Simple as that.
It was all forest once, in the whole country, at least by the water. Even here in Hvalfjörður, it was trees. But the trees were cleared to make pastures and to keep the Icelanders warm, and then there were no trees, and so it remains in most of the country. Because of this history…,
… horses are now trees. Stick a pale of hay in the middle of a forest clearcut 1000 years ago, and there you have it, a grove. As I’ve said. before, in this country everyone is an artist.
Icelandic horses came over with the first settlers. They know a thing or two. Here in Eiðar, you can see the technique for getting at the good stuff: you can strrrrrrretch that border, but you never, technically, really, for honest and for true, cross the line.
Also in Eiðar, you can see just how flexible this rule is below. The Icelander on the left has one hoof back behind the line, and the one on the right has the line running right down his midline.