The Dakota Group...
Ripple marks in ancient beach sand preserved in Dakota Sandstone on Dinosaur Ridge near Morrison.
The Cretaceous Sea Coast
The coastal plain which sustained the great plant eaters came under threat about 110 million years ago. The interior of the North American continent sank, allowing seas around the Arctic to move south and those in the Gulf of Mexico to advance north. Eventually they joined in the center, then began spreading west. Colorado remained above sea level for a time, about 100 million years ago, when it served as the west coast of the Cretaceous Sea, or the Western Interior Seaway (or the Western Interior Cretaceous Sea). The Sea's westward expansion would eventually engulf Colorado, as well as the eastern two-thirds of Utah. What saved the western third was the presence of an eroding mountain range.
Rivers and streams coming off the slopes of the Utah range would carry the sand which became the tan Dakota sandstone forming the crest of Dinosaur Ridge. The sand would be carried as far as Kansas. As the coastline moved west the river mouths retreated, leaving the sand deposits in their wake. Coastal currents moved the sands up and down the shores. Conditions 100 million years ago proved just right, not only for turning the sand into sandstone, but also for preserving the ripple marks made in the shallow waters by waves, created by winds or tides.
Conditions along the shore also were perfect for preserving the tracks of dinosaurs as they moved along the coast, as well. Iguanodons, a family of duck-billed dinosaurs, part of a family (as a biological classification) called iguanodontids, which, in turn, were part of an order named ornithopods (for "bird feet"), had replaced Brontosaurus. The number of dinosaur tracks (of iguanodons and other dinosaurs) found at Dinosaur Ridge has contributed to the rise of the terms "megatracksite" and "dinosaur freeway." Tracks found at other sites in Colorado, New Mexico, and Texas, in all, an area of some 30,000 square miles, has suggested that some sort of migration might have been involved.
The Dakota Group has been divided into two formations. The lower of the two, the Lytle Formation, laid just on top of the Morrison Formation, consists of white to yellowish-gray sandstone, probably was deposited between 110 and 103 million years ago. The upper, the South Platte Formation, is the tan or yellowish-brown beach sandstone, the Dakota sandstone, was deposited between 105 and 100 million years ago. It is also the formation which displays the tracks seen on Dinosaur Ridge.
Once the Cretaceous Sea made its final advance through Colorado, around 100 million years ago, the results were "permanent." Colorado would remain submerged for the next 30 million years. It would be the uplift of the Rocky Mountains, around 70 million years ago, which would raise Colorado above sea level again, forcing the Sea to retreat.
(1) Andrew M. Taylor, Ph.D., "Guide to the Geology of Colorado," Cataract Lode Mining Company, (Golden, CO 1999), pp. 78-79.
(2) Kirk R. Johnson and Robert G. Raynolds, "Ancient Denvers: Scenes from the Past 300 Million Years of the Colorado Front Range," Denver Museum of Nature & Science, (Denver 2003), p. 14.
(3) Martin Lockley, "Fossil Footprints of the Dinosaur Ridge and Fossil Trace Areas, 2nd ed." Friends of Dinosaur Ridge and the University of Colorado at Denver Tracker Research Group, (Golden, CO 2003), pp. 23 & 36.
(4) Harald Drewes and John Townrow, "Trailwalker's Guide to the Dinosaur Ridge, Red Rocks and Green Mountain Area, 2nd Edition" Friends of Dinosaur Ridge, (Morrison, CO 2005), p. 43;
Taylor, "Guide to the Geology of Colorado," p. 79.