LOW-ANGLE FAULTING 7 
overfolds. If this be the true explanation, it would add to our list 
this remarkable structure of the Alps as a most pronounced and 
complicated case of low-angle faulting. 
Similar structures have been reported from Spain, Euboea, the 
Balkans, and the island of Timor; in the last case an extensive 
sheet of shallow water strata, ranging in age from Triassic to 
Eocene, has been thrust over what appear to be deep-sea deposits 
of nearly the same age.* 
Detailed studies elsewhere—practically the world over, indeed— 
are bringing to light overthrust faults of great displacement along 
gently inclined planes. This sort of faulting seems, therefore, to 
constitute a phenomenon of a definite, independent type. It seems 
to belong to a genus of its own, distinct from the ordinary reverse 
fault, though the two are no doubt connected by composite types 
that bind them together. The common reverse fault is defined 
by displacement along planes neighboring 45° or a little less, and is 
confined to more limited movement on these planes, while the 
great overthrusts slide along planes that approach horizontality 
and involve displacements of astonishing magnitude. ‘Though each 
great low-angle overthrust is commonly attended by a retinue 
of reverse faults of lesser magnitude—a fact which suggests that 
there may be a kinship between them—nevertheless an inspection 
of any good section, as in the Scottish Highlands, shows a radical 
difference between the two types. Some distinctive feature seems 
to be added to simple straight compression to form the low-angle 
overthrusts. 
PREVIOUS INVESTIGATIONS 
Willis has divided thrust faults into four classes, the break- 
thrust, stretch-thrust, shear-thrust, and erosion-thrust. Of these 
the shear-thrust and the erosion-thrust are low-angled overthrusts, 
while the other two classes belong to the more common group of 
reverse faults. The shear-thrust is a class to cover the conspicuous 
Scottish Highland type, while the erosion-thrust covers a special 
case of alternate competent and incompetent strata in which the 
upper competent formation carrying the thrust is first removed 
1G. A. F. Molengraaff, ‘‘ Folded Mountain Chains, Overthrust Sheets, and Block 
Faulted Mountains in the East Indian Archipelago,’ Compte Rendu, Congrés Géol. 
Int. (Toronto, 1913), pp. 689-702. 
