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Earthen dams are mostly fractured rock.
They are sealed on the upstream side with about 8-10 feet of clay. The clay stays moist and flexible.
The visible erosion shown on the daylight videos was soil eroding.
Rock is used when it’s a resource that is available. There are plenty of earthen dams in the SE US that are nothing more than a compacted clay core with silty loam soil as the shell on either side.