This picture is a view upstream of Hoover Dam near Las Vegas. The boundary between white rock and red rock indicates the highest water level that has been seen in the reservoir behind the dam. The water has since gotten much lower, exposing all that white rock.
Presumably that white rock used to be red just like the never-submerged rocks above that point. So what turned them white?
Could be one of two things. Either the water leeched the color out of the rocks by dissolving the minerals that color the soil, or the white is a result of deposition of material onto the rock. I tend to favor the latter explanation. It’s probably dissolved salts that are coloring the rock.
Wiki says:
At lower water levels, a high-water mark or “bathtub ring” is visible in photos that show the shoreline of Lake Mead. The bathtub ring is white because of the deposition of minerals on previously submerged surfaces.
wiki source: Bryan Walsh (2008-12-04). “Dying for a Drink”. Time. Time Inc… http://www.time.com/time/magazine/article/0,9171,1864440,00.html?imw=Y. Retrieved 2009-08-26.
Water absorbed by the rock when it is submerged dissolves soluble salts (including, but not limited to, NaCl, gypsum, and calcite).
When the water level of the lake drops, the salt-laden water is wicked to the surface and evaporates, leaving these salts deposited on to the surface of the rock.
