The easternmost section of Grand Canyon cuts north–south through the Marble Platform of Paleozoic rocks for more than 60 miles. Early canyon explorer John Wesley Powell thought the smoothly polished, almost vertical walls looked like marble, so he named this section Marble Canyon.
In 1969, the area became Marble Canyon National Monument to protect the river from proposed dam sites. In 1975, Marble Canyon was added to Grand Canyon National Park. Geologically speaking, it is part of Grand Canyon, though its historic appellation remains.
The rocks of Marble Canyon introduce the highest layers found throughout the rest of the canyon: the Kaibab, Toroweap, and Coconino Formations. In about five miles, past 467-foot high Navajo Bridge, reddish slope-forming Hermit Shale makes its appearance. Formed by an ancient swamp, Hermit Shale bears insect and plant fossils, especially ferns. (Hermit Shale becomes a predominant layer farther west, where it erodes to form sloped shoulders on buttes and canyon rims.)
The first sizable rapids that river runners encounter, House Rock Rapids, lies at the mouth of the tributary Rider Canyon, about 17 miles downriver from Lees Ferry. Like most rapids, House Rock Rapids was formed when flash floods through this tributary canyon pushed debris into the Colorado, creating a spillover.
In 1890, when engineer Robert Brewster Stanton attempted a second survey to find a railroad route through the canyon, the expedition’s photographer, Franklin Nims, was nearly killed in a fall. Stanton and his crew halted their journey to evacuate Nims via Rider Canyon.
© Kathleen Bryant from Moon Grand Canyon, 5th Edition