DRAINAGE OF IRRIGATED SHALE LAND. 



39 



VIII and IX. All the lands represented had become water-logged, 

 alkalied, and unproductive. They have been reclaimed by drainage 

 to the extent that the crop yields are normal again. Before drainage 

 the orchard shown in the background of Plate VIII, figure 1, was 

 dying, while the land now in alfalfa was barren. Since the comple- 

 tion of the drainage system no trouble has been experienced on this 

 tract. All the improvements shown by Plate IX, figure 2, were made 

 after the drainage system was installed and the benefits to the land 

 evident to the owner. 



CONCLUSION. 



Outcroppings of shale and lands immediately underlain by shale, 

 as treated in this bulletin, are found in northern New Mexico, in 

 southeastern Arizona, in large areas of Colorado, in the eastern 

 portion of Utah, in the extreme eastern part of Idaho, in Wyoming, 

 Montana and in the western parts of Nebraska and the Dakotas. 



Shale is an important factor in the movement of underground 

 water, especially in those areas where uplifts and displacements have 

 occurred. 



Three different ways by which shale becomes a factor in the move- 

 ment of seepage water have been considered: (1) Over the top of 

 the undisturbed and impervious strata; (2) between the layers; and 

 (3) through joints, faults and cleavage planes. 



The minor features of the surface of the underlying shale are fre- 

 quently quite irregular and are masked by the overlying soil. They 

 can be determined only by soil borings. 



The source of the seepage water is deep percolation, resulting from 

 irrigation and from seepage losses from canals and laterals. 



Artesian conditions exist usually where the seepage water moves 

 through the shale, although the pressure may be low owing to a 

 large number of areas of leakage in the confining medium. 



There is a relation between the seepage areas and the topography 

 of the underlying shale. The affected areas usually occur near shale 

 ridges and points. This is due to the fact that there is greater 

 porosity in the shale ridges than in the deeper zones, the former hav- 

 ing sustained the effects of weathering and therefore being more 

 shattered and fractured and the joints more open and greater in 

 number. Furthermore, the soil covering is shallowest over the 

 ridges. 



The deeper zones carry most of the water, owing to continuity and 

 greater area of cross section, and the general movement of the water 

 is parallel with the main jointing systems of the shale. 



Practically all the shales run high in alkali salts, and the seepage 

 waters leach out large quantities. Consequently many of the waters 

 discharged from drainage systems in shale carry a salt content as 

 high as 2 and 3 per cent, in which are many nitrates. Because of 



