Oho, whilst you might get a shallow cup out of granite or sarsen using a stone tool, I reckon it'd be very time consuming to get a tightly defined groove. Not impossible, but it would take a long while. Making a bullaun sized cup to serve as part of a trilithon is one thing, belting a shallow cup on a recumbent is another, but neither of these would get you the kind of marks shown on the pics Slumpy has posted. That's my take on it anyway.