﻿GEOLOGY OF THE NEW YORK CITY AQUEDUCT 203 



The second reason is related to the probable later faulting move- 

 ments. It is evident from the map [Folio 83] that the formations 

 in the vicinity of Bryn Mawr are bulged up. One would expect 

 the trough which contains the schist and limestone of Grassy Sprain 

 valley to continue uninterruptedly southwestward and join with 

 Tibbit brook valley. But a cross fold has bulged the formations 

 up so much that for a distance of a mile erosion has removed all 

 of the formations except the gneiss. Bryn Mawr station is about 

 central on this bulge. Evidence of such a movement is readily 

 seen on the gneiss along the northerly margin where it slopes down 

 toward the limestone. The movement had developed a little shear- 

 ing and has tilted the minor folds downward toward the north at 

 angles varying from 30 to 8o° from the horizontal. This angle 

 becomes somewhat more accentuated as the limestone is approached, 

 and it is believed that it may pass a short distance into the limestone 

 border. There is, however, no great amount of crushing evident in 

 the gneiss and this may hold also in the limestone. 



The fact that Sprain brook crosses the formations along this 

 northerly margin and flows for 2 miles in a southeasterly direction 

 may indicate a still later movement, probably faulting. There is no 

 surface evidence of it except the abnormal course of the creek. 

 But, if there is such a fault, it also crosses the siphon line in the 

 same zone, i. e. in the vicinity of the limestone-gneiss contact, not 

 far from the location of the present course of the brook. 



Therefore it seems reasonable to conclude that the critical zone 

 is near the contact, probably on the limestone side, and in the 

 vicinity of the present course of Sprain brook. It is also probably 

 cut deepest here by erosion. If this zone is in good enough condition 

 to stand tunneling the rest of the line ought to be. 



Conditions indicated by borings 



All rock formations stand very steep. They vary from 8o° to 

 90 . This means that very few beds can be explored by one hole, 

 and that any weakness or crevice is likely to make a showing in 

 excess of its true proportions. 



The cores show considerable crushing. Some of the fractures 

 are not healed, although weathering from circulation is not present 

 on all of them. The micaceous layers are most affected by circula- 

 tion. Some beds of this variety are considerably weakened even at 

 depths of over 200 feet. Occasional seams have been encountered 

 that give no core at all for several (even 20 or 30) feet. But the 



