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



requires the mapping — or at least the gaining of a correct idea of the 

 surface topography — of the underl^dng shale. This necessitates a 

 large number of borings and very careful subsoil examinations, ow- 

 ing to the sudden changes and extreme irregularity in the shale 

 topography. On some tracts the surface of the underlying shale is 

 fairly regular and not a great many subsurface examinations are re- 

 quired, but in many instances the shale has been eroded into bad-land 

 topography (Pis. IV, V, and VI, fig. 1). The auger at one point may 

 strike the top of a shale knoll, dome, or point immediately underlying 

 the covering of soil, while 20 feet away in any direction it will not en- 

 coimter shale at a depth of 10 to 20 feet. (See profiles C and D, 

 fig- 8, p. 34). Very narrow ridges often are found, the tops of 

 which are near the ground surface, but a short distance on either side" 

 the shale is too deep to be reached by tile drains. A common occur- 

 rence in the shale formation is a very deep and quite wide arroyo 

 running through a rather smooth area of shale, but with no' indication 

 of this deep depression, owing to the covering of soil. 



Where only small areas are affected the shale knoll, or point, 

 contributing the seepage water may be located sometimes by ascer- 

 taining from the landowner the spot where seepage first made its 

 appearance. The damaging features of the underlying shale topog- 

 raphy often can be quickly ascertained by a line of borings closely 

 spaced along the upper edge of the affected area. Where large areas 

 are affected, and where an idea must be gained of the topography of 

 the shale underlying the entire district, perhaps the most rapid 

 method is to begin with a boring at each of the extreme corners of 

 the tract, then to subdivide the distances between these holes, con- 

 tinuing the process until a sufficient number of borings have been 

 made to determine the general features, after which those spots or 

 localities which seem to overlie points or ridges can be more care- 

 fully examined. In cases where the conditions are very complex, 

 and it is desired to map the underlying shale, it is best to space the 

 majority of the holes regularly over the area in question. 



The topography of the underlying shale usually resembles the 

 present surface topography in a very general way. In the bottom 

 or older portions of a valley that are more or less even and uniform, 

 the underlying shale usually is also fairly smooth. On the slopes 

 the more pronounced ridges are often underlain by shale ridges 

 which represent the general trend of the system of shale topography. 

 Frequently outcroppings of shale will give an idea of the under- 

 lying features. In some cases large underlying shale draws can be 

 located by noting the gaps and openings in hills forming the rim 

 of the valley. A knowledge of the kind of shale being dealt with 

 and its characteristic topographic features is of value in gaining 

 a general idea of its surface. 



