The southern foothills of Alberta are composed of much compressed, imbricately faulted Paleozoic and Mesozoic rocks in an elongate belt between the Great Plains and the Rocky Mountains. Within the belt structural shortening evidently has been achieved mainly by thrust faulting, accompanied by the development of drag folds, most of which are not large. The major thrust faults lie in well defined sliding zones, and they generally transect the bedding at relatively low angles. This is true for supposedly competent as well as incompetent beds, although there is apparently some refraction of fault planes between beds of differing competence. At and near the surface the faults commonly dip steeply, but this is a result of rotation and is not indicative of the dips the faults had when formed.
Structures that originated as folds and were later faulted may not exist. Turner Valley anticline, which is commonly regarded as a faulted fold developed from a protofold, might be an exception, but the evidence is by no means conclusive.
In this area oil and gas have been found in Cretaceous, Mississippian, and Devonian rocks. The reservoirs tapped to date are associated with thrust faults and it appears that, with the possible but unproved exception of the Turner Valley pool, all the traps are fault traps. Migration of hydrocarbons might have started very early, but was not completed before the close of the Laramide revolution. There is no reason to suppose that all hydrocarbons migrated at once, or in any one direction, or that the process of migration was uninterrupted.