More than 100 million years ago, tectonic forces compressed the earth's crust in this region and pushed layers of sedimentary rock eastward along major thrust faults, stacking flat slices of rock to build high mountains. Molten rock intruded beneath the surface and cooled into large granite masses known as the Boulder Batholith, now exposed on the east side of Deer Lodge Valley. About 50 million years ago, the crust began to pull apart, and the Anaconda-Pintler and Flint Creek Mountains were separated from Deer Lodge Valley by the gently east-sloping Anaconda detachment fault. Hard metamorphic and igneous rocks from deep in the crust were brought up to form the mountains, while the sedimentary rocks that once lay above them slid eastward and downward along the fault and now lie buried beneath the valley. As the valley dropped and the mountains rose, the valley filled with thousands of feet of younger material eroded from the mountains. Although the Anaconda detachment fault is no longer active, other similar but steeper normal faults still are, so the mountains may still be growing. In 1852, Francois Finlay discovered gold on Gold Creek near the northern end of the Flint Creek Mountains, the first gold found in Montana, setting off a rush to the area in 1858 before the mines played out by 1863 and miners moved on to richer strikes elsewhere in the territory.