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 became home to many different dinosaur species, which rose and went extinct before the sea finally receded from Montana about 65 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 shallow, warm sea, probably no more than a few hundred feet deep, supported abundant aquatic life, including oysters in dense banks along the shore, ammonites feeding on large clams offshore, and sharks hunting in the shallows. For several million years, long-necked Plesiosaurs and snakelike Mosasaurs stood at the top of the food chain; though not dinosaurs, they were air-breathing marine reptiles and carnivores that ate almost anything they could seize and swallow, and their fossils 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, as an interconnected 4,000-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.