Inland Floodplain Environment

Photo provided by Julius Csotonyi & Alexandra LefortPhoto provided by Gastropods, bivalves, and crustacean burrows from Lehman, 2010Photo provided by Wann Langston with the femur of Alamosaurus (UT Austin VPL)Photo provided by UT Dallas crew excavating the gigantic articulated neck of AlamosaurusPhoto provided by Hadrosaur skin impressionsPhoto provided by North America 65 Million Years Ago / Ron BlakeyPhoto provided by Texas Brazos River: modern analogue for Javelina Fm
Inland Floodplain Environment

 72-55 Million years ago

72 million years ago the Rocky Mountain uplift had elevated the Big Bend Region hundreds of feet above sea level. Around this same time the climate started to cool and sea level continued to drop as ice formed at the poles. The combination of these events causedthe retreat of the ancient seashore hundreds of miles to the east of Big Bend. Seasons developed with distinct wet and dry periods. Dinosaurs still lived here, and they were even bigger than those of the earlier Aguja Formation.

 The largest flying creature to ever exist, the pterosaur Quetzalcoaltlus, thrived here, flying high above the heads of the terrifying predator Tyrannosaurus and the long-necked giant Alamosaurus. 65 million years ago, a meteor impact not too far south of Big Bend in Mexico led to the extinction of the dinosaurs.

Geologic Setting:

As the Rocky Mountains rose higher and higher to the west, the compression of North America caused the land here in Big Bend to be warped downward, creating a broad, river-filled basin. This forested floodplain supplied many slow-moving meandering rivers that carried lots of mud, sand and silt. The landscape resembled today’s Brazos River valley in southeast Texas.

Over millions of years, the sediments carried by the rivers filled up the basins, creating the Javelina and Black Peaks rock formations. The layers of mud and sand surrounded and preserved the skeletons to create fossils. Today the Javelina and Black Peaks Formations are represented by thick, fluvial channel sandstones, floodplain mudstones as well as colorful paleosol (ancient soils that resemble badlands) horizons in reds, blacks, and purples.

Fossils are not as numerous in the Javelina and Black Peaks Formations as they are in the underlying Aguja Formation.  Invertebrate fossils are rare but include fresh water snails and crustacean burrows. Vertebrate fossils include those from fish, turtles, pterosaurs, dinosaurs as well as small mammals. Plant fossils are also present in the Javelina Formation, including fan palms as well as conifers and flowering plants.

Featured Fossils:

Bravoceratops (1), one of the largest of the horned dinosaurs, was discovered in Big Bend in 2013. Bravoceratops would have lived along the riverbanks, searching out plants to eat and keeping a watchful eye out for the giant predator, Tyrannosaurus (2). As the predators got larger over time, so did their plant-eating prey. An adult Bravoceratops was as long as a dump truck. Alamosaurus (3) is another giant that has been unearthed in Big Bend National Park. Alamosaurus limb bones indicate that these long-necked dinosaurs were one of the largest dinosaurs to ever exist. Size estimates suggest that Alamosaurus would have been 80 ft (25 m) long and weighed up to 65,000 pounds (29,000 kg).

Quetzalcoatlus (4) soared overhead and likely migrated across the globe as fossils of this giant flying reptile have been found in Texas and Romania. The most common dinosaur in Big Bend at this time was Gryposaurus (5), a duck-billed (or hadrosaur) dinosaur that ate a plant-based diet. Unlike most plant-eaters of their time, hadrosaurs are unique because they stood on two legs (bipedal) instead of four. It is likely that Gryposaurus lived in large herds and were social animals. Other reptiles that thrived alongside the dinosaurs were the alligator-like Brachychampsa (6) and the turtle Baena (7). The presence of alligators and turtles is another clue that Big Bend was a much wetter place 72 – 55 million years ago.

 A frequently forgotten Cretaceous critter is Meniscoessus (8), a tiny mammal that lived beneath the feet of the giant dinosaurs. Meniscoessus is a member of an extinct group of mammals known as multituberculates. Multituberculates are known for their intricate and distinctive crowns of their teeth. The diversity of tooth shapes allowed Meniscoessus to feed upon many different plants and animals and thrive alongside the giant dinosaurs.


One of the most unique features of the Javelina & Black Peaks Formations of Big Bend National Park is that they preserve the most well known extinction event of all time, the end of the dinosaurs. At 65.5 million years ago the dinosaurs go extinct, a time called the K-Pg (Cretaceous - Paleogene) boundary. This extinction event was triggered by a giant meteor hitting Earth, leaving behind the huge Chicxulub crater along the Yucantan Peninsula in Mexico. By 63 million years ago (Paleocene time) the dinosaurs were gone (except the avian dinosaurs, which evolved into birds) but the ancient mammals that managed to survive the vast extinction event began to flourish.

The long-necked sauropod dinosaur Alamosaurus was discovered in Big Bend in 1940 by famous paleontologist Barnum Brown. It was thought that sauropods went extinct at the end of the Jurassic period 145 million years ago, but the Alamosaurus fossils from Big Bend from the Late Cretaceous Javelina Formation (72 million years ago) helped prove otherwise. More specimens of Alamosaurus have been discovered in Big Bend National Park than anywhere else in the world. A full-size skeleton replica of Alamosaurus can be seen at the Perot Museum of Nature and Science in Dallas, TX.

Quetzalcoatlus, the largest flying creature to ever be discovered, was found in the Javelina Formation of Big Bend National Park. This giant flying reptile, or pterosaur, was found in 1971 by Douglas Lawson (seen below), a graduate student from the University of Texas at Austin. The wingspan of Quetzalcoatlus is estimated to be almost 40 feet long. This fossil is named after the Aztec feathered serpent god Quetzalcoatl. 

Where to see the rocks in the park:

The Javelina Formation seen to the west of the Highway 118 park entrance sign

The Javelina Formation can be seen to the west of the park entrance sign near the Maverick Entrance Station (the western park entrance) along highway 118. As you drive south from Study Butte towards the park, the outcrops straight ahead preserve the Javelina Formation. Along the western portion of the Chimneys Trail more Javelina Formation can be seen up close. For those folks who enjoy the less-maintained roads, check out the Glenn Spring Road around Chilicotal Mountain for a chance to see the Javelina Formation. As you travel south on the Glenn Spring Road from the northern entrance, keep your eyes out for the Camp Chilicotal campsite, approx. 3.6 miles down the road. This site offers views of the Javelina Formation to the east, which continues as you drive further south on Glenn Spring road. On the road between mile 3.6 and 7.1 check out views of the Javelina Formation to the east, at the base of the Chilicotal Mountain. If you continue on south, the Javelina Formation continues to outcrop on the right (west) between the turn-off for the Black Gap Road (mile 8.6) and where the Glenn Spring Road meets the River Road (mile 8.5). 

The Black Peaks Formation can be best seen from the Fossil Discovery Exhibit, where a sighting scope has been installed to point directly at the hills for which the Black Peaks were named (Check out our page about Where to see the K-Pg Boundary in Big Bend). The Glenn Springs 4-wheel drive road also reveals outcrops of the Black Peaks Formation south of the Robber’s Roost backcountry campsite. To get to this section of the park, enter the Glenn Spring road from the northen entrance and travel 7.1 miles south. Then turn right (west) on to the Juniper Canyon Road. Follow this road for 2.9 miles until you reach the Robber's Roost campground. The rocks outcroping to the south are Black Peaks Formation.