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.
[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 4½ 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)]
[Illustration: Fig. 18 B]
[Illustration: Fig. 18 C]
[Illustration: Fig. 18 D]
[Illustration: Fig. 18 E]
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.
A different mode of origin than that given in the caption for figure 18
is depicted on a poster in the Visitor Center, which shows the pothole
being formed by a waterfall having an apparent flow rate of several
cubic feet per second. Potholes can be formed in this manner in places
where sufficient streamflow is available, either continuously or
following rainstorms, but I believe the process depicted in figure 18 is
a more likely mode of origin for Pothole Arch.