THE FOLDING OF SUBJACENT STRATA BY GLACIAL 



ACTION 



FREDERICK W. SARDESON 

 Minneapolis, Minn. 



A number of upthrust folds of stratified rocks subjacent to glacial 

 deposits occur at several places in Minnesota. Formerly they were 

 considered as well-recognized glacial phenomena, and accordingly 

 I took no special interest in them. More recently doubt has arisen' 

 as to their origin, and this has led to my making a closer study of 

 some of these occurrences. Proof of their true nature as glacial 

 phenomena can now be given. The folds are more or less regular 

 arches from 2 to 30 feet wide, and half as high as wide. The strata 

 of which they are composed belong to the Galena (Trenton) series, 

 or to other formations in Minnesota, which lie as a rule horizon- 

 tally throughout their extent. The folds and similar displacements 

 are both exceptional and hmited to the superficial part of stratified 

 formation, so that no doubt need be entertained as to their being 

 the result of surface agencies. 



For example, three such folds were seen in quarries and gradings 

 at St. Paul, Minn., along the 800 foot terrace which runs on the 

 north side of the river from the city to Fort SneUing. This terrace 

 is a limestone bench nearly cleared of glacial deposits by the 

 glacial River Warren. The surface of the limestone lies now gen- 

 erally within the reach of the winters' frosts — i. e., less than 8 feet 

 deep — and the folds in it either lie within reach of frost or may have 

 been in that position at some time in the past. Their origin might 

 therefore be supposed to be due either to the action of frost in water- 

 filled joints of the limestone, or, on the contrary, to the mechanical 

 action of a glacier. Saturation and drying of the strata might also 

 be supposed to have caused them. It was, in short, difficult to 

 prove which agency had caused them. 



I H. L. Fairchild, "Ice Erosion Theory a Fallacy" Bulletin of the Geologica 

 Society of America, Vol. XVI, pp. 12-23. 



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