Canyon Cutting
When the Colorado River was diverted into its new course through the Grand Valley past the Monument, the stream channel seems to have been only about 600 to 800 feet higher than it is today, but the present divide in Unaweep Canyon is now about 2,500 feet higher than the channel. The difference of 1,700 to 1,900 feet was caused by the additional uplift of the Uncompahgre arch during the Pleistocene.
Thus, the Grand Valley and its tributary canyons, such as those of Colorado National Monument, were cut since the abandonment of Unaweep Canyon, probably mainly during the Quaternary Period. This suggests that the cutting of the Monument’s canyons began only about 2 million years ago, but that much of the canyon cutting occurred only a few hundred thousand years ago. Indeed, the canyons are still slowly being deepened, lengthened, and widened.
As you stand on any of the lookout points and gaze down into the canyons of the Monument, you may well wonder how such immense chasms could have been cut by such puny streams that are dry most of the time. The streams flow only for short periods after heavy thundershowers or after rapid melting of snow. If you are lucky enough to see them flow, you will notice that the water is red or brown because of the suspended mud, silt, and sand. If the flow is large, you may see or hear pebbles and cobbles rolling along the bed. Accordingly, the streams and their cutting tools are slowly deepening the channels. But, you may ask, how does this account for such wide, broad-bottomed, cliff-walled canyons? Such streams act mainly as storm sewers to carry off the rock debris formed by other types of erosion.
When cutting first began, the Monument’s canyons were narrow, steep, and V-shaped. When the top of the hard, dark Proterozoic rocks was reached, however, downcutting slowed just as it had earlier in Unaweep Canyon. While the streams were thus hung up, other erosional processes caused the cliff walls to recede away from the streams, forming broad, flat-bottomed, U-shaped canyons.
Recession of the cliffs away from the middle of the canyons probably was caused partly by undercutting of the soft Chinle Formation by wind and in places by streams. This allowed slabs of the overlying Wingate Sandstone and younger rocks to break off and fall into the canyons—eventually to break up and to be carted off as sand and mud by streams.
But other processes are probably the ones chiefly responsible for the present shape and width of the canyons. The summer sun heats the cliff faces until they are hot to the touch, but in the present desert climate of the Monument the rocks cool rapidly after sundown. Oftentimes the hot cliff faces are chilled rapidly by summer thundershowers. Repeated heating, cooling, wetting, and drying causes expansion and contraction of the rocks so that thin layers break off and fall. This process goes on slowly even in winter on sun-facing cliffs, but it does not occur on the cliffs that face away from the winter sun.
Even more important, perhaps, is the alternate freezing at night and thawing by day on sun-facing cliff faces during the winter. Water in cracks near the cliff faces alternately freezes and melts, gradually prying off slabs of rock. Canyon walls that are shaded from the sun most of the winter, however, stay cold or frozen much of the winter; hence, they are not subject to repeated heating and cooling or freezing and thawing. Thus, you will notice that because of talus accumulation many such canyon walls are sloping rather than vertical.
To illustrate the above conjectures concerning the cutting and shaping of the canyons, let us consider several canyons that trend in different directions. We have seen in [figure 12] that the left side of northeastward trending Red Canyon is a nearly vertical cliff that faces the sun most of the winter; whereas, the right side, which is shaded most of the winter, slopes gently enough to be climbed at many places. The sides of Ute Canyon, which trends more nearly northward ([fig. 52]), slope about equally, as would be expected. However, the west arm of the Canyon, which trends slightly southeastward, has sides whose slopes differ markedly ([fig. 35]).
This brings us to the remarkable transformation of the original V-shape of Unaweep Canyon to the beautiful U-shape of the present canyon, which is shown in [figure 36]. The abandonment of Unaweep Canyon discussed earlier removed the gigantic storm sewer that for millions of years carried off the products of vigorous erosion of the canyon walls by the processes just described. Rock materials that now fall from the cliffs of the inner gorge in hard Proterozoic rocks and that fall from the overlying softer sedimentary rocks simply pile up at the foot of the cliffs to form a canyon equally as U-shaped as those cut by glaciers in the high mountains. Indeed, Unaweep Canyon has been mistaken for a glacial canyon by many, including some geologists.
FALLEN ROCK, in west arm of Ute Canyon below waterfall, from Fallen Rock Overlook. Note that the sun-facing left side of canyon, containing the fallen block of Wingate Sandstone, is a vertical cliff; whereas, the shaded right side can be readily climbed. The right side contains a trail near the middle of the picture, but it is hidden by vegetation. (Fig. 35)
UNAWEEP CANYON, looking southwest from rim of inner gorge cut in hard Proterozoic rocks, just to the right side of first cattle guard on Divide Road, near middle of sec. 16, T. 14S., R. 100 W., about 5 miles northeast of drainage divide shown in [figure 34]D. Drainage divide is just around the corner to the right of the most distant part of the canyon visible. Slope above vertical cliff on right consists of Chinle Formation, Wingate Sandstone, and flat crest of Entrada Sandstone (Kayenta Formation is absent). Paved road in canyon is Colorado Highway 141. (Fig. 36)