The Red-Capped Hills of Eastern Montana

Striking red rock caps many of the yellow layered hills of eastern Montana. Some of the enigmatic rock looks volcanic, so it has incorrectly been called scoria. Lewis and Clark attributed the red rock to the burning of coal beds, calling the red areas "burnt hills". Geologists call the rock clinker, and it primarily occurs within the Tertiary Fort Union Formation which contains many coal beds sandwiched between soft sedimentary rock. Clinker develops when coal burns from the surface into a hill, where it cooks, fuses, and melts the adjacent rock, forming new, completely different types of rocks. Sandstone is baked to a brick-like rock. Shale may be fused like a ceramic in a kiln. Other rock may melt to look like ropy hardened lava. As the coal burns, a void is produced, and the overlying rock breaks up and collapses into it. As a result, angular blocks of baked rock develop. The heat from burning coal rises, so most of the clinker develops above the burning coal bed. Some clinker beds are 100 feet thick, but clinker only extends a foot or two below where a coal bed burned. Clinker typically has developed many cracks allowing water to infiltrate through the cracks rather than run off. This infiltration recharges the groundwater and protects the underlying rock from erosion, producing hills with red clinker caps. Before a coal bed can ignite and burn it must be dry and exposed to air. Streams and flash floods erode the hills of Fort Union Formation in eastern Montana. In the process, coal beds are left above the water table where water can drain out of them. While exposures are fresh, coal is also exposed to oxygen. Lightning, spontaneous combustion, chemical reactions, and range fires ignite the coal. Burning trees rooted in coal beds can also start coal fires. Scientists have determined that coal has been burning in eastern Montana for at least four million years, but each burning coal bed eventually extinguishes naturally. The fire starts where the coal is exposed along the side of a hill. As it burns into the hill, the overlying rock breaks up and collapses which provides void space. This allows air deeper into the hill and keeps the coal burning underground. Eventually, too much overlying rock collapses to allow air to enter, and the fire goes out. Coal still smolders underground in a few places in eastern Montana today.