The Madison Range rises from the east side of the Madison Valley along an active fault at the base of the range, where the mountains began to slowly rise about 50 million years ago and where activity continues today. On August 17, 1959, a magnitude 7.5 earthquake struck the southern end of the Madison Range near Hebgen Lake, triggering a massive landslide that dammed the Madison River and created Earthquake Lake; fault scarps as much as 21 feet high significantly changed the landscape and remain visible along the zone. In the northern Madison Range, prominent peaks including Lone, Pioneer, and Fan mountains are supported by resistant igneous rock formed from magma that pushed upward along layers of sedimentary rock, while glaciers and landslides carved the peaks into their present shapes. Sphinx Mountain is topped by 3,000 feet of gravelly conglomerate rock made from gravels eroded from surrounding highlands and deposited in an ancient valley that now forms part of the highest peaks. Mountain streams also deposited sediments into the Madison Valley, creating alluvial fans. The range later formed a dramatic backdrop to the fur trade in the American West: beginning in the 1830s, American fur trappers, or mountain men, caught beaver in streams flowing into the Madison River, but by the 1840s both the Blackfeet and the beaver had all but disappeared from the area. Their legacy remains in the names of many geographic features in the region.