A thin marine limestone and calcareous mudstone unit (Portland Point Member) in the Hamilton Group grades laterally into sandstones of the Cooksburg Member when traced eastwards toward a palaeoshoreline. Sedimentological analysis of the Cooksburg Member shows it to consist of a complex of shallow- marine deposits together with sediments of tidal flat and beach environments. These sediments formed during a brief transgressive period during regional Devonian regression. This transgression comprised two phases of sea level rise, an initial slow rise of about 7 m followed by a more rapid rise of about 11 m. Calcareous sandstones with fossil debris, and bioclastic limestone and calcareous shale of the Portland Point Member were deposited offshore on a continental-shelf-like area. Towards the south, in relatively deeper water, calcareous fossiliferous siltstones were deposited. The whole area was subject to complex water movements of tidal and wave origin. This sedimentary assemblage can be causally related to a rise of sea level having two principal side effects. First, the rate of alluvial deposition on the floodplain lying to the east was increased, and second, the nearshore marine profile of equilibrium was disturbed. The result was rapid deposition of sands nearshore to form the Cooksburg Member, whereas the reduced supply of terrigenous detritus to the shelf allowed concentration of bioclastic debris to form the Portland Point Limestone.