Mayburgh henge immediately comes to mind; as you know, said to built from millions of rounded stones taken from the River Eden.

On a field trip to Gardom's Edge with John Barnatt, we visited a cairn which he had previously excavated. Every stone within the cairn was an unbroken, rounded stone. At the Gardom's Edge enclosure, we saw burnt stones lying about. They were all rounded.

And no, I haven't mentioned the 'R' word.

:o)

Baz