ZION CANYON
The brilliant Vermilion Cliffs which form the bottom two thousand feet of Zion’s walls and the contrasting White Cliffs which rise an additional thousand feet are hard, resistant sandstones. Wherever these rocks are found, they form conspicuous perpendicular cliffs because of their hardness, yet even they show the results of the constant attacks of weather and erosion over long periods. From once continuous layers, they have in many places gradually been cut and dissected by running water until now numerous canyons expose their secrets.
It was relatively late in the last chapter of the earth’s history (Cenozoic Era) that the cutting of Zion and associated canyons was made possible by gradual though tremendous crustal movements in the region. The then broad, low-lying country was raised several thousand feet to about its present altitude. This was the means of giving power to the streams, including the masterstream—the Virgin River. Steadily and relentlessly these active agents of erosion, heavy-laden with their tools, the muds, sands, and pebbles, have been cutting notches and then canyons. Zion Creek itself, which is one of the largest tributaries of the Virgin River, has cut downward through all of the layers now exposed in the walls of its canyon, and the rock fragments derived from these have been the means of grinding and gouging. As the stream has struggled in its course, these sides have slowly but surely receded through the combined efforts of rain, wind, frost and plants. They have been undermined and otherwise attacked, but as yet represent comparatively little progress in widening. Beautiful Zion Canyon, therefore, has been created as the result of crustal movements bringing into action the effective cutting power of running water, assisted by all the ever-working forces of disintegration and decay.
GLACIERS
(PLEISTOCENE EPOCH)
“What part did glaciers play in cutting the Grand Canyon?” This is a question asked almost daily on the rim of that great chasm. Everyone has heard of the mighty ice sheets which somewhere back in geologic history covered a large part of eastern and midwestern United States. Many people have seen beautiful Yosemite Valley in the far west, and have been told how it was carved by the action of glaciers. It is not altogether strange, therefore, that visitors to the Grand Canyon should associate the cutting power of ice with that tremendous gash in the earth’s surface. But geologists agree that the glaciers of the Ice Ages had no direct part in the story of Grand Canyon. No scratches or gouges made by ice are to be found on the canyon walls, no great rock piles formed at a glacier’s front are in evidence, and lastly, the very V-shape of the canyon itself is vastly different from the usual flat-bottomed valley scooped out by moving ice.
During the last epoch of geologic history, climatic conditions the world over altered very materially. Upon five different occasions snow accumulated in the north to form extensive ice sheets which advanced steadily toward the equator and then retreated with a later change in conditions. These were not the only ice ages in history, but they were of such a comparatively recent date that their influence is strongly felt today in many parts of the world. At the time of greatest expansion the glaciers reached New York in the east, Missouri in the mid west, and Washington in the far west. Contemporaneous with these main ice sheets, furthermore, were large glaciers throughout the Rocky Mountains and the Sierra Nevadas. In the mountains of Utah and even in the San Francisco Peaks, just south of the Grand Canyon, are found evidences of glaciers of this period. Evidently then, climatic conditions in the Southwest as elsewhere were greatly different from those of today. It is believed that there was much more rainfall and less evaporation, as shown by the extensive lakes which existed in this region at that time. Furthermore, the later melting of the glaciers must have supplied a vast quantity of water to the Colorado River causing it to be far larger and more powerful than today.
SAN FRANCISCO MOUNTAIN VOLCANIC FIELD
(PLIOCENE AND PLEISTOCENE EPOCHS)
VIEW OF VOLCANOES FROM NORTH
Over a large part of the great plateau of northern Arizona are found sheets of hard lava and cone-shaped craters. The center of this volcanic activity is located in the San Francisco Peaks just north of the town of Flagstaff, but lavas and cones are found westward beyond Williams and to a considerable extent in every other direction. Looking south from Grand Canyon these peaks may be seen to rise high above the plateau and are a very beautiful sight. From Flagstaff and Williams not only the mountains but also great sheets of lava resting on the limestone surface are conspicuous.