Complex sentence: Sand cays are formed when tidal action, wind and sea birds deposits, over a long period ot time, coral debris and sand on to reef flats, usually on the leeward side of the reef, but occaisonally on the windward side.
Split into simpler sentences: Sand cays are formed when tidal action, wind and sea birds deposit coral debris and sand onto reef flats over a long period of time. These deposits usually occur on the leeward side of the reef, but occasionally made on the windward side.