The canyons and adjoining terraces are spectacular illustrations of erosion. They show with diagrammatic clearness the work of running water, rain, frost, and wind, of ground water and chemical agencies active throughout a long period of time. The horizontal tables and benches, broken by vertical lines that in distant view appear to dominate the landscape, are normal features of erosion of plateau lands in an arid climate. The tabular forms are the edges and surfaces of hard strata from which softer layers have been stripped. The vertical lines mark the position of fractures (joints)—lines of weakness which erosion enlarges into grooves and miniature canyons. As they entrench themselves in horizontal layers of rock that vary in resistance to erosion, the master streams and their tributaries are developing stairlike profiles on their enclosing walls. Cliffs in resistant rocks and slopes in weak rock constitute risers and treads that vary in steepness and height with the thickness of the strata involved. Thus near the south entrance to Zion Park the edge of a layer of hard conglomerate is a vertical cliff, its top a platform. Above this platform a long slope of shale, broken by many benches developed in hard beds, extends upward to the great cliff faces of West Temple and the Watchman. In front of Zion Lodge a slope of weak shales leads upward to a cliff of resistant sandstone above which a slope of shale extends to the vertical wall of Lady Mountain. In Bryce Canyon the rim road is on the highest tread of a giant rock stairway that, as viewed from Rainbow Point, leads downward in steps 30 to 400 feet high to the flat lands 3,000 feet below. [Illustration: _View of multicolored formations from Bryce Point_ (_Union Pacific Railroad photo_)] The streams at work in the parks, though relatively small, have steep gradients, including rapids and waterfalls, and are supplied with disintegrated rock material swept from the ledges by torrential rains about as fast as formed. They are therefore powerful agents of erosion, especially in times of flood. The fresh, sharp, angular profile of mesas, ridges, and canyon walls and the extensive areas of bare rock are maintained by the rapid down-cutting and prompt removal of rock waste. The resulting land forms reflect the aridity and the topographic youth of southern Utah and contrast strongly with the rounded hills, broad valleys, plant-covered slopes, and deep soils of more humid regions. _Geologic History._—A large part of geologic history is revealed in the canyon walls of Zion and Bryce Canyon National Parks. Just as Grand Canyon is the best known record of ancient geologic history, Zion Canyon records most clearly the events of medieval geological time, and Bryce reveals much of modern geologic history. The story of Zion begins where that of Grand Canyon ends, and ends where Bryce begins. The rocks exposed in these three national parks incorporate the records of a billion years.