CHAPTER ill 



RELATIVE VALUES OF DIFFERENT SOURCES OF INFORMA- 

 TION AND STAGES OF DEVELOPMENT 



In the earlier stages of work topographic features were of most 

 concern, and they largely controlled the selection of reservoir sites 

 and possible lines for tlie aqueduct to follow. It was, however, 

 at once recognized that tunnels would be unavoidable and studies 

 as to the types of rock formations to be encountered were begun 

 It was also early appreciated that the soil or drift cover is very 

 unevenly distributed over the rock surface and that, especially in 

 the chief valleys requiring pressure tunnels, it would be necessary 

 to determine the profile of the rock floor. At this point wash bor- 

 ings were begun. But the natural limitations of the wash rig^ for 

 penetrating drift of all kinds left the information still too indefinite. 

 The wash rig can not penetrate hard rock. It can not wash up 

 anything but the finer matter, and a boulder of very moderate size 

 is almost as effectual a barrier as true rock ledge. By a combination 

 of washing and chopping or by the use of an explosive to break or 

 dislodge an obstruction some progress in unfavorable material may 

 be made, but the wash rig alone, in a drift-covered region, gives 

 only negative results. It is certain, for example, that bed rock lies 

 at least as deep as the wash rig has penetrated, but it is not certain 

 that it is bed rock instead of some other obstruction. Except in 

 areas of special drift conditions,- therefore, the wash rig was insuf- 

 ficient. To rely upon the process at random was clearly impossible, 

 and to determine whether or not the results of a particular locality 



1 A " wash rig" is a device composed essentially of two iron pipes, one 

 within the other, and so mounted that the inner one can be worked up 

 and down in sort of a churning fashion while water under considerable 

 pressure is forced through it to the bottom and out again by the larger 

 pipe to the surface, carrying up with the current the displaced sand and 

 clay. As progress is made with the inner pipe the outer one is from time 

 to time driven down and the process renewed and repeated till the hole is 

 finished. 



2 One of the most notable areas of special drift conditions is repre- 

 sented in the Walkhill valley Vsee discussion in pt 2] where there were 

 developed large deposits of modified drift, stratified gravel, sand and clays, 

 lying immediately upon the bed rock floor. In this the wash bore 

 process was eminently satisfactory, and the rapid progress made by it 

 together with its economy made this an especially attractive method of 

 exploration. 



[25] 



