Montana Governor and First Lady's Math and Science Initiative
Red-Capped Hills of Eastern Montana
Mile Post 192 on Highway I-94 at the Bad Route Rest Area.
Striking red rock caps many of the hills of eastern
Montana. Some of the rock looks volcanic, so it has
incorrectly been called scoria. Lewis and Clark attributed
the red rock to burning coal beds and called the red areas “burnt
hills.” Geologists call the rock clinker. It occurs within the
Fort Union Formation which contains 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 hardened lava. The heat from burning coal rises, so
most of the clinker develops above the burning coal bed. Some
clinker beds are 100 feet thick. Clinker beds are porous allowing
water to infiltrate into them rather than run off. The 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 in
eastern Montana. In the process, coal beds are left above the
water table where water can drain out of them and the coal
is exposed to oxygen. Lightning, spontaneous combustion,
chemical reactions, and range fires ignite the coal. Burning trees
rooted in coal beds can also start the 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. As the fire burns into the hill, the overlying rock breaks up and collapses, 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.
Red-Capped Hills of Eastern Montana
