Ambiguous wear pattern on a granite erratic.

slowburner · 13-09-2013 12:44pm #1

I would appreciate opinions on this subject. I'm wearing both moderator hats on this one.
My feeling is that this is a quern - used for grinding grain over a prolonged period.
This relatively large (c. 3.5 x 2m) erratic granite boulder has been moved from its unknown original position to the edge of a field. The face which would have lain uppermost is now 90º to its original situation.
It may not be easy to see but there is a marked depression (c. 60cm diam) at the left side of the boulder.
The presumably glacial striations have clearly been interrupted by the formation of the saddle.
I can't imagine any natural process that could cause such a depression - hence the reason for posting here.
The first photo is of the boulder face with the anomalous depression to the left.
The second picture is of the underside of the stone (before it was moved) and the third picture is shown to indicate the curvature of the depression which is consistent with wear patterns on quern stones.

slowburner · 13-09-2013 9:16pm

Rereading the post above, I may not have made the question terribly clear.
It is this: what natural process could be responsible for creating a roughly circular depression that is notably smoother than the surface on the surrounding stone?
Water pooling on granite for a prolonged period causes fragmentation, so water can be ruled out of the equation.

Ambiguous wear pattern on a granite erratic.

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