Tectonic events beginning over 1 billion years ago influenced the type and structure of rock that formed in western North Carolina, guiding where mountains, valleys, streams, and rivers developed. Rock mined in Buncombe County includes material about a billion years old. The region's mountains consist mostly of igneous rock such as granite, sedimentary rock such as sandstone, and metamorphic rock such as gneiss. Approximately 270 million years ago, North America and Africa collided, pushing up the Blue Ridge Mountains. At that time these mountains were among the highest in the world, rising over 12,000 feet, but after hundreds of millions of years of erosion, the highest peak east of the Mississippi River, Mount Mitchell, stands 6,684 feet high and lies 10 miles to the north.