Toward the end of the Mesozoic Era (in the latter part of the Cretaceous Period), the metamorphic basement rocks of Yellowstone lay covered by the vast blanket of flat-lying sediments. Today, these sedimentary rocks are exposed along the Snake River and its tributaries in the south-central part of the Park, over much of the Gallatin Range in the northwest corner, and at several places in the north-central and northeastern parts (pl. 1). Elsewhere, either they are hidden from view beneath volcanic debris—ash and lava—that later buried them, or they have been removed by erosion. But wherever exposed, the original horizontal layers of sedimentary rocks have been severely twisted and broken by later mountain-building movements. [Illustration: CROWFOOT RIDGE in the southern Gallatin Range, as viewed from the road along the Gallatin River near the northwest corner of Yellowstone National Park. The rocks, chiefly Paleozoic limestone, sandstone, and shale, were deposited in broad shallow seas that covered all of the Yellowstone region several hundred million years ago. The original layers were horizontal, but they have since been tilted and broken by giant mountain-building forces originating deep within the earth. (Fig. 9)] [Illustration: MOUNT EVERTS, as viewed toward the northeast from the road south of Mammoth Hot Springs. The mountain, about 1,500 feet high above the plain, is formed by gently tilted sedimentary rocks of Cretaceous age, chiefly sandstone and shale of the Frontier, Cody, and Everts Formations (fig. 5). The conspicuous rimrock at the top of the mountain to the right is composed of the Yellowstone Tuff. When the tuff was deposited (by explosive eruptions from the south), there was no valley along the edge of the mountain. (Fig. 10)] [Illustration: FAUNAL SUCCESSION in sedimentary rocks. The different animals are now preserved as fossils, which are diagnostic of the period in which the animals lived. (Fig. 11)] Man CENOZOIC QUATERNARY and TERTIARY Mammals CRETACEOUS MESOZOIC JURASSIC Dinosaurs TRIASSIC PERMIAN Reptiles PENNSYLVANIAN Amphibians MISSISSIPPIAN PALEOZOIC DEVONIAN Fishes SILURIAN Sea scorpions ORDOVICIAN Nautiloids CAMBRIAN Trilobytes PRE-CAMBRIAN Soft-bodied creatures [Illustration: LIMESTONE OF MISSISSIPPIAN AGE along Pebble Creek at the Pebble Creek campground, northeastern Yellowstone National Park. (Fig. 12)]