Some hammocks are dominated by types of vegetation that grow in relation to the amount of water or type of soil present. The majority of hardwood hammocks create a thin poor soil covering the limestone called humus, made of decaying plant matter and moisture trapped by the structure of the trees. When peat forms the layer atop the limestone of a tree island, bayheads develop, dominated by bay trees such as sweetbay magnolia (Magnolia virginiana) and others like swamp holly (Ilex decidua), wax myrtle (Myrica cerifera), and cocoplum (Chrysobalanus icaco).[46] Willowheads, dominated by willow trees (Salix caroliniana), take hold where the hydroperiod is long, usually around solution or alligator holes, and may surround the holes, giving them a donut appearance from above.[47]