Rivers, like people, do not always choose their courses wisely. After a
few million years of downcutting through the soft sedimentary rocks,
mainly what is now called the Mancos Shale, the ancestral Colorado and
Gunnison Rivers found themselves cutting through the hard Proterozoic
rocks in a deep gorge athwart the slowly rising Uncompahgre arch, which
greatly slowed the downcutting power of the combined streams. Note in
figure 34_A_ that while the mighty ancestral Colorado and Gunnison
Rivers were in this frustrating predicament, a young upstart tributary
began cutting northward from what is now the mouth of the Dolores River
(fig. 34_D_). Although the combined main rivers could lower their
channel only very slowly because of the hard rock in Unaweep Canyon, the
tributary was able to cut downward and headward quite rapidly through
the soft Mancos Shale. It eventually cut around the northwestward
dipping Uncompahgre arch and headed southeastward toward the ancestral
Colorado River near the present site of Palisade.
Then occurred an act of piracy that put to shame the mightiest exploits
of Blackbeard and Captain Kidd. In latest Pliocene or earliest
Pleistocene time additional uplift of the Uncompahgre arch, an unusually
large flood, or both, caused the ancestral Colorado River to overflow
its banks and spill across a low shale divide into the headwaters of the
tributary. Some ponding may have preceded the spillover. With this
enormously increased supply of water, the tributary cut down rapidly
through the soft shale and captured the entire flow of the Colorado
River, but the ancestral Gunnison River still flowed through Unaweep
Canyon, as shown in figure 34_B_. Stream capture of this type is
appropriately called “piracy.”
But the piracy had not ended. Note in figure 34_B_ that the “new” river
sent out several tributaries, one of which headed for and, with the aid
of yet additional and greater uplift, soon captured the ancestral
Gunnison River, as shown in figure 34_C_. This second act of piracy left
Unaweep Canyon really “high and dry” except for small streams that
carried off what little water the canyon received from local rain and
snow. While these piracies were taking place, the Book Cliffs and the
edge of Grand Mesa gradually retreated away from the valley because of
erosion, and more of the Uncompahgre arch was uncovered.
The rising Uncompahgre arch, whose renewed uplifts in latest Pliocene or
earliest Pleistocene times played such an important role in the ultimate
abandonment of ancestral Unaweep Canyon, was asymmetric in that the
crest was not in the middle but was near the southwest side. Although
sharp, locally faulted monoclines are found on both sides of the arch,
including the part within the Monument, in general, the northeastern
flank has a rather gentle northeastward dip; whereas, the southwestern
flank of the arch also is bordered by normal faults of considerable
vertical displacement.[31] Thus, after abandonment, the minor drainage
in Unaweep Canyon continued to flow northeastward from a new divide near
the southwestern border, and ancestral West Creek began cutting
northeastward toward the new divide. The drainage pattern depicted in
figure 34_C_ differs slightly from my earlier interpretation and results
from additional fieldwork.[32]
[Illustration: PROBABLE DRAINAGE PATTERNS AND LAND FORMS NEAR THE
MONUMENT AT FOUR SUCCESSIVE STAGES OF DEVELOPMENT. Solid drainage
lines taken from the Moab and Grand Junction, Utah-Colorado,
topographic maps of the Army Map Service; dashed drainage lines are
my own ideas. _A_, just prior to piracy of ancestral Colorado River;
_B_, after piracy of ancestral Colorado River and just prior to
piracy of ancestral Gunnison River; _C_, abandonment of Unaweep
Canyon after renewed uplift of Uncompahgre arch and piracy of
ancestral Gunnison River; and _D_, present drainage pattern after
additional uplift and piracy of East Creek. Modified from Lohman
(1961, 1965a, 1965b). (Fig. 34)]
[Illustration: A, just prior to piracy of ancestral Colorado River]
[Illustration: B, after piracy of ancestral Colorado River and just
prior to piracy of ancestral Gunnison River]
[Illustration: C, abandonment of Unaweep Canyon after renewed uplift
of Uncompahgre arch and piracy of ancestral Gunnison River]
[Illustration: D, present drainage pattern after additional uplift
and piracy of East Creek.]
At the time of abandonment, ancestral Unaweep Canyon was a V-shaped
canyon resembling Glenwood Canyon just upstream from the city of
Glenwood Springs, Colorado. The reasons for its change in shape and
appearance to the beautiful U-shaped canyon we find today and the
profound effect the abandonment of Unaweep Canyon had on the deepening
of the Grand Valley and the canyons of the Monument will be brought out
in the section “Canyon Cutting.”