the other 1500 cairns were probably destroyed by the quarrying. I particularly liked the elliptical circle that Dave found, the builders must have been aware of the workings of the solar system, with the larger stones representing the planets ;^)
You wouldn't be ripping the piss there would you!? There were at least 15,000 cairns up there - though many people might say I was a bit screwy. But you've gotta admit it was weird that the cairns were all shaped like burial mounds - something truly significant about that I reckon.