.
The dam was built against undisturbed native limestone.
The face had soil overburden, and that is mostly what erodes first, but the limestone can also erode, especially if it is exposed to weather for a long period.
It “chalks.”
Found a video of the dam being built. The fill looks to be alluvial in nature with a lot of fist sized, rounded rocks in it.
https://www.youtube.com/watch?v=hJ8Um5KJfF0