The Tertiary history of the southern Rocky Mountain Trench is inferred from a study of the distribution, stratigraphy, fabric, lithologic composition, structure, and palynology of the Miocene St. Eugene Formation in southeastern British Columbia.The St. Eugene Formation consists of flood-plain and fan facies and represents the upper part of up to about 1500 m of sediments which accumulated in the proto-Rocky Mountain Trench upon cessation of Laramide deformation and after initiation of extension and block faulting in the eastern Cordillera during Eocene or early Oligocene time. Deep Tertiary basins in the southern Rocky Mountain Trench are bounded on the east and west by high-angle faults parallel to the Trench margins and on the north and south by faults transverse to the trend of the Trench. Block faulting of a half-graben style was probably contemporaneous with sediment deposition, but at least 600 m of displacement on the east boundary fault postdates deposition of the St. Eugene Formation. Although there is no present seismic activity along the Rocky Mountain Trench north of latitude 49°N, Holocene fault scarps and earthquakes in a zone along the Rocky Mountains of the United States attest to the continuation of block faulting south of 49°N.The St. Eugene microflora includes at least 39 genera of ferns, gymnosperms, and anthophytes. Phytogeographic reconstruction based upon the habitats of extant counter-parts indicates floral elements growing on poorly drained lowlands, adjacent slopes, and montane uplands; thus, there was moderate to high relief in southeastern British Columbia during St. Eugene time. The climate apparently was temperate, with warm summers, mild winters, and abundant, uniformly distributed precipitation. This contrasts with the present climate of the southern Rocky Mountain Trench which is semiarid with hot summers and cold winters, and suggests that the mountain barriers which presently restrict cool, moist, Pacific maritime air masses to the coast were lower during the Miocene, or that the polar seas were relatively warm.