The Tertiary Kishenehn Formation, exposed along the valley of the North Fork of the Flathead River in southeastern British Columbia and northern Montana, consists of about 8,600 feet of relatively fine-grained lacustrine deposits, overlain by more than 7,000 feet of conglomerates and breccias. Landslide blocks of Proterozoic and Paleozoic rocks occur as clasts several hundred feet across in the upper unit. These blocks were derived from the ancestral Clark Range to the east. The Kishenehn was deposited in a half-graben on the downthrown (west) side of the Flathead fault, concurrently with late movement of the fault, upon an irregular surface eroded in gently dipping Mesozoic and Paleozoic strata. Late movement of the Flathead fault resulted in eastward tilting of the Kishenehn and underlying strata to form the east flank of the MacDonald dome, a broad anticline west of the Flathead Valley.

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