The hinge belt between the Paleozoic Rocky Mountain geosyncline of central Idaho and the cratonic shelf of southwestern Montana is a critical zone with respect to stratigraphic changes. Structural data and isopach patterns indicate that this zone was recurrently uplifted during the Paleozoic. The Skull Canyon Disturbance during the Cambrian or Early Ordovician gave rise to a sharp angular unconformity truncating Upper Precambrian (Belt) rocks. Additional isolated uplifts were produced in Devonian and Early or Middle Mississippian times.
The hinge is thought to be a zone of weakness in the crust which was especially responsive to early stresses in the geosyncline. This same zone became the site of the greatest deformation during the Laramide orogeny, yielding along a prominent belt of major overthrusts.