214 G. L. COLLIE WISCONSIN SHORE OF LAKE SUPERIOR 



rial accumulates to form true shoals. The most extensive shoal deposit 

 in the region is that off from the mouth of Bad river. The shoal ex- 

 tends out into the lake for a distance of 5 miles, its position being marked 

 by the clianged color of the water above it and by the discoloration of 

 the water during storms. On the average, the water on this shoal is 

 shallower by 20 feet than the surrounding waters. 



The major portion of the shoal lies to the west of the river mouth, 

 carried to that position by the prevailing direction of the currents. 



Though the watershed of lake Superior covers a relatively small area, 

 3^et its slopes are so steep and there is so much loose debris on the sur- 

 face that the conditions for the transportation of sediment into the lake 

 are very favorable. The debris thus brought in is carried long distances 

 by the strong currents of the lake. The writer has seen the lake discol- 

 ored at Devils island b}'^ sediments brought in by the Nemadji and Saint 

 Louis rivers 1)0 miles away. Tbis wide carriage of debris tends to form 

 shoals over vast areas. Especially is this true in a limited and narrow 

 area such as that represented by the head of lake Superior. 



Wave Erosion and its Topography 



CA VERNS 



Caverns occur on tbose portions of the islands and mainland which 

 are well exposed to wave action. The best examples of this feature are 

 found on Devils island. Tbe caverns are formed in those layers of rock 

 exposed to maximum wave-action. As the vertical range of the Avaves 

 is not great, tbe caverns rarely exceed a height of 15 feet above the level 

 of the water. In many cases they are gouged out to quite a deptb below 

 the surface. Tbe caverns extend back to a maximum distance of 100 

 feet. Usually their depth is mucb less than this. In a number of in- 

 stances the caverns are ])ractically continuous, their broad, low i)ortals 

 being separated only by a thin wail of rock, or possibly a few interven- 

 ing i)illars. The amount of clay in the sandstone is the chief factor in 

 determining the form and size of the cavern. The more silicious por- 

 tions of the rock are cavernless as a rule. On the distribution of the 

 clay depends, in large part, the nature of the architecture, the presence 

 of pillars, the form of the i)ortal, the detailed fretwork, all of which add 

 so much to the charm of these occurrences. 



The erosion of these caverns is slow, as the water carries little sedi- 

 ment with which to do effective work. The retreat of tbe coast, how- 

 ever, is accomplished chiefly through the falling in of tbe cavern roofs. 

 The process is an exceedingly slow one. The waves plunge into the 

 caverns with terrific force, but they do little except to wash out the soft 



