2o8 WILLIAM H. HOBBS 



Backfolding occurs where relatively competent rock formations 

 are locally replaced by weak material, or where a surface of dis- 

 junction transects the formation. The backfold develops in front of 

 and eventually above the zone or surface of weakness. 



The evolution of an anticline to late stages of underturning is 

 directly determined by the measure of lamination of the competent 

 member and the toughness of the laminae. Relatively strong rocks 

 devoid of lamination fail by cross shear ("break slide") and at a 

 much earlier stage of the anticline than tough well-laminated rocks 

 which, on the contrary, fail on disjunctive surfaces which are nearly 

 or quite parallel to the lamination ("stretch slide"). 



After failure of an anticline has occurred, a new resolution of the 

 external forces takes place with components parallel and perpen- 

 dicular respectively to the surface of failure (slide); this surface 

 tends to be extended in either direction along surfaces of least 

 resistance — usually bedding planes — and is thus flattened as it 

 recedes from the competent arch. With the advent of the slides, 

 migrations of the severed parts of the fold are greatly facilitated, 

 and movements of the rock slices take place in such a sense that 

 the lower are driven in beneath the upper — underthrusting. 



The large friction on slides incident to underthrusting is reduced 

 by a weak formation superior to the competent member. Friction 

 breccia (mylonite) generally marks the position of the slide which 

 develops special features at its forward end. Here are found 

 secondary folds (drag folds) and secondary slides (listric surfaces), 

 the former being underturned and attenuated in the underhmb, 

 while the latter are concave upward (shovel-shaped) and converge 

 into the major slide below. The lower of two contiguous slices 

 may be ruptured and involved in the complex of curved wedges at 

 the front of the slice immediately above. 



September 15, 19 13 



