Finding Theropod Tracks in the Moab Member
– by Mary Ecsedy, June 23, 2005
Pre-Dinosaur Theropods in Arches National Park
Although you'd never guess it by the signs and stores on Moab's Main St., the paleontology resources in the Colorado Pleateau are extensive and significant. There are nearly 300 million years' worth of exposed geology strata here, and many of the sandstone layers were once environments that were rich in life. Ever since we read Fran Barnes books about how to look for prehistoric tracks (all kinds, not just dinosaurs) in the region's sandstone layers, we have kept a sharp lookout for them on our hikes.
On April 15, 2005, after dropping our income tax returns in the mail, I went out hiking alone to Arches National Park, climbing up to the top of the steep cliffs high above the Visitor's Center. I was hiking across the Moab Tongue Member of Entrada sandstone, which is generally white or light beige. I noticed a dark brown patch of rock on a ledge and I looked closer. It had once been mud, and was sharply outlined against the lighter stone around it. It was uneven. It had a curve. Three curves... they were toes. It was a three-toed Theropod dinosaur track! And there was another! And another!
I was absolutely thrilled! We've been looking for tracks for several years, and these were well-worth the effort it took to find them. The tracks were very large, and seemed too close together to have been made by one individual. They were eroding free from the sandstone around them, on a ledge located about 300 feet from the fossilized, muddy brown deposits of an ancient oasis or desert dry lake.
Possibly the animals were hunting along the water's edge, and their footprints in the light Moab Tongue quickly filled in with the muddy water. The edges were very sharply defined. The prints may have been covered over quickly with more sand, and they eventually fossilized. Now there they were, exposed once more to the light of the sun. Unlike the Megatrack site north of town, where thousands of these three-toed prints may be found at the very top of the Moab Tongue layer, these were located well below that final surface by several tens of feet.
There are 3 tracks eroding free from the left edge of the sandstone ledge, close together.
The Moab Tongue was deposited about mid-way through the Jurassic approximately 175 million years ago. It was once the shoreline dunes of the ancient Sundance Sea.
We think of the past as no longer "here". The past is long-gone, we tell ourselves. But standing over those tracks in the bright desert sunshine, I realized that isn't true. The past hasn't gone anywhere. It's right at our feet.
References
(1) Navajo Sandstone; a Canyon Country Enigma, by Fran Barnes, Canyon Country Publications Series No. 55, 1998.