Mount's Bay () is a bay on the English Channel coast of Cornwall, England, stretching from the Lizard Point to Gwennap Head. In the north of the bay, near Marazion, is St Michael's Mount; the origin of name of the bay. In summer, it is a generally benign natural harbour. However, in winter, onshore gales present maritime risks, particularly for sailing ships. There are more than 150 known wrecks from the nineteenth century in the area. The eastern side of the bay centred around Marazion and St Michael's Mount was designated as a Marine Conservation Zone in January 2016.
Geography and geology
Mount's Bay is the biggest bay in Cornwall. Its half-moon shape is similar to that of Donegal Bay in Ireland and Cardigan Bay in Wales, although, unlike the aforementioned bays, Mount's Bay is relatively sheltered from the prevailing Atlantic westerlies. However, it is a danger to shipping during onshore southerly and south-easterly gales.
The coast is about from Lizard Point to Gwennap Head. Heading north and west from Lizard Point, the serpentine and hornblende schist cliffs reach a maximum height of at Vellan Head and are only broken by small streams and coves such as at Kynance, Gew-grade and Mullion Cove. After Gunwalloe Fishing Cove the cliffs have the softer look of Devonian Meneage Formations of greywacke and mélange, with erosion a problem either side of the naturally dammed ria of Loe Pool. West of Porthleven there are high Devonian slate and granite cliffs to Rinsey Head after which the cliffs are topped by Pleistocene periglacial head and have eroded to form sandy beaches such as those at Praa Sands and Kenneggy. These beaches are in deficit and the cliff line is retreating. With the exception of the harder Devonian dolerite and gabbro of Cudden Point, the low, eroding cliffs and beaches continue to Mousehole. This part of the bay is the most populated with the towns of Penzance and Marazion and the villages of Newlyn and Mousehole. Beyond Mousehole the granite cliffs, rise to 60 m, and are broken by small streams such as at Lamorna Cove and Penberth.
There are small sand dune systems at Church Cove and Poldhu Cove, Porthleven Sands, Praa Sands and from Marazion to Eastern Green, Penzance. The former sand dunes of the Western Green are now covered by Penzance promenade. All, but Marazion to Penzance, are examples 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.