SOME EXPERIMENTS IN FOLDING 511 



found in the less brittle materials deformed at greater depth. Field 

 observations only in part substantiate this idea. Since the mechan- 

 ical principle is doubtless correct, the anomalies must be explained. 

 Some suggestions as to the cause of high-angle faulting were obtained 

 from these experiments on folding. 



High-angle thrust faults. — By using layers of paraffin and vase- 

 line, which would fold before breaking, thus representing the con- 

 ditions of folding not far from the earth's surface, fairly numerous 

 high-angle faults were developed. The conditions determining these 

 steep faults were found to be somewhat varied. In one or two cases 

 a crack developed in the upper side of an asymmetrical anticline, 

 due to tension, and this crack changed into a thrust fault with its 

 dip well over 45° (Fig. 17). Secondly it 

 was found that high-angle thrusts may 

 develop on the limbs of anticlines. In 

 such situations the transmitted forces 



operate nearly parallel to the sloping 



J. r , 1 T 1 Til r 1- 1 J Fig. 17. — Break thrusts on 



surface of the limb, and therefore should . . , j 1 v u c 



' _ stretched lower umb of anti- 



a fault develop at 45° to the apphed dine. The higher up on the 



forces, it may be inclined as much as 90° fold, the steeper the angle of 



from the horizontal. In several cases a ^'^^^''''; Thrust near crest has 



angle of 57 . 



fault developed on the side of an anti- 

 cline, and as compression continued, the fault ceased to grow, while 

 the anticHne became more tightly folded. In such cases the angles 

 of the faults were low at first, but gradually increased as the sides 

 of the anticlines grew steeper, and in one case they approached 

 verticality. 



Low-angle thrust faults. — In the other direction the very great 

 potency of rotational stress in lowering the angle of thrust faulting 

 even down to horizontality, as exemplified in the great overthrusts, 

 has been pointed out by the senior author in a paper on "Low Angle 

 Faulting."^ This rotational stress may be developed in a variety 

 of ways. 



Faulting in sand and clay. — While testing the qualities of the 

 sand used in enclosing the models, in order to determine what effect 

 they would have on the deformation, some rather surprising results 

 came to light. A layer of dry sand 3 inches thick, with flattened 



^ R. T. Chamberlin and W. Z. Miller, op. cit. 



