ECOLOGY OF ISLE ROYALE. 9 



glacial ice movement upon the valley slopes, whicli tend to be gentle 

 on the southeastern side, the corresponding shores of the bays and 

 coves are usually at a low angle, except possibly where faulting has 

 taken place, or a wave cut terrace has been developed. The northern 

 sides of the bays are comparatively abrupt, and there is thus a tendency 

 for the cliffs to occur mainly upon the northern slopes and shores. The 

 larger bays are the submerged portions of the valleys, mark the 

 location of the less resistant rocks, and are inherited topogi'aphic fea- 

 tures; but many of the minor coves and the rocky headlands have been 

 carved by the activity of the present lake. The beaches are only de- 

 veloped at the heads of the coves and bays, and are very largely com- 

 posed of shingle and gravel. The only extensive sand beacli seen was 

 at the head of Conglomerate Bay. The character of the material com- 

 posing these beaches clearly shows its local origin, and emphasizes the 

 isolation which prevents long shore transpoi'tation of such material. Thus 

 only floating material is liable to extensive long shore dispersal, a signifi- 

 cant fact that bears upon .the dispersal of the snail life along the shore. 



During severe storms, the wave action upon this coast is quite in- 

 tense and even the waves of the summer storms are quite active, as may 

 be seen by referring to Fig. 3. The blue deep lake water comes close 

 up to the shore, so that generally no breaker line is developed off shore. 

 In several places there are numerous reefs or islands (usually the iso- 

 lated continuations of the rock ridges), which tend to bi"eak the force 

 of the waves rolling in from the open lake. 



No effort was made to study the life of the open lake, only the shallow 

 water of the bays and coves being examined. The major environmental 

 features of the coast are the Lower, Middle and Upper Beaches; but these 

 are only differentiated clearly at the heads of the coves and harbors. 

 The Lower and Middle Beaches are only seasonal expressions of the same 

 phenomena, but ecologically they are fairly distinct. 



The Loiccr Beach. This beach extends from the shallow water to 

 the upper limit of the summer waves. The submerged portion is not 

 sharply defined above on account of the changes in level of the water 

 surface, due to waves, the periodical and seasonal fluctuations, and the 

 atmospheric pressure (seiches). In time there has been a downward 

 migration of the entire beach zone, a tendency which is in part counter- 

 acted by the northward elevation of the land. This is the zone domi- 

 nated by water, ice, and wave action. It is certainly a sharply defined 

 tension line upon an exposed coast which clearly suggests that it is not 

 probable that many forms of animals have made the transition from 

 fresh water to the land under such conditions. If we consider the 

 shore habitats as including all stages from a rock cliff to the sand beach, 

 the lower beach and the protected shores are the most favorable aquatic 

 habitats upon such shores. 



Upon the sloping rock, shingle, gravel and sand beaches is found a 

 varied fauna. In winter, when the bays are frozen over, a calm is pro- 

 duced which must be favorable to the preservation of the aquatic life 

 upon this stormy coast. 



The general character of the sandy beach at the head of Conglomerate 

 Bay is shown in Fig. Jj. The life of the submerged portion of the shore 

 is quite limited, except on the beaches and protected portions. The vege- 

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