DEAIN'AGE 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 lie near 

 the surface. 



RELATION OF UNDERGROUND WATER TO SHALE. 



Water moves through the pores and laminae of shale so slowly that 

 solid shale is of negligible value as a water-carrying medium. The 

 displacements and uplifts, 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 (PI. 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- 



