Broken Arch (fig. 16) was formed near the top of the Slick Rock Member and is strengthened and protected by the more resistant overlying Moab Member, which forms the upper half of the span. The crest is only 6 feet thick at the thinnest point and is not broken as the name seems to imply.
Double Arch (fig. 17), "one" of the most beautiful in the park, is in The Windows section near the east end of the road. The southeast arch, which is 160 feet wide and 105 feet high, is the second largest in the park, but the west arch measures only 60 feet wide and 61 feet high. In common with most arches in The Windows section, these two arches of the Slick Rock Member rest upon bases of the weak, easily eroded Dewey Bridge Member. More rapid erosion of the Dewey Bridge undercut the arches and hastened their development.
[Illustration: DOUBLE ARCH, in The Windows section. (Fig. 17)]
[Illustration: PROBABLE STEPS IN FORMATION OF POTHOLE ARCH. _A_, Original pothole probably formed in relatively level bed of sandstone, such as this one, which is in an older rock unit--the White Rim Sandstone Member of the Cutler Formation, a unit not present in Arches. This pothole, which contains 4 feet of water, is in nearby Canyonlands National Park (Lohman, 1974, fig. 17), just north of the edge of the White Rim, about 4-1/2 miles north of the confluence of the Green and Colorado Rivers. Photograph by E. N. Hinrichs. _B_, Pothole is being deepened by solution while cliff is receding toward pothole by weathering. _C_, As erosion continues, pothole and cave in cliff face are growing deeper. _D_, Pothole Arch formed by union of vertical pothole and horizontal cave. _E_, Telephoto view of Pothole Arch from park road near stop 14. Visible span is 90 feet across and 30 feet high. (Fig. 18)]
The cause of the wavy bedding in the Dewey Bridge Member, as shown in figure 17 but as better shown in the frontispiece, is not known for sure but generally is regarded to be the result of irregular slumping during or just after deposition of the sediments in a body of water, caused by the weight of overlying sediments.
The last example I shall take up is Pothole Arch (fig. 18), which differs from all the other examples in that this arch is roughly horizontal rather than vertical. Most park visitors, including me, were not aware of this interesting feature until after publication of the pamphlet "The Guide to an Auto Tour of Arches National Park," which, as previously noted, may be purchased at the Visitor Center. Pothole Arch caps a ridge high above the road half a mile northwest of Garden of Eden, so only those who happened to look up at the right place were aware of its existence.