Graben

graben [′grä·bən]

(geology)

A block of the earth's crust, generally with a length much greater than its width, that has dropped relative to the blocks on either side.

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Graben 

a portion of the earth’s crust that is sunken relative to surrounding areas along steep, often vertical fractures, usually faults. The largest system of graben in the world is found in East Africa (Great Rift Valley). In Western Europe the largest graben is the valley of the Rhine River. Grabens on a planetary scale are called rifts. Grabens with additional fractures at their edges, which form a series of steps, are called complex grabens.