On-The-Rocks Field Trip: Dinosaur Trackway

Title: On-The-Rocks Field Trip: Dinosaur Trackway–A Mountain Bike Adventure Along the Purgatoire River

Trip Leaders: Joanna Wright, CU Denver, and Matt Morgan, Colorado Geological Survey

Date: May 11, 2002

Publication: The Outcrop, April 2002, p. 18

The Purgatoire River rises in the Rocky Mountains west of Trinidad, Colorado, and flows northeast across the southern High Plains for a distance of about 150 miles to its confluence with the Arkansas River just east of Las Animas. Although a sparsely populated and little known area, the Purgatoire Valley is one of the most beautiful locations on the High Plains. This field trip will focus on one small stretch of this valley known as Picket Wire Canyonlands, an area containing the largest dinosaur tracksite currently known in North America, perhaps the world. The Picket Wire Canyonlands lie within the Comanche National Grasslands, approximately 23 miles south of the town of La Junta in southeastern Colorado. The area, currently managed by the U.S.D.A. Forest Service, is rich in history, flora and fauna, and above all has a unique paleontological heritage. The dinosaur tracksite is located in the northern portion of the Canyonlands, and the track-bearing rock is situated both within, and on the banks of the Purgatoire River.

It was not until the 1930s that the Purgatoire site and several other important dinosaur tracksites in the western USA were discovered. Even then, the full potential and significance of the tracksites was not realized. The Purgatoire Tracksite was studied in detail for the first time between 1982 and the present and has been described in scientific literature. In fact, the Purgatoire site alone was shown to yield four track-bearing layers containing at least 1300 footprints, representing a minimum af a hundred animals, mainly sauropods and theropods. Sauropods are the group of large, long-necked creatures that are commonly referred to, in general, as Brontosaurs. Theropods are the two-legged, carnivorous dinosaurs such as Allosaurus. These tracks represent a larger number of dinosaurs than has been found at almost any other single dinosaur site in the Late Jurassic Morrison Formation. The fossil evidence and rock record also show that the tracksite area represents a lake shoreline environment preserving evidence of a very distinctive 150 million year old lake ecosystem.

Tracks are the evidence of living animals and provide biological insights into the trackmaker’s anatomy and behavior. First, the tracks tell us how big the animal was, how many toes it had, if it was a four-footed quadruped or a two-footed biped. These characteristics, along with the age of the strata, usually help us to identify the type of animal that made the tracks. This, however, is just the beginning of the biological evidence that can be gleaned from tracks-plan to come on the field trip to discover the subtle clues about behavior that is suggested by the tracks!

The field trip will depart at 8:00 am at the Highlands Ranch RTD Park & Ride, C-470 and Qniversity Blvd.; participants will drive 2.5 hours to the bike trip starting point near La Junta. The mountain bike ride is 6 miles each way (into and out of a canyon, strenuous, may be hot) and is interesting in itself–rock art and other heritage sites will be enjoyed along the way. Bring lots of water and a lunch, and consider bringing an extra bike tire tube. It is anticipated that the field trip will finish at the Park & Ride at approximately 6:00 pm. For reservations contact Fred mark, e-mail (preferred): fmark@csd.net, or 303-674-1682.

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