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Diggin' For Dinosaurs - Purgatoire River & Dinosaur Ridge

Updated on March 30, 2010

When you first wander about the site, it seems that the tracks fall more or less randomly. They can be felt underfoot when crossing the cold waters of Purgatoire and are strewn about the side of the waterway amid sheets of river mud that dry and curl under the baking sun. On closer inspection, though, it becomes clear that at least some of the Apatosaurus tracks follow the same path: Five distinct trackways run parallel to one another for some distance. Scientists say it is unlikely that such parallel tracks are a chance occurrence and theorize that the subadult animals must have been moving together along the shoreline. These five trackways have the distinction of being the first published evidence of such gregarious behavior among apatosaurs.

Even after surviving for millions of years, the Purgatoire site constantly changes. The limestone layers holding the tracks are quite hard, but the soft shale layers between them are easily eroded by the normal flow of the river and by occasional floods. When sections of shale get swept away, the remaining limestone fractures into large blocks; the blocks then break away from the bank and come to rest looking like giant sugar cubes tossed along the river. On the one hand, such changes sometimes reveal cross sections of footprints and show how underlying layers were compacted by the massive dinosaurs, a process called dinoturbation. They also may expose entirely new layers of footprints. On the other hand, the erosion is ultimately destroying a world class paleontological site. The Forest Service, which manages this part of Comanche National Grassland, recently took measures to divert the river away from the tracks to prevent further damage.

Dinosaur Ridge, Colorado

The grand panorama from atop Dinosaur Ridge, at the eastern end of Colorado's Front Range, appears to be as far from the influence of the sea as one can get. To the east lie the seemingly endless High Plains. To the west, slabs of rust colored sandstone and conglomerate the size of city blocks thrust from golden prairie, pointing toward the granite massifs of the Rockies.

But as high and dry as this land looks, abundant evidence exists that it once was as low and damp as the Gulf Coast of Louisiana ... and a veritable paradise for dinosaurs.

The evidence for a lower, wetter environment is as subtle as ripple marks in stones, where tides once ruffled the bottom of a lagoon, and as dramatic as a gigantic, slanted slab of sandstone with dinosaur footprints. The tracks were left by carnivores and herbivores passing along the shore of a vast inland sea that split North America in half 100 million years ago.

Seeing these and other signs of lost worlds does not require an arduous journey to forlorn badlands. It's all conveniently exposed just 15 miles west of Denver along the Alameda Parkway, a two lane road that angles up and over the ridge near the small town of Morrison. The town gave its name to what may be the most renowned rock formation in the world.

Continued in: Diggin' For Dinosaurs -Dinosaur Ridge

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