of bay dune systems which develop where there is a limited supply of sand trapped within the shelter of two rocky headlands. Church and Poldhu Coves are SSSI and also have associated climbing dunes which occur when sand is blown inland of the main dune system. Evidence of higher sea-levels in the past can be seen at Marazion where the town is built on a raised beach. A second example is the road between Newlyn and Mousehole. Sea levels rise and fall as the ice sheets advance and retreat, and raised beaches now mark the interglacial periods when sea levels were higher. Gwavas Lake is an area of relatively calm water that is situated outside the current harbour area of Newlyn. Post-ice age Offshore surveys have found submerged erosional plains and valleys containing deposits of peat, sand and gravel. The deposits indicate cyclical changes from wetland, to coastal forest, to brackish conditions have been occurring over the past 12,000 years as sea levels rose. With the melting of ice-sheets and glaciers after the last ice age, sea levels reached their present levels about 6,000 years ago during what is known as the Flandrian Marine Trangression. Either side of Penzance, on the beaches at Ponsandane and Wherrytown, evidence of a 'submerged forest' can sometimes be seen at low tide in the form of several partially fossilised tree trunks. Divers and trawlers also find submerged tree trunks across Mount's Bay and the forest may have covered a coastal plain 2 to 5 kilometres further south than today. The samples of peat and wood around Penzance have been radiocarbon dated and indicate that the forest was growing from at least 6,000 to around 4,000 years ago when rising sea levels finally killed the trees. Artefacts dating from the Mesolithic (10,000 to 5,000 BCE) have been found indicating some occupation contemporary with the forest. Marshes formed and were overlain by sand, gravel and by sand dunes which formed natural barriers to the sea. Storms sometimes destroyed the barriers depositing sand and gravel over peat beds in Marazion Marsh, and in the foundations of many buildings in Wherrytown and Long Rock. The remains of these natural barriers can still be seen at Eastern Green and the dunes to the seaward of Marazion Marsh. The submerged forest in the intertidal area between Wherrytown and Long Rock is of national importance and is a County Geology Site.