Channels eroded into the sea cliff at the distal margin of the Canterbury Plain have a composite origin. Large- and medium-size composite channels formed initially by overland flow from a swamp. Subsequent seepage through outwash gravels enlarged them to produce wide, flat-floored valleys with irregular margins. Smaller channels have been and are being formed by seepage and mass movement, but they stagnate when an armor is formed that cannot be breached by seepage flow. All the channels show characteristics that have been described as criteria of channels formed by ground-water sapping, but the large channels were eroded predominantly by surface runoff and subsequently modified by ground-water sapping. The New Zealand composite channels resemble some small Martian valleys that are incised into cliffs and valley walls on that planet.