Although some two-wheel-drive cars or trucks have traversed the White Rim and Shafer Trails, they may encounter trouble with deep sand, washouts, or fallen rocks, so four-wheel-drive vehicles are recommended. In the summer these trails should not be attempted without plenty of water, and two vehicles traveling together provide an added margin of safety. All vehicles should carry emergency equipment including a shovel, tow chain or rope, jack, tire tools, and other necessary items. Geologists and uranium prospectors working along the White Rim Trail have obtained good drinking water from small springs that flow from the base of the White Rim Sandstone in many places (Neal Hinrichs, U.S. Geol. Survey, oral commun., Feb. 1973). After rains, runoff gathers in large potholes in the White Rim Sandstone in some places and affords emergency drinking water. Several such potholes filled with water are shown in figure 17. Some potholes occur also in the Cedar Mesa Sandstone in the Needles district. GRAND VIEW POINT About a mile southwest of The Neck, the road crosses Grays Pasture—the widest and flattest part of Island in the Sky. The drive over this flat grassland yields not the slightest hint of the awesome cliff-walled chasms on either side of the island. Some 5 miles southwest of The Neck, both the island and the road branch like a Y. At a point 0.4 mile north of the Y, Mesa Trail leads one-quarter mile east to Canyon Viewpoint Arch, which frames the Colorado River canyon and the La Sal Mountains (fig. 18). This arch, at the very top edge of the cliff, is composed of the lower part of the Navajo Sandstone. The only other arch of Navajo Sandstone in or near the park that I know of is the small one shown in figure 33, but of course there may be others. [Illustration: NATURAL TANKS, filled with runoff from rain, serve as emergency sources of drinking water. Largest tank in foreground contains 4 feet of water and small fresh-water shrimp. So-called tanks, or potholes, are formed partly by water dissolving the calcium carbonate cement and partly by wind or water removing the resulting loose sand grains. View is north toward Junction Butte from point about a mile south of the White Rim Trail. Red rocks in hill on right are in lower part of Moenkopi Formation. Photograph by E. N. Hinrichs. (Fig. 17)] [Illustration: CANYON VIEWPOINT ARCH, framing Colorado River canyon at east end of Mesa Trail 0.4 mile north of Y in Island in the Sky road. Arch is in lower part of Navajo Sandstone. (Fig. 18)] [Illustration: INDEX MAP showing localities where most of the photographs were taken. Arrows point to distant views. Numbers refer to figure numbers. (Fig. 19)]