40 MICHIGAN SURVEY, 1905. 



been no Beaver on Carp river within the memory of several of the trappers 

 in the region, and yet several of the bends of the river were found to be due- 

 to the remains of old dams which were once evidently of considerable size. 

 Now a slight raising of the water level in this valley, such as would be caused 

 by these dams if they were in repair, would cause the plain to be flooded, 

 the trees to be killed off and either a pond or meadow would result according^ 

 to the height of the dam. It thus seems probable that the present flood- 

 plain societies of Carp river have reoccupied the flood-plain since the Beaver 

 were killed off, which suggests that the history of the meadow on Little 

 Carp river will be similar. 



6. SUMMARY. 



In order to get at the relation between the physical changes and the 

 biota of the region as a whole, it will be necessary to summarize the present 

 conditions, the biota, the processes that are dominant in the different 

 habitats, and the changes which they are bringing about. 



/. Beach. The beach consists of tilted strata of rock that are exposed 

 to the forces of weathering and the action of waves. It may be divided inta 

 three parts on the basis of the processes acting upon it. The Lower Beach 

 is exposed to the pounding of the waves, that tend to break up the rock 

 and carry the fragments back in the undertow, building them up in a sub- 

 marine terrace. The Middle Beach is exposed to the action of waves only 

 during the winter months; during the summer months the forces of w^eather- 

 ing predominate and tend to form a soil. This soil tends to be removed 

 during the winter with the exception of a small amount that accumulates 

 in the edges of the joint planes. The Upper Beach is, at present, above the 

 reach of both summer and winter waves and is chiefly exposed to the forces 

 of weathering, which have formed a slight residual soil. 



The biota of the beach as a whole is adapted to the conditions that prevail 

 in this habitat, and may be divided into three groups that are closely asso- 

 ciated with the physiographic divisions. There is practically no life on 

 the Lower Beach, owing to the pounding of the waves. The life on the 

 Middle Beach consists for the most part of annuals that can obtain a foot- 

 hold during the summer on the slight soil in the crevices, and a few asso- 

 ciated animal forms. The flora of the Upper Beach, in response to the more 

 favorable soil conditions and the absence of wave action^ consists of a number 

 of annuals and perennials; while the more favorable food conditions are 

 likewise marked by an increase in the animal forms. 



The changes that are taking place are due to the action of waves and 

 weathering. The waves by cutting into the beach cause it to retreat inland,, 

 but the submarine terrace built up at the same time tends to cause this 

 retreat to proceed more slowly in its later stages, as the growing terrace 

 carries the breaker line off shore and tends to lessen the effect of the summer 

 and winter waves. This is, in part, counteracted in the Porcupine Moun- 

 tains by the sinking of the coast line, and the beach as a whole is moving 

 inland. As the force of the waves is diminished, the process of weathering 

 increases in comparative importance, the beach tends to become broken up, 

 and the area of the habitats frequented by the crevice forms increases. 



11. 1. Arhor Vitae Swamp. The narrow area back of the beach, at the^ 

 foot of the north slope is not well drained, and the soil is covered by a thick 

 layer of humus. 



The forms adapted to these conditions are plants and animals found in 



