Next among the main events leading to the formation of landforms in the
park was the raising and additional buckling and breaking of the Plateau
by Earth forces partly during the Late Cretaceous but mainly during the
early Tertiary. After uplift and deformation, the Plateau was vigorously
attacked by various forces of erosion, and the rock materials pried
loose or dissolved were eventually carted away to the Gulf of California
by the ancestral Colorado River. Some idea of the enormous volume of
rock thus removed is apparent when one looks down some 2,000 feet to the
river from any of the high overlooks farther south, such as Dead Horse
Point (Lohman, 1974, fig. 15). Not so apparent, however, is the fact
that younger Mesozoic and Tertiary rocks more than 1 mile thick once
overlaid this high plateau but have been swept away by erosion. In all,
the river has carried thousands of cubic miles of sediment to the sea
and is still actively at work on this gigantic earth-moving project. In
an earlier report (Lohman, 1965, p. 42) I estimated that the rate of
removal may have been as great as about 3 cubic miles each century. For
a few years the bulk of the sediment was dumped into Lake Mead, but now
Lake Powell is getting much of it. When these and other reservoirs
ultimately become filled with sediment—for reservoirs and lakes are but
temporary things—the Gulf of California will again become the burial
ground.
According to Cater (1970, p. 65-67), who made an intensive study of the
salt anticlines, collapse of their crests seemingly occurred in two
stages—the first stage following Late Cretaceous folding; the second
following uplift of the Plateau later in the Tertiary. Solution and
removal of salt by ground water played the leading role in the ultimate
collapse.
[Illustration: TILTED BLOCK OF ROCKS IN CACHE VALLEY GRABEN, viewed
to the east toward Cache Valley from point on gravelled side road to
Wolfe’s cabin, about half a mile east of paved road. Steep slope on
left composed of Jurassic Morrison Formation, hogback on top formed
by Dakota Sandstone of Late Cretaceous age, and gentle slopes to
right composed of the Mancos Shale of Late Cretaceous age. (Fig.
11)]
As shown by Dane (1935, pl. 1, p. 121-126), collapse of the Salt Valley
and Cache Valley anticlines was accompanied by considerable faulting and
jointing, particularly along their northeast sides; by the upward
intrusion of two large areas of the Paradox Member of the Hermosa
Formation, one just northwest of the park and one in the middle of Salt
Valley south of the campground; and by two downdropped masses of rock
known to geologists as grabens (pronounced gräbǝns)—one just northwest
of the park and one called the Cache Valley graben, which extends both
east and west from Salt Wash. The Cache Valley graben has preserved from
erosion the youngest rock formations in the park, as shown in figure 11.
The remarkable jointing of the rocks on the northeast limb of the Salt
Valley anticline is shown in figure 12. All the arches in this section
of the park were eroded through thin fins of the Slick Rock Member of
the Entrada Sandstone, and some, like Broken Arch, figure 16, are capped
by the Moab Member.
Differences in the composition, hardness, arrangement, and thickness of
the rock layers determine their ability to withstand the forces of
fracturing and erosion and, hence, whether they tend to form cliffs,
ledges, fins, or slopes. Most of the cliff- or ledge-forming rocks are
sandstones consisting of sand deposited by wind or water and later
cemented together by silica (SiO₂), calcium carbonate (CaCO₃), or one of
the iron oxides (such as Fe₂O₃), but some hard, resistant ledges are
made of limestone (calcium carbonate). The rock column (fig. 4) shows in
general how these rock formations are sculptured by erosion and how they
protect underlying layers from more rapid erosion. The nearly vertical
cliffs along the lower reaches of Salt and Courthouse Washes and the
Colorado River canyon upstream from Moab consist of the well-cemented
Wingate Sandstone protected above by the even harder sandstones of the
Kayenta Formation. (See figs. 21, 22.) To borrow from an earlier report
of mine (Lohman, 1965, p. 17), “Vertical cliffs and shafts of the
Wingate Sandstone endure only where the top of the formation is capped
by beds of the next younger rock unit—the Kayenta Formation. The Kayenta
is much more resistant than the Wingate, so even a few feet of the
Kayenta * * * protect the rock beneath.” In some places, as shown in
figures 19 and 20, the overlying Navajo Sandstone makes up the topmost
unit of the cliff.
[Illustration: JOINTED NORTHEAST FLANK OF SALT VALLEY ANTICLINE,
viewed westward from an airplane. Light-colored wedge in middle
background is Salt Valley bordered on extreme left by Klondike
Bluffs. Dark-colored fins and pinnacles on left, of Slick Rock
Member of the Entrada Sandstone, form Devils Garden. Sharp pinnacle
above valley is the Dark Angel. (See fig. 57.) White bands of
sandstone extending to foreground are composed of Moab Member of the
Entrada. Note vegetation in the joints. Photograph by National Park
Service. (Fig. 12)]
Last but far from least among the factors responsible for the grandeur
of Arches National Park and the Plateau in general is the desert
climate, which allows one to see virtually every foot of the vividly
colored naked rocks, and which has made possible the creation and
preservation of such a wide variety of fantastic sculptures. A wetter
climate would have produced a far different, smoother landscape in which
most of the rocks and land forms would have been hidden by vegetation.
On the Plateau the vegetation grows mainly on the high mesas and the
narrow flood plains bordering the rivers, but scanty vegetation also
occurs on the gentle slopes or flats.
The combination of layers of sediments of different composition,
hardness and thickness, the bending and breaking of the rocks, and the
desert climate, has produced steep slopes having many cliffs, ledges,
and fins with generally sharp to angular edges, rather than the subdued
rounded forms of more humid regions.