the Hudson and its Tributaries. 305 



without, presents certain novel and interesting problems and 

 makes solid rock a fundamental necessity. In the locations 

 the geologist is of well-nigh indispensable service to the 

 engineer. 



In the course of its line from the reservoir to New York, 

 the aqueduct has to cross the following principal depressions: 

 Eondout Cr. 160' A.T. ; Wallkill River 150' A.T. ; Moodna 

 Or. 90' A.T. ; Hudson River A.T., which it readies at 

 El. 400+ ; Sprout Br. 145' A.T. and Peekskill Cr. 60' A.T. 

 Before the final line was selected several tentative ones were 

 explored, giving us the records of depressed channels not on 

 the final line. At the outset wash-borings alone were used, 

 but when later tested by the diamond and calyx drills they 

 were found to be entirely unreliable. On the basis of their 

 records a bowlder might be taken for the bed-rock as easily as 

 not. The sections subsequently plotted on wash-borings, there- 

 fore, show merely that the bed rock is presumably deeper yet. 

 To this extent they are, however, of value. 



The geological section inevitably crossed is complex both in 

 number of formations and in their structural relations. 

 Beginning on the north in the flat Hamilton beds of the 

 Devonian, the tunnel in passing beneath Rondout Creek pene- 

 trates the full section of the Helderberg series as shown in 

 fig. 4 together with the Shawangunk grit, in and west of the 

 mountains of the same name. It passes beneath Bonticou 

 Crag, three or four miles north of Lake Mohonk, and thence 

 through or over Hudson River slates until it reaches the 

 Archean granite of Storm King mountain, here thrust up on 

 the slates by a reversed fault. The tunnel dips under the 

 Hudson in the granite entirely, and rises on the east bank in 

 the same rock. Thence it continues over or through the sedi- 

 mentary gneisses, marbles, etc., of the Grenville,* but at 

 Peekskill Creek also cuts the Poughquag quartzite and Wap- 

 pinger limestone of the Cambro-Ordovician. These formations 

 have all dips from flat to vertical ; are folded often in a 

 violent way ; and are faulted in a very complicated manner. 

 Several of the tentative lines had more to do with the Wap- 

 pinger limestone than the one finally selected and therefore 

 this formation appears along the more northerly routes. 



The General T)rainage Relations. — The relations of the 

 tributary streams to the Hudson north of the Highlands are 

 in some respects peculiar. Those on the west bank present 



* After consultation with Dr. C. P. Berkey, who has done much detailed 

 mapping in the Highlands, and after going over together the exposures both 

 in this locality and in the eastern Adironclacks. this name, hitherto current 

 in the more northerly region, is employed, in the belief that the formations 

 are essentially equivalent. 



