The Folding of Subjacent Strata by Glacial Action
The Journal of geology
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
... 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 horizontally throughout their extent. The folds and similar displacements are both exceptional and limited 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 8oo00 foot terrace which runs on the north side of the river from the city to Fort Snelling. This terrace is a limestone bench nearly cleared of glacial deposits by the glacial River Warren. The surface of the limestone lies now generally 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 waterfilled 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.