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In page Geology of the Grand Canyon area:
"Subsequent erosion lasting 300 million years stripped much of the exposed sediments and the mountains away.[8] This reduced the very high mountains to small hills a few tens to hundreds of feet (tens of meters) high.[2] Geologist John Wesley Powell called this major gap in the geologic record, which is also seen in other parts of the world, the Great Unconformity.[8] Other sediments may have been added but, if they ever existed, were completely removed by erosion. Such gaps in the geologic record are called unconformities by geologists. The Great Unconformity is one of the best examples of an exposed nonconformity, which is a type of unconformity that has bedded rock units above igneous or metamorphic rocks.[9]
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