The Kamloops Group is redefined as an assemblage of Lower to Middle Eocene volcanic and sedimentary rocks widespread in south-central British Columbia. In the type area west of Kamloops, the basal Tranquille Formation consists of 500 m of lacustrine and deltaic sediments, pillowed flows, and hyaloclastites. Elsewhere, basal coal-bearing nonvolcanic fluvial and lacustrine units occur, such as the Coldwater Formation at Merritt, the Chu Chua Formation at Barriere, and the Shorts Creek Formation west of Vernon. Overlying these formations are dominantly volcanic units. At the type area, the Dewdrop Flats Formation includes over 1000 m of interstratified basaltic andesite flows, andesitic flow-breccia sheets and cones, basaltic tuff rings, and an andesitic composite cone. Elsewhere, flat-lying basaltic andesite flows about 600 m thick with local flow breccias are common.The basal sediments accumulated in separate fault-bounded basins initiated immediately before the onset of volcanism. These volcanic rocks filled the basin and formed a widespread volcanic blanket, which was disrupted by continued fault movement. The numerous basins are linked by a throughgoing fault network with up to 12 km of net right-lateral strike-slip displacement.