R. T. CHAMBERLIN AND W. Z. MILLER 
bo 
INTRODUCTION 
In the literature of structural geology it is commonly stated 
that rigid materials subjected to non-rotational strain tend to 
fracture along planes which are inclined approximately 45° to the 
direction of applied force. This conclusion has been developed 
partly from a mathematical analysis of stress and strain relations 
and partly from results observed in the familiar practice of crush- 
ing cubes of building stone to determine their strength. That 45° 
is the angle at which rigid materials normally fracture under direct 
compressive stress appears to be very generally accepted. This 
angle, therefore, has come to be regarded by structural geologists as 
the theoretical angle at which thrust faulting, under ordinary con- 
ditions, should occur. 
But if the actual angles of dip of a large number of thrust- 
fault planes in the earth be tabulated and averaged, it is found that 
the mean inclination is less than 45° from the horizontal. Accord- 
ing to Leith an average compiled from folios of the United States 
Geological Survey gives a dip of 36° for planes of thrust faults 
and 78° for planes of normal faults.‘ An inspection of numerous 
cross-sections from various other countries gives results in fair 
agreement with these figures. The average dip angle of thrust- 
fault planes, as they occur in nature, is considerably less 
than 45°. 
While the most prevalent type of thrust-fault plane, that of the — 
ordinary reverse fault, dips somewhat less steeply than 45°, it 
still does not depart widely from that governing angle. Neverthe- 
less, in notable variation from this, field studies in the last few years 
have brought to the attention of geologists impressive evidence of. 
the prevalence and the great importance of what may well be called 
a different genus of fault, namely, the great low-angle overthrust. 
Its generic characteristics are the very low inclination of its fault 
plane and the extraordinary horizontal displacement often attained. 
Such low-angle overthrust faulting has been well described, as it is 
strikingly shown in the Northwest Highlands of Scotland, where the 
Moine, Ben More, Glencoul, and other remarkable thrusts form 
1C. K. Leith, Structural Geology, 1913, p. 55. 
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