440 New York State Museum 



Washington. There were several advances and retreats 

 of the ice front, forming glacial and interglacial stages 

 to which special names have been given. It was during 

 the retreat of this ice sheet that the Great Lakes began 

 their development. Other temporary Pleistocene lakes 

 of large size were developed, as in North Dakota and 

 Minnesota (Lake Agassiz), Nevada and Utah (Lake 

 Bonneville) and northwestern Massachusetts (Lake Bas- 

 com). Great Salt Lake, Utah, is today a small remnant 

 of Lake Bonneville. During the Pleistocene there was a 

 great subsidence of the northeastern Atlantic coast and 

 marine waters spread over the St Lawrence valley and 

 Lake Ontario area and through the Lake Champlain and 

 Hudson valleys. Marine shells and skeletons of whales 

 have been found in the sediments deposited. The dura- 

 tion of the Pleistocene or Glacial period has been esti- 

 mated at 500,000 to 1,000,000 years. The area covered 

 by the ice received a variety of deposits (gravels, sands 

 and clay, in the form of till or boulder clay, drumlins, 

 kames, eskers, sandplains etc.) composed of materials 

 stripped from the regions over which the ice advanced. 

 With the retreat of the ice sheet postglacial or recent 

 time was inaugurated. 



New York State has a variety of glacial deposits (con- 

 tinental) but there are also marine and brackish water 

 deposits of gravel, sand and clay found in the St Law- 

 rence, Champlain and Hudson valleys. These deposits 

 contain fossil shells of animals that live in the sea today, 

 and some of these are illustrated in figure 61. It was dur- 

 ing the period of the Champlain subsidence that the sea 

 coast acquired nearly its present position. Following the 

 subsidence was the very recent gradual elevation which 

 expelled the Champlain sea. Marine and brackish waters 



