DRAINAGE OF IRRIGATED SHALE LAND. 



25 



the total acreage affected by seepage water, as all of the lands im- 

 mediately adjoining were becoming affected. The surface soil is 

 mostly a dense blue adobe and the subsoil shale which in some 

 cases approaches the surface, although generally it is at a fairly 

 uniform depth of from 3 to 5 feet below. Much of the shale is 

 a flakelike material that has been washed in. This shale forma- 

 tion comes under type No. 2 (p. 3), and constitutes a part of the 

 pressure fold which is west of the tract. 



The source of the seepage water was thought by the residents to 

 be the irrigation canal which ran through the wet area and, ac- 

 cordingly, a wood-stave pipe was installed, as indicated in figure 2, 

 to prevent seepage from this canal, but conditions became worse 

 soon after its installation. This indicated that the open canal had 

 been an actual benefit in this particular case in that it had carried 

 away part of the surface water. In any event, the canal could not 

 have been the source of all the water, since the seepage area was 

 well defined for a considerable distance above the canal. The land 

 to the south and east of this tract is lower, and the hogback on the 

 west is a very steep, sharp-crestecl ridge which does not carry water, 

 as a large irrigation tunnel through it near the base proves. The 

 only other possible source is from the north, and in this direction a 

 mile of unirrigated virgin land intervenes. From all indications 

 the water follows the joints formed between the nearly vertical layers 

 of shale which were formed by the uplift to the west. The move- 

 ment of the water must take place at considerable depth, for there 

 are no seepage areas in this intervening mile. 



Since the underlying shale is rather uniform and not characterized 

 by any distinct features of topography, a uniform drainage system 

 was laid out as indicated on the map. InsuiScient funds and other 

 reasons have prevented the installation of all of the drains, but 

 2,768 feet were constructed. They were effective immediately, and 

 the results on that portion of the tract are very gratifying. Some 

 8-inch tile were used, but most of it was 6-inch. They were placed 

 7 feet deep, and relief wells were installed where the solid shale 

 formation was encountered. The work was done by contract and a 

 considerable portion of it was difficult, as it was necessary to use 

 cribbing. 



EXAMPLE II. 



This is a 10-acre tract of which not over one acre was affected 

 (fig. 3). The source of the damage was the large amount of irriga- 

 tion water applied on the lands to the northwest near the rim of the 

 valley, in which the soils are comparatively porous. The soil on 

 the tract, is very dense adobe. At the time of drainage the land was 

 in fruit trees and a few of the trees were dead on the affected area. 



