The Hoover Dam was conceived by Herbert Hoover while he was Secretary of Commerce in 1921 as a means of flood control and to provide Southern California with water. It wasn't until 1929 and after Hoover had bacame president that work on the dam was approved. Construction of the dam across Black Canyon was started in 1931 and took just 5 years to be completed by a Depression era workforce.The hard rock in Black Canyon is of volcanic origin and known as "andesite breccia."
Hoover Dam Resources:
http://www.usbr.gov/lc/hooverdam/History/articles/hhoover.html
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