GEOLOGY OF THE NEW YORK CITY AQUEDUCT 23 



have been made use of in coming to a consistent understanding of 

 the conditions. 



But the amount of accumulated data is no more remarkable than 

 the difficulties that have been encountered in obtaining it. For 

 example, in the Moodna valley it has taken three to four months' 

 time to put down a single hole to bed rock — the average time con- 

 sumed for each of the 15 holes exploring the deepest portion of 

 the valley was about 60 days. The chief trouble is caused by heavy 

 bouldery till. In one case a boulder was penetrated for 35 feet, 

 lying a hundred feet above bed rock. 



The extreme of such difficulty is, of course, encountered in the 

 Hudson river itself, where the drill has to contend with: (i) the 

 rise and fall of the tides, (2) the river currents, (3) a maximum of 

 90 feet of water, approximately 700 feet of silt, gravel, till, boulders, 

 etc., fining the old preglacial gorge. The heavy steamboat and 

 towing traffic has been a serious element in the problem. Probably 

 never anyw^here have drillmen had to face so nearly insurmount- 

 able obstacles. In two years only two holes reached below a depth 

 of 600 feet below sea level. A third, now in progress, has pene- 

 trated a depth of 768 feet without entering rock. 



