The map (fig. 1) and cross section (fig. 10) of the park show that in general the major features of the landscape lie at three different and distinctive levels. A recently erected plaque on Grand View Point appropriately refers to these levels as the “Three Worlds.” The high plateaus, or mesas, in and adjoining the park dominate the skyline—in fact, the central one, between the Green and Colorado Rivers, is appropriately named Island in the Sky. If you stand on either the east or the west shore of this towering cliff-bordered island, you can look across a sea of fantastic erosional forms to a similar cliff-bordered shore at about the same level. Closer inspection of the sea of rocks on either side shows relatively flat benches or platforms about halfway to the bottom; below these are the generally steep sided or cliff-bordered canyons of the two rivers and their larger tributaries. From some vantage points along the shore, such as Dead Horse Point (fig. 15) or Green River Overlook (fig. 23), you can see the deepest level of all—the channels and flood plains of the Green and Colorado Rivers. What caused the “Three Worlds” and the formidable cliffs supporting the high mesas or forming towering monoliths like Angel Arch or Druid Arch (figs. 43, 54)? Differences in the composition, hardness, arrangement, and thickness of the rock layers determine their ability to withstand the forces of fracturing and erosion and hence their tendency to form cliffs, ledges, or slopes. Most of the cliff- or ledge-forming rocks are sandstones consisting of sand grains deposited by wind or water and later cemented together by silica (SiO₂), calcium carbonate (CaCO₃), or one of the iron oxides (such as Fe₂O₃), but some hard, resistant ledges are made of limestone (calcium carbonate). The rock column (fig. 9) shows in general how these rock formations are sculptured by erosion and how they protect underlying layers from more rapid erosion. The nearly vertical cliffs supporting the highest mesas consist of the well-cemented Wingate Sandstone protected above by the even harder sandstone of the Kayenta Formation. To borrow from an earlier report of mine (Lohman, 1965, p. 17), Vertical cliffs and shafts of the Wingate Sandstone endure only where the top of the formation is capped by beds of the next younger rock unit—the Kayenta Formation. The Kayenta is much more resistant than the Wingate, so even a few feet of the Kayenta * * * protect the rock beneath. In some places remnants of the overlying Navajo Sandstone make up the topmost unit of the cliff. [Illustration: Petroglyph] How to See the Park