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Another problem I can see is deeply weathered reddish saprolite zones EXPOSED beneath the spillway chute, where the initial problem occurred. There are some significant problems with the Paleozoic age metavolcanics, referred to in the old days as “Amphibolite Schist.” This material is what the dam and spillways are founded on, where it was most recently exposed by the downcutting of the Feather River. But, up on the hilltops you have weathering surfaces that have been exposed for as much as 9 million years (a long time in a dynamic terrain like California). Because it’s been sitting there for such a long, long time the material is deeply weathered, with bands of saprolite. Saprolites are the almost disintegrated remains of a parent igneous rock, which looks good on first glance, but when you “bit” into it, you find you can excavate it with your hands… It would appear that the lion’s portion of the first big holes in the floor of the service spillway were founded on saprolite, which was rapidly removed by uplift and erosion.