armor

Armor forms in a riverbed when the current flushes away silts, sands, and smaller gravels to such an extent that only heavier materials remain. These coarse gravels and stones, under the endless pushing of the water, may gradually settle into a smooth and durable surface called armor, which is capable of resisting the disturbance of powerful floods. In theory, however, no armor can last forever, for a flood mighty enough to tear it out and rearrange its component materials must inevitably come. The terrestrial equivalent of armor is desert pavement.

William deBuys