THE AGE OF MAMMALS
THE BEGINNING of the Cenozoic Era 65 million years agogive or take a few million yearsmarked the beginning of a long span of geologic time during which mammals became the ruling land animals. Remains of some small primitive mammals have been found in Mesozoic rocks (p. 50), but these tiny newcomers did not have a chance to flourish until the formidable dinosaurs died out.
The Cenozoic Era is divided into the long Tertiary PeriodThe Age of Mammalsand the short (about 2 million years) Quaternary PeriodThe Age of Man. The Tertiary in turn is divided into five epochsthe Paleocene, Eocene, Oligocene, Miocene, and Pliocene (fig. 61). Events during parts of the Tertiary Period had an important bearing upon the Monument even though no rocks of this period now occur in the area.
THE BROAD INLAND BASINS that were formed late in the Cretaceous Period received sand, silt, and mud brought in by streams from the uplifted or folded areas. These materials became compacted into the Wasatch Formationthe red or pink rock from which Bryce Canyon National Park was sculptured. One such basin lay just northeast of the Monument. The Monument probably was covered by some of these stream deposits after the main basin was partly filled.
The mammals that roamed the area during the Paleocene Epoch were primitive, but more advanced forms appeared later, in Eocene time. Some of their fossil remains have been found in the Wasatch Formation in Plateau Creek Valley north of Grand Mesa and near Rifle, about 60 miles northeast of Grand Junction. The entire 5,000 feet of the Wasatch may be seen along U.S. Highway I-70 between the towns of DeBeque and Grand Valley, and much of it helps support towering Grand and Battlement Mesas.
IN EOCENE TIME the northern part of the Colorado Plateau sagged downward and gradually filled with water until it became a huge lake, now known as Lake Uinta. The waters in it teemed with plants and animals, particularly micro-organisms such as algae, whose remains, coated with calcium carbonate, settled to the bottom along with the sand, silt, and mud washed into the lake by streams. These sediments compacted into the remarkable Green River Formation which contains, among many rock types, large deposits of rich oil shale.
The light-colored Green River Formation, which is about 3,800 feet thick, may be seen from U.S. Highway I-70 in the upper part of the towering Roan Cliffs on the northwest side of the Colorado River between DeBeque and Rifle. It also underlies the volcanic caprock of Grand and Battlement Mesas. John R. Donnell, of the U.S. Geological Survey, estimated that the oil shale in the Piceance Creek Basin, northwest of the Colorado River alone, contains more than one trillion barrels of oil. The Monument was at or near the south shore of this lake, and may have been covered with a few hundred feet of the Green River Formation.
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 heremore 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.
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).
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.
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.
GRAND AND BATTLEMENT MESAS, respectively east and northeast of the Monument, are capped by several resistant thick flows of dark basaltic lava. The molten rock welled up through fissures at the east end of Grand Mesa and flowed westward and northwestward over the eroded surface of Eocene rocks. Radiometric dating of a sample of the basalt indicated an age of 90-1/2 million years plus or minus half a million years, placing the event in the Miocene Epoch of the Tertiary Period (fig. 61).
A small remnant of the lava on the crest of the Roan Cliffs just southwest of the present town of Grand Valley indicates that the flows crossed this part of the ancestral Colorado River Valley and may have pushed the young stream westward.
The lava flows are about 800 feet thick on the eastern part of Grand Mesa but are only about 200 feet thick above the western rim of the mesa. As the ancestral Gunnison River is believed to be pre-Miocene in age, it is not known whether or not the lava flows crossed the old river valley and reached as far west as the Monument.
DURING most of the Pliocene Epoch the ancestral Colorado River did not flow past what is now Grand Junction; instead, it joined with the ancestral Gunnison River about 10 miles southeast of the present city, and the combined streams flowed southwestward across the slowly rising Uncompahgre arch through what was later to be called Unaweep Canyon (fig. 36). Southwest of the canyon, near the site of the present town of Gateway, the ancestral Colorado River was joined by the combined flows of the ancestral San Miguel River and the previously diverted ancestral Dolores River, then it flowed northwestward to what is now the mainstem of the Colorado River. I have attempted to show my ideas of this ancient drainage system as it may have existed in middle to late Pliocene time in figure 34. But the stage was set for more spectacular drainage changes to follow.
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 34A 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. 34D). 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 34B. Stream capture of this type is appropriately called "piracy."
But the piracy had not ended. Note in figure 34B 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 34C. 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.26 Thus, after abandonment, the minor drainage in Unaweep Canyon continued to flow northeastward from a new divide near the south western border, and ancestral West Creek began cutting northeastward toward the new divide. The drainage pattern depicted in figure 34C differs slightly from my earlier interpretation and results from additional fieldwork.27
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."
Last Updated: 8-Jan-2007