Hi, Littlestone,
I think that Silbury is as high as it is, and has a flat top because it needs to be seen from all approaches, and must be known to be constructed. Approaching it from the ground, it sits squat and hard; welcoming and aloof all in the one eyeful. Only when you get up onto the Ridgeway does it loose its majority. To be viewed from there, it would need something to make it stand out from those lovely rolling Wiltshire hills; thus, the flatness of its constructed bonce leaves one in no doubt that it was designed rather than naturally occuring, and its height puts it there to be viewed from this part of the landscape. (at least, that's my belief).
Peace
Pilgrim
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