Although the effects of the Laramide Revolution were not as profound at Dinosaur as they were east of it, they were quite important. The rocks were lifted to form the southwest flank of Split Mountain—a small arch, or anticline, on the south side of the Uinta Mountains. This mountain building explains the pronounced southward tilt of the Dinosaur Ledge and other rock layers visible in the quarry area. As the land rose, streams flowed more rapidly, cutting deeper into the rocks and carrying away the debris. Gradually thousands of feet of this debris—shale, sandstone, and clay—were stripped away through erosion. Finally all the material on top of the Morrison sandbar weathered away. Some 140 million years after burial the fossil bones were exposed by the agent that had buried them so long ago—running water! All that remained was for them to be found, and that was the luckiest chance of all. Just suppose they had been uncovered a million years ago—only a second in geologic time. No one would have been present to discover them, and through the years they could well have crumbled into dust and been blown away. Why Did Dinosaurs Become Extinct? At Dinosaur National Monument only Morrison rocks of the upper Jurassic Period contain the fossil bones of dinosaurs. After Morrison time, the Cretaceous seas invaded this area. More than 5,000 feet of sandstone, shale, and mudstone were formed from sediments deposited in these seas. Elsewhere in North America and the rest of the world, the diversity and numbers of dinosaurs actually increased. Entirely new groups evolved and achieved success in the battle for survival. The climax of reptile development seems to have come near the end of Cretaceous time in the Mesozoic Era. As the dinosaurs ruled the continents, so did other strange reptiles dominate the seas. Had you been able to see this ancient world, you would surely have been convinced that the dinosaurs and other reptiles would rule forever. But it was not to be. The dinosaur hordes were wiped out and the reptiles reduced to the position of relative insignificance they occupy today. Such a profound and sudden change in the evolutionary trend of life must have had a cause, and scientists have sought it. Several theories have been proposed to explain extinction of dinosaurs, and they are most interesting. At the end of Cretaceous time, some of our great mountain ranges were formed. It was a time of earthquakes and of volcanoes that belched forth clouds of ash and rivers of molten rock. Some people would say these catastrophic events killed all the dinosaurs. The scientist shakes his head. If these events killed dinosaurs, why not the other animals that lived with the dinosaurs. And what of those parts of the world that had no volcanoes, what killed dinosaurs there?