Devils Tower, known as Bear's Lodge to Northern Plains Tribes, rises high above the Belle Fourche River, grasslands, and ponderosa pine forests as a major landmark of the Northern Great Plains that has attracted people for thousands of years and still holds many meanings for American Indians, local ranchers, rock climbers, and thousands of visitors from near and far. The Tower and the Missouri Buttes to the northwest formed about 50 million years ago when magma was forced up from below the earth's crust, moved along cracks and weak spots, and pooled beneath the land surface, where it cooled, hardened, shrank, and cracked into the 4, 5, 6 and 7-sided columns seen today. Over time, ancient rivers washed away about 1.5 miles of the softer sedimentary rock layers that covered the Tower, and erosion is still slowly wearing away its hard igneous rock. Although many columnar rock features exist in the world, the Tower is unique because of its size and type of rock, phonolite, and its columns are the tallest, with some more than 600 feet high, and the widest, measuring 10-20 feet in diameter. Some geologists estimate that the last rock column fell 10,000 years ago.