﻿GEOLOGY OF THE NEW YORK CITY AQUEDUCT 



211 



Sc h i s t" 



Crush -a^orvc^ 



Fig. 37 Sketch of the geologic structure at shaft 13 on the New Croton aqueduct. 

 Interpreted from field observations 



5 It would appear probable that this zone was penetrated at the 

 worst possible level, i. e. near enough to its Wholly decayed upper 

 part to furnish no resistance at all to the overlying sand and gravel, 

 and not deep enough to reach the more substantial (although prob- 

 ably crushed) rock that may reasonably be expected to prevail at 

 no very much greater depth. 



The chief point is that the weak spot has a reason and is not an 

 accidental thing that might be expected just anywhere. But it must 

 be admitted, in spite of this fact, that a casual examination of the 

 locality would not make one suspicious of its existence, and it is 

 surprising that the spot could have caused so much trouble. 



From the above it will be seen that in several respects the Bryn 

 Mawr case is somewhat similar to this. They both indicate fault- 

 ing; they are in the same type of rock; they both show or indicate 

 caving tendencies. 



On the other hand, there are certain elements of difference some 

 of which are capable of very materially modifying any conclusion 

 that might be based upon the simple facts of likeness. For exan> 

 ple — it should be expected (i) that the fault movement at shaft 13 

 would be the greater because of lying in the more prominent lines 

 of such displacement of the region, (2) being a thrust movement, 

 the crush effect is probably more prominent at shaft 13 than at 

 Bryn Mawr, (3) occurring at greater elevation above probable cir- 

 culation outlet, the opportunity at shaft 13 for extensive and rather 

 deep decay is the greater, (4) being cut so near the surface (160 

 feet), its condition there is not necessarily a reliable guide to the 

 seriousness of decay at a greater depth. 



