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



ravines usually are hard and solid, for the small streams have carried 

 away the loose shale ; but the ridges are covered to some depth with 

 loose, broken, flakelike shale which has been formed by weathering. 

 With the exception of some of the higher ridges and knolls, the 

 shale in the river valleys has been overlain with a covering of soil 

 which varies greatly in depth. In general, the surface of these val- 

 leys has a configuration corresponding somewhat to the underlying 

 shale surface, the minor irregularities of which are masked by the 

 overlying soil. In the design of a drainage system the locations of 

 these minor and abrupt irregularities must be determined by a large 

 number of subsoil borings. This is made difficult by the flakelike 

 covering over the solid shale, for this flakelike shale is found in other 

 places at various depths in the soil, where it has been washed, and 

 is not underlain by the solid formation. 



UNDERGROUND WATER. 



The collective medium for the underground water is the soil, espe- 

 cially those higher and more porous portions where irrigation is 

 heavy, and also those higher exposed portions of broken shale in 

 which canals and reservoirs have been constructed. 



PRESSURE CONDITIONS. 



The underground water usually exists under pressure. This fact, 

 together with the moisture retentiveness of the soil, renders drainage 

 difficult. After having stated that the mantle of soil in the higher 

 land acts as a collective medium, it may seem inconsistent to say 

 that it serves as the confining agency in the artesian conditions that 

 exist at lower levels. However, the existence of artesian conditions 

 does not necessarily require that the confining strata be wholly 

 impervious, but only that they be less pervious than the water- 

 bearing stratum. The top formation may be penetrated by consid- 

 erable quantities of water, so that the leakage is large, and yet be 

 available as a confining agent. This loss merely causes a reduction 

 in pressure and volume. If it were not for the leakage, the head 

 which the water derives from the highest zone of intake would con- 

 tinue under the entire region, but owing to this leakage there -is a 

 gradual diminution as the distance from the source increases. 



The fact that the soil is less porous and offers greater resistance 

 to the movement of underground water than does the shale causes 

 the soil to act as a confining agent, the efficiency of which increases 

 with its thickness. There is little need that cover beds of highest 

 impervious character be very thick, but when the degree of imper- 

 viousness is inferior the element of thickness, in itself, is not without 

 consequence. This is true especially where low pressures exist. The 

 thicker covering offers more frictional resistance as the degree of 

 consolidation increases with depth. 



