Visitors to the Grand Canyon, and especially those who hike to Plateau Point at the end of Bright Angel Trail, will see a major change in rock type when looking into the inner canyon. The steep walls reveal metamorphosed basement rocks, but resting on these are the horizontally-bedded fresher looking Tapeats Sandstones. The linear boundary between them is known as the “Great Unconformity”. There are many other unconformities to be found in the Grand Canyon, but this one is by far the most dramatic. It can be traced as far as the eye can see – and beyond. It is found on most continents:
“The Great Unconformity is well exposed in the Grand Canyon, but this geomorphic surface, which records the erosion and weathering of continental crust followed by sediment accumulation, can be traced across Laurentia and globally, including Gondwana, Baltica, Avalonia and Siberia, making it the most widely recognized and distinctive stratigraphic surface in the rock record.”
In a recent paper, two geologists have suggested that the processes involved in forming the Great Unconformity provided a trigger for the Cambrian Explosion.
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