Rigphoto Yosemite rock stains as Realneo banner

This image is posted here in response to Norm's Question of the Day on Realneo [1] - the question being what caused the stains on the rock formations in Dulce.  I had my opinions - mainly relying on water and organic growth - but before I responded to Norm I thought it purdent to go to Google Images and look  for other examples and explainations.   I found this spectacular image of rock stains in Yosemite on Flicker.  The image belongs to Rigphoto and I link to it here [2].