Mount's Bay

Between Cornwall's 'toe' at Land's End and its 'heel' at the Lizard lies the magnificent sweep of coastline of Mount's Bay.

The interesting shoreline of coves, reefs and sandy beaches was formed by the selective erosion of different types of rock.

On this page you are guided to these rocks on a walk from Lamorna Cove (1) to Marazion (8).

The Holiday Geology Guide, available from the BGS Bookshop, gives more details, including a guide to the coast between Praa Sands and Porthleven.

Geological map of Mount's Bay
Lamorna Cove Mousehole Penlee Wherry Longrock foreshore Great Hogus St Michael's Mount Marazion foreshore

  1. Lamorna Cove
    Lamorna Cove: granite quarries by the sea. South-west of Penzance, a charming wooded valley leads down to Lamorna Cove, which once echoed with the din of granite quarrying operations.
  2. Mousehole
    Mousehole: named after the granite cave Mousehole ('Mowzul') is a typical Cornish fishing village. It is situated on the contact between the granite and the surrounding slaty rocks.
  3. Penlee
    Penlee: magmas from the deep. Much of the foreshore consists of purplish brown spotted slate but the more massive greenish-coloured rock has a volcanic origin. It was a basaltic magma which was squeezed into fractures in the Earth's crust.
  4. Wherry
    Wherry: where tin miners worked under the sea. One of the most ingenious and audacious mining enterprises undertaken in Cornwall was carried out on a reef between Penzance and Newlyn, but little evidence remains. A miner from Breage, Thomas Curtis, became interested in attempts to work rich veins of tin ore (cassiterite) from these rocks and in 1778 constructed a wooden shaft, caulked with pitch, and sealed on the rock of the sea floor.
  5. Longrock foreshore
    Longrock foreshore: an ancient submerged forest. On theforeshore between Penzance and Marazion you may be lucky enough to see a submerged forest. During glacial periods, when sea-level was low, trees grew on what had been sea floor surrounding St Michael's Mount.
  6. Great Hogus
    Great Hogus: scene of an ancient underwater volcano. The low reefs of Great Hogus consist of volcanic agglomerate, a rock made up of chunks of basaltic lava, which were explosively fragmented during a violent submarine eruption.
  7. St Michael's Mount: the granite isle. St Michael's Mount has defied erosion in comparison to the surrounding softer sedimentary rocks. A small, roughly circular intrusion of granite forms the Mount and the southern foreshore. It is similar to other Cornish granite instrusions and, although considerably smaller, is equally tough and resistant. Good examples of mineral veins can be seen on the southern foreshores but only with prior permission from: The Manager, St Michael's Mount, Manor Office, Marazion, Cornwall, TR17 0EL.
  8. Marazion foreshore
    Marazion foreshore: folded rocks. On the foreshore, just east of Marazion, you will see typical sedimentary rocks such as mudstone and sandstone in which the original sedimentary layers or beds have been crumpled and deformed by folding into complex patterns by the energy of the colliding continents.