iQo6] TRANSEAUBOGS OF THE HURON RIVER VALLEY 35 



ing of stagnant bodies of ice in old glacial drainage channels after 

 their abandonment; (2) the differential settling of fluvio-glacial 

 deposits; and (3) unequal deposition of glacial material in moraines 

 and till plains. 



Marl and peat deposits are commonly associated. The former 

 are of interest in so far as they aid in the filling of the lake basins. 

 Both are formed through plant agencies. 



Peat deposits may be classified under two general heads: (i) those 

 connected with glaciation, and (2) those associated with coastal plain 

 phenomena. In North America the bulk of the deposits come under 

 the first head. Their geographic distribution approximates that of 

 the Pleistocene glaciers. Near the southern border the peat areas are 

 scattered, but they become more nearly continuous and more inde- 

 pendent of depressions as we go northward. The same effect is 

 brought about in mountainous regions by increased altitude. In 

 the tundra, peat accumulates because of the low temperature and in 

 spite of the scant vegetation. In temperate regions a vigorous vege- 

 tation and areas of stagnant water render peat accumulation possible. 

 In the southern coastal plain swamps, peat is formed in stagnant 

 water because of the luxuriant vegetation and in spite of the high 

 temperature. 



During peat formation two processes are involved: (i) erema- 

 causis and (2) putrefaction. The former is essentially an oxidizing 

 process, brought about in the presence of air by certain fungi and 

 bacteria. Its products are of direct value as food materials for 

 plants. Putrefaction is carried on in the absence of oxygen and is 

 essentially reduction; the organisms involved are anaerobic bacteria, 

 and the products are of no value to the higher plants as food materials. 

 The accumulation of peat depends upon the scarcity of oxygen below 

 the water level, the acidity of the ground water, and the occurrence 

 of low temperatures. 



Peat varies in color beneath the various plant societies, being light 

 brown in the youngest (bog sedge) and dark brown in the oldest, the 

 darkest and most thoroughly decayed form being known as "muck." 

 As disintregration proceeds it brings about a decrease in water capac- 

 ity, a decrease in volatile combustible matter, and an increase in 

 the amount of ash. 



