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]
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)
