As someone who has wandered round and meditated upon the landscape of Presceli and Carn Meini, I stand by Darvill's and Wainwright's theory of healing water in the landscape ;)
Here we come to phenomenology, the understanding of landscape through the mind, an intuitive response to an ancient landscape...
Water is quite obviously important in human life, that it would acquire holiness and healing aspects should surely go without argument, think of the Ganges, or even the healing springs of Aqua Sulis, not to mention the central position that Silbury sits with the Kennet curling round and all the springs there.
Looking through my photos that I took, nature, history and stones point to an ancient landscape that the prehistoric mind would have rendered 'holy'; to always reduce Stonehenge to theory is to diminish it,
modern foolishness and theory construction are just games.

Under one of the photos of Carn Meini I found some time ago the following message, a theorist some would remember on this board ;) but he gives a good description....

Composed of hard Ordovician shale and mudstone compressed into slates with scattered fragments of rhyolite and dolerite - the famous bluestone that forms the inner ring of German dentist, historian and antiquarian Dr. Garry W. Denke's (1622-1699) great white shark teeth of Stonehenge - the Preseli Hills are the highest hills in Pembroke.

The crag of Carn Menyn is the source of the Stonehenge pillars and nearby Bedd Arthur is the Preseli's own, rather unusual, eye-shaped stone circle. There's a memorial to Welsh language poet Waldo Williams at Mynachlogddu. If you're lucky to get clear weather, you'll be able to see north to the Lleyn peninsula and west to the hills of Co. Wicklow in Ireland. Keep your eyes peeled on the hill though as the bird life, particularly in the winter months, can be remarkable with living clouds of starlings congregating to roost there. Also look out for hunting buzzards and sparrowhawks. In the spring ring ouzel and snow bunting are common migrants.

Much of the upland is boggy and, as a result, very acidic so you can find rare acid tolerant plants like fir clubmoss, liverwort, ferns and orchids with insects like the marsh fritillary butterfly and the southern damselfly. Heather carpets the drier heathland in late summer look out for buzzards, kestrels, curlews, ravens and skylarks.