In 1869, John Wesley Powell was studying layers of rock in the Grand Canyon when he noticed an unconformity in the layers. Around a billion years were missing, and the problem turned out to be global. There are exceptions, of course – for example, faulting moving younger rock on top of older rock – but in general, the farther down you go, the older the rocks are. But sometimes as you go down through the layers of the rock, you find a break where l... [3998 chars]
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