Let's look at stone quantities. Each of the burial cairns has a diameter of c 20m and a height of c 4 m. Reduce to a box of dimensions 15 m x 15 m x 3 m. The volume of stone contained in each one is 675 sq m, giving a solid rock weight of 1822 tonnes. Allow for "open work" air spaces occupying 20% of each mound, and the weight of stone contained in each mound becomes c 1458 tonnes. Using a figure of 2.7 tonnes per cubic metre, the total weight of stone in the three mounds is therefore approx 4,374 tonnes.
Add to that the weight of stone contained within the defensive mounds. The total length of the defensive embankments is between 950 m and 1000 m. The embankments vary quite substantially in their cross-sections, but let's take an intelligent guess at a base 5 m wide and a height of 3 m. Reduce to a box of dimensions 1000 m x 2 m x 2 m. That gives a solid rock volume of 4,000 sq m and a weight of 10,800 tonnes. Reduce by 10% to allow for incorporation of earth and rubble -- and we arrive at a total of 9,720 tonnes for the embankments.
So our grand total for stone required in the construction of this site is around 14,094 tonnes. That's a lot of stone. Where did it come from? It's pretty obvious -- from the on-site quarries on the hilltop. Nobody with any sense would want to carry stone uphill from the surrounding lower moorlands anyway, and economy of effort must have been just as important in prehistoric times as it is today.
So what is the physical evidence of prehistoric quarrying on the site? There is actually a great deal, and I have been up there today to have a look.