Except for Old Faithful, the Grand Canyon of the Yellowstone is probably the best known and most talked about and photographed feature in the Park (fig. 41). Although not so deep or wide as some of the other great canyons in America, its sheer ruggedness and beauty are breathtaking. Here the aptness of the name “Yellowstone” can be fully appreciated and understood, for the viewer is at once engulfed in a sea of yellow hues streaked and tinted with various shades of red and brown. [Illustration: GRAND CANYON AND LOWER FALLS of the Yellowstone River, as viewed upstream (southwest) from Artists Point on the south rim. The yellow-hued rocks lining the canyon walls are soft, hydrothermally altered rhyolite lavas. The rocks at the brink of the falls consist of less altered and therefore more resistant rhyolites. The falls, 309 feet high, formed at the contact between the hard and soft rhyolite units. (Photograph courtesy of Sgt. James E. Jensen, U.S. Air Force.) (Fig. 41)] [Illustration: DEVELOPMENT OF GRAND CANYON. Profiles along the floor of the Grand Canyon of the Yellowstone as it appears today (C) and as it appeared at two older stages in its development (A and B). Note particularly the various kinds of rocks through which the canyon has been cut, and how rock differences have influenced the location of the two falls. Diagonal lines indicate unaltered rhyolite; large dots, rhyolite with much volcanic glass; small dots, hydrothermally altered rhyolite; and circles and dots, Absaroka volcanic rocks. (Based on information furnished by R. L. Christiansen and G. M. Richmond; vertical scale is exaggerated about 10 times.) (Fig. 42)] C: As it appears today Yellowstone Lake Hayden Valley Grand Canyon of the Yellowstone Upper Falls Lower Falls Inspiration Point North edge of Yellowstone caldera Confluence of Lamar and Yellowstone Rivers B: A stage somewhat before 300,000 years ago Profile of Yellowstone River today A: A stage somewhat before 600,000 years ago Profile of Yellowstone River today