Yongle Islands(atoll)is situated in the west of Xisha Archipelago of the South China Sea.It consist of seven bigger separated reefs.The central lagoon of theatoll is about 40min depth.These reef-flats appear a similar in geomorphic features

and can be divided into several zones

including outer-slope

outer-crest

outer-flat

island(sand

gravel or rock island)

inner-flat

inner-slope.Each zone has its own organisms and sediment features.Two lines of beachrocks occur outside the Quanfu Island.This indicatesthat the sand island has moved three times at least.Shiyu Island

consisting of cemented rock

appears to be an anticline structure.Obviously

it is a deposi-ting anticline.