In my random web wanderings I came across a great blog "Walking the Wall" by a Emma Nicholas and Brendan Fletcher who walked the entire length of the Great Wall of China while sponsored by the Powerhouse Museum of Sydney, Australia. I recommend perusing the whole thing as there is quite a bit of material on culture, history and natural history of China. Particularly amazing to me was this: considerable portions of the wall were constructed using rammed earth. These portions of the wall are covered with biocrusts which apparently protect the wall from erosion (Photo: Nicholas & Fletcher)
Check out their post "The Living Wall", containing more photos and some info about the crusts provided by Dave Eldridge.