466 GEORGE ROGERS MANSFIELD 
Rocky Mountain geosyncline.—Southeastern Idaho forms a 
part of a great geosyncline in which sediments were deposited with 
few interruptions of magnitude from early Cambrian to Upper 
Cretaceous times. This great trough extended from the Arctic 
Ocean southward through the Great Basin and was in general an 
area of subsidence or a negative element’ on which the sediments 
had accumulated in great thickness. On the west during the same 
interval a relatively persistent land mass or positive element had 
separated the geosyncline from the Pacific Ocean, and on the 
east a less persistent barrier at times had separated it from the 
interior sea. 
The geosyncline served to localize the deformation and had a 
directive influence upon it. The tangential pressure which pro- 
duced the folds and overthrusts was normal to this structure and, 
in southeastern Idaho, came from the west southwest. 
Initial dips within the geosyncline and differences in the charac- 
ter of the sediments doubtless tended still further to localize 
the folds and thrusts and to determine their character. 
Favorable formations —Many of the Paleozoic formations are 
massively bedded and would act as competent strata under 
deformation. A number of formations, however, contain shaly 
members. Some of the limestones, too, are thin bedded. Such for- 
mations exposed to deformation in the zone of fracture would 
furnish horizons in which thrust planes might originate. The 
Bannock overthrust zone is complex and no one formation has 
yet been identified as the source of the thrust plane. 
The Mesozoic formations are generally weaker and less well 
consolidated than are the Paleozoic rocks. Lying with favorable 
initial dip and in great thickness athwart the direction of maximum 
compression, the Mesozoic rocks crumpled under the accumulating 
compressive stress and permitted the more or less folded Paleozoic 
rocks with some accompanying or overlying Mesozoic rocks to 
override them. They thus generally form the basement over 
which the great thrust block of the Bannock overthrust moved 
and on which it now rests. Although it has been customary in 
« Bailey Willis, “A Theory of Continental Structure Applied to North America,” 
Bull. Geol. Soc. America, Vol. XVIII (1907), pp. 389-412. 
