The N. Yukon-lower Mackenzie area lies in the northwestern corner of Canada, between the 65th parallel and the Arctic coast, the 125th meridian and the Alaska boundary. Strata of every Paleozoic and Mesozoic system as well as rocks of Precambrian and Tertiary age are present in the area. Dominant rock types are as follows: Precambrian and Lower Cambrian - clastics; Middle and Upper Cambrian - carbonates; Ordovician and Silurian - carbonates, black shales, and bedded cherts; Middle Devonian - carbonates and shales; Upper Devonian - clastics; Mississippian - carbonates; Upper Pennsylvanian and lower Permian - clastics; Triassic - shales and limestones; Jurassic and Cretaceous - clastics; Tertiary - clastics. Most of the eastern part of the area has formed part of the cratonic interior of the continent since Precambrian time. Basins have occupied parts of the western half and southern margin of the area at many times, and these regions have generally exhibited relatively high tectonic activity. Mountain-building movements took place in the British Mountains-Barn Mountains area, probably during late Middle Devonian or Upper Devonian. Upper Devonian sediments believed to be in part derived from this disturbed area form a depositional body that is in many respects similar to the Catskill delta. An uplift of the ancestral Rockies type appeared in the northern Richardson Mountains during the Pennsylvanian. The uplift was flanked by a complementary basin. Laramide deformation strongly affected the western and southern parts of the area. Faulting of the Canadian Rockies type occurred in the area of the northern Ogilvie and northwestern Richardson mountains. The southern Richardsons are formed by an anticline of the Wyoming Rockies type.