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Why do rift valleys often have flat floors and steep sides?

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Snow-capped, rugged Atlas Mountains encircle a green valley with scattered trees, terraced fields, and a few small buildings in Morocco. — valley The rugged Atlas Mountains surround a valley in Morocco.

Rift valleys are characterized by steep sides and relatively flat floors due to the mechanics of crustal extension and normal faulting.

The steep sides are created when the Earth’s crust fractures and pulls apart at divergent plate boundaries. As these crustal blocks move away from each other, a central block—known as a graben—slides downward along steep fault planes. This displacement creates the dramatic, high-walled margins that define the valley's edges.123

The floors of these valleys are typically flat because they act as basins that accumulate material over time. This flatness is largely the result of volcanic deposition and the settling of marine or lacustrine sedimentation, which fills in the low-lying areas created by the down-dropped crust. While many rift valleys are symmetrical, some are asymmetrical with one steep, faulted side and one gentler side formed by the tilting or flexing of the lithosphere.24