A River Ran Through It

By editor

Ekalaka, Carter County, Montana

If you had ventured to the place now called Medicine Rocks 61.5 million years ago, you would have found yourself submerged beneath a vast, turbulent river system, its waters thick and breathing with life quite unlike our dry, open prairie today. Instead of the arid wind that shapes the sandstone pillars now, you would have needed gills to survive in the rushing currents that once carved this ancient landscape. This river, part of a sprawling network flowing across what is now the Northern Great Plains, carried immense loads of sand and mud, depositing underwater sandbars along its deep channel. These sedimentary processes, preserved in the very stone beneath our feet, reveal a chapter of Earth's history writ large by water and time.

The story begins in the late Paleocene epoch, roughly 61.5 million years before the present. At that time, the region was far warmer, and the climate more humid, supporting a profusion of plant and animal life. Geologists classify the sandstone here as part of the Fort Union Formation, a sequence of sedimentary rock layers that stretch across parts of Montana, Wyoming, and the Dakotas. The sandbars you see etched in the stone record the river’s ancient pulse, each layer a frozen ripple or sand dune shaped by currents long vanished.

As these sandbars moved downstream, they carried with them fragments of the eroding riverbank -- bits of wood, leaves, and other organic debris. When buried rapidly under mud deposited by slower-moving waters or subsequent floods, these materials became trapped within the shifting sediments. Over millions of years, mineral-rich groundwater percolated through these layers, cementing the grains of sand together and transforming loose sediment into solid sandstone. This lithification preserved not only the sandy bedforms but also the delicate impressions of tree limbs and trunks. Often, the wood itself decayed, leaving behind hollow cavities outlined by dark mineral halos formed by iron oxides. These “ghosts” of ancient forests embedded in stone invite us to imagine the once-living trees swaying in prehistoric currents.

Wind and weather have since taken up the sculptor’s work. The sandstone at Medicine Rocks is soft and porous, easily shaped by the persistent prairie winds that carve the mottled pillars, arches, and buttresses standing here today. The cross-bedding visible in the rock -- thin layers of sandstone inclined at angles within thicker beds -- records the shifting currents of the ancient river, a feature familiar to geologists as evidence of sediment transport in water.

The colors of the sandstone tell another geological tale. The predominant tan hue arises from limonite, an iron oxide mineral that stains the rock’s surface. Yet streaks and patches of red also appear, marking the presence of hematite, another iron oxide carried downstream from the Black Hills of South Dakota, hundreds of miles to the east. These mineral signatures map the journey of sediments across what was once a dynamic river basin.

One of the remarkable features of Medicine Rocks is the presence of mudrock seams embedded within the sandstone. These seams consist of chunky white sediment, deposited when the river temporarily slowed and mud settled out of suspension. Their irregular layering interrupts the general sandstone matrix, further evidence of the river’s changing moods and the episodic nature of sedimentation.

The erosional forces at work here continue to shape the landscape. Wind-driven abrasion carves out pits and caves in the rock faces, some large enough to walk through. Over centuries, these hollows may bore entirely through the sandstone, creating natural windows that frame the surrounding prairie. This fragile environment, however, is slowly disappearing. Each gust of wind strips away microscopic grains of sand, reshaping formations that took millions of years to form. Visitors are rightfully cautioned against defacing or removing stones, for such acts accelerate natural erosion and rob the landscape of its geological heritage.

The significance of Medicine Rocks extends beyond its geological features. The site has long held cultural importance for Native American tribes, particularly the Lakota and Cheyenne, who regarded the formations with reverence and incorporated them into their oral histories and spiritual practices. The unusual shapes and mysterious hollows inspired stories and ceremonies, linking the living landscape with the ancestors.

The scientist and explorer William Clark, of the famous Lewis and Clark Expedition, once wrote about the Missouri River region’s striking geological formations, noting how "the rocks and land forms tell a story older than memory." Though he did not visit Medicine Rocks specifically, his reflection captures the enduring fascination geologists and naturalists share with these ancient sediments.

In the early 20th century, geologists studying the Fort Union Formation at Medicine Rocks contributed significantly to understanding North America’s Paleocene environment. Albert Silberling, a pioneering paleontologist of Montana, described the area’s fossil-bearing strata, uncovering evidence of early mammals that roamed the plains soon after the extinction of the dinosaurs. These discoveries placed Medicine Rocks within a larger narrative of Earth’s recovery and renewal following the Cretaceous-Paleogene extinction event 66 million years ago.

Standing amid the sandstone pillars today, one cannot help but feel a connection to the immense epochs that shaped this land. The river that once surged here is long gone, its waters transformed into stone and wind-carved forms. Yet the rocks preserve the memory of water’s passage, the slow dance of sediment and mineral, and the relentless forces of erosion. Medicine Rocks invites us to read these natural archives with patience and care, reminding us that the surface of the earth is both ancient and ever-changing.

See also

• A River Ran Through It at Ekalaka, Carter County
• The Hell Creek Formation at Jordan, Garfield County
• Paleocene Mammals and Albert Silberling at Forsyth, Rosebud County