The Work of Erosion.
The Witch of Endor and Cerberus.
Photograph by J.K. HILLERS, U.S. Geol. Survey.
The Work of Corrasion.
Paranuweap Canyon of the Virgen River, Southern Utah. 20 to 30 feet wide and 1500 feet deep and 18 miles long.
Photograph by J.K. HILLERS, U.S. Geol. Survey.
The Grand Canyon may be likened to an inverted mountain range. Imagine a great mountain chain cast upside down in plaster. Then all the former ridges and spurs of the range become tributary canyons and gulches running back twenty or thirty miles into the surrounding country, growing shallower and shallower as the distance increases from the central core, just as the great spurs and ridges of a mountain range, descending, melt finally into the plain. Often there are parts where the central gorge is narrow and precipitous, just as a mountain range frequently possesses mighty precipices. But it is an error to think of great canyons as mere slits in the ground, dark and gloomy, like a deep well from whose depths stars may be sighted at midday. Minor canyons sometimes approach this character, as, for example, the canyon of the upper Virgen, called Parunuweap, fifteen hundred feet deep and no more than twenty to thirty feet wide, with vertical walls, but I have never been in a canyon from which stars were visible in daylight, nor have I ever known anyone who had. The light is about the same as that at the bottom of a narrow street flanked by very high buildings. The walls may sometimes be gloomy from their colour, or may seem so from the circumstances under which one views them, but aside from the fact that any deep, shut-in valley or canyon may become oppressive, there is nothing specially gloomy about a deep canyon. The sun usually falls more or less in every canyon, no matter how narrow or deep. It may fall to the very bottom most of the day, or only for an hour or two, depending on the trend of the canyon with reference to the sun’s course. At the bottom of the Kanab where it joins the Grand, the sunlight in November remains in the bottom just two hours, but outside in the main gorge the time is very much longer.
The Grand Canyon and Terrace Plateau Region.
This covers an area in Southern Utah and Northern Arizona 144 miles square, containing more marvels than any other part of the globe of equal extent. In the lower right-hand comer are the Yosemite Valley and the Gorge of Niagara on the same scale. The scale of this reduction is about 14 miles to one inch. The vertical and horizontal scales are the same. The black ribbon on the left at Virgen and Beaver Dam Mountains is to represent Archæan formation. The Shiwits Plateau and Uinkaret Mts. are basaltic. The Kaibab and the Colorado Plateau are Carboniferous. Paria Plateau and Vermilion Cliffs are Triassic. The Markagunt and Paunsagunt Plateaus are Tertiary. Cardenas reached the canyon about at letter “A” of the word “Cañon.” Tourists now come to the river about half way between the letters “L” and “O” of the word “Colorado.” The largest settlement is the charming little Mormon city of St. George, on the Virgen, north of the letter “A” of the word “Arizona.” Grand and Marble Canyons form a continuous gorge about 300 miles long, a complete barrier to travel for this distance as its walls are precipitous and reach a height of between 5000 and 6000 feet—4000 being about the average. The lowest portion is toward Lee’s Ferry, upper right-hand side of the map, where the walls begin at a couple of hundred feet and rise very rapidly, the river cutting down also. The relief map from which this photograph was made was modelled by E.E. HOWELL for the U.S. Geological Survey and is 6 x 6 feet.
The walls of a great canyon, and usually a small one, are terraced; seldom are they wholly vertical for their entire height, though occasionally they may approach this condition on one side or the other, and more rarely on both sides at once, depending on the geological formations of the locality. Owing to the immense height of the walls of such canyons as those on the Colorado, the cliffs frequently appear perpendicular when they are far from it, just as a mountain peak often seems to tower over one’s head when in reality it may be a considerable distance off. In the nature of the formation and development of canyons, they could not long retain continuous vertical walls. What Powell calls the “recession of cliffs” comes into play. The erosive and corrasive power of water being the chief land sculptors, it is evident that there will be a continual wearing down of the faces of the bounding cliffs. The softer beds will be cut away faster than the harder, and where these underlie the harder the latter will be undermined and fall. Every canyon is always widening at its top and sides, through the action of rain, frost, and wind, as well as deepening through the action of its flowing stream. Erosion is this power which carves away the cliffs, and corrasion the one which saws at the bottom, the latter term, in geological nomenclature, meaning the cutting power of running water.[[3]] This cutting power varies according to the declivity and the amount of sediment carried in suspension. It is plain that a stream having great declivity will be able to carry more sediment than one having little, and in a barren country would always be highly charged with sand, which would cut and scour the bed of the channel like a grindstone. As Dutton says, a river cuts, however, only its own width, the rest of a canyon being the “work of the forces of erosion, the wind, frost, and rain.” That is why we have canyons. The powers of erosion are far slower than those of corrasion, especially in an arid region, because they are intermittent. Where rocks take a polish, as in Marble Canyon, the scouring and polishing work of corrasion is seen in the shining bright surface as far as the water rises. This all belongs to the romance of the Water-gods, those marvellous land sculptors.