A combined echo sounder and seismic profiling survey in Miramichi Bay has detected two prominent seismic reflectors. The upper reflector probably represents a marine terrace, above which are recent sediments. The sea bed consists mainly of sands in the barrier island and Outer Bay area, whereas the Inner Bay consists mainly of sandy muds and muds. A discontinuous 'gas' reflector in the muds of the Inner Bay masks all deeper seismic reflectors where it is present. The lower seismic reflector is probably the Pennsylvanian bedrock surface. Between the upper and lower reflectors, proglacial sediments and glacial till are probably present. The bedrock surface has been eroded into deep, linear channels by a pre-Pleistocene drainage system which may have been subsequently overdeepened by glacial scouring. These channels may have been eroded along lines of weakness in the Carboniferous sediments representing structures, such as joints, fractures, or faults in the bedrock. These structures may be related to the extension of the Catamaran Fault Zone in the pre-Carboniferous basement rocks beneath Miramichi Bay.