FOLDING OF SUBJACENT STRATA BY GLACIAL ACTION 231 



paring Figs. 2 and 3, and noting that the direction of movement 

 is from left to right in Fig. 2, and from right to left in Fig. 3, one 

 can see how upfolding may be begun, and to what extent it could 

 quickly develop in strata subjacent to a glacier which moved over 

 a till- and gravel-covered surface. Why the disturbance ceased 

 just where it did in either of these cases is not evident. 



SE 



m 



^ o ■ ■ 



/ . 



O o 



^a^.^.•,^ 



'^:.si.i:^c. 





- -a- -- -^ - - 



- ~ - - -o- - o. 



,^^ 



Fig. 3. — Profile showing a thrust fault in strata under till. 



I imagine no other cause for the starting of the fold represented 

 in Fig. 2 than that seen in the fault, Fig. 3. The stratified rocks 

 in both places are fresh and not oxidized. In some other cases 

 the cause may have been a little different, especially in the ones 

 mentioned as seen in St. Paul, where there was much evidence of 

 leaching and oxidation by surface water, and where the folds were 

 of limestone strata alone. Those were each immediately over a 

 vertical joint of the rock and contained some residuary clay. From 

 these circumstances it appears that they may have been begun by 

 the sliding of superficial strata toward a widened joint which had 



