There are a variety of different types of reefs. The reef ecosystem has never existed in fresh water. They are built upon the skeletal remains of earlier organisms. Reefs need not be made up of corals at all. The most well-known type of reef is a barrier reef.
Barrier reefs can be enormous.
They form parallel to coast lines. Other types include atolls, apron reefs and patch reefs. Atolls fringe submerged volcanoes. Apron reefs form on shallow undersea slopes, away from land masses. The coral bed at the Falls is a patch reef.