146 NATURE STUDY REVIEW [12:4— April, 1916 



From a geological standpoint the bluffs are rich in meaning. 

 Records of great processes in the formation of the earth's crust can 

 be clearly traced. The main ridge is of igneoiis rock of the trap- 

 pean variety, which rose through a fissure in the earth's crust in 

 what the geologists term the Jurassic Period. This process is said 

 to have occurred ^0,000,000 years ago and is similar to that which 

 produced the Giant's Causeway in Ireland and the Cliffs of 

 Kawaddy, India. The present appearance of the bluffs did not 

 result from this process alone, however. It remained for future 

 processes to disclose the Palisades to view. The deposits which 

 covered them were worn away by erosion, and thus the transforma- 

 tion which gave us the majestic cliffs began. As the igneous rock 

 cooled before being exposed to the air it shrunk and broke off in 

 sharp perpendicular formation; this occurred along the whole 

 ridge. North and south along the present bed of the Hudson 

 occurred an immense "fault" or slip in the earth's crust. This 

 caused the Hudson to flow in a canyon which recent borings show 

 to have been 300 feet deep. The gorge extended beyond Sandy 

 Hook, the coast at that time being many miles southwestward of 

 its present location. 



The next transformation came, with the ice fields of the glacial 

 period. The ice moved across the cliffs obliquely from the north- 

 west carrying with it millions of tons of rock and debris which were 

 deposited here and there about the bluffs forming an intra- 

 morianic drift. The crunching of the ice over the flat rocks of the 

 top ground deep grooves into the igneous rock, leaving imprints of 

 its passage clearly discernable to-day. 



With the melting of the ice the ridge was surrounded by water to 

 the depth of 200 feet. As this sea subsided, the Hudson began 

 filling its gorge with silt. This process is still going on — it is now 

 necessary to dredge the river often — and the river is known as a 

 "drowned river." 



The Palisades rock is dark gray and blue-black in color, very 

 hard and heavy. Chemically it consists of 55 per cent, silica 

 though under a microscope it discloses myriads of crystals, the 

 feldspars, pyroxine and magnetite predominating. The ridge has 

 a perpendicular thickness of 1,000 feet. 



Historically the Palisades are rich in associations. Of the pre- 

 historic dwellers we have no trace. Tribes of the great Delaware 

 nation lived there when the first white men in September, 1609, 



