DRAINAGE OF lEEIGATED SHALE LAND. 



than 4 per cent of water.^ Their value as a water-carrying medium 

 may be considered negligible. 



The second type is to be found along the sharp-crested and well- 

 defined " hogbacks." These usually are steep ridges which affect all 

 of the formations immediately adjacent to them, the general uplift 

 having tilted the layers of shale until they dip sharply and, in fact, 

 in some places are nearly vertical (see Plate I). The pitch of these 

 layers, however, decreases with depth and the distance from the hog- 

 back. In the uplifting process they have slipped, broken and shat- 

 tered in a very complex manner. 



The third type usually is found in folds of great extent, where the 

 displacement has not been so intense and the layers or strata do not 

 vary far from the horizontal. The layers, however, are by no means 

 continuous and unbroken. If in the elevation of land areas the only 

 forces are those acting in vertical lines so that the strata remain 

 horizontal, cleavage may not be introduced, but if there are great 

 forces acting horizontally, cleavage may be developed. " Whole moun- 

 tains of strata may be cleft from top to bottom in thin slabs along 

 planes parallel to each other." ^ This is well illustrated along the 

 sides of a deep cut in an irrigation ditch shown in Plate II, figure 1, 

 which cuts across a number of these cleavage planes. Plate II, figure 

 2, also is a good example of this. " The planes of cleavage seem to 

 have no relation to the strata, but cut through them, maintaining their 

 parallelism, however the strata may vary in dip. Usually the cleav- 

 age planes are highly inclined and often nearly perpendicular.^ 



Owing to the fissile nature of the shale, the compression also has 

 caused shearing planes along the bedding planes. These, in turn, 

 have become broken and in excavation the shale comes out in large 

 flakelike pieces (Plate III, fig. 1). In some instances where the 

 pressure has been intense, fault planes have developed. Plate III, 

 figure 2, illustrates one of these, showing the shale layers to have 

 slipped about 18 inches. The extremely broken and shattered condi- 

 tions along this fault plane show clearly that it would carry water 

 quite freely. 



SURFACE TOPOGRAPHY. 



During remote times the shale formations were subject to erosion 

 and formed a topography of their own, similar to that of the exposed 

 shale which can be seen at the present time. Erosion has produced 

 " bad land " topography differing in character according to the local 

 conditions. On the higher slopes, where erosion was especially 

 vigorous, the shale is cut by deep, V-shaped ravines, the sides of 

 which are very steep or nearly vertical (PI. IV). Sometimes knolls 

 or domes occur, as illustrated by Plate V. The bottoms of the 



1 U. S. Geol. Survey, Water Supply and Irrigation Paper No, 160, p. 72, 

 2E31ements of Geology, Le Conte, p. 189. 



