CHAPTER X 



GEOLOGY OF SPROUT BROOK 



Three creeks unite to form an inlet at the sharp bend in the 

 Hudson immediately above Peekskill. The middle one of these 

 is known as Sprout brook. It occupies a deep and narrow valley 

 tliat is well marked for lo miles in its lower course and is trace- 

 able as a physiographic feature of less prominence to the north mar- 

 gin of the Highlands. Its persistence indicates some important 

 structural control in erosion. 



Geology 



This valley lies in the midst of the most typical gneisses and 

 granites of the Highlands region. And in addition several of the 

 " iron mines " of Putnam county lie on its western flank. The 

 rocks are complex granitic and quartzose gneisses and granites. 

 Foliation and banding and bedding wherever this appears is parallel 

 to the axis of the valley. The most notable geologic feature is the 

 occurrence of a broad belt of crystalline limestone throughout the 

 lower 4 miles. It is undoubtedly chiefly this limestone, which is less 

 resistant to weather than the gneisses, that controls the form and 

 size of the valley. As to geologic relations, this is one of the most 

 iriteresting formations of the region. It is coarsely crystalline, fiiU 

 of silicious impurities at many places and carries small igneous in- 

 jections and dykes, and so far as the bedding can be followed, 

 stands almost on edge. Although an actual contact is not seen, at 

 several places the limestone and gneiss approach within a few feet 

 of each other and it is certain that no other formation can come 

 between them. This is more certainly indicated in the northerly 

 extension of the valley where the limestone gradually disappears 

 leaving only the gneisses and granites. That there may be a fault 

 contact must be admitted, but of this there is no good evidence in 

 the field. 



Such relations and character show that this limestone is similar 

 to the smaller interbedded occurrences noted frequently with the 

 gneisses in the Highlands and elsewhere. If it is of that type then 

 it is the largest representative yet found in that series. But it is 

 also in these characters similar to the Inwood limestone of more 

 southerly areas. The overlying Manhattan schist which is lacking 



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