LOW-ANGLE FAULTING 25 
H and L. But the point Z is in the line of least resistance, since it is nearest 
to the region of application of the compressing force, and hence the mass of 
material to be moved is less than if the break were to occur at L. After passing 
the central rigid mass the line of least resistance follows the upper bed of 
minimum rigidity C till another fold is reached where it passes through the 
upper rigid bed D. Erosion of the latter might determine the point at which 
the line would emerge at the surface. 
The requisites of this type of overthrust are thus seen to be an 
alternation of formations of pronounced difference in competency, 
affected by slight folding and possibly by moderate erosion of the 
upper competent layer on the anticlines. before the faulting com- 
mences. 
RESISTING |. = 
BLOCK |}; 
PRESSURE 
BLOCK 
Fic. 8.—Arching up of competent layers when there is too much difference in 
the strength of the layers. The competent layers were composed of pure plaster, the 
weak layers of damp sand. 
Experimentation: A single strong layer amid much weaker 
material.—To test experimentally some of these conclusions rela- 
tive to the influence of a marked difference in the competency of 
neighboring layers upon the nature of their deformation, the crush- 
ing machine was loaded in the first set of tests with a single heavy, 
competent layer of plaster in the midst of weaker sandy layers above 
and below. When pressure was applied, the strong layer which 
carried most of the thrust commonly warped upward into a very 
low swell and then cracked at the top of the swell. With further 
compression the broken beds continued to arch up, leaving an 
open space below (Fig. 8). It did not appear to make much 
difference in its effectiveness whether the strong brittle bed were 
the top layer, or whether it were lower down in the series. If lower 
down and it was sufficiently competent, it still controlled the defor- 
mation and carried up the overlying weaker material with it. 
1 Tbid., pp. 151-52. 
