[crystalk]

So maybe they even had stones cut to this size and shape, to fit the boat hull...I wonder if anyone has ever analyzed the stone to see if it might be from Europe, Greenland, or Iceland...but it was always said to be not from the local outcropping on the hill (now called Runestone Hill) where it was found...

[TexanToTheCore]

Most likely the stones would be from the entry point on the northern coast. If someone cares to dig around the hill they may find other ballast stones, as they were frequently dumped when not needed. It may also be an indication that there is the ruins of a boat nearby.
By the way, this method of keeping a saling boat upright is still used, I know of several designs that are very shallow and are kept upright by pouring concrete with rebar into the hold. It eliminates a centerboard or keel. I think the last one I saw was a "Friendhsip Sloop", 18 feet and a beauty from stem to stern, with very little draught.