DEPARTURE OF ICE-SHEET FROM TPIE LAURENTIAN LAKES. 27 



ClIAKACTKR AND PuOClUHSS OF TIIK Ui'LIFT F0LL0\VIN(5 RECESSION OF THE ICE-8IIEET 



A wave-like uplift from the Clianiplain t^ubsideiiee advanced to the area of lakes 

 Iro(iU()is and IIudson-Chanii)lain after it was nearl}- or (juite completed in the area 

 of lake Warren. 



In previous discussions of the relationships of these glacial lakes* I have stated 

 more fully than can be attemi)ted in this paper the stages of advance of the gradual 

 upward movement of their areas in its progress from south to north and from 

 southwest to northeast, pari passu witli the recession of the ice-sheet in the same 

 directions. Closely following the ice-border in its retreat, there ensued an uplift 

 of the northern i)art of the region covered by lake Warren to a total amount of 

 400 to ()00 feet, of which the greater part, ranging from seven-eighths of the wiiole 

 400 feet at Duluth to about two-thirds of the whole 600 feet at lake Nipissing, had 

 taken place ])efore the time of the Algonquin or Nipissing beach. The continua- 

 tion of this uplift during the time of accumulation of the lower beaches of lake 

 Algonquin probably raised the area of the watershed between lake Nipissing and 

 the ^lattawa river to so great an altitude as to forbid outflow there previous to the 

 removal of the ice barrier from the Mattawa and Ottawa basins on the east. 



Like the uplift of the lake Agassiz area, first at the south, later in its central 

 part, and latest at the north, so the region of the Laurentian lakes appears to have 

 been elevated nearly or quite to its present level, lirst from Duluth east to lake 

 Ni])issing and Butialo, and afterward, while the ice barrier of lake Iroquois was 

 retreating, the basins of lakes Ontario and Champlain were raised approximately 

 to their present altitude. The maximum gradient of the earlier part of the uplift 

 of the Saint Lawrence basin was about five feet per mile from south to north upon 

 portions of the ^lichigan upper peninsula ; and an equally large differential move- 

 ment gave to the Iroquois beach an ascent of nearly 300 feet in 55 miles from south 

 to north between Kome and the latitude of Watertown, New York. The correla- 

 tive maxinunn northward uplifting of the shorelines of lake Agassiz is found by 

 Mr J. B. Tyrrell along the eastern base of Riding and Duck mountains, in the 

 north central portion of that lake's entire extent, being about three feet per mile. 

 After the lake Iroquois area had received the greater part of its re-elevation from 

 the ChAmplain subsidence, the more northern Ottawa and Saint Lawrence valleys 

 were ui^raised to a maximum amount exceeding 500 feet at Montreal. From south 

 to north and northeast a wave of epeirogenic upward movement advanced upon 

 the region of the Laurentian lakes and to ^lontreal, nearly contenqwraneous with 

 the uplift of the valley of the Red river of the North, of Manitoba, and of the 

 country tiience northeast to Hudson bay. 



The acting President (leclared the scientific program closed. 



Professor Josej)!! Le Conte moved that the thanks of the Society he 

 extended the trustees and officers of the Packer Institute for the use of 

 rooms, and to the Local Committee of the American Association for the 

 Advancement of Science for the i)reparations and facilities for the meet- 

 ing. It was unanimously voted. 



* Biilktin Geol. Society of America, vol. 11, 1890, pp. 2.'i8-2(J'i; vol. Ill, 1891, pp. 481-187 and 508-511. 



