From the appearance of the rugged, mountainous terrain of Yellowstone National Park, it is difficult to visualize a time when this region lay close to sea level, at times even below sea level. Yet the evidence is clear that from the Cambrian Period to the latter part of the Cretaceous Period, a span of about 500 million years, vast stretches of western lands were flooded repeatedly by broad shallow seas that often reached from Canada to Mexico (fig. 8). During these great floodings, widespread horizontal beds of sand, silt, clay, limy mud, and other sediments were deposited on the ocean floors, along the adjoining beaches and wide tidal flats, and across the broad flood plains of large rivers that emptied into the seas. All of these ancient sediments have now hardened into compact well-layered sandstones, shales, and limestones (figs. 9 and 10). These _sedimentary rocks_ have been divided into 25 or more distinct formations in the Yellowstone region (fig. 5), where they locally attain a combined thickness of more than 10,000 feet. The first Paleozoic sea to reach the Yellowstone region, some 550 million years ago, brought with it the earliest abundant signs of life on earth. Small hard-shelled animals that lived mainly on the shallow sea bottom are now preserved as fossils in rocks deposited during the Cambrian Period. Many of these animals were _trilobites_, long-extinct organisms resembling today’s crabs and spiders. Each younger set of rocks or formations contains a different group of dominant fossils, each diagnostic of that period of geologic time in which they lived (fig. 11). [Illustration: MIDDLE PERMIAN SEAS. Distribution of sea (blue) and land (red) during the middle part of Permian time (approximately 250 million years ago). Only a part of the Yellowstone National Park area (black) was flooded during this period. (Fig. 8)] Fossils indicate the kind of environment in which the animals lived (fig. 12). Some species thrived in the open oceans; others thrived only along the beaches and in nearby lagoons. Still others, such as the incredibly large dinosaurs of the Jurassic and Cretaceous Periods, could survive only on the land or in swamps. From studies of the fossils and of the physical characteristics of the rocks in which they are now found, the shoreline patterns of the shifting seas can be determined. Studies show that the seas advanced and retreated across the Yellowstone Park region at least a dozen times during the Paleozoic and Mesozoic Eras.