OCR Text |
Show 18 INTRODUCTORY. traces, and these can be unraveled. It was a period of slow uplifting, reaching a great amount in the aggregate; and it was also a period of stupendous erosion. The uplifting was, however, unequal. The comparatively even floor of the old lake was deformed by broad gentle swells rising a little higher than the general platform. In consequence of their greater altitudes, these upswellings at once became objects of special attack by the denuding agents, and were wasted more rapidly than the lower regions around them. Here were formed centres, or short axes, from which erosion proceeded radially outward, and the strata rising very gently toward these centres, or axes, from all directions, were\ bevelled off. As erosion progressed, so also did the local upliftings, thus maintaining the maximum erosion at the same localities. It is a most significant fact that the brunt of erosion throughout the Plateau Country is directed against the edges of the strata and not against the surfaces. This is directly traceable to the fact that the strata are nearly horizontal, the dips rarely exceeding four or five degrees, and even then only where a great monoclinal flexure occurs. The rains wash and disintegrate most rapidly where the slopes are steepest, and where the strata are flat the steepest slopes are the valley sides and chasm walls. Thus the battering of time is here directed against the scarps and falls but lightly on the terrepleim. . Ordinarily, the local uplifts have one diameter longer than the others, and we may call the greatest the major axis. The strata dissolved away in all directions from this axis, and after the lapse of long periods the newest or uppermost stratum encircled the centre of erosion at a great distance from it, the next group below encircled it a little nearer, and so on. This has been the history of each of the subdivisions of the central part of the Plateau Country. Upon the western and northern sides of the Colorado five of these centres are now easily discerned. By far the largest and probably the oldest is around the Grand Canon; a second lies east of the Kaiparowits Plateau; a third is found about 50 miles south-southwest of the junction of the Grand and Green; the fourth is the Henry Mountains, and the fifth is what is known as the San Rafael Swell, lying between the |