Is the rock on left in the picture of Alcatraz below white from birdcrap?

Question

http://www.cyberlights.com/lh/calif/images/alcatraz/alcatraz01.jpg

My coworker says it is. I say it isn't. I think it's white because of the chemical makeup of the rock. I don't see why the birds would only c**p on one part of the island.

Answer 1

Sorry to tell you this, but Alcatraz rock is mainly greywacke sandstone with some guano on top.  In other words, decades (if not centuries) of bird s h i t.  During construction, a lot material was scraped off and top soil was imported from the mainland, and I guess it wasn't put down on the left side (too steep).