The Incredible Story Behind Billings’ Beautiful Rimrocks

The Rimrocks are one of the things that make Billings feel like Billings. They rise above the city in unmistakable sandstone cliffs, and no matter how many times I see them, I am still in awe. At sunset especially, when the rock catches the fading light, the Rims can make an ordinary drive home feel pretty extraordinary.

Their beauty is not an accident. The Rimrocks are the visible result of ancient sediment, shifting coastlines, and tremendous erosion over geologic time.

Billings' Rimrocks Are Cliffs of Eagle Sandstone

The Rimrocks, often called the Rims, are sandstone cliffs made of the Eagle Sandstone, according to the Montana Bureau of Mines and Geology's geologic map of the Billings area.

That map identifies the Eagle Sandstone as an Upper Cretaceous geologic unit. The Late Cretaceous Period ended about 66 million years ago, meaning the rock exposed in the Rims formed long before Billings existed.

The Sandstone Formed Along an Ancient Coastline

The sandstone did not begin as rock. It began as loose sand deposited in an ancient coastal environment.

A geological field guide focused on the Eagle Sandstone in Billings identifies the Rims as sandstone cliffs and describes the lower Eagle Sandstone here as containing shoreface accretion surfaces associated with a barrier bar. In plain language, this means the sand accumulated in a coastal setting before it was eventually buried and hardened into stone. You can read the geologic description in this Billings Eagle Sandstone field guide.

Research presented by the American Association of Petroleum Geologists also documents a tidal point bar deposit in the Cretaceous Eagle Sandstone at Billings, further supporting the area's ancient coastal history.

Layers in the Rimrocks Preserve That Ancient Story

The Rims don't look like one solid, featureless wall of rock because the sand that formed them was deposited over time in different layers.

The Montana Bureau of Mines and Geology describes the Eagle Sandstone as fine-grained and cross-bedded in places. Cross bedding is a pattern of slanted layers that forms as sand moves and settles under flowing water or wind. Those details preserve evidence of the conditions that existed when the sediment was first laid down.

Erosion Exposed the Cliffs We See Today

The Eagle Sandstone had to be exposed before it could become the landmark Billings residents know today.

The Yellowstone River Valley is a landscape shaped by erosion. Over a long period, the Yellowstone River system cut into the surrounding terrain, leaving sandstone cliffs exposed along the valley margins. The field guide identifies the Rimrocks as a sandstone cliff landform, while the Billings-area geologic map shows Eagle Sandstone at the surface across the areas surrounding the city.

The Rims are therefore erosional cliffs, not a mountain range. They are the remaining exposed edges of ancient sandstone after the surrounding landscape was worn down.

Billings' Rimrocks Continue to Be Shaped by Weather

The Rimrocks were formed over millions of years, and weathering still affects the exposed sandstone today. Rain, wind, temperature changes, and water freezing and thawing in cracks can gradually break down rock at the surface.

That slow process helps create the Rims' rugged ledges, layers, and uneven faces. It is also why they're much more than a pretty backdrop. They're a visible piece of Billings' natural history.

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The Rims Make Billings Feel Like Home

The next time you see the Rimrocks glowing in the sunset, it is worth remembering what you are looking at. Those cliffs are remnants of ancient coastal deposits, transformed into sandstone and exposed by erosion over immense spans of time.

That is a pretty remarkable thing to have right outside our windows.