GEOLOGY OF THE NEW YORK CITY AQUEDUCT 1 55 



place has presented greater difficulties in penetrating this drift man- 

 tle. Boulders of such immense size occur that they have to be 

 drilled like bed rock. In one of the holes a boulder 30 feet 

 through was penetrated and 100 feet more of drift found below. 

 Progress in such ground is extremely slow and costly. This is 

 so much the more so where as in this case there are long stretches 

 with unusually deep cover. 



A glance at the accompanying profile and cross section will show 

 a very deep and wide valley. Many of the borings are more than 

 300 feet in drift which almost wholly obscures the ancient topog- 

 raphy. The present ]\Ioodna is about half as deep and occupies 

 the extreme eastern margin of the older gorge. There is a sec- 

 ondary gorge on the west separated from the main channel by a 

 sharp divide. A few other smaller notches in the line represent 

 smaller tributary or independent stream courses. One of these 

 of much interest is known as Pagenstechers gorge. 



The rock floor at all points except two in the central jMoodna 

 valley including its two nearest tributaries is Hudson River shales, 

 slates and sandstones of considerable variation, sometimes much 

 brecciated. The two exceptional borings are no. 8/AzJ4 and no. 

 16/A44 on the west flank of the westerly tributary gorge, and 

 they are in pegmatite and granitic gneiss which is in all probability 

 the narrow southerly extension of the Snake hill ridge. Here 

 again neither quartzite nor limestone were found on the flank, a 

 condition that seems to support the view of a double fault along 

 the Snake hill ridge. 



In striking contrast with the broad central Moodna are the two 

 narrow and very deep notches farther to the east, the first in 

 slates and the second (Pagenstechers) in Highlands gneiss. 



Special features 



Course of the Moodna. The chief interest centers around the 

 Moodna channel. There are several unusual conditions, for 

 example : 



The rock floor along the profile is almost flat for a distance of 

 nearly half a mile in spite of the fact that there would seem to 

 be every reason for a different form. The differences in hard- 

 ness of rock floor alone would encourage diff'erential erosion; and, 

 since the structure of the formations, the strike, is almost parallel 

 to the supposed course of the stream, the influence of different 

 beds would be at a maximum. Furthermore, the deep gorge of 

 the Hudson, into which the stream flowed is only 2 miles away; 



