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. The Irish ships, of which I have spoken, thus come into the Lake, waiting outside the bar till the tide lifts them over. The Irish ships, being built to traverse the ocean from their country, are large and stout and well manned, carrying from thirty to fifty men. The Welsh ships, which come down from that inlet of the Lake which follows the ancient course of the Severn, are much smaller and lighter, as not being required to withstand the heavy seas. They carry but fifteen or twenty men each, but then they are more numerous. The Irish ships, on account of their size and draught, in sailing about the sweet waters, cannot always haul on shore at night, nor follow the course of the ships of burden between the fringe of islands and the strand. They have often to stay in the outer and deeper waters; but the Welsh boats come in easily at all parts of the coast, so that no place is safe against them. The Welsh have ever been most jealous of the Severn, and will on no account permit so much as a canoe to enter it. So that whether it be a narrow creek, or whether there be wide reaches, or what the shores may be like, we are ignorant. And this is all that is with certainty known concerning the origin of the inland sea of sweet water, excluding all that superstition and speculation have advanced, and setting down nothing but ascertained facts.