The Highwood Mountains to the northeast are made of resistant volcanic rocks that rise above softer surrounding sedimentary rocks and contain an unusual abundance of the dark igneous rock shonkinite, which was named for exposures near the community of Shonkin on the north side of the range. Belt Butte, northeast of the highway, is recognizable by its belt of Cretaceous sandstone and gave its name to Belt Creek, the town of Belt, and the Big and Little Belt mountains; the Belt Supergroup of western Montana was in turn named for the Big and Little Belt mountains because of that sandstone belt around Belt Butte. The Little Belt Mountains to the south were bowed up by multiple blister-like pockets of magma that arched the overlying sedimentary layers into domes, and the domes and mountains are cored by igneous rock formed about the same time as the Highwood Mountains, about 50 million years ago. The Great Falls coal field extends through the Armington area, where discontinuous coal formed from plant material that accumulated in Early Cretaceous swamps and was compacted over millions of years into medium-grade bituminous coal. Coal mined here powered Great Northern Railway locomotives, fueled the smelter at Great Falls, and heated homes throughout central Montana, but mining declined after 1950 when coal from the Great Falls field could not compete with diesel, oil, and natural gas.