Chalmers — Shore Lines of the St Lawrence Valley. 177 



formed. A great mass of literature bearing on the origin and 

 development of these old water lines and of the lake basins 

 themselves has appeared from time to time in the scientific 

 journals during the last two or three decades, and geologists 

 are by no means in accord as to their origin. In the United 

 States the hypothesis of an ice dam in the St. Lawrence Yalley 

 during the latter part of the glacial period is generally accepted, 

 but most of the Canadian geologists are inclined to regard that 

 of an oscillating land barrier to the north and northeast of the 

 Great Lakes as the most probable cause. 



In regard to the latter theory it may be stated that there is 

 a belt of granite or Archean rocks about fifty miles wide 

 crossing the St. Lawrence at the Thousand Islands connected 

 with the Adirondacks to the south, and with the great Archean 

 area to the northwest, towards which it widens out as we pro- 

 ceed in that direction. This Archaean neck, as it is sometimes 

 called, seems to have been an oscillating axis in the Post-Ter- 

 tiary period and up to the present day. As, however, the shore 

 lines of the St. Lawrence Yalley and Great Lakes are of post- 

 glacial age, it is only its later Pleistocene and recent history 

 with which we are concerned. At this stage the St. Lawrence 

 Yalley below the Thousand Islands and the region of the Great 

 Lakes would be at a lower level than at present, as already 

 stated, and the axis referred to must have been higher. This 

 valley would then be a gulf or arm of the larger Gulf of St. 

 Lawrence and the barrier described would form, on its east 

 side, the shore of the Champlain sea, and on its west a great 

 fresh-water lake, or series of lakes, would be held in, the floor 

 of which is now represented to some extent in Southwestern 

 Ontario by plains and terraces 1100-1200 feet above the sea. 

 These water levels, as already pointed out, can be seen at Strat- 

 ford and on the watershed between Ontario and Simcoe Lakes, 

 also at corresponding heights in the Lake Superior basin, 

 as described by Bell and Lawson.* 



The sequence of events which occurred in the St. Lawrence 

 Yalley and basin of the Great Lakes brought about by the 

 changes of level which followed may be thus summarized : — 



1. A subsidence of the Archean rocks immediately to the 

 north of the Great Lakes with correlative upward movements 

 to the east and to the west. These changes of level seem to 

 have reached a stationary position, temporarily, when in the 

 lake region the 890-892 feet shore line was formed ; and in 

 the St. Lawrence Yalley, the 890-895 feet terrace. 



2. Another subsidence of the axis referred to then followed, 



* The Geological History of Lake Superior ; Trans. Can. Institute, Memo- 

 rial volume, 1849-99, by Dr. R. Bell ; Sketch of the Coastal Topography of 

 the North Side of Lake Superior, by Dr. A. C. Lawson. 



