Striking red rock caps many of the hills of eastern Montana. Though some of the rock looks volcanic and has incorrectly been called scoria, geologists call it clinker. Lewis and Clark attributed the red rock to burning coal beds and called the red areas "burnt hills." Clinker occurs within the Fort Union Formation, which contains coal beds sandwiched between soft sedimentary rock. It develops when coal burns from the surface into a hill, cooking, fusing, and melting adjacent rock and creating completely different types of rock. Sandstone is baked into a brick-like rock, shale may be fused like a ceramic in a kiln, and other rock may melt to look like hardened lava. Because heat from burning coal rises, most clinker develops above the burning coal bed, and some clinker beds are one hundred feet thick. Clinker beds are porous, allowing water to infiltrate rather than run off, which recharges groundwater and protects 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. As streams and flash floods erode the hills in eastern Montana, coal beds are left above the water table where water can drain out. Lightning, spontaneous combustion, chemical reactions, range fires, and burning trees rooted in coal beds can ignite the coal. 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, overlying rock breaks up and collapses, allowing air deeper into the hill and keeping the coal burning underground, until too much rock collapses for air to enter and the fire goes out.