lO NEW YORK STATE MUvSEUM 



All the strata which are shown in the Rochester region were laid 

 down in the Mississippian sea. This sea in eastern New York was 

 limited by a barrier which separated the waters of the Atlantic from 

 those of the interior basin. As early as Medina time tliere was a 

 subsidence of the land at least along the western side of the barrier, 

 and as the Mississippian sea gradually transgressed toward the east, 

 the deposits from the Medina to the close of the Salina overlapped 

 the next older formation. There was a slight uplift at the close of the 

 Clinton and the effects are noticeable in the central portion of the 

 State where the upper portion of the Niagaran does not overlap the 

 Clinton. Finally the Salina period was brought to a close by the sub- 

 mergence of the barrier to the east and we have once again the waters 

 of the Mississippian sea mingling with those of the Atlantic. 



FORMATIONS 



In ascending order 



Medina formation. This formation takes its name from Medina, 

 N. Y. at which place an excellent section of the upper part is shown 

 along Oak Orchard creek. It will best serve our purpose to describe 

 the Medina of the Rochester region under two divisions. 



Loiver Medina shale. This division consists of an extensive series 

 of soft, red shales, with occasionally a small amount of interbeddcd 

 silicious material. The total thickness of the red shales is about 900 

 feet. The greater portion of the outcropping edge of the formation 

 is concealed beneath Lake Ontario so that only about 100 feet are 

 shown within the limits of tlie mi[). Along the Oswego river these 

 shales are seen at numerous points. They are here more silicious 

 and follow the Oswego sandstone, the lowest member of the Upper 

 Siluric. The Oswego does not outcrop anywhere in western New 

 York, but from borings we know that it is present with a thickness 

 of about 85 feet. 



The beds of this division are well shown in the gorge of the 

 Genesee river from below the lower falls nearly to Charlotte. The 

 rock as here exposed is made up of red arenaceous shales in which 

 occasionally are found thin beds of sandstone. From below the 

 lower falls the bed of the Genesee is entirely excavated in these red 

 shales. Upon exposure to the atmosphere, they break up into small 

 angular fragments, which in course of time disintegrate and form a 

 soft .reddish soil, which often becomes covered with vegetation. At 



