The Red-Capped Hills of Eastern Montana

By editor

Glendive, Dawson County, Montana

As one travels the sweeping plains of eastern Montana, especially along the route of the Northern Pacific Railroad, the eye is arrested by a peculiar sight: hills and buttes topped with a vivid red crust, striking in contrast to the muted grays, tans, and ochres of the surrounding badlands. These red-capped hills, scattered across the landscape near Glendive and beyond, represent a natural phenomenon wrought by fire and time deep beneath the earth’s surface. They are no ordinary rocks, nor simple iron-stained formations; rather, they owe their existence to the slow, subterranean combustion of coal beds that have smoldered here for millions of years.

The red caps are clinker, a term used by geologists to describe rock that has undergone intense baking and transformation through heat. Unlike volcanic scoria or lava, clinker originates when coal seams ignite underground, usually ignited by natural causes such as lightning strikes, spontaneous combustion, or fires burning roots within the coal seams themselves. In eastern Montana, these coal seams belong to the Fort Union Formation, a sedimentary sequence laid down during the Paleocene epoch, roughly 66 to 56 million years ago. This formation is rich in lignite coal, formed from ancient swamp vegetation, sandwiched between layers of shale, mudstone, and sandstone.

When these coal beds catch fire--a process that may last thousands or even millions of years--they generate intense heat that bakes the overlying sedimentary rock. Sandstone fuses into a brick-like hardness, shale vitrifies much like ceramic in a kiln, and other materials partially melt, creating a hardened, erosion-resistant cap. The clinker thus formed often measures tens of feet thick and retains its red or reddish-brown hue from iron oxidation intensified by heat. Because clinker is porous, rainwater seeps into these caps rather than running off, replenishing groundwater and slowing erosion beneath. This process sculpts the landscape, leaving behind mesas and buttes crowned with these distinct red caps, rising prominently above the softer, eroded badlands.

The red-capped hills are not only geological curiosities but also ancient landmarks. The Lakota people, whose presence in this region predates European settlement, called these formations "the burning hills." These hills served as navigational aids across the open prairie, their fiery appearance a constant guide in the vastness of the plains. Similarly, the explorers Lewis and Clark, traversing the region in 1805 during the Corps of Discovery, noted these red hills. William Clark wrote in his journal on September 9, 1805: "We passed some hills which appeared to be burnt… the rock is a red color, supposed to be caused by the burning of the coal beds beneath." Their observation confirmed the indigenous knowledge and hinted at the fiery processes beneath the surface.

The badlands of eastern Montana, where these clinker hills occur, owe their sculpted forms to the erosive forces acting upon the Hell Creek and Fort Union formations. The Hell Creek Formation, famous for its rich fossil beds containing the remains of Tyrannosaurus rex, Triceratops, and other late Cretaceous fauna, lies stratigraphically below the Fort Union Formation. The rivers, coulees, and intermittent streams that dissect this terrain expose the layers of sedimentary rock and underlying coal seams. As the softer rocks erode, the harder clinker caps resist, forming the distinctive red-capped hills.

The persistent underground fires that bake these caps have been burning for an astonishing length of time. Scientific investigation into the region’s burning coal beds reveals that such fires have persisted intermittently for at least four million years. These natural coal fires ignite when the coal beds become exposed to dry air, often after erosion lowers the surrounding land or streams cut into hillsides, draining the water and drying the coal. Lightning strikes, spontaneous chemical reactions within the coal, and even wildfires igniting burning roots can set these seams aflame. As the fire progresses, the rock above fractures and collapses, allowing fresh air to feed the burning coal, sustaining the underground blaze.

Though these fires burn slowly, they eventually extinguish themselves when they reach less combustible rock or are starved of oxygen. Over time, the burning moves through the coal seam, baking the overlying rock into clinker, creating the red caps visible today.

The presence of coal and its burning in this region also had historical significance. Fort Union Trading Post, established in 1828 at the confluence of the Yellowstone and Missouri rivers, became the most important fur trade post on the upper Missouri River. The fort operated until 1867 and today is preserved as a National Historic Site near present-day Williston, North Dakota, just east of the Montana border. Traders and trappers who passed through were no doubt familiar with the red-capped hills, which marked the landscape they traversed and traded within.

The geological processes that produced the red-capped hills introduce us to a slow, fiery transformation beneath our feet, one that defies the immediate calm of the prairie landscape. As I stood amid these hills, feeling the dry wind carry the scent of sagebrush and the faint mineral tang of baked rock, I was reminded how the earth’s surface carries the memory of deep time and elemental forces--fire and water shaping the land across ages. The clinker caps, resilient and vivid, reveal a story of subterranean combustion and the relentless work of erosion, a narrative that extends back millions of years and continues to unfold.

In the words of geologist Charles D. Walcott, who studied the fossil-rich formations of Montana: "The burning coal beds and their clinker caps are among the most curious and instructive phenomena in the geology of the western plains." His observation invites us to regard these hills not merely as curiosities but as chapters in the grand history of this land, written in stone and fire.

See also

• The Red-Capped Hills of Eastern Montana at Glendive, Dawson County
• The Hell Creek Formation at Jordan, Garfield County
• Returning to the Plains at Glendive, Dawson County