They are probably natural, but along the Avenue there is another large triangulate stone opposite its male counterpart that also has 2 large cupmarks nearer ground level. Its in a photo taken by Jaquetta Hawkes, the old b/w photos always shows up more interesting detail that digitals never do.
And, very rare for down south, but we have a megalithic stone with concentric circles, see Rhiannon's link at Knowlton Henges, it was found in the south henge. Plus, of course the "footsteps" stone found up on the Mendips at Pool Farm.