The Grey Havens - Middle-earth: What exactly happened at the end of the First Age?

What exactly happened at the end of the First Age?

From: The Tolkien Less FAQ by William D.B. Loos

The Noldorin Elves had made war on Morgoth (referred to as "the Great Enemy" by Aragorn in "A Knife in the Dark") to recover the three Silmarils, which he had stolen, and had been totally defeated. The Valar then used their full power against Morgoth. In the resulting cataclysm Beleriand, the land in which the tales of the The Silmarillion took place, was destroyed and sank under the Sea. There are thus various references to "lands under the waves".

On the The Lord of the Rings map, Beleriand would have been far to the west, beyond the Blue Mountains (Ered Luin), which also appear at the far right of the The Silmarillion map. It is difficult to make an exact correlation because the mountain range was much altered, having been split when the Gulf of Lune created. Nogrod and Belegost, the ancient dwarf-cities, are located on the The Silmarillion map, and existed as ruins in the Third Age, but where they fall on the The Lord of the Rings map is not known (they were said to be "near Nenuail", which is only slightly helpful). Lindon was definitely the same land as Ossiriand, where Beren and Luthien once dwelt. [The Atlas of Middle-earth includes a map showing how Eriador and Beleriand lay relative to each other.]

Here you can find informations concerning Middle-earth. Actually, I didn't know where to put this pages, so here they are.