STUDIES OF SPECIAL LACUSTRAL HISTORY. 101 



approximate!}' parallel with it. 01)servations on the aniount of deforma- 

 tion that this l)eaeh has suffered, are not as extended as could be desired, 

 l)ut near its western extension there is evidence of a change of level of 

 about one foot per mile. 



Recent observations by F. B. Taylor ^ in the region adjacent to Lake 

 Superior on the south, have shown that ancient beaches may be clearly 

 recognized at many places between Duluth and Sault Sainte Marie. The 

 facts recorded by Ta3'lor supplement in a very interesting manner the 

 work of Lawson on the northern side of the same basin, although faither 

 study is necessary before the entire history of the great predecessor of Lake 

 Superior can be Avritten. At the south, the highest beach has an eleva- 

 tion of from 512 to 588 feet above Lake Superior, or from 1014 to 1190 

 feet above the sea. 



Taylor suggests that when the entire outline of the highest beach at 

 the north shall have been traced, it will be found that there were straits 

 connecting the Superior basin with that of Hudson Bay. This would 

 imply a submergence of a very large portion of the North American con- 

 tinent to a depth of over a thousand feet. 



The erosion produced by the movement of ice trheets man}^ htmdreds 

 of feet thick, over the Laurentian basin, modified and subdued the pre- 

 vious relief, and the debris left when the ice melted covered the country 

 M-itli a sheet of superficial deposits to such a depth that the character 

 of the underlying hard-rock toi)ography is only occasionally revealed. 

 The depth of these glacial deposits over great areas, as in Michigan 

 and Wisconsin, is from one to two hundred feet, but is probably of less 

 average thickness in Ohio and Ncav York. All pre-glacial drainage channels 

 were either obstructed or obliterated and a new surface given to the land. 

 The drainafre was thus reiuvcnatcd and is still immature. The effects of 

 glacial plantation and of glacial deposition, in forming the 1)asins of the 

 present Laurentian lakes, has been pointed out in discussing the origin of 

 lake basins. 



In this brief sketch I have endeavored to show that the history of 

 the Laurentian basin includes a study of the hard-rock topograi)hy as it 

 existed previous to the Glacial epoch ; the disturlwnces and changes in 

 drainage produced by the ice invasion and by movements of elevation 

 and depression ; the obstruction of the ancient waterways by glac-ial 

 dejiosits ; and the origin of new cliannels of discharge, as tlie glaciers 



' " A roconnoi.ssance of the abaiidoned .sliurc liiR-s of the south coast of Lake Suiicrior," 

 in Am. Geol., Vol. 13, 1894, pp. 305-383. See al-so more recent papcns in llif same journal. 



