Physiographically, the Bermuda platform is an atoll, in the sense that it has a peripheral reef and island development enclosing a central lagoon. On the north side, the peripheral reef is an irregular ledge of massive coral (The Ledge Flats) lying at depths of 2 to 6 meters. It is thus not comparable to the barriers of the Pacific atolls, especially since the only representatives of a coralline algal ridge are a few small, circular, algal cup reefs (called boilers and breakers) which are exposed at low tide. On the south side of the lagoon, the Bermuda islands, composed primarily of calcareous eolianite deposits, form an effective lagoonal barrier. In addition, there is a line of algal cup reefs which form another barrier a few hundred to a few thousand meters of Bermuda's southern shore.