Crossposted from r/asoiaf, all credits to u/wildrussy
We've all heard the tales, and all the Maesters agree.
There was once a great land bridge that connected the "heel" of Essos to Dorne, called the Arm of Dorne. The first men crossed this land bridge into Dorne, and from there migrated into the rest of Westeros.
The old tales say the first men were led by a great King, sometimes called the First King, sometimes called High King of the first men, Garth.
But these tales must be folly, for they contradict themselves. They say that everywhere Garth walked, the land bloomed, but Dorne is famously barren. Dornish tales make no mention of the Greenhand at all. It seems that Garth never set foot in Dorne.
More mysteries abound, because the oldest runic records of the First Men appear in the Stormlands, leading Maesters to believe that the First Men must have taken one look at Dorne and kept walking. This seems plausible enough, but something struck me while looking at a map of the Broken Arm:
Here we can see where the Arm of Dorne once was quite clearly, but there's an unwelcome intruder (or rather, protruder) in our midst. Cape Wrath extends eerily close to the Arm of Dorne; the islands off its coast even look like they're islands in the same chain. Could it be that Cape Wrath was once connected to Essos as well?
It was in the process of investigating this that I came across this gem in the World of Ice and Fire:
There is also much to suggest that the Sea of Dorne was once an inland freshwater sea, fed by mountain streams and much smaller than it is today, until the narrow sea burst its bounds and drowned the salt marshes that lay between. - The World of Ice and Fire - Dorne: The Breaking
If you'll redirect your attention to the map above, you'll see that in order for the Sea of Dorne to be an inland sea, Cape Wrath would have had to connect to the Arm.
It is absolutely incredible to me how well hidden this crucial piece of information is. Yandel drones on and on about how the Hammer of the Waters isn't real and the Arm sank due to climate change. He practically puts you to sleep before he offhandedly mentions "Oh, by the way, there's evidence to suggest the Stormlands connected to Essos". Very sneaky, George, but I've CAUGHT you. You can't get anything past this old weasel. I've got a problem with compulsive fixations and too much fucking time on my hands and I worked it out, you sly old goblin.
Anyway, I have no earthly idea what the significance of this is. It seems to indicate that the Garth legends could be true. Beyond that, I have no clue why George was such a sneaky little bandit about how the Arm of Dorne was a "Y" shape and the First Men crossed into the Stormlands directly.
I think theorizing about these books might be driving away my last scraps of sanity.