14 BULLETIN 502, U. S. DEPARTMENT OF AGRICULTUEE. 



the amount of water found, and whether or not pressure conditions 

 exist. It is very important that none of these shale points that con- 

 tribute water be missed, for, although the system may develop a con- 

 siderable quantity of water from the points tapped, any remaining 

 one will furnish enough water to these retentive adobe soils to prevent 

 complete reclamation. 



In the drainage of those lands underlain by shale that has no dis- 

 tinctive topographic features, but which is smooth and at a fairly uni- 

 form depth below the surface of the soil, a system should be employed 

 ' which has branches spaced at regular intervals and extending up the 

 slope. However, the determination of the interval required for the 

 branch lines necessitates a careful examination of the shale. Ex- 

 perience in drilling often will enable one to determine whether or not 

 the shale is traversed by systems of small crevices; the drill takes 

 hold with more difficulty in shale that does not contain them than in 

 shale that does. The nature of the borings brought out on the auger 

 is indicative also of the presence or absence of such crevices. If the 

 borings are fine-grained and more or less compact, few or no crevices 

 are probable; but flaky, mealy borings indicate the contrary. By 

 those unfamiliar with the mode of occurrence of water in shale those 

 zones containing water often will be overlooked, for the small lumps 

 and layers of shale between the cracks' are impervious in themselves, 

 and the auger, passing through these, will bring up fragments of 

 perfectly dry shale, while the free water frequently found between 

 these dry borings will be thought to have collected there in pulling 

 the auger out through the upper wet portions of soil. 



The existence of pressure often can be detected after having struck 

 cracks in the shale containing water, by placing the ear near the hole ; 

 a hissing sound caused by the escape of the water from the small 

 crevice into the larger opening is an almost certain indication of 

 pressure conditions. The intensity of the pressure can be estimated 

 by the rapidity and height of rise of the water in the well. Often the 

 water will rise several feet within a few minutes, and again it may 

 require a day or two for it to attain its maximum height. Where the 

 w^ater rises only a few inches, and that very slowly, the pressure is 

 slight or negligible. High pressure is indicated by a rapid and high 

 rise of the water. Of course, the height and rapidity of rise depend 

 somewhat on the season, there being generally a seasonal fluctuation 

 of the water table. In seasons of high water table, pressure strong 

 enough to cause these wells to flow on the surface is encountered fre- 

 quently. 



The borings will indicate certain spots and streaks where the pres- 

 sure seems high and the water free ; drainage lines should be located 

 to tap these areas. Figure 1 is an example of the application of the 

 information obtained from the borings. It becomes apparent at once 



