I do not think it can be the end of a supergrid because the trees they are deciduous. It is my experience that the forest which is about the rim of each supergrid is one that is coniferous.
Not all the outer grids are full grids so world border can be pretty much anything.
"Since all men count themselves righteous, and since
no righteous man raises his hand against the innocent,
a man need only strike another to make him evil."
many rivers start that way, so this is no sg border, but when looking on your minimap, you should see some sort of friction going thru, this would indicate some grid border.