FOLDING OF SUBJACENT STRATA BY GLACIAL ACTION 227 



Two of the same kind of folds have been disclosed more recently 

 in Minneapolis, and these lie in such relations as to show very dis- 

 tinctly their origin from the mechanical action of glacial ice. Since 

 I am now convinced that the folds seen in other places have, with 

 one exception, not been produced by the action of frost, nor by 

 the saturation and drying of the rock strata, I shall describe, for 



. ih. 



Fig. I. — Limestone strata upheaved by the freezing of a stream, which now 

 flows under them. 



the sake of comparison merely, the one case of undoubted upheaval 

 of a hmestone fold by frost and weathering, and then, in particular, 

 the two newly discovered cases of undoubted glacial folding. The 

 case of folding by frost and weathering occurs in the old city quarry 

 at Camden Place, in the northern part of the city of Minneapolis. 

 Here the lowest limestone bed of the Galena (Trenton) series hes 

 near the ground surface, within the reach of frost and humid agen- 

 cies. A small spring issues from it on the face of a quarry wall, 

 and above the spring the strata and subsoil are seen to be strongly 

 arched (Fig. i). The origin of this structure is evidently as fol- 

 lows: The stream once flowed on the hmestone 's surface; annual 

 frosts have heaved the rock-bed so as to let the stream into deeper 

 courses; weathering has aided in loosening the stones and in form- 



