About a quarter mile west of the Y, a left fork of the road goes about a mile and a half to Green River Overlook, which provides a superb view of Stillwater Canyon of the Green River, the Orange Cliffs beyond, and the Henry Mountains in the extreme distance (fig. 23). Note that here the White Rim Sandstone is much thicker than in preceding views. The prominent butte enclosed by the loop of the river is known as Turks Head and is better seen from the air (fig. 24). The light-colored band near the base of the cliffs in the background of figure 24 is characteristic of the bleached upper part of the Moenkopi Formation in this part of the park. According to F. A. McKeown and P. P. Orkild (U.S. Geol. Survey, unpub. data, Feb. 16, 1973), petroliferous material or odor generally occurs in this bleached zone and in the basal beds of the Moenkopi. The campground just north of Green River Overlook has no water at this writing (1973), but water from wells in Taylor Canyon will eventually be piped to nearby parts of Island in the Sky. UPHEAVAL DOME Five miles northwest of the Y we come to Upheaval Dome, one of the most unusual geographic and geologic features of the park. Viewed from the air (fig. 25), it resembles somewhat a volcanic or meteor crater and has been called such by some. Because beds of salt are known to underlie the park, some have suggested that the salt may have thickened and welled upward to form a salt dome, similar to domes along the Gulf Coast (Mattox, 1968). However, only 1,470 feet of salt was encountered in an oil test just east of Upheaval Dome (Robert J. Hite, U.S. Geol. Survey, oral commun., Feb. 13, 1973); so although salt may have played a role, Upheaval Dome clearly is not a salt dome with dimensions similar to the Gulf Coast types. It may be related to a mound on the deep-seated Precambrian rocks (Joesting and Plouff, 1958, fig. 3; Joesting and others, 1966, p. 13, 14, 17), but the exact origin of the dome is not clear. The central part has the structure of a dome, in that the strata dip downward away from the middle. A ringlike syncline, or downward fold in the rock layers (fig. 26), surrounds the dome, beyond which the strata resume their nearly flat position. The white rock in the bottom of the craterlike depression is not salt, but jumbled large fragments of the White Rim Sandstone. Surrounding that are slopes of the Moenkopi and Chinle Formations, cliffs of the Wingate Sandstone, a circular bench of the Kayenta Formation, and outer ramparts of the Navajo Sandstone. Upheaval Canyon leads to Stillwater Canyon of the Green River at the upper left. [Illustration: STILLWATER CANYON AND GREEN RIVER, looking southwest from Green River loop of river. Brown material covering nearby parts of the White Rim is lower part of Overlook. Orange Cliffs in background, Henry Mountains on right skyline, Turks Head in Moenkopi Formation. (Fig. 23)]