Between sixty and ninety million years ago, eastern North Dakota lay beneath the warm, subtropical Western Interior Seaway, a body of water less than 500 feet deep that stretched from the Arctic Ocean to the Gulf of Mexico. Sediments washed into this sea formed three rock layers across eastern North Dakota, with sandstone deposited along the changing shoreline and shale and limestone forming in deeper water on the seafloor. Fossils preserved in these rocks, including mosasaur vertebrae from the Niobrara Formation found about eight miles north of here near Little Yellowstone Park, provide evidence of marine life that inhabited the seaway. Mosasaurs were giant reptiles related to monitor lizards, not dinosaurs, and they lived in shallow seas until 65 million years ago, hunting fish, turtles, mollusks, and shellfish. About 13,000 years ago during the Wisconsinan Ice Age, torrents of meltwater from the ice sheet covering Canada and eastern North Dakota carved the Sheyenne River Valley in a relatively short time. Floodwaters flowing from glacial Lake Souris into glacial Lake Agassiz filled the valley from rim to rim, cutting a trench as deep as 330 feet and a mile wide. The area's hillsides also bear glacial erratics, boulders carried hundreds of miles by ice and left behind as the glacier melted and meltwater swept away finer debris.