LOW-ANGLE FAULTING 27 
mixtures of clay and plaster were used. Fairly good results were 
obtained by making the stronger layers of equal parts of plaster and 
clay and the weaker layers of one part of plaster to two parts of 
clay. There was then a working difference in competency, but at 
the same time not too great a difference to cause simple arching 
or the troublesome dovetailing. 
Fracturing was found to take place at different angles across 
different beds. In general, it followed lower angles across the 
softer layers than across the stronger and more brittle ones. Per- 
haps the most typical result obtained is shown diagrammatically in 
> 
BLOCK AB =e BLOCK 
“de 
Vs 
Fic. 10.—Drawing of experiment which shows how the inclination of the fracture 
plane may vary greatly in crossing beds of different competency. Because of the 
operation of a rotational strain following the fracturing of the brittle competent layers, 
there resulted nearly horizontal shearing through the weak, clayey layer. The strain 
ellipses are drawn upon the fracture line to illustrate the variable nature of the 
strain. 
_ Strong layers (.S) composed of equal parts of plaster and clay; weak layers (W) 
of one part plaster and two parts clay. 
Fig. ro. When pressure was first applied in this experiment, the 
stronger layers carried most of the thrust, while the softer layers 
yielded and accommodated themselves so far as was necessary by 
compacting. With increasing strain the upper strong layer frac- 
tured at an angle averaging 20°, and this plane of fracture was 
projected through the overlying clay. With this fracturing a 
strong rotational strain developed below. This caused almost 
horizontal shearing through the soft clayey layer, where the pre- 
vailing angle of the fault plane is found to be less than 5°, and as a 
result of these shearing stresses the strong plaster layer below was 
faulted at 12°. 
Bedding, therefore, where there is sufficient difference in the 
relative competency of the strata, may be an important factor in 
