GEOLOGY OF THE BROADALBIN QUADRANGLE 59 



Tribes Hill from the Broadalbin sheet because it was removed 

 during this time of erosion. 



Another downward movement brought a return of marine con- 

 ditions when the Black River-Trenton (Ordovicic) limestones were 

 laid down. The profusion of animal life in the ocean of that time 

 is proved by the abundance of fossils embedded in the rocks of 

 Black River-Trenton age. After this the waters became muddy 

 when the Canajoharie and Utica black shales were deposited and 

 then the alternating sandstones and shales of the Frankfort (Ordo- 

 vicic). Although the Frankfort is the youngest Paleozoic forma- 

 tion of the quadrangle, it is quite certain that still later sediments 

 were here deposited but have since been removed by erosion. 



After Paleozoic sedimentation there was a great uplift, most 

 likely at the close of the Paleozoic era, which raised the region high 

 above the s'ea level. Thus another vast erosion cycle was estab- 

 lished to extend through all the millions of years to the present 

 time. There is no evidence whatever that the region was ever 

 again submerged below the ocean level. The Paleozoic sediments 

 have been completely removed from those portions of the quad- 

 rangle where the Precambric rocks are now exposed, while they 

 have suffered great erosion over the Paleozoic area itself. 



Postpaleozoic erosion must have been vigorous during the long 

 time of the Mesozoic era and there is good reason to think that, 

 by the close of that era, the whole region was reduced to the con- 

 dition of a fairly good peneplain (part of the well-known Creta- 

 ceous peneplain of the Appalachians) and that at the close of the 

 Mesozoic the peneplain was upraised. According to this the pres- 

 ent major topographic features are the result of erosion since this 

 late Mesozoic uplift or rejuvenation of the region. 



Another feature of great importance in the history of the quad- 

 rangle is the faulting or fracturing of the earth's crust which oc- 

 curred sometime after the deposition of the Frankfort, since that 

 formation is involved in the faulting. The exact date of this fault- 

 ing is not known but it probably took place at the time of the great 

 uplift at the close of the Paleozoic. 



The most recent event of special interest in the history of the 

 quadrangle was the existence of the great ice sheet during the 

 Glacial epoch. Extensive superficial deposits and rock scorings 

 bear testimony to the vigorous glaciation of the quadrangle. From 

 the geologic standpoint this ice sheet was present only very recently 

 and covered most of New York State. 



