Fjallsárlón or Fjallsarlon is actually a very young natural phenomenon and does not have a long history in terms of being a lagoon. Here is a short history lesson on this area and facts about Fjallsarlon.
When Iceland settlement started in 874 it was said that the land was all grown with trees between the ocean and the mountains. The glaciers in Iceland were at the time also much smaller than they are today. During the first centuries after Iceland’s settlement there were 3 big farms in the area of Breiðamerkursandur the area where you now have Fjallsarlon and Jokulsarlon. Neither lagoon existed. But in the area was a big forest called Breiðamörk (Breidamork). It means broad forest.
The farm on the east side of this area was called Fell, on the west side the farm was called Fjall, and in the middle was the third farm with same name as the forest and called Breiðamörk.