OCEANS AND LAKES 



265 



by mountain torrents are likely to be composed of coarse ma- 

 terial and to have very steep fronts. (3) The slowly decaying 



FIG. 282. An undercut cliff on the shore of Kelleys Island, Lake Erie. 

 (H. E. Wilson.) 



remains of plants accumulate upon the bottoms of lakes, 



especially about their shallow borders, and tend to shoal them. 



(4) Various kinds of shell-building animals inhabit the waters 



of most lakes, and at death 



their shells help to fill the 



lake basins. (5) Winds 



blow some fine material 



into all lakes and much 



into many. (6) Certain 



minerals are deposited 



from the waters of many 



FIG. 283. Sketch of the Bowsprit, Point 

 aux Barques, Michigan. The island was 

 formerly a part of the mainland, from 

 which it was isolated by wave erosion. 

 The waves are undercutting the island, 

 and will destroy it in time. 



lakes, especially in arid 



regions. (7) Most lakes 



in humid regions fill their 



basins above the level of 



the lowest point in the rim, and so have outflowing streams. 



Such streams wear the outlets lower, rapidly when they are of 



large volume and the rock is soft, but very slowly when the 



B. & B. QEOL. 15 



