LACiOON AM) MAKSII 1 )i;r()Sl'rs AM) ISLAM) SI'ITS 211 



grass, which readily hohls (lol)ris carried in the water, has huilt up a 

 shoal within a few feet of the surface, (^n this new shoal a(|uatic veg- 

 etation such as reeds, rushes, sedges, and wild rice take root, and these 

 hring about true marsh formations. The last stage is the meadow con- 

 dition, determined hy the accumulation of debris sufficient to allow tlie 

 hind vegetation to gain a foothold. The east side of Chequamegon bay 

 is entering U})on the })reliminarv stage of swamp formation. The aver- 

 age dei)th of the water is only 10 feet, a depth which i)ermits the growth 

 of eel grass. Already this type of vegetation has gained a widespread 

 distribution in these shallow waters. 



ISLAND SPITS 



The formation of s[)its is chieiiy confined to the islands of the Apostle 

 group. Spits form invariably on the south end of islands in this region. 

 This distribution is due to the direction of the i)revailing winds and 

 currents, which set in from the northeast or the northwest, as a rule. 

 Spits are not found on all of the islands. A combination of factors is 

 necessary to allow their formation. Among these factors may be men- 

 tioned the following : Size of the island. JNIany of the islands are too 

 small to furnish sufficient debris for spit formation. Another factor is 

 the ])resence of formations that can be attacked readily by erosive 

 agents. At the same time the debris must be of such a character that 

 it can be transported with some degree of ease by the waves and cur- 

 rents. For example, on those islands in which glacial debris is abun- 

 dant, other things being equal, spits will occur. On the other hand, if 

 the waves must attack tlie consolidated sediments for material, the spits 

 are either absent or insignificant in size. Usually the cliff debris found 

 along the shores of the islands is in the form of coarse blocks, much too 

 large to be transported. Ultimately these blocks will be reduced l)y 

 the waves to transportable sizes. As yet this kind of work has not gone 

 on to any great degree, for Potsdam sandstone debris is not very com- 

 mon in the spit deposits. 



Another factor that has hel[)ed to determine location of spits is the 

 slope of the lake bottom. Frequently the slope is so great that a long 

 period of time must elapse before enough sediment is collected to reach 

 the surface of the water, even though other conditions be favorable. 

 Another condition that controls the formation of spits is the position of 

 the islands with reference to each other, and also with reference to the 

 prevailing currents. Islands which lie in the lee of others, and are thus 

 protected, in a measure, from the waves and currents, are free from these 

 deposits, as a rule. The most exposed islands, like Outer, Michigan, 

 and Cat islands, possess spits, while other less exposed islands are with- 



