The Big Belt Mountains on the skyline east of Helena Valley are primarily composed of Belt Supergroup rocks deposited about 1.4 billion years ago in a large, shallow sea that covered much of western Montana and extended into Canada, where sand and mud later became hard rock such as quartzite under thick layers of younger sedimentary rock. The crags to the left are part of the Boulder Batholith, a huge mass of granite extending south of Butte, formed when molten rock cooled below the earth's surface from 80 to 75 million years ago. In the foreground toward Helena Valley, beds of sandstone and shale were hardened by adjacent hot magma that cooled to form the granite. During the Ice Age, glaciers partly covered the mountains near MacDonald Pass, leaving round boulders north of the highway when they melted about 14,000 years ago, and meltwater turned Ten Mile Creek into a raging torrent that deposited boulders up to 4 feet across in the valley. The road between Elliston and Helena began as a toll road built by Constant Guyot in 1866 and was known as the Frenchwoman's Road for his wife, who collected tolls and offered meals and lodging at their cabin near Elliston until she was found murdered there in 1868; after her husband fled, Elijah Dunphy took over operations and hired Alexander MacDonald to maintain the road and collect tolls. The mineral-rich Boulder Batholith covers 350 square miles and includes the mining areas of Helena, Butte Basin, and Unionville. Granite quarried in the mountains north of U.S. Highway 12 and east of here was used in many prominent historic buildings in Helena beginning in the 1890s. MacDonald Pass is one of three mountain passes crossing the Continental Divide west of Helena; Mullan Pass is named for John Mullan, who built the first engineered road in Montana in 1860, and Priest's Pass, to the north, was the main route over the Continental Divide west of Helena for a few years in the 1920s.