Lakes, like mountains, are temporary things. Even as lakes are forming,
sediment begins to fill them until ultimately they are obliterated. So
it was with Lake Uinta. Sometime after this lake dried up, the Earth’s
crust again became restless. The gentle folds that were formed late in
the Cretaceous were lifted higher and bent more sharply, and the flanks
of some folds were wrinkled and broken (figs. 27, 28). The sharply bent
or broken rocks along the northeastern border of the Monument are
thought to have been deformed mainly at this time, but in part both
earlier and later. That pronounced folding of the rocks followed the
deposition of the Eocene Green River Formation is clearly shown along
the Grand Hogback monocline between the towns of Rifle and Meeker,
Colorado, where the once flat lying beds of the Green River and Wasatch
Formations now stand vertical.
The folds and faults along the northeastern border of the Monument,
which are shown on the geologic map (fig. 8), are discussed briefly
here—more details are given later in “Trips through and around the
Monument.” The folded and faulted northeastern border of the Monument,
which is shown in figure 29 and in several ensuing photographs, is
believed to have resulted from renewed uplift of the area southwest of
the folds and faults, including the Monument. The Redlands fault (figs.
8, 29, 37, 38, 40, 41) generally is a normal fault but locally is a
reverse fault, as discussed on page 92 and as shown in figure 40 and in
the cross section of figure 8. This fault has a maximum vertical
displacement of 700 or 800 feet, but dies out in scissors fashion at
each end. Beyond the end of the Redlands fault in the upper right of
figure 29 may be seen another unbroken monocline. A close-up view of the
northwestern end of this fold in shown in figure 30.
[Illustration: COMMON TYPES OF FAULTS. Top, normal, or gravity,
fault which generally results from tension in and lengthening of a
segment of the Earth’s crust, which allows the lower block to
subside. However, some normal faults, particularly some that are
vertical or nearly so, may result from uplift of the upper block.
Low-angle reverse faults generally are called overthrust faults or
simply overthrusts. In both the normal and reverse faults note
amount of displacement and repetition of strata. Displacement of
such faults may range from a few inches to many thousands of feet,
and in overthrusts may reach many miles. From Hansen (1969, p. 116).
(Fig. 28)]
If we proceed about a quarter of a mile northeast of the point from
which figure 30 was taken, walk about 50 feet north, and look to the
northwest, we see quite a different structure, for here the gentle lower
fold of the Lizard Canyon monocline has become the east end of the
Kodels Canyon fault (fig. 31).
[Illustration: LADDER CREEK MONOCLINE AND REDLANDS FAULT, telephoto
view looking northwest from point near Little Park Road east of the
Monument. No Thoroughfare Canyon in foreground, which is bordered on
the left by northeastward-dipping beds of Wingate Sandstone at
northwest end of Ladder Creek monocline. The old Serpents Trail, the
lower part of which is barely visible, ascends this dipping block of
rock. The dark Proterozoic rocks form the flat-topped bluff to the
right and are exposed by the Redlands fault which lies just above
the sharply upturned remnants of the Wingate Sandstone. (Fig. 29)]
[Illustration: LIZARD CANYON MONOCLINE, looking southeastward across
mouth of Lizard Canyon from southeasternmost loop of Rim Rock Drive
just before it ascends Fruita Canyon. Note gentle lower bend at
lower left and sharper upper one at upper right. Lower bend changes
to Kodels Canyon fault in Fruita Canyon behind camera station. Grand
Mesa forms left skyline. (Fig. 30)]
[Illustration: KODELS CANYON FAULT, looking northwest across mouth
of Fruita Canyon from point on Rim Rock Drive just described in
text. Here, along a normal fault dipping steeply northeastward, the
350-foot cliff of Wingate Sandstone at upper left has been sheared
and squeezed into only a few feet of broken rock overlain by a steep
slope of the Kayenta Formation covered by piñon and juniper. The
thinner cliff at right is the Entrada Sandstone which belongs high
atop the cliffs at left. Book Cliffs form distant skyline at right.
(Fig. 31)]
If you doubt that figure 31 shows a fault, a glance at figure 32 in the
next major canyon eight-tenths of a mile to the northwest should
convince you. Here, on the northwest side of Kodels Canyon, the Wingate
was not thinned but was rent completely asunder by the vertical Kodels
Canyon fault (fig. 32). Kodels Canyon is not readily accessible to
visitors.
The Lizard Canyon monocline, Kodels Canyon fault, and other structures
are clearly shown in the stereoscopic pair of aerial photographs in
figure 33.
Another structural feature within the Monument is the Glade Park fault
(fig. 8), which lies mainly south of the Monument but just cuts across
the south end of No Thoroughfare Canyon in the latest addition to the
Monument. It is well shown both from the air and the ground in figures
58 and 59. It is unique among all the major faults in the area in that
the rocks south of the fault subsided with respect to those on the north
side.
[Illustration: KODELS CANYON FAULT, looking northwestward across
canyon of same name. Base of Wingate cliff on left is just about
opposite the top of the Wingate on right. Here, nature was kind to
the geologist, for the vertical displacement (rise of left side with
respect to right side) is virtually the thickness of the Wingate
Sandstone—about 350 feet. The Wingate on the right is lighter
colored than that on the left seemingly because rockfalls removed
desert-varnish-coated rocks and exposed the true color of the
sandstone. (Fig. 32)]
[Illustration: GEOLOGIC STRUCTURES AT FRUITA ENTRANCE TO COLORADO
NATIONAL MONUMENT. The stereoscopic pair of aerial photographs may
be viewed without optical aids by those accustomed to this
procedure, or by use of a simple double-lens stereoscope, such as
the folding ones used by the armed forces during and after World War
II. Geologic details may be identified by comparing photographs with
the geologic map, figure 8. If viewer is unable to see stereoscopic
pairs in three dimensions, looking at either photograph alone will
convey a good idea of the geologic structure. The monocline near top
of the photographs may be seen on the right-hand side of the highway
in figure 43. Photographs taken in 1937 by U.S. Soil Conservation
Service, hence, alinement of then unpaved Colorado Highway 340
differs from the paved present highway. (Fig. 33)]
At this point in our story it might be well to point out that the
folding and faulting of the rocks just described occurred when thousands
of feet of younger rocks covered the area. Additional folding and
faulting, drainage changes, and gradual removal of the overlying rocks
occurred during the remainder of the Tertiary and Quaternary Periods, as
will be discussed further.