CHAPTER 13
THE HIGH AREAS OF THE EARTH
190.Young Plateaus. -Sometimes large areas of horizontal rock are elevated high above the sea, forming lofty plains whose surfaces are often irregular, owing to previous erosion. Such areas are called plateaus. The descent from a plateau to the lower land is usually steep. Areas of this kind, where streams are present, suffer rapid and deep erosion, since the grades of the streams are steep because of the eleva- tion.
If there is not much rain there will be few streams, and these will have deepand steep-sided troughs. Such troughs render the area very difficult to cross. The valleys are too narrow for habitation or for building roads, and the deep troughs of the streams are too wide to bridge. Thus the uplands are isolated.
If these high areas are in a warm latitude, they are desirable for habitation on account of their cool climate, due to the elevation; but if in temperate latitudes, their bleak surfaces are too cold.
As the river troughs wear back, the harder rocks stand out like huge benches winding along the course of the rivers. From the different benches slopes formed from the crumbling of the softer strata slant backward. Thus the general outline of the stream sides will be something like that of a flight of stairs upon which a carpet has been loosely laid.
An excellent example of a region of this kind which has been eroded by a strong river gaining its water from a distant region is that of the Colorado Ca?on Plateau. Here is found the grandest example of erosion on the face of the earth. The rocks are of various colors, andCOLORADO PLATEAU.
The river has cut a deep canon through the plateau.
the gorge is nearly a mile deep and in places some fifteen miles in width. Words are inadequate to express the grandeur of the panorama spread out before one who is permitted to see this gigantic exhibition of the results of erosion. Wonderful, grand, sublime, are mere sounds which lose themselves in the ears of one who looks out upon this overpowering display of Nature"s handiwork.
The region is very dry, and the river receives few and short branches for many miles of its course. The valley is widening much more slowly than it would if this were a land of considerable rainfall, and as yet the river fills the entire bottom of the gorge. The valley is in the early stages of its development and has just begun the vast work of wearing down the region. The side streams are small and the interstream spaces broad.
191.Dissected Plateaus. -If a plateau has been elevated for con- siderable time in a region of abundant rainfall, the streams extend their courses in networks, thoroughly dissecting the area and leaving between their courses only narrow remnants of the upland. The valleys are still deep, but the intervening uplands are of small extent. Travel- ing over the region in any direction except along the stream courses is a continual process of climbing out of and into valleys.
There is very little level space that can be used for cultivation, and on account of the steepness of the slopes it is very hard to build roads. The river valleys are so narrow that unless the roads are perched highTHE ENCHANTED MESA.
With old Indian village in the foreground.
up on the sides, they are liable to be swept away at the time of flood. Farming in these regions is very discouraging because of the difficulty of transporting crops and of finding anything but a steep side hill on which to grow them.
Railroads can get through only by following the principal valleys, and here, on account of the narrowness, the engineering of the roads is difficult. Unless the region is rich in minerals, it can support only a small population, and that will of necessity be poor. As soon as the forests are cut off, the soil rapidly washes down the hillsides and leaves naught but bare surfaces. Regions of this kind are found in the Allegheny and Cumberland plateaus, extending from New York to Alabama.
A BUTTE.
192.Old Plateaus. -If a plateau remains elevated for a great length of time, the dissecting rivers are able to widen their valleys and wear away all the interstream spaces, except where these are very broad. Thus the riversbring the surface down to a comparatively low level, with here and there a re mnant which has not been worn away, but which shows in its steep sides the edges of the rock layers which former-ly spread over the whole
region. If these residual
AN INDIAN HOGAN.
masses are large, they are called by the Spanish name mesas, meaning tables, and if small, buttes, from the French word which means land- marks.
Some of these mesas are so high and so steep that it is impossible to climb them, and others are simply low, flattopped hills. A traveler in New Mexico and Arizona will see many of these mesas, which, like the lonely Indian huts or hogans, are but scattered remnants of what was formerly widespread.
On old plateaus travel is easy. There are no deep valleys, and one can easily pass around the mesas, which only add charm to what would otherwise be a most monotonous landscape. When these mesas areCLIFF DWELLINGS.
A protected retreat in a mesa.
high, they are sometimes occupied by a few Indian tribes who have fled to them for protection, as the medieval barons when hard pressed fled to their isolated castles.
193.Broken Plateaus. -The force which has uplifted the plateausis not always uniform enough in its action to lift large areas without fracturing the rock layers. Thus plateaus are found which have the rock layers broken and displaced. The layers on one side of the break may stand thousands of feet higher than those on the other side.