The eastward flow of the Thames being at first checked, and finally almost or quite stopped by the formation of these banks, the water turned backwards as it were, and began to cover hitherto dry land.  And this, with the other lesser rivers and brooks that no longer had any ultimate outlet, accounts for the Lake, so far as this side of the country is concerned.

At the western extremity the waters also contract between the steep cliffs called the Red Rocks, near to which once existed the city of Bristol.  Now the Welsh say, and the tradition of those who dwell in that part of the country bears them out, that in the time of the old world the River Severn flowed past the same spot, but not between these cliffs.  The great river Severn coming down from the north, with England on one bank and Wales upon the other, entered the sea, widening out as it did so.  Just before it reached the sea, another lesser river, called the Avon, the upper part of which is still there, joined it passing through this cleft in the rocks.

But when the days of the old world ended in the twilight of the ancients, as the salt ocean fell back and its level became lower, vast sandbanks were disclosed, which presently extended across the most part of the Severn river.  Others, indeed, think that the salt ocean did not sink, but that the land instead was lifted higher.  Then they say that the waves threw up an immense quantity of shingle and sand, and that thus these banks were formed.  All that we know with certainty, however, is, that across the estuary of the Severn there rose a broad barrier of beach, which grew wider with the years, and still increases westwards.  It is as if the ocean churned up its floor and cast it forth upon the strand.

Now when the Severn was thus stayed yet more effectually than the Thames, in the first place it also flowed backwards as it were, till its overflow mingled with the reflux of the Thames.  Thus the inland sea of fresh water was formed; though Silvester hints (what is most improbable) that the level of the land sank and formed a basin.  After a time, when the waters had risen high enough, since all water must have an outlet somewhere, the Lake, passing over the green country behind the Red Rocks, came pouring through the channel of the Avon.

Then, farther down, it rose over the banks which were lowest there, and thus found its way over a dam into the sea.  Now when the tide of the ocean is at its ebb, the waters of the Lake rush over these banks with so furious a current that no vessel can either go down or come up.  If they attempted to go down, they would be swamped by the meeting of the waves; if they attempted to come up, the strongest gale that blows could not force them against the stream.  As the tide gradually returns, however, the level of the ocean rises to the level of the Lake, the outward flow of water ceases, and there is even a partial inward flow of the tide which, at its highest, reaches to the Red Rocks.  At this state of the tide, which happens twice in a day and night, vessels can enter or go forth.