Above the Great

'Then over the black gneiss are found 800 feet of quartzites, usually in very thin beds of many colours, but exceedingly hard, and ringing under the hammer like phonlite. These beds are dipping and unconformable with the rocks above; while they make but 800 feet of the wall or less, they have a geological thickness of 12,000 feet ... these quartzites vary greatly from place to place along the wall, and in many places they entirely disappear. Let us call this formation the variegated quartzite.'

Again, it was Powell who was first to note the geology of these rocks and, just over a decade later, Charles Doolittle Walcott divided them into lower Unkar

Charles Doolittle Walcott, 1850-1927, American palaeontologist and third director of the United States Geological Survey (from 1894), secretary of the Smithsonian Institution (from 1907), discovered the Burgess Shale in 1909.

(2073 m/6800 ft) and upper Chuar (1587 m/5200 ft) groups within the Grand Canyon Series (today called a Supergroup). It was Walcott who also provided the first geological map and sections, in which he agreed with Powell's measure of 12,000 feet (3660 m) for the total thickness of the 'Series'. Powell belonged to the generation of geologists who had been persuaded by Murchison's bluff and bluster to accept an enlarged Silurian extending down from the base of the Devonian to the Precambrian. Consequently, he thought the rocks to be Silurian in age, but Walcott, who belonged to a new generation that embraced new evidence and ideas, correctly reassigned them to the Precambrian by rather long-distance comparison with rocks in the Lake Superior region. Walcott was right and their age has now been confirmed on a more secure basis by radiometric dating. They are overlain by the mid-Cambrian-age Tapeats Sandstone (some 510 million years old) and thus altogether the Grand Canyon Supergroup spans over 700 million years of Earth Time. The only rocks within the Supergroup that are suitable for radiometric dating are the lavas found at the top of the Unkar Group and their age has been calculated at around 1100 million years old.

One of the most important aspects of this early geological history is the fundamental question of how and why sedimentary rocks accumulate on continents. Globally, most deposition occurs in the oceans of the world, especially around their edges where they lap onto the continental margins.

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