DRAINAGE OF IRRIGATED SHALE LAND. es 
In making the examinations a small stake should be driven at 
each hole and the stakes numbered consecutively. If a map is to be 
made, the depth to shale should be kept in a notebook. Otherwise 
the depth to shale should be marked on each stake in order to facili- 
tate locating the drains along the ridges and other higher portions 
of shale. 
The weathered, flakelike covering of the shale ridges, mixed with 
soil, frequently makes it difficult to determine the true depth to the 
solid shale formation, as pockets of coarse shalelike material occur 
in the soil and may be far from any shale formation; consequently 
examinations of borings from small auger holes frequently are mis- 
leading. Where there is any doubt, holes should be continued for 
some depth after encountering the shale to make sure that it is the 
solid formation. Occasionally, where the exact depth to shale seems 
uncertain, it can be ascertained best by excavating a pit. 
LOCATION OF DRAINS. 
Attempts to accomplish drainage by running tile lines across and 
along the upper edge of the affected area (see lateral A, fig. 9, p. 35), 
thereby cutting into a number of shale ridges and points contributing 
water, have proved unsatisfactory; (1) because of the absence of 
an impervious stratum that can be reached by the tile, the lack of 
which permits some of the water to pass freely below the tile to a 
point farther down the slope; (2) these ridges do not discharge water 
at their points alone, but frequently along the sides also. This is 
due to the fact that pressure exists and the water is supplied at 
various points from the continuous shale formation beneath the shale 
ridge. 
These complex conditions existing in the shale make it imperative 
that the drainage lines should follow as nearly as possible along the 
backbone of the shale ridges or cut through the knolls and other high 
portions. Usually the lines should extend for some distance up the 
ridge or other formation above the affected area. Not only is it 
necessary to run the tile lines along the shale ridges to secure the 
best results, but trenching in the shale is far easier than in wet adobe 
soils. Where quite broad shale ridges are encountered, one line will 
be insufficient; on such ridges a tile line should extend sone each side 
(lateral C, fe. Jp) 30)) - 
Where a number of shale ridges that are supplying the seepage 
water are encountered the main line should, if possible, run along the 
points of these ridges, with a branch follgeme up each ridge (lateral 
B, fig. 9, p. 35). As not all shale ridges, or points, furnish seepage 
lee a Taber of deep holes should be put down into the shale, not- 
ing the degree of hardness, whether or not soft layers are encountered, 
