 The north coast of Cornwall falls on the Celtic Sea in the Atlantic Ocean, which also surrounds the Isles of Scilly, the south coast falls on the English Channel and the county is bounded by the River Tamar, forming the border with Devon, to the east.Cornish geology consists mainly of rocks from the Devonian and Carboniferous geological periods.Granite forms a large part of these, with mineralisations of tin, copper, lead and arsenic having been mined in the area.