62 NEW YORK STATE MUSEUM 



No part of the region is wholly free from faulting effects, except 

 perhaps a part of Long Island. The Catskills also are very little 

 affected — so little that this type of structure has not require con- 

 sideration in the vicinity of Ashokan reservoir. But all parts of 

 both the northern and southern aqueduct system have had this 

 feature to consider. 



Further discussion of the specific local problems introduced by 

 faulting and folding is given under the problems of part 2. A con- 

 siderably more extended comment on the age of fault movement 

 is given under the heading " Postglacial faulting." 



4 Outline of geologic history 



Most of the general features of geologic history have been 

 involved more or less in the foregoing discussion. It is im- 

 possible to wholly separate matters that are so intimately inter- 

 related even though it is convenient to think of or consider one 

 phase at a time. But it may serve a useful purpose to summarize 

 the steps of progress as illustrated by local geology from the earliest 

 geologic time to the present. 



a Earliest time. (Prepaleozoic, Agnotozoic, Proterozoic, or 

 Azoic Era). There is little doubt that the oldest rocks known in 

 this region are representatives of a time of regular sedimentation. 

 Conditions favored the deposition of silicious detritus of variable 

 composition with an occasional deposition of lime, nearly always in 

 very thin beds. What these sediments were laid down upon or 

 where they came from are unsolved questions. The remnants of 

 them that are still preserved are the basis of the " Grenville series " 

 as interpreted in this area, and are the basal (oldest) members of 

 the " Fordham " or " Highlands gneisses." 



How long ago this series was deposited is not known. It can be 

 stated only approximately even in the rather flexible terms used in 

 historical geology. It is older than any Paleozoic strata (Pre- 

 cambric), probably very much older. It is even possible that this 

 series is as much older than the Cambric as that period is compared 

 to the present. In short, it is not known, and there is apparently 

 little immediate likelihood of finding out even to which of the sev- 

 eral subdivisions of the Prepaleozoic this series belongs. It is cer- 

 tain that before the Cambric sandstones of the Paleozoic era had 

 begun to form, this older series was disturbed by crustal move- 

 ments, folded, metamorphosed, intruded by igneous injections, ele- 

 vated above the water (sea) level of that time and eroded by sur- 

 face agencies. These movements and steps there is no doubt of. 



