GEOLOGY OF THE NEW YORK CITY AQUEDUCT 63 



When subsidence^ again depressed the area beneath the sea the 

 deposition of sands that we now call Cambric (Poughquag) quartz- 

 ite began. 



b Early Paleozoic time. With the sedimentation upon this old 

 crystalline rock floor a long time of apparently continuous deposi- 

 tion began which ultimately resulted in the accumulation of several 

 thousand feet of sandstones, limestones, and sandy or clayey shales 

 that are now known as the Cambro-Ordovicic series (Poughquag- 

 Wappinger-Hudson River series). But at the close of Ordovicic 

 time or late in that period another crustal revolution began. The 

 whole region was again compressed into mountain folds, faulted, 

 sheared, metamorphosed, elevated above sea level, and subjected to 

 erosion. This corresponds to the Green mountains folding of 

 Vermont. 



With the next subsidence and a return of sedimentation a new 

 series began to form. The break marking the occurrence of all 

 these changes, known locally as the Postordovicic unconformity, 

 represents a considerable portion of Siluric time. 



c Middle Paleozoic time. The earliest deposits of this series, 

 which continued to accumulate through late Siluric and all of De- 

 vonic time, were heavy conglomerates very unevenly distributed 

 over the new rock floor. These are the so called Shawangunk con- 

 glomerates, a formation that within the boundaries of this imme- 

 diate area and within a distance of 20 miles varies from a thickness 

 of more than 300 feet to almost nothing. But for the most part, 

 sedimentation was regular and fairly continuous and of immense 

 volume. The whole series of conglomerates, sandstones, shales, 

 grits and limestones belonging to the later Siluric and the Devonic 

 are included. Not all are believed to be marine however. The 

 Catskill and Shawangunk conglomerates may well be of continental 

 type. 



Long after the deposition of all of these strata another crustal 

 disturbance, for at least the third time, repeated the process of 

 mountain-folding and erosion. This was the time of the Appalach- 

 mn mountain-folding. In this region it caused a wonderfully com- 

 plex development of folds and faults that are especially important 

 and determinable as to type and age in the Rondout cement region. 

 The movement, of course, affected all of the older formations as 



1 There may possibly be an intermediate stage, practically a duplication 

 of the whole as given above, between the very oldest and the Cambric, 

 represented in the " later crystallines," but this may as well be neglected 

 for the present. 



