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Comb Ridge. A formation of Navajo Sandstone that was once part of the largest erg, or sand desert, that the world has ever known. It was deposited mostly in Triasic and Jurasic time: early dinosaur ages, on what was the westen part of what's now North America, when it was closer to the equator, and prevailing winds were mostly from the east. Other members of this family of formations extend west to the red and white Aztec sandstones around Las Vegas in Nevada. Nice piece here about the possible provenance of Navajo Sandstone grains.