CHAPTER IV 

 GENERAL GEOLOGY OF THE REGION 



It will save much repetition and it is believed will altogether 

 serve a useful purpose in maintaining unity of treatment to give 

 an outline of the geologic features of the region in advance of the 

 discussion of special problems. It is intended only for those not 

 sufficiently familiar with the general geology to follow subsequent 

 matters. 



The region includes some of the most complicated and obscure 

 sections of Xew York geology. It is simple in almost no one of the 

 larger branches of the subject. In physiography there is the long 

 and involved history and the results of long continued erosion of a 

 variable series of formations in different stages of modification as 

 to structure and metamorphism and attitude, modified still fur- 

 ther by subsidences and elevations, depositions and denudations, 

 peneplanations and rejuvenations, glaciation and recent erosion — 

 all together introducing as much complexity as can well be found in 

 a single area. 



In stratigraphy the whole range of the eastern New York geologic 

 column is represented from the oldest known formation up to and 

 including the Middle Devonic — a succession of at least 25 distinct 

 formations which may for convenience be treated in groups that 

 have had similar history. Each of these formations has a constant 

 enough character to map and regard as a physical unit. Even this 

 classification ignores the great range of petrographic variability 

 shown in such formations as the Highlands or Fordham gneisses. 

 All but two or three of these formations will be cut by the tunnels 

 of the aqueduct. 



In petrography the range is even greater — so great, in fact, that 

 only an enumeration of the variations will be attempted. They 

 include elastics, metamorphics and igneous types ; stratified and un- 

 assorted, coarse and fine, detrital and organic, marine and fresh 

 water, homogeneous and heterogeneous, argillaceous, calcareous and 

 silicious sediments, unmodified and thoroughly recrystallized strata ; 

 acid and basic and intermediate intrusions ; massive and foliated 

 crj'stallines — of many varieties or variations in each group. 



In tectonic geology an equal complexity prevails. There are regu- 

 lar stratifications, cross-beddings, disconformlties. overlaps and un- 

 conformities ; interbeddings, lenses and wedges : flat, warped, tilted 



[29] 



