The Colorado River dug this 12.5-mile-long chasm through gray Mississippian and green-gray Devonian limestones, brown Ordovician dolomite, and light and chocolatebanded Cambrian quartzites to reach the foundation of pink Precambrian granite. This natural masonry, exposed and sculpted over the past 40 million years, dwarfs sensational highway and railroad design in the 2,000-foot-deep chasm. The Denver & Rio Grande Railroad first blasted, tunneled, and bridged its way through the south wall of the canyon in 1887, followed by a northside 1890 wagon road that evolved into U.S. 6, then I-70.
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