DRAINAGE OF IRRIGATED SHALE LAND, 5 
The water pressure usually is low because of the frictional resist- 
ance of the shale, and because of the leakage through the cover bed 
which causes seepage areas where the shale ridges or points le near 
the surface. 
RELATION OF UNDERGROUND WATER TO SHALE. 
Water moves through the pores and lamine of shale so slowly that 
solid shale is of negligible value as a water-carrying medium. The 
displacements and uplhfts, however, which caused these strata or lay- 
ers to be traversed by faults, cleavage planes and joints, or which 
caused them to assume sharp dips, have left this shale in a compara- 
tively permeable condition. 
Those hard, calcareous shales mentioned earlier (type 1, p. 2), 
which have not been disturbed and which are poorly laminated, 
do not carry water to any great extent and for all purposes of this 
discussion may be considered as impervious strata, the principal 
movement of the underground water being laterally over the shale 
surface. 
in the second type of shale, as mentioned on page 3, the water 
is carried principally between the nearly vertical shale layers, as 
illustrated by Plate I, and the principal direction of its movement is 
of course parallel with the strike, especially in the deeper and less 
fractured zones. These water carriers have no regularity in spacing, 
which may vary from a small fraction of an inch to several inches. 
They are partly surface phenomena and diminish in number rapidly 
with depth and probably are better developed in the hills than in 
the valleys. Since the pitch of the strata usually decreases with 
depth, the surface of a valley cuts across a less number than does 
the surface of a hill. 
In the third type of shale referred to on page 3 the important 
water carriers are the nearly vertical planes of cleavage (Pl. II) 
which cut at close intervals across the more or less horizontal strata. 
The distance between these planes may be only a few inches, but 
usually the larger and more important ones are a much greater dis- 
tance apart. They vary in length from a few feet to hundreds of 
feet. While they influence the general trend of the direction of 
movement of the underground water, they may not be the imme- 
diate cause of seepage areas, for these planes are connected with 
each other and with the sloping surface of the shale by zones and 
widespread areas of shale that has been shattered and broken by 
shearing along its bedding planes. 
All shales that have been subjected to intense pressure and dis- 
placements are traversed by numerous fissures or joint cracks, and 
_ these openings carry a large portion of the water. The prominent 
_ joints may extend several hundred feet, but even though the con- 
