Climatic changes may have divided the New World from the Old.  Late in the Bronze Age the route was still open, if the report by Plato has any truth. But, if so, these visitors   to America were  traders and prospectors for tin. 

The period from about the start of the Common Era to the time of Columbus - with the exception of the Viking settlement in Newfoundland - could well be  the only time there has not been traffic from Europe to America. During the Dark Ages, it may be that new world sailors made landfall off the Irish coast from time to time, giving rise to golden-skinned, beardless visitors from the paradisial Islands in the West.

Early immigrants on the Northern route to the New World could  have dismantled their boats and trekked overland, or, if solitary adventurers, carried their coracles on their backs, and walking staves in their hands. Such world-wide travelers have been imagined as responsible for the carving of related petroglyphs, signs and symbols common to all continents, such as the basic maze, great snake, the "Birthing Goddess" glyph, the spiral, the fylfot. 

These symbols could be the signs of a people traveling outside of history, pursuing some vision quest in search of gods, for instance; perhaps in search of  the Tuatha De Danaan of Ireland, half of whom were supposed to have taken themselves off  to where the herds could be seen traversing the plains without interruption, without seeing the beginning or end of them, across the Plains of Joy in the Isle of the Blessed.

Admittedly, such a vision of paradise might be expected from a cattle - loving, pastoral people such as the Irish. Yet the description of the vast, endless herds of the otherworld in the Western Isle are very reminiscent of descriptions of the great bison herds that roamed  the plains of America.