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GEOLOGY OF THE POUGHKEEPSIE QUADRANGLE 97 



account of reduced resistance, to the base-leveling forces of the 

 time. 



Late Tertiary uplift. Late in the Tertiary cycle, probably 

 during the latter part of Pliocene time, it seems probable that an 

 elevation occurred which rejuvenated the whole river system. The 

 Hudson began the construction of its present gorge and its tribu- 

 taries began to deepen their valleys within their former confines. 

 It has been suggested that the temporary shifting of the St Law- 

 rence drainage through the valley of the Mohawk gave the main 

 stream a tremendous advantage. It was able to sink its channel 

 at a very rapid rate. The larger tributaries were able to deepen 

 their gorges near their mouths and for some distance back from 

 the Hudson before the glacier invaded the land. 



Buried river channels. Borings have been made at different 

 points across the Hudson river and its tributaries in connection with 

 the location of the aqueduct of the great metropolitan reservoir in 

 the Catskill mountains. These have yielded important data regard- 

 ing the preglacial channels of these streams. Professor Kemp has 

 summarized and discussed these data in an interesting paper. ^ 



Borings across the Hudson have been made at Pegg's Point, at a 

 point one-half of a mile north of that place, at New Hamburg and 

 at Danskammer within this quadrangle, and at Storm King just 

 south of Newburgh. 



The most northerly line of borings is known as the " Tuff 

 crossing." From this, only wash borings were secured. The river 

 here is only 2200 feet wide. 



At Pegg's Point the river narrows still more. A diamond drill 

 was sunk 720 feet from the west shore and reached the slate at 

 223 feet below sea level. Another sunk 440 feet from the east bank 

 reached the limestone at 92 feet. The distance separating these two 

 borings is 1040 feet. Professor Kemp believes that a deep and 

 relatively narrow gorge lies between. Several lines of wash borings 

 at this place gave depths to supposed bed rock varying from 139.5 

 feet to 256 feet in what would perhaps be thought of as the deepest 

 part of the river. 



At New Hamburg the river is 2300 feet wide. Drill borings on 

 each bank found the slate beneath the limestone. At the point of 

 boring on the east bank it was reached at 220 feet; on the west at 

 351 feet. Only wash borings were made in the river bottom. These 

 ranged from 130 feet to 263.5 ^^^^ below tide. 



1 Buried Channels beneath the Hudson and its Tributaries. Amer. Jour. 

 Sci. Ser. 4. 26:301-23. 



