High marsh is a tidal marsh zone located above the Mean Highwater Mark (MHW) which, in contrast to the low marsh zone, is inundated infrequently during periods of extreme high tide and storm surge associated with coastal storms. The high marsh is the intermittent zone between the low marsh and the uplands, an entirely terrestrial area rarely flooded during events of extreme tidal action caused by severe coastal storms. The high marsh is distinguished from the low marsh by its sandy soil and higher elevation. The elevation of the high marsh allows this zone to be covered by the high tide for no more than an hour a day. With the soil exposed to air for long periods of time, evaporation occurs, leading to high salinity levels, up to four times that of sea water. Areas of extremely high salinity prohibit plant growth all together. These barren sandy areas are known as "salt pans". Some cordgrass plants do survive here, but are stunted and do not reach their full size.