Coincidently, I spent yesterday up on Bodmin Moor looking at 19th century stone quarrying. Apart from it being a fantastic day weather wise with views as far as Exmoor and Lundy, we were shown a large rock that had been prepared to be cut for two mill stones.
The grooves were similar to the ones on your photo from Aberdeen and those used in Ireland and elsewhere. It is generally accepted that this was the way granite was cut here in the south west pre 1800. Grooves cut about 6 inches long and 2 or 3 inches deep and 3 to 4 inches across are known as wedge and groove and are thought to date back to Medieval times.
On Dartmoor commoners were allowed to take moorstones but not quarry, hence the number of stones lying around still with grooves in them.
I have read many versions about how stones were split using this technique...some mention filling the grooves with water that would then freeze and split the rock.....I feel it would need to be very cold to do that! The most plausible description I have come across is that wooden wedges were inserted into the grooves, water was then used to expand the wood overnight to split the stone. I would love to see this done as I do find it hard to believe wood could split granite.
Did they use the same technique in prehistoric times? I would have to say most Cornish menhirs and circles tend to be made up of stones that are the right shape for the job...they had plenty to choose from. If they were cutting them, would they not be a much more uniform shape?
I can't comment on other parts of the country, but I don't think the grooves I see down here in the West Country are prehistoric.
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