Although all the arches in the park were carved from the Entrada Sandstone, slight differences in their mode of origin or placement within the Entrada allow them to be grouped into three classes: (1) vertical arches formed in the Slick Rock Member alone or in the Slick Rock and Moab Members, (2) vertical arches formed mainly in the Slick Rock Member but partly in, and with the aid of, the incompetent underlying Dewey Bridge Member, and (3) horizontal arches, or so-called pothole arches, formed from the union of a vertical pothole and a horizontal cave. Hereinafter, the three members will be referred to alone, without reference to the Entrada.
“BABY ARCH,” just southwest of Sheep Rock in Courthouse Towers area. For details, see text. (Fig. 15)
Before giving examples of arches in each of the three classes, it is appropriate to remark that the arches and other erosion forms in the park represent but a fleeting instant in geologic time. Many of the pinnacles or piles of rock may be the broken remains of former arches, and many of the arches we see may be gone tomorrow, next year, or a few hundreds of years and, certainly, before many thousands of years. On the other hand, many new arches will form by the processes described above as the geologic clock ticks on.
BROKEN ARCH, reached by a ½-mile trail leading northward across field that separates Fiery Furnace from Devils Garden. White thin-bedded unit at top is the Moab Member, which rests upon the massive salmon-colored Slick Rock Member. Opening is 59 feet wide and 43 feet high. (Fig. 16)
Examples of Arches
Tunnel Arch ([fig. 14]) is a good example of an arch eroded entirely within the massive Slick Rock Member. Just southwest of Sheep Rock ([fig. 31]) is an unnamed opening in the lower part of the Slick Rock Member which I call “Baby Arch,” because it is one of the newest ones visible from the park road ([fig. 15]). It is only 25½ feet wide and 14 feet high and penetrates a wall 14 feet thick. Note that the breakthrough probably began along the prominent recessed bedding plane at the base of the arch. Its youthfulness is also indicated by the sharp, angular breaks in the ceiling and by the pile of freshly fallen rocks. Some visitors have asked park personnel why they have not cleared away such debris! Despite its youthfulness, the ceiling has already taken on the shape of an arch.
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.