Grand Canyon Unconformities –and a Cambrian Island
I have discussed the Grand Canyon myself at https://paulbraterman.wordpress.com/2018/03/09/why-science-needs-philosophy-cont-and-why-it-matters-with-examples-from-geology/ , and strongly recommend this beautifully written and illustrated guide to its unconformities
A prominent ledge punctuates the landscape towards the bottom of the Grand Canyon. It’s the Tapeats Sandstone, deposited during the Cambrian Period about 520 million years ago, when the ocean was beginning to encroach on the North American continent, an event called the Cambrian Transgression. Above the ledge, you can see more than 3000 feet of near-horizontal sedimentary rocks, eroded into cliffs and slopes depending on their ability to withstand weathering and erosion. These rocks, deposited during the rest of the Paleozoic Era, are often used to demonstrate the vastness of geologic time–some 300 million years of it.
But the razor-thin surface between the Tapeats and the underlying Proterozoic-age rock reflects the passage of far more geologic time –about 600 million years where the Tapeats sits on top the sedimentary rocks of the Grand…
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