Confronted with the claim of a Norse origin for some of the stone holes, my first question is "how do you know the holes were made by the Norse?" You can't directly date a hole in stone, after all, and there are several possible reasons for creating holes in stones in the first place. The leading "mainstream" explanation for most of the holes, as described in this 1998 article by Tom Trow, is that they were created for the purposes of blasting the rocks apart so the pieces could be used for the foundations of buildings. Trow (pp. 127-128) quotes an older resident's description of how and why the stone holes were created, by hand, with iron or steel chisels:
"Gee, whiz, I had to crank the grindstone for ’em to sharpen them chisels. You know, you had a chisel this long [showing about a foot and a half]. And then it was about as big as your finger. And then it was sharp, you know, sharpened, and then you took it like this and then you held it on the stone and then you gave it a crack and, you know, they were experts at turning it, see? And then they turn it, and then they gave it another crack, and that’s the way, after a while, the chisel went down in the stone. And then they dug the scrap out and they kept on drilling until it was about this deep [showing about eight inches]. Then they put powder. . . . in there, black powder, and then if it was a big stone you’d have to make another hole over here and another hole over there. And then they set fire to it and blasted it. And it cracks nice."
Trow's informant (Emil Mattson) was born in 1897, and therefore would have been describing creating stone holes using hand chisels in the early 1900's.
If I understand Voyles correctly, he claims that one can differentiate stone holes created in the 1800's and 1900's from those created by the Norse because of differences in (1) weathering and (2) form. With respect to weathering, Voyles states that the edges of what he interprets as medieval stone holes show significantly more weathering on their edges than those that were obviously made by modern drills. On this page he shows what he describes as aging differences between a "modern" stone hole and one created by the Norse.
"The medieval stoneholes are slightly triangular because it is not possible to make a perfectly round stonehole with a hand chisel. Later modern drilling could make perfectly round holes, and this is how old and new can be distinguished from one another."
I think Voyles is correct that are groups of holes made using different techniques. My issue comes with equating "triangular" or "hand-chiseled" with "medieval." A little searching online turned up some descriptions of early American stone quarrying methods that are useful, I think. The first quote is from the 2005 book The Art of Splitting Stone: Early Rock Quarrying Methods in Pre-industrial New England 1630-1825 (Mary Gage and James Gage) in a section titled "Hammer Drilling or Triangular Hole Method" (page 48):
". . .Triangular holes are documented at archaeological sites in Massachusetts, New Hampshire, Delaware, and Great Britain. Archaeological, historical, and experimental evidence has demonstrated that the triangular holes were cut with a straight edge chisel being rotating approximately 60 degrees between hammer strikes. These holes come in a wide variety of diameters and depths.
Frederick Pohl in his book The Lost Discovery has documented and researched several triangular hole sites found in stone outcroppings along the tidal waters and bays on Cape Cod . . . Mr. Pohl felt a straight-edged chisel rather than a plug or star drill was used to drill/cut the triangular hole. To verify his theory, he conducted an experiment. He and two other people, "taking turns leisurely cut a hole 1 1/4 inches deep in five minutes. We found we could not make a round hole with a straight-edged chisel. All our attempts resulted in triangular holes with the corners rounded.""
Gage and Gage point to other interesting accounts of using hand chisels to create stone holes. A 1904 book by Halbert Powers Gillette titled Rock Excavation: Methods and Cost is quoted in this report by James Gage:
"Hammer Drilling. – The common weight of hammer for one-hand drilling is 4 ½ lbs; for two-hand or three-hand drilling 10 lbs. The striking face must be flat or slightly rounding, and smaller than the stock of the hammer. The hole is started on a solid and squared surface, with a short drill, for the longer the drill the less effective the blow. Light blows are struck at first. The bit is turned one-eighth of a revolution after each blow to insure keeping the hole truly circular. But in spite of this precaution most hand-drilled holes are three-cornered, or “rifled.”"
Based on the fact that we've got multiple accounts demonstrating that hand chisels were commonly being used to produce holes in rocks in the northern United States at least into the early 1900's (including in Minnesota), and that such methods typically produced triangular holes . . . on what basis could one claim that some triangular/irregular stone holes were created by the medieval Norse? What is the positive evidence? How could you separate out the Norse holes from all the others that we know were being created?
As Voyles suggests, weathering could be a way to go. If similar tools were used to create similar holes, there's still at least the theoretical possibility that one could differentiate stone holes created in the 14th century from those created in the 19th or 20th centuries by looking at the weathering of exposed surfaces. As Voyles stated in his comments on my blog about the Sauk Lake Altar Rock, "multiple hand-chiseled holes in this rock can be compared to the mineral or mica decomposition of the late 1800's stoneholes to see that they are extremely aged by comparison." I think there's potentially a significant difficulty here, however, if we depend on macroscopically-observable "weathering" to assign relative age. The descriptions of steel chisels "dancing" around in the holes as they're created suggests to me that the edges of hand-chiseled holes might be battered as they are created (rather than smoothed gradually by mechanical weathering). To my eye, for example, the triangular hole shown by Gage and Gage in Figure 20 (page 49) appears to have a battered/smoothed lip. Perhaps there would be a way to look at some microscopic characteristics of the stone and say something about aging. You'd have to have good controls for that, however. If I were trying to build a case that some of these stone holes were created by medieval Norse, I'd start working on that aspect and looking for other ways to differentiate them from modern holes.
That's my two cents on stone holes for today.