Normal faults are common in areas of the Earth's crust where thick piles of sedimentary layers have accumulated. Such sedimentary basins sag by nature -- that's why the area "accommodated" deposition in the first place. This sagging can result in stretching around the edges of the basin. Geologists call the stretching force by the term extension. Extensional tectonics, such as has affected the prominent Basin and Range province of the American West, forms block-faulted mountains and valleys, bounded by normal faults. The blocks that stay up, forming mountain ranges, are called horsts, and those that go down, forming valleys, are called grabens. In the following animation, and in the others in this section, one side of the fault moves down and the other side stays put, or moves up.
