CHAPTER VII 

 THE WALLKILL VALLEY SECTION 



Between the Rondout and Wallkill valleys the aqueduct is to 

 follow a tunnel at hydraulic grade which so far as can be seen will 

 cut only Shawangunk conglomerate and Hudson River slates. No 

 doubt there are many complicated small structures which because 

 of the nature of the slates can not be reconstructed. The work 

 of tunneling is not advanced far enough to add anything. But 

 in the Wallkill valley, where it is necessary again to plan a pressure 

 tunnel several hundred feet below grade, a considerable amount 

 of exploration has been carried on.^ 



These explorations [see sketch map fig. 8] are distributed along 

 several lines crossing the valley at intervals between Springtown, 

 about 3 miles north of New Paltz, and Libertyville, which is about 

 an equal distance south. 



The geology is simple. Only Hudson River slates form the rock 

 floor, and so far as can be judged no other formation is likely to 

 be cut by the tunnels. There are no doubt many complicated struc- 

 tures, both folds and faults, as indicated by the high dips, but again 

 because of the nature of this rock it is impossible to discriminate 

 closely enough between different beds to determine exact relations. 

 The point of greatest practical importance lies in the fact that 

 the rock is fairly uniform and, although much disturbed is of 

 such nature that crevices and joints or fault zones are almost as 

 impervious as the undisturbed rock. This is because of the tend- 

 ency of a formation of this composition to heal itself with fine, 

 compact clay gouge. In fact, the mechanical disturbance produces 

 or develops the cement filling contemporaneously with the move- 

 ment. It is chiefly a mechanical filling, whereas the heaHng of a 

 harder and more brittle rock like a granite or a limestone requires 

 more chemical assistance. 



An additional practical question involves the estimate of depth 

 required to avoid any possible buried Prepleistoeene gorges and 

 maintain a safe cover to guard against undue leakage or rupture. 



1 Explorations on the Wallkill division are carried on under the direction 

 of Lawrence C. Brink, division engineer. The final construction is in charge 

 of James F. Sanborn, division engineer, with headquarters at New 

 Paltz, N. Y. 



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