The Rimrocks: A Seventy-Million-Year Shoreline

Stand at the base of the Rimrocks on the north edge of Billings and look up. The sandstone wall above you is roughly eighty million years old. It was once a beach.

During the Late Cretaceous, a vast shallow sea called the Western Interior Seaway stretched from the present Gulf of Mexico all the way to the Arctic Ocean. The land that is now southern Montana lay along its western shoreline. Sand and silt accumulated in bars and deltas. Over tens of millions of years, that sediment compressed into the pale, fine-grained Eagle Sandstone that forms the Rimrocks today. The sea retreated, the land buckled and rose, and then, about a million years ago, the ancestral Yellowstone River, considerably larger than it is now, cut downward through the valley floor, leaving the sandstone layer standing as cliffs above the new floodplain.

The Rims run for miles along the north and east edge of Billings, rising several hundred feet above the valley floor. The easternmost section, called Sacrifice Cliff, is associated with a Crow Nation account of the smallpox epidemic of the eighteen thirties. The story, as the Apsaalooke tell it, describes two warriors returning home to find their camp devastated by disease. Grief-stricken, they blindfolded a horse and rode over the edge. The Crow place name for this spot translates as where the faded horse went down.

Early builders quarried this same sandstone for foundations and facades. The Parmly Billings Library, now the Western Heritage Center on Montana Avenue, is built of it, the ancient seabed turned into walls you can still touch today.