Where fault lines in the Earth’s crust run parallel, the land between them may subside into a long trough or rift valley called a graben (the German word for ditch) as the adjacent blocks, lying on the opposite sides of each fault, are thrust upward into horsts. For instance, the San Luis Valley is part of the Rio Grande Rift system, extending from central Colorado southward through New Mexico and west Texas into northern Mexico. This valley, which has been called the highest, largest mountain desert in North America, formed long ago when a large graben sank along deep bounding faults.

Barbara Kingsolver