For over sixty million years during the Cretaceous Period, much of eastern Montana lay underwater beneath a vast inland sea. As the Rocky Mountains formed to the west, a broad, flat coastal plain developed that was home to many different dinosaur species, and over the sea’s long existence many dinosaur species rose and became extinct before the water finally receded from Montana about sixty-five million years ago at the end of the Cretaceous Period. Sediments deposited underwater and along the coast later formed the sandstone rimrocks in the Yellowstone River valley between Columbus and Billings. The sea was shallow and warm, probably no more than a few hundred feet deep, and supported abundant aquatic life, including oysters along the shore, ammonites feeding on giant clams offshore, and sharks hunting in the shallows. For several million years, plesiosaurs and mosasaurs stood at the top of the food chain there; although not dinosaurs, they were air-breathing marine reptiles and carnivores that ate nearly anything they could catch and swallow. Fossils of both have been found throughout eastern Montana. For centuries, the sandstone rimrocks along the Yellowstone River guided travelers between the mountains and the buffalo country to the east. In 1912, Good Roads enthusiasts and county officials created the Yellowstone Trail, one of the first interstate highways in the United States, an interconnected four thousand-mile network of county roads marked by chrome yellow signs with black arrows that linked Plymouth Rock, Massachusetts, and Seattle, Washington, with a branch to Yellowstone National Park. Later redesignated U.S. Highway 10, it was bypassed by Interstate 90 in 1971.