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