Great Wall erosion cause

This is the picture for my friend Quin.
The possible cause of Great Wall destruction process caught here :)

My wallpapers

Here is some photo I have shot.

               
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| |  | | ___| | ___ ___  _ __ ___   ___| |
| |/\| |/ _ \ |/ __/ _ \| '_ ` _ \ / _ \ |
\  /\  /  __/ | (_| (_) | | | | | |  __/_|
 \/  \/ \___|_|\___\___/|_| |_| |_|\___(_)