How is it possible for marine conditions to exist in the St. Lawrence lowlands while freshwaters still occupied the Lake Ontario basin?  This question has been asked many times, and many theories have been proposed.  Originally it was thought that glacial ice still existed at the St. Lawrence, Lake Ontario divide, however it has since been determined that this was likely not possible.  It is now widely accepted that the ice sheet had long since left the area, and isostatic rebound and water levels controlled the extent of the Champlain Sea.
   It is suggested that the maximum extent of the marine intrusion was just past the Duck-Galloo Sill (SW of Kingston) meaning that these waters did in fact flow into the Lake Ontario basin.  The reason marine environments were never established in this region was due to the predominance of freshwater outflow, and the large abundance of this freshwater would have prevented the existence of marine organisms. (Pair,Karrow,Clark, 1988)
   The maximum extent of the Champlain Sea into the Lake Ontario basin was reached about 11 700 years B.P., and existed until 11 400 years B.P. when uplift of the regional Sills separated the water bodies for good.