Here is a link to the 1st Age map by Fonstad.
 
1st Age Map
It shows the Inland Sea of Helcar in place just east of the Great Gulf, across the Straits of the World. It was my understanding that when Thangorodrim was broken, it would have caused a major geologic upheaval resulting in the partial drainage of the Inland Sea by the Great Gulf. The boxy mountain ranges which would be Mordor may have been formed during the volcanic processes caused by this event.