[ctrager]

I recently did a backpacking trip in the Grand Canyon that included Horseshoe Mesa. There are old copper mines there, and beautiful green and blue copper-bearing rocks visible. I asked (in a facebook forum), why just in that one place and not other places in the Grand Canyon, given how the rock layers in the Grand Canyon tend to repeat themselves for miles and miles. The url is what somebody posted as the answer. It was so satifying to have a question and have it be so thoroughly answered.