240 GEOLOGY OF EASTERN WISCONSIN. 



In the valley of the Mink river in Door county, at White Clay lake 

 in Shawano county, and, in lesser quantities, at other points, similar 

 accumulations occur, usually associated with peat. 



PEAT. 



When the glacier retired from our state, it left its debris in the 

 form of drift heaped up in an irregular way over the surface, giving 

 rise to numerous depressions which soon filled with water, resulting 

 in lakes of various forms and sizes. It is perhaps not too much to 

 say that within our district, these numbered thousands. In most cases 

 they soon filled to the brim and then began to overflow their margin 

 at some point, thus forming a channel, which was rapidly cut deeper 

 and deeper, at the same time draining the lake. As the water became 

 shallower, vegetation sprang up in the form of reeds, flags, rushes and 

 the so-called water mosses, which, on dying, fell to the bottom of the 

 lake, and being prevented by the water from complete decomposition, 

 accumulated as a peat deposit. As the draining continued ,these lakes 

 became marshes, and a new class of vegetation sprang up, varying ac- 

 cording to the character of the marsh formed. 



In the region now occupied by prairies and by oak openings, the 

 marshes were occupied generally by members of the grass or sedge 

 group, accompanied with those mosses that are usually found associ- 

 ated in this group. As the vegetation thus produced died with the 

 succession of seasons, it was added to the accumulating peat deposit. 

 In the more heavily timbered regions of the state, the marshes usually 

 came to be occupied by the swamp-frequenting conifers, the most 

 abundant, of which is the tamarac. In association with these there 

 is everywhere to be observed a luxuriant growth of minor vegetation, 

 among which the Sphagnum mosses are most efficient in peat produc- 

 tion. These have the property of dying below while growing densely 

 above, and thus they contribute to the rapid accumulation of vegeta- 

 ble debris, and for this reason they take foremost rank as agents of 

 peat formation. They are not confined in their association to the ar- 

 boreus vegetation named, but in the region under description are most 

 abundant in that connection. 



In regard to the amount of peat formed in these several ways, the 

 order will be the reverse of that in which they are named. The ac- 

 cumulation appears to have been much more rapid in the tamarac and 

 similar swamps, than in the open marshes, and as a result the deposits 

 of these marshes are almost un iversally found to be deeper than those 

 of the other class. The amount of accumulation thai took place in 

 the open marshes, after they became such, was undoubtedly much 



