(aka Boulder Dam)
Hoover Dam (Boulder Dam) is on the Nevada-Arizona border, east-southeast of Las Vegas. It dams the Colorado River in Black Canyon & the water ponded up behind it forms Lake Mead. The dam itself is impressively big at 726 feet tall. Built over 6 years in the 1930s, the dam was intended to serve as a flood-control structure, to create a water supply, and to generate electricity.
The rocks on either side of Black Canyon at Hoover Dam include volcanics, siliciclastic sedimentary rocks, and intrusive igneous rocks. The light-brown rocks dominating the photo below are part of the Hoover Dam Tuff (inf.), consisting of volcanic tuffs (welded ash-flow tuffs) of early Middle Miocene age (~14 million years). The dark brown layer in the lower right corner of the photo is a latite sill, intruded after deposition of the tuff beds. Note that the sill is offset by a subvertical fault. There's a whole series of very obvious faults lining the canyon walls downstream from Hoover Dam.
Above: view of southeastern side of Black Canyon from atop Hoover Dam. Looking SE.