Page:Popular Science Monthly Volume 7.djvu/413

Rh  —Longitudinal valleys, having a direction the same as the strike. Of the first order, three varieties are noticed: a, diaclinal, those which pass through a fold. (Fig. 3.) b, cataclinal, valleys that run in the direction of the dip. (Fig. 4.) c, anaclinal, valleys that run against the dip of the beds. (Fig. 7.) Of the second order, we have, also, three varieties: A, anticlinal valleys, which follow anticlinal axes. (Fig. 5.) B, synclinal valleys, which follow synclinal axes. (Fig. 6.) C, monoclinal valleys, which run in the direction of the strike between the axes of the fold—one side of the valley formed of the summits of the beds, the other composed of the cut edges of the formation. (Fig. 8.)

Many of the valleys are thus simple in their relations to the folds; but, as we may have two systems of displacements, a valley may belong to one class, in relation to one fold, and to another in its relation to a second. Such we designate as complex valleys.

Again, a valley may belong to one class in one part of its course and to another elsewhere in its course. Such we designate as compound valleys. It will be further noticed that valleys may have many branches, but, in relegating a valley to its class, we consider only the stem of the valley proper, and not its branches.

A great diversity in the features of all these valleys is observed. Most of these modifications are due to three principal causes: First, a greater or lesser inclination of the rocks. Second, the texture of the beds—that is, their greater or lesser degree of heterogeneity. The third class of modifying influences is found in the eruptive beds.

The last-mentioned agencies are not found in the region under immediate discussion.

The explanation of the cañons of Green River will assist us in understanding the origin of the lateral valleys and cañons. The streams were there before the mountains were made; that is, the streams carved out the valleys, and left the mountains. The direction of the streams is indubitable evidence that the elevation of the fold was so slow as not to divert the streams, although the total amount of elevation was many thousands of feet. Had the fold been lifted more rapidly than the principal streams could have cut their channels, Green River would have been turned about it, and all the smaller streams and water-ways would have been cataclinal.

Thus it is that the study of the structural characteristics of the valleys and cañons teaches us, in no obscure way, the relation between the progress of upheaval and that of erosion and corrasion, showing that these latter were pari passu with the former, and that the agencies of Nature produce great results—results no less than the carving of a mountain-range out of a much larger block lifted from beneath