Centered on the state of Nevada and extending from southern Oregon to western Texas, the Basin and Range Province is an immense region of alternating, north-south-trending, faulted mountains and flat valley floors. It has no counterpart elsewhere in the U.S. The province was created about 20 million years ago as the Earth's crust stretched, thinned, and then broke into some 400 mountain blocks that partly rotated from their originally horizontal positions. These mountains of late Precambrian and Paleozoic rock continue to erode and fill the intervening valleys with fresh sediment. - USGS