202 G. L. COLLIE — WISCONSIN SHORE OE LAKE SUI'EKIOK 



without the intervention of earth movements as a necessary accompani- 

 ment. 'J'he dei)th of the stream gorges rarely exceeds 100 feet, and the 

 energy of the streams is expended chiefly in deepening the channel rather 

 than in any attempt to widen it. The rivers are not graded, and there 

 is a constant succession of falls and rapids. The most important fall in 

 the region is that of the Montreal river within lialf a mile of the Superior 

 shore. There is a })lunge of 100 feet in a series of cascades, the hist one 

 of which is 40 feet high. The fall is probably an inherited one, coming 

 from the Keweenaw-Potsdam contact, which lies at some distance out in 

 the lake at present. 



There is no objection to the supposition that the sandstone once covered 

 the foothills of the Penokee range, and that the fall began at the contact 

 of the two formations as a result of its passage from the harder rocks of 

 the Keweenawan to the weaker la3'ers of the Potsdam. 



On the wliole the toi)ography of the sandstone belt is not marked by 

 any conspicuous features ; the plain is smooth and without marked prom- 

 inences. Stream erosion is responsible for the only notable topographic 

 characters. 



The topography of the Apostle islands is similar to that of the sand- 

 stone belt on the mainland. The inner islands are more rugged than 

 the outer ones, the latter being fiat, table-like areas of rock without much 

 glacial debris resting on them. On the whole, the former are covered 

 with a mu(;h heavier mantle of drift than are the latter. The trend of 

 the islands and of the channels separating them corresponds to the 

 general trend of the pre-C51acial valleys and of the shoreline on the main- 

 land. This indicates that the islands have resulted from the drowning 

 of i)re-Glacial valle3\s and the consequent isolation of the higher portions 

 of the land surface. Well developed valleys, })rol)ably of pre-Glacial 

 origin, are found on the mainland at the ])resent time, which are con- 

 nected with each other by sags over the intervening divides. A slight 

 depression of the land would cause the flooding of these valleys by the 

 waters of the lake and the formation of several new islands. Should 

 the lake level rise 30 feet, the promontory which lies between Buffalo bay 

 and Frog bay would be cut off from the mainland and an island would 

 result. If the lake level- should rise 50 feet. Detour point would be cut 

 off and an island formed, and the same is true of Bark point and Siskiwit 

 point. In such cases it is natural that the trend of the islands should 

 agree with that of the mainland. 



On the contrary, should the lake level be lowered, not only would the 

 islands be increased in size, but a number of them would disappear as 

 islands and would again become mainland ridges and promontories. A 

 lowering of the lake to the amount of 10 feet would cause Sand island 



