GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK IO5 



geologist for two reasons: (i) it shows with accuracy the struc- 

 ture of this most interesting down-faulted block with quality and 

 condition of rock that could otherwise not be determined, and (2) 

 it illustrates an engineering problem where uncertainty in the 

 interpretation of the conditions discovered led to the adoption of 

 a new plan, avoiding the whole question. 



HISTORICAL GEOLOGY 

 General Historical Outline 



In the Adirondacks no formations are known older than the Gren- 

 ville. This seems to be true also of southeastern New York, and 

 there is no great doubt but that the formations designated as Gren- 

 ville in these areas are essentially of the same age as those of 

 the type areas in Canada. Unless this correlation is accepted, tlicre 

 is no possible way of determining even approximately the position 

 of the oldest formations in this district with respect to the standard 

 Precambrian system. On account of the very great similarity in 

 character and the relations to other units, there is no serious doubt of 

 the correctness of this name for the oldest metamorphic rocks of 

 the district. 



Granville sedimentation. The series was sedimentary and 

 undoubtedly of very large development. There must have been a 

 basement on which these sediments were laid down, biit thus far 

 no trace of it has been detected. Probably the members of the series 

 which are now found represent only a fraction of the whole series, 

 presumably the uppermost members, and the igneous invasions ha-ve 

 destroyed not only the basement, but also considerable portions of 

 the original sedimentaries. 



Judging from the quality of the formation still preserved, in which 

 numerous limestones of varied prominence occur, the deposits 

 must have developed under conditions favoring much shifting of 

 sedimentation between lime deposits and clastic sediments. Perhaps 

 they were marginal sea deposits into which the fluctuating streams 

 brought much detrital matter. In which direction the land areas of 

 that time lay, or what the general geographic distribution of land 

 and sea was. is not indicated by the data now available. If, how- 

 ever, the Manhattan-Inwood series is considered as part of the 

 Grenville, its heavy development toward the south would seem to 

 indicate that the land areas probably lay to the west. 



The largest development of limestone clearly belonging to the 

 Grenville age occupies Sprout Brook valley and may be traced for 



