252 New York State Museum 



in eastern United States and the southern part of Can- 

 ada. Many oscillations of land and water occurred 

 during Middle Ordovician times. There was a great 

 spreading of the continental seas early in the epoch 

 (Mohawkian) which included a great inundation from 

 the Arctic ocean. This Arctic inundation reached its 

 greatest extent during Trenton time. The submer- 

 gence of North America at this stage in the Ordovi- 

 cian was more extensive than at any other time ex- 

 cept early Mohawkian (Lowville sea). Floodings 

 from the Atlantic ocean in the Lower and early Mid- 

 dle Ordovician epoch were restricted to eastern North 

 America (Alabama to Newfoundland only) and in the 

 Cordilleran area there was a seaway from the Great 

 Basin region to the Arctic ocean. At the close of the 

 Middle Ordovician there was a vast but not complete 

 withdrawal of the marine waters from the middle and 

 northern areas of the continent. Some marine waters 

 perhaps remained in the southern portion of the east- 

 ern interior and Appalachian areas in which a nearly 

 complete, but now only partly accessible, record of 

 marine deposition between the two epochs may occur. 

 When the Middle Ordovician sea had nearly vanished 

 a new invasion of the sea spread over the continent, 

 from the Gulf of Mexico in Upper Ordovician time. 

 This sea advanced northeastward along the western 

 side of Appalachia, spreading northward into the Ot- 

 tawa region, and westward into Indiana. 



Ordovician deposits have a maximum thickness of 

 several thousands of feet with a limestone value of 

 about 8000 feet, that is, limestones plus the equiva- 

 lence in limestone of the elastics which are more rap- 

 idly deposited. The greatest thickness of limestones 



