The sequence of dunes is often interrupted by the appearance of slacks or troughs in between the dune ridges where the water table comes close to the surface. In these places the plant species are once again adapted to the particular conditions. There are two types of slacks according to the average level and seasonal changes in the water table: dry slacks where plants such as sea holly um maritimum) and sea. spurge(Euphorbia paralias) colonise and wet slacks where plants such as creeping willow(Salix rapens) and bog myrtle(Myrica gale) thrive. The other major disruption to the normal psammosere sequence is the occurrence of blow outs where an individual dune has