The Perky Ent is right. I also posed this question and got much the same answer. The whole of Beleriand sunk beneath the sea, but a few mountain chains and the area now known as Lindon are still above sea level. I know of a link that shows a pretty good draw up of it. Try
this .
The brown line is the coast of Beleriand, and the puke green one shows the coast after the breaking.
Hope that helps.