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Longman Dictionary of Contemporary English
rift valley
noun
EXAMPLES FROM CORPUS
▪ Further evidence contradicting the traditional symmetric rift valley model comes from observations of their morphology and surface structure.
▪ Luisenlund Bronze Age monoliths and the rift valley of Ekkodal.
▪ Upland masses exist on either side of a central rift valley.
Wiktionary
rift valley

n. 1 (context geology English) A graben structure between two tectonic plates which are opening relative to each other. 2 (context geology English) A mid-oceanic ridge system with central valleys.

WordNet
rift valley

n. a valley with steep sides; formed by a rift in the earth's crust

Wikipedia
Rift valley

A rift valley is a linear-shaped lowland between several highlands or mountain ranges created by the action of a geologic rift or fault. A rift valley is formed on a divergent plate boundary, a crustal extension, a spreading apart of the surface, which is subsequently further deepened by the forces of erosion. When the tensional forces were strong enough to cause the plate to split apart, a center block dropped between the two blocks at its flanks, forming a graben. The drop of the center creates the nearly parallel steeply dipping walls of a rift valley when it is new. That feature is the beginning of the rift valley, but as the process continues, the valley widens, until it becomes a large basin that fills with sediment from the rift walls and the surrounding area. One of the best known examples of this process is the East African Rift. On Earth, rifts can occur at all elevations, from the sea floor to plateaus and mountain range in continental crust or in oceanic crust. They are often associated with a number of adjoining subsidiary or co-extensive valleys, which are typically considered part of the principal rift valley geologically.