The sea's boundary is God's command

The flowing of the waters and the sea's confinement within its boundaries both come from that first legislation: "Let the waters be gathered into one gathering" (Genesis 1:9). The water that kept flowing in would have overflowed the places that received it, always moving on and filling one place after another, until it flooded the whole continent in an unbroken sweep. So it was commanded to be gathered into one gathering. That is why the sea, even when it rages under the winds and its waves rear up to an enormous height, the moment it touches the shore, dissolves its force into foam and turns back. "Will you not fear me, says the Lord, who set the sand as a boundary for the sea?" (Jeremiah 5:22). The sea, whose violence cannot be withstood, is bridled by the weakest thing of all—sand. What else kept the Red Sea from sweeping over all Egypt, which lies lower than the sea itself, and joining the ocean that borders Egypt, if it was not bound by the command of its Creator? That Egypt is lower than the Red Sea has been proved to us in practice by those who wanted to join the two seas to each other—the Mediterranean and the Indian Ocean, in which the Red Sea lies. That is why they abandoned the attempt: first Sesostris the Egyptian, who began it, and then Darius the Mede, who later wanted to carry it through. I have said all this so that we may understand the power of the command: "Let the waters be gathered into one gathering." That is, let no other gathering be produced from this one, but let what is gathered remain in that first collection.

Then the one who said that the waters should be gathered into one gathering showed you that the waters had been many, divided among many places. The hollows of the mountains, split by deep ravines, held their own collections of water. And in addition, many broad and level plains, no smaller in extent than the greatest seas, and countless valleys and basins hollowed out in every kind of shape—all of them, filled with water at that time, were emptied by the divine command, as the water from every direction was driven together into one gathering. And let no one say: if there was water above the earth, then surely all the hollows that now hold the sea must already have been full. Where, then, were the gatherings of the waters going to form, if the hollows were already occupied? To this we will answer that the vessels were prepared at that very time, when the water had to be set apart into one mass. For the sea beyond Cadiz did not yet exist, nor that great ocean untried by sailors, which embraces the island of Britain and the western Iberians. But at that time, when the wide expanse was created by God's command, the masses of water were gathered together into it.

As for the objection that our account of the world's making contradicts experience—since not all the water appears to have run together into one gathering—there is much to say, and it is obvious to everyone. Perhaps it is even ridiculous to argue against such objections. Surely they do not mean to bring up marsh water and rainwater collected in pools, and think they are refuting our argument with these? No, he called the greatest and most complete running together of the waters "one gathering."