220 



GEOLOGY OF EASTERN WISCONSIN. 



form of fresh water lakes, wliich are to be regarded simply as the 

 expanded predecessors of our present great lakes. As tliis water grad- 

 ually accumulated and advanced upon the land, it washed out the 

 finer material of the Bowlder Clay, carrying it backward into the still 

 water, where it was redeposited, while it left along the beach the 

 coarser sand, gravel and other material, forming a beach deposit. 

 This deposit, to distinguish it from those which subsequently fol- 

 lowed, is here designated Beach Formation A. It is abundantly 

 exposed along the shore of Lake Michigan, above and below Milwau- 

 kee, where it overlies the Bowlder Clay and underlies a subsequent 

 deposit of red clay. The accompanying figure illustrates its relations 



and relative thickness. 



Fia. 12. 



M//.Wy4l//f££ C/TK 



J/i ^^. 





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2</*. 



ere/ o/' £aA'e ^icfi.iyan 



Sections showing tlio relations and magnitude of the Bowlder Clay, Beach Formation A, and 

 Lower lied Clay, at and near Milwaukee. 



The deposit consists chiefly of sand, gravel and bowlders, with in- 

 cluded layers of clay. The material is thoroughly stratified, and ex- 

 hibits most beautiful and abundant examples of cross and oblique 

 laminae, which indicate the nature of its origin. The sand is usually 

 white or yellow, but sometimes ferruginous or dark colored. The 

 constituent grains are of all sizes, from afineness that renders the mass 

 compact and almost plastic, and quite indistinguishable at the dis- 

 tance of a few feet from the clay bands, to a coarseness that is only 

 arbitrarily distinguished from gravel. It is banded and laminated in 

 the greatest variety of forms. Horizontal, oblique, undulating, and 

 even contorted laminations, are present in variety. 



The gravel exhibits every gradation, from sand to that which is so 

 coarse that it is scarcely less than a bed of bowlders. It is chiefly 



