fossils preserved by the Western Interior Seaway as well as rare fossils of more recent megafauna. Its worthwhile to visit the Amistad National Recreation Area Visitor Centre and explore more.
During the Cretaceous Period south west Texas was submerged under an ancient seaway that was rich in marine life such as mosasaurs, plesiosaurs, ammonites, sharks, clams, oysters, fish, foraminifera, bryozoans, bivalves, gastropods, echinoids and marine plankton that are important in the formation of the Texas oil reserves.
The seaway that submerged south east Texas as well as large parts of North America during the Cretaceous Period was called the Western Interior Seaway. The Western Interior Seaway was a shallow tropical inland sea that stretched from the Gulf of Mexico (America) to the Arctic Ocean splitting North America into the two landmasses of Laramidia on the west and Appalachia to the east.
A key feature of the Amistad National Recreation Area is what lays beneath in its karst scenery formed by the process of dissolution. Limestone is primarily composed of calcium carbonate and is dissolved by the action of water. Water naturally picks up carbon dioxide from the atmosphere forming a weak carbonic acid. This weak acidic water interacts with the limestone made of calcium carbonate causing it to dissolve along areas of structural weakness such as cracks, joints, and bedding planes leading to the formation of karst features such as caves, sinkholes, and underground drainage systems known as a karst landscapes.
The high water quality and clarity of the Amistad National Recreation Area is attributed to the limestone in much in the same way that the limestone filtered water of Kentucky underwrites Bourbon's reputation for having a clean and sweet flavour.
The Great Plains Physiographic Province where the Amistad National Recreation Area is located spans over 725,000 Km2 of sedimentary bedrock including sandstones, shales, limestones and conglomerates. The province is bordered by the Rocky Mountains to the west and the Central Lowlands to the east and has a number of distinct geological formations like the Black Hills, Badlands, Devils Tower and Nebraska Sand Hills.
The oldest exposed rock in the Amistad National Recreation Area is the Lower Cretaceous Devils River Limestone and is known for its now extinct box, tube and ring-shaped marine living rudist bivalves. This formation, along with the slightly younger Salmon Peak Limestone are among the North America's most complete Lower Cretaceous exposures. Overlying these formations is the Upper Cretaceous Del Rio Clay, Buda Limestone, and Boquillas Formation that each reflect a progressively deeper marine environments. Notable fossils from these formations include Cretaceous oyster called Ilymatogyra arietina.
More recently the Amistad National Recreation Area holds deposits from the Cenozoic Era following the Cretaceous-Paleogene (K-Pg) extinction event that saw the demise of the dinosaurs. These deposits include Uvalde Gravel, Pleistocene river terraces and Holocene alluvium and although fossils are rare they have yielded Pleistocene Epoch megafauna including the remains mammoths, bison, horses, camels, antelope, mountain deer and short-faced bears.
Among the natural artefacts of the Amistad National Recreation Area is its impressive collection of ancient petroglyphs and pictographs especially at Panther Cave. These rock art images were created by Indigenous peoples over 4,000 years ago belonging to the Pecos River Style of rock art found throughout the Lower Pecos region.
The cave’s name comes from a striking depiction of a large leaping panther painted along the shelter wall. The site features hundreds of intricate images, including human-like figures, animals, and abstract symbols, believed to have held ceremonial or spiritual significance. The pigments were made from natural materials like minerals and plant extracts, allowing the images to survive for millennia. Panther Cave is today protected as an important archaeological and cultural heritage site offering insight into the lives, beliefs, and artistic expression of the region’s earliest inhabitants. Access to the cave is limited to preserve its fragile rock art legacy.
The Amistad National Recreation Area in south west Texas offers a remarkable window into the geological and fossil record of the region. From its ancient marine limestones formed during the Cretaceous Period to its more recent deposits from the Cenozoic Era this landscape reflects a history shaped by sea level change, evolving lifeforms, and geological processes like karstification. The fossil record preserved within both the limestone formations provides invaluable insight into the marine ecosystems that once thrived in the warm sea waters of the Western Interior Seaway as well as dryer terrestrial Pleistocene megafauna.