TOrociKAl'lI V OK 'llll'; I-AKI-: J'.OTTOM 203 



to be joined to tlie mainland. A reduction in level of oO feet would re- 

 sult in the ol)lit(n'a.tion of the followini!; islands : Ilaspherry, York, Oak, 

 and ]>asswood. 1 f tlie lake sliould he lowered (JO feet, Bear island would 

 he joined to Raspherr}', Otter to Oak, South Twin to Rocky, Michigan to 

 Ahideline, and Madeline to tlie mainland. The islands are simply a 

 phase in the history of the lake, their existence or their o1)literati()n de- 

 pending on relatively slight lluctuations of the lake level. 



A brief statement regarding the topography of the lake bottom should 

 be given, in addition to what has been said regarding the land surface. 

 Along the Michigan and Minnesota shores there is a sudden descent 

 from the shoreline to the onedumdred-fathom line. This is due to the 

 fact that the Keweenawan rocks, which form the shores, dip at high 

 angles and i)lunge rapidly to considerable depths. The reverse is true 

 on the Wisconsin shore. Here the rocks are not the stee})l3^ di})})ing 

 members of the Keweenawan synclinal, but are composed of Potsdam 

 sandstones in horizontal beds. These Potsdam sediments have filled 

 the western end of the syncline in large part and deei)l3^ buried the 

 Keweenawan. On this account there is a marked contrast between the 

 Michigan and Wisconsin shores, both in respect to trend and in respect 

 to to])ograph3\ The twenty-fathom line a})proaches the Minnesota shore 

 within a mile or two. In Wisconsin, on the contrar3% it passes outside 

 the Apostle group at a distance of 20 miles from the mainland. On ap- 

 l)roaching Michigan it returns in close proximity to the shore again. 

 The fifty-fathom line parallels the twent3''-fathom line approximatel3^ 

 It passes far out around the Apostle islands, at a distance of 30 miles 

 from the mainland. As it approaches the Minnesota shore, however, it 

 returns close to the shore once more. 'J'he arrangement of these con- 

 tours indicates that a plateau of rock extends to some distance into the 

 lake, and, in addition, it shows that the plateau is an extension of Chip- 

 pewa point. The extent of this platform gives an intimation of the 

 widespread distribution of the Potsdam sediments. It also indicates 

 the degree to which the ancient Lake Superior synclinal was filled at its 

 western end. 



Recent Changes in Laki^: Level 



Before discussing the shore formations it will be necessar3^ to consider 

 certain of the recent changes in the level of the lake, because these 

 changes have a bearing on the deposition of the shore formations and 

 on their extent. As is well known, the Pleistocene stages of the lake 

 were higher than the present stage by several hundred feet. The low- 

 ering of level has been one of the marked features of the lake hist()r3'', 

 and this has not been accomplished without oscillations, for there have 



