[Illustration: Closeup A shows one of the highly fossiliferous layers within the limestone.] [Illustration: Closeup B shows some of the fossils and their casts. Most of the fossils are of a variety of shelled sea animals (brachiopods) that lived on the ocean floors approximately 300 million years ago.] The first mountain-building episode Near the close of the Mesozoic Era the earth was subjected to a series of intense crustal disturbances that geologists call the Laramide orogeny (orogeny means mountain-building). The origin and nature of the forces that bent and cracked the crust are unknown, but current theories being developed about sea-floor spreading and continental drift may shed light on this major upheaval that began about 75 million years ago. A significant effect of the Laramide orogeny was the uplift and contortion of many of the mountain ranges within what we today call the Rocky Mountains. At the onset of the crustal disturbance, the gently rolling landscape of the Yellowstone region began to warp and flex into large upfolds (_anticlines_) and downfolds (_synclines_) (fig. 13). Gradually the mountain-building pressures increased, finally reaching such magnitude that the limbs of the folds could bend and stretch no further; thereupon, the rock layers broke and were shoved over one another along extensive _reverse faults_. The severely crumpled rocks within the Park area can now be seen only along the north edge and in the south-central part along the Snake River. In both places, the folds and faults are especially well displayed by the layered Paleozoic and Mesozoic sedimentary formations (fig. 9). One of the most prominent Laramide structural features is a large anticline in the north-central and northeastern parts of the Park (fig. 14, section B-B′); the road from Mammoth to the Northeast Entrance crosses much of this feature (pl. 1). Although originally forming a high mountain mass, the anticline has been eroded so extensively that it no longer appears mountainous (fig. 18). It displays a broad core of Precambrian gneisses and schists and is bounded along its southwest margin by a large reverse fault. Along the fault, the ancient gneisses and schists have been shoved over rocks as young as Late Cretaceous, a movement amounting to 10,000 feet or more. The Cretaceous rocks are those that are now exposed at Mount Everts (fig. 10).