GEOLOGY OF THE NEW YORK CITY AQUEDUCT 1 57 



The cnitical point therefore in the whole argument is as to the 

 origin of the valley, i. e. is it essentially a stream valley? Or is it 

 as to present rock floor form wholly a glacial valley? 



If it is a stream valley then no doubt full account must be taken 

 of the proximity to the Hudson, and the poissibility of developing 

 a temporary graded condition and some adequate allowance must 

 be made for its work during the subsequent continental elevation 

 and the deepening of that river to several hundred feet below the 

 known bottom of the Moodna. In short, one wo'uld expect a nar- 

 row deeper notch in the Moodna floor as a result of this rejuvena- 

 tion. But on the contrary if in preglacial time the stream were 

 not so powerful and had not been able to keep pace, and if the 

 ice movement can be assumed to have concentrated along this line 

 to such efficiency as to gouge out a groove 3000 feet wide almost 

 flat to a depth of 300 feet only guided in direction by the original 

 Moodna, then one may readily abandon the idea of a deeper notch. 



One or the other of these types of origin must be the chief 

 factor in reaching a reasonable opinion as to the presence of an 

 inner notch. 



In any attempt to choose between these factors, one is led to 

 reconstruct the preglacial drainage lines. When this is done it at 

 once appears as mo.st probable that there was at that time as now 

 a considerable area tributary to the Hudson with a stream course 

 very much like the present Moodna. In other words a fair sized 

 stream is assured. Once such a stream is granted and the effects 

 of its work reckoned in full knowledge of the adjacent Hudson, 

 and its probable behavior is studied in the light of the data ob- 

 tained in exploration of the valleys of other tributaries, it becomes 

 more and more diflieult to wholly eliminate the inner gorge idea. 

 It seems to the writer probable that the valley owes its erosion 

 chiefly to the preglacial stream. But the channel has suffered sub- 

 sequent widening and smoothing by ice especially in its upper and 

 broader portion, below which there may yet be a notch. One must 

 admit that the results of boring prove the notch to be very nar- 

 row, less than 150 feet, or else not there at all. In reaching an 

 opinion as to the possibility of one so narrow, it is worth while 

 to note that the Esopus, which is a larger stream, has cut down 

 at Cathedral gorge to a depth of from 50 to 80 feet with almost 

 vertical sides and only about 150 feet wide. This gorge further- 

 more is cut in almost horizontal strata of such character that 

 there is no special structural tendency in them to contract the 

 stream. At the Moodna on the contrary, in addition to the smaller 



