428 O. B. HOPKINS STRUCTURAL FEATURES OF PLAINS OF ALBERTA 



Eockies have overridden the Cretaceous, leads naturally to the supposi- 

 tion of similar causes, but operating on a smaller scale. The similarity 

 is quite marked in the following respects : 



(a) The disturbance of the formations leading to crushing and crum- 

 pling is more intense on the north side of the hills — that is, on the side 

 from which the thrust was applied. 



(h) Folds and faults have been developed at right angles to the direc- 

 tion of the thrust — that is, in a general east-west direction. 



(c) Thrust faults are the dominant structural features, and such anti- 

 clines as are formed are commonly thrust faulted on their southern limb. 



(d) Thrust faulting has led to the formation of small blocks which 

 dip generally to the north — that is, in the direction from which the 

 thrust was applied. 



The analogy is so close that we are forced to look for a thrusting force 

 from the north-northeast, whether it is granted that the deformation is 

 surficial or not. 



Development of Complex Structure by Movement of Ice-sheet 



If the surficial nature of the disturbance is granted, the surficial nature 

 of the force and its direction leads naturally to the assumption that the 

 thrusting force was the great ice-sheet because it was the only competent 

 force. It might be questioned whether the movement of the ice-sheet 

 w^as a competent force, as it is not usually conceded such power for per- 

 forming work. However, it is unquestionably the most potent surficial 

 force and probably the only one that can be called upon to explain such 

 results. 



The hills present in the area today were undoubtedly hills in Pleisto- 

 cene time and formed obstacles to the forward movement of the ice-sheet. 

 The ice piled up against and finally overrode the hills. As the pile of ice 

 increased, the pressure against the north side of the hills probably caused 

 the soft shaly strata to buckle and fold, accompanied by movement along 

 some horizontal bed of soft material, thus developing a horizontal thrust 

 plane. As this continued inward toward the center of the hill, a sub- 

 sidiary fault would develop along the small buckles, thus causing the 

 thrust faults observed at Mud Buttes (see figure 10). When the blocks 

 thus formed had been rotated to such a position that the pressure was 

 more or less at right angles to the bedding, instead of parallel to it, the 

 strata would be capable of transmitting the thrust, causing the extension 

 of the flat thrust plane and the further development of other blocks. In 

 this way the complex structure observed was probably developed. 



