OK so enough myth, legend and history, it's time for some science...
So why were these lands inundated? What changed?
The sea did.
In times gone by, namely the ice ages, much of the water of the world was locked up in the ice sheets of the far North and South. Winds carried water vapour north where it fell as snow onto the glaciers. Over the years more and more fell and it eventually compacted into ice. This ice didn't melt, and if and when it did it wasn't enough to shrink the constantly replenished ice. Because ice is white it reflected light and heat back into space so it was very hard, once started, to stop the ice sheets growing and growing.
More and more of the world's water became locked in ice and the oceans shrank, land that is now far below sea level would have been above sea level during glacial periods. A lovely interactive map here, shows here the presence of ice across Britain and the changing sea line. 20 thousand years ago Britain was connected to Europe all along the South coast and up much of the East coast across to Scandinavia. In the west, Ireland was linked to Scotland, Wales and Cornwall. It wasn't until 9 to 10 thousand years ago that enough ice melted to raise sea level enough to make Britain an island.
The ice had another effect on Britain. Ice sheets are heavy and the weight of it pushed Scotland and the North down. Britain tipped like a set of scales and the far south of England was raised up. This took a long time to happen but raised beaches can be seen in many places around a Cornwall. They are odd rock shelves at the back of a beach and clearly show that once the sea was much higher with respect to the land.
Of course this ice is gone now and the scales are still adjusting. The South is sinking and the North is regaining it's previous heights. At the moment London is sinking by about 3 mm a year.
Evidence that Cornwall used to extend further out to sea is everywhere. If you know what to look for, it is painted on every map. Cornwall has some large tidal rivers, in particular the Fal, Fowey and Helford. The rivers that feed into the estuaries are tiny in comparison to the estuaries. The thing is these rivers are actually rias or flooded river valleys. As sea level rose it came up the valleys between the hills.
I love these stretches of river and I once took a kayaking course on the Fal. Exploring little inlets with heavily wooded shores was a joy. Many of Cornwall's most famous gardens lie on the shores of these rivers such as Trelissick, Tregothnan, Glendurgan and Trebah. They also featured in Daphne Du Maurier's novels in particular Frenchman's Creek.
So how far did the ice get? Well it covered the Welsh mountains and reached the coast of Cornwall and even reached the edge of the Scillies. Deposits of glacial head can be found in many places along the North coast such as at Godrevy. Glaciers are not clean pieces of ice, they pick up everything loose iin their path. They transport bits of rock and rubble over long distances and then when they melt and dump it. The head is rough bits of unsorted rock all jumbled together with silts, clays, gravels and anything else the glacier left behind.
The Irish sea Glacier would have sat on dry land and at this time the sea level would have been 130 metres lower than at the present time. Cornwall was mostly ice free however and would likely have been inhabited by some hardy Ice Age folk. Maybe they wondered about all the land lost under the ice. Maybe they had legends of cities the ice ate but didn't quite believe them.
As more ice melts in the Artic and Antartic the sea level will continue to rise and the coast line of Cornwall will to change. More land will be lost.
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