About 80 million years ago this area was near the shore of a great seaway called the Western Interior Seaway that stretched from the present-day gulf coast to the Arctic Ocean. Rivers draining highlands to the west carried sediment into the seaway and near-shore currents concentrated the sand creating barrier islands. As the sea level alternately rose and fell, the barrier islands migrated, forming an extensive layer of chiefly fine-grained sand across much of central Montana. The sand was eventually buried, compacted, and cemented into the rocks we see here in the rims and that geologists have named Eagle Sandstone. When observed from the south, the rimrocks reveal low angle, westward-dipping cross beds called accretion surfaces. These surfaces record the deposition of sand washed over the barrier and deposited on the other side by waves, causing the bar to accrete or grow shoreward.
The seaway was shallow, warm, and probably no more than a few hundred feet deep. Yet it was home to a wide variety of aquatic life. Oysters lived in dense banks along the shore, while tentacled ammonites fed on clams that lived in the shallow water offshore; sharks also cruised the shallows preying on whatever animals appeared tasty to them. For several million years, two predators were at the top of the food chain in the sea: the long-necked Plesiosaurs and the snakelike Mosasaurs. Neither were dinosaurs, but were air-breathing reptiles adapted to living in the oceans. The rims near the confluence of the Yellowstone River and Alkali Creek is one of Montana’s historic hot spots. Native Americans utilized the area for at least 8,000 years before William Clark passed through the area in July 1806. Although known as Crow Country, the Blackfeet, Lakota, and Northern Cheyenne Indians were also frequent visitors to the area. Near here on May 31, 1823, Blackfeet warriors ambushed and killed seven Missouri Fur Company trappers under the command of Michael Immel and Robert Jones. The rims provided a spectacular backdrop for the rip-roaring settlement of Coulson in the 1870s and for Billings from the early 1880s, just after the arrival of the Northern Pacific Railway.
Slide toward Today to fade the historical sheet and see the modern map underneath.