The bedrock that the actual dam is built on is "hard amphibole" https://kb.osu.edu/dspace/bitstream/handle/1811/58502/SES-SeniorThesis_AndersonDennisM_1976_tex

The bedrock that the actual dam is built on is "hard amphibole" https://kb.osu.edu/dspace/bitstream/handle/1811/58502/SES-SeniorThesis_AndersonDennisM_1976_text.pdf?sequence=4 https://kb.osu.edu/dspace/bitstream/handle/1811/58502/SES-Se... This material will erode, but not so quickly that the main body of the lake wouldn't be shut off. The "30 foot" figure is the depth of the water above this bedrock. Had the emergency spillway failed, this is what would have been released without any controls. The main spillway was built on alluvium. I've posted elsewhere in the comment thread that cavitation was the primary cause of the concrete erosion.