ECOLOGY OF NORTHERN MICHIGAN. 25 



little life, but near the upper limit of summer wave action a few straggling 

 annuals come in that characterize the Middle Beach above, while near the 

 upper limit reached by the winter waves and ice a few perennials of the 

 Upper Beach occur. The biotic tension lines between these divisions also 

 undergo occasional fluctuations, since the position of the tension line between 

 the Lower and Middle Beaches is determined by the severity of the summer 

 storms, while between the Middle and Upper Beaches it is determined by 

 the height of the winter waves and ice. 



Owing to the action of the waves, the beach as a whole is retreating inland, 

 but as it retreats a wave cut and wave deposited terrace is being formed, 

 and this by shallowing the water carries the breaker line off shore and lessens 

 the effect of the waves, as illustrated in Fig. 7. The conditions of the Lower 

 and Middle Beaches thus become more favorable, resulting in a progessive 

 downward movement of the biota of the Middle and Upper Beaches respect- 

 ively that decidedly narrows the beach zones. 



On the other hand, the sinking of the coast in this region counteracts, 

 in places, the tendency of the submarine terraces to decrease the efficiency 

 of the waves. The deepening of the water near shore caused by this sinking 

 of the coast brings the breaker line nearer shore, thus increasing the efficiency 

 of the waves to such an extent that the beach habitats are being forced back 

 into the adjacent swamps. 



Station II. Substation 1. Back of the beach, at the foot of the first 

 range, there often extends for considerable distances a narrow belt of arbor 

 vitae swamp. This swamp owes its origin, according to Wright ('05, p. 37), 

 to the general sinking of the beach of Lake Superior referred to in the dis- 

 cussion of the previous station. Gilbert ('97) in a discussion of this problem 

 states that a general canting or tilting of the Great Lake Basins toward 

 the southwest is taking place, as is indicated by the inclination of the beach 

 lines of post-glacial lakes and by the drowned mouths of the rivers along 

 the coast. It is evident that the rivers of the Porcupine region are drowned, 

 as they are widened near their mouths into broad estuaries with adjacent 

 swamps, and evidence that the tilting of the lake basin that is causing this 

 is going on at the present time may be found in the submergence of 

 standing trees along the lake shore, and in the discrepancy in the length of 

 the section lines between different surveys, (Wright, '05, p. 37). 



This sinking of the coast makes the conditions of a narrow zone just back 

 of the beach too wet for some of the mesophytic forest forms that occupy 

 the higher parts of the ridge, but at the same time it makes the conditions 

 favorable for some of the forms of the arbor vitae swamp, so that this area 

 is inhabited by certain forms characteristic of each of these habitats. The 

 tree cover consists principally of the Arbor Vitae, with considerable Balsam 

 Fir and some White Spruce and Paper Birch; the soil cover is characterized 

 by the Running Pine, Lycopodium davatum, and occasional clumps of Blue 

 Flag. The molluscan fauna is very poorly represented, and the only shells 

 collected were a few specimens of Punctum pygmaewn and Zonitoides milium. 

 A single spider was taken here, Dolomedes tenebrosus. The mammals are 

 those of the forest, such as the Southern Varying Hare, Southeastern Red 

 Squirrel, etc., with the exception of the Lake Superior Chipmunk which 

 was occasionally seen near the beach. 



If the coast continues to sink, this swamp will approach more closely the 

 conditions of the swamps of the river valleys and will -become occupied 

 more exclusively by the biota of the ordinary arbor vitae swamp, while 

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