28 R. T. CHAMBERLIN .AND W. Z. MILLER 
determining low-angle faulting. As shown in Fig. to, in which the ~ 
ellipsoids representing the axes of strain are drawn upon the beds, 
the lowering of the angle in this way seems to be the result of 
shearing stresses and rotational strain. A difference in competency 
is thus one means of developing rotational strain, and the type of 
faulting described above comes under the category of rotational 
strain thus produced. The difference in competency may not be 
solely because of a difference in the kind of rock, but it may result. 
also from a very unequal distribution of bedding planes which are 
planes of weakness. It is to be noted that an abundance of strongly 
marked, closely spaced bedding planes, in addition to making the 
competency of the formation less with respect to more massive 
adjacent formations, also makes splitting parallel to the bedding of 
the bedded rocks much easier than breaking across the bedding. 
With less resistance offered in that direction, fault planes crossing 
very thin bedded shales will be lowered to a certain extent toward 
parallelism with the bedding. The more bedding planes and the 
more pronounced they are, the lower the angle of faulting. 
General discussion.—The Lewis overthrust in the Glacier 
National Park of Montana, according to the well-known explana- 
tion of Willis, who would classify it as an erosion thrust, appears to 
be a case of low-angle faulting controlled by bedding.’ The fault 
plane, where observed, is located in the Proterozoic Altyn limestone, 
whose bedding it appears to parallel closely. Above the fracture 
plane, for the most part, are rigid, brittle, competent strata. What 
lies below the fault plane is not known, since the oldest formation 
in the vicinity is the Altyn limestone just above the fault plane. 
This has been overthrust upon the Cretaceous. In the explanation 
given by Willis, the sequence of events is, first, gentle folding by 
which there was developed a low, unsymmetrical anticline whose 
gentler west limb had a nearly constant westerly dip. Erosion 
then removed the crest of the fold, thus leaving the west limb a 
thick sheet of competent strata, unweakened by secondary flexures 
t Bailey Willis, ‘““Mechanics of Appalachian Structure,’ U.S. Geol. Surv., 13th 
Ann. Rept., Part II (1893), p. 223 and Pl. LIV, Figs. 6 and 7; ‘‘Stratigraphy and 
Structure, Lewis and Livingston Ranges, Montana,” Bull. Geol. Soc. Amer., XIII 
(1902), 331-43. 
