No More Learning

I make this inquiry rather :
If there were any reason why, before the outlet was opened at Byzantium, the bed of the Eux
ine (being deeper than either that of the Propontis or of the adjoining sea) should not gradually have become more shallow by the deposit of the rivers which flow into it, allowing it           either to have been a sea, or merely a vast lake greater than the Palus Meeotis?