Ribbon (also called barrier) reefs only occur in the northern part of the Great Barrier Reef extending from Cooktown to eastern Torres Strait, a distance of 670 kilometres.
They form along the edge of the continental shelf and, in some places, the outer edge is so suitable for coral growth that the reefs grow high and form narrow walls.
This almost continuous barrier is only broken by passages between the individual reefs. Some of these channels, however, are deep enough for ships to pass though.