Over the years, looking out for cup-and-ring marked rocks in Argyll, I've found a number of these deep, bowl shaped depressions in boulders or bed-rock. All are next to long-standing farmsteads, and I have just recently found out what they are: Knocking Stones.
Knocking stone, Balnakailly, Bute.
Basically it's a large mortar used for crushing grains. The accompanying pestle would be a heavy wooden stick called a mell. I haven't found much information on how knocking stones relate to querns - what were their relative merits? Would a household with a functioning quern ever find a use for a knocking stone?
Most knocking stones would have been large - but movable - rocks with bowls ground out of them, and these would have been disposed of when newer, easier methods of processing grain came in. It is only those few that were opportunistically shaped out of conveniently at-hand boulders and bedrock that have remained in situ.
A 'portable' knocking stone that has been re-purposed as a flower pot.