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At the end of the last ice age, what are now the British Isles were joined to the European mainland as a mass of land extending north west from the modern-day northern coastline of France, Belgium and the Netherlands. Ice covered almost all of what is now Scotland, most of Ireland and Wales, and the hills of northern England. From 14,000 to 10,000 years ago, as the ice melted, sea levels rose separating Ireland from Great Britain and also creating the Isle of Man. About two to four millennia later, Great Britain became separated from the mainland. Britain probably became repopulated with people before the ice age ended and certainly before it became separated from the mainland. It is likely that Ireland became settled by sea after it had already become an island.

What happened to British Isles area when the ice melted after the last ice age?
sea levels rose