A sedimentological study of the Tertiary sediments of the Mandamus - Dove River area in North Canterbury, New Zealand, indicates that fluvial conglomerates and coal were the first sediments deposited on a Cretaceous peneplain developed on greywackes of the Torlesse Group. Subsequent cross-bedded glauconitic sands were deposited by north-flowing longshore currents following marine transgression in the area. Rapid build-up of bedded tuffs produced by volcanic activity to the north-east diverted longshore current sedimentation from the area and permitted an environment suitable for the growth of an algal—crinoidal—bryozoan biostrome. Cessation of volcanic activity and continuing basin subsidence halted biostrome growth and allowed renewed deposition of detritus by marine currents.