Readers may recall back in January that I posed a question about Bly Tunnel and the geology. On the inlet side of the lake the tailing piles is a granitic type of sand, which is typical of Sierra Nevada. In April, I asked Michael Clynne of U.S.G.S. to explain some of the geology of the Eagle Lake basin. As it turns a portion of the basin is a spillover of Sierra Nevada. The Brockman Flat Lava flow occurred some 125,000 years ago, prior to the formation of the lake. It extended to what is the area of Bly Tunnel. In essence the lava flow capped over the Sierra Nevada, thus answering my question.
