STRUCTURAL GEOLOGY 
169 
impinging sands at the line where surface of intermontane plain 
meets steep mountain side. In the long distances, the railroad 
traverses lowland plain which on careful inspection is discovered 
not to be miles deep with mountain debris, as Gilbert claimed, 
but to be worn out on the basset edges of upturned strata, with 
soil only a few inches thick. A special desert geology was 
unknown at the time the Lake monograph was written. 
Keyes. 
• 
Thrust at Crow's Nest. In the Front Ranges round about the 
Glacial ,National Park, Montana, evidences of reverse faulting 
are many. The development of thick, stiff and unyielding lime¬ 
stone plates followed by great thicknesses of plastic shales enables 
identical compressive forces to express their activities very differ¬ 
ently from what they do farther south. 
Where, in Colorado, the Rocky Mountains are bulged up into 
huge but simple arches, and in New Mexico into multiplex but 
open folds, in Montana and Alberta somewhat more intensified 
movement or more concentrated power causes the unsymmetrical 
fold to rupture and glide over itself. The transformation of 
folding into thrusting appears to be due only partly to increased 
compressive activity. It seems partly the result of the character 
and succession of the rock terranes. 
North of Glacial National Park, where the southern line of the 
Canadian Pacific Railroad crosses the Rockies at the low Crow’s 
Nest Pass, overthrust phenomena are displayed with such wonder¬ 
ful clearness that the structures are satisfactorily viewed from a 
distance of many miles. East of the Pass Crow’s Nest Mountain 
rises out of the plains as a gigantic flat-topped butte. Lithologic¬ 
ally, the substructure of the surrounding plains and of the pedicel 
of the mountain itself is composed of soft shales of great thick¬ 
ness. Hard limestone forms the walled capping of the butte. It 
is Carbonic in age; while the shales are of Cretacic date. The 
plane of juncture of the two is sensibly inclined, and crumpled, 
and the upper surface of the shales below shows all signs of 
severe over-riding. 
Superposition of the older Carbonic limestone over younger 
Cretacic beds might prove puzzling were it not for the fact that 
in the main mountains to the west the same phenomena are also 
finely displayed over a broad field. The distance of over-riding 
