| DRAINAGE OF IRRIGATED SHALE LAND. 3 
than 4 per cent of water.t. 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 I, 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 ITT, 
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 (Pl. IV). Sometimes knolls 
or domes occur, as illustrated by Plate V. The bottoms of the 
1U. S. Geol. Survey, Water Supply and Irrigation Paper No. 160, p, 72, 
2Hlements of Geology, Le Conte, p, 189. 
