In general, the oldest rocks in South Dakota formed more than 2 billion years ago, during the Precambrian. They consist of granites and metamorphic rocks and are found in the core of the Black Hills and in two small areas in eastern South Dakota.
In the Black Hills of western South Dakota, great sheets of granite intruded the igneous and metamorphic rocks. The Harney Peak Granite was carved to form Mt. Rushmore. Many thousands of pegmatites were also intruded during Precambrian time. In the southern Black Hills, these pegmatites have been mined for their crystals of feldspar, quartz, mica, beryl, and lithium minerals. Some have also been mined for tin and tungsten.
In northeastern South Dakota, the Precambrian Milbank Granite is exposed and quarried at the surface. In southeastern South Dakota, the Sioux Quartzite is exposed and quarried at the surface.
By the end of the Precambrian (570 million years ago), South Dakota had been deeply eroded and worn to a nearly flat plain interrupted by low knobs of granite and ridges of resistant quartzite. The top of Precambrian rocks in South Dakota slopes generally from east to west across all but the southwestern part of the state.