You'll be mist

As fog blankets the Midlands, even unto Nottingham and beyond, I ponder why the old stone circles went out of use.

There are many theories as to why civilisations change. When we had an Empire, it was all about invasion hypothesis - that each race (Beaker People, Celts, Romans, Angles, Vikings) was superseded by the next, better-equipped, conquering people. But in this allegedly gentler age, that has been in its turn superseded by the idea that the people basically stayed where they were, and technologies and habits were acquired by communication and copying rather than conquest - a kind of Iron-Age liberal interventionism, if you will.
But for myself, I reckon that what really tended to happen  was that the menfolk would invade, kill or enslave the pre-existing menfolk, and take the women along with the farms and other possessions. It's not nice, but it seems to fit in with our human condition. And  male behaviour.

But back to the Stone Circles. I reckon it wasn't that a new Celtic belief system that came in, removing the stone circles in a kind of Cornish iconoclasm. Nor that the Celts wiped out the Beaker People. I reckon it was simple. Stone circles have solar alignments - you get together at midsummer or midwinter and watch the sun rise. But imagine if every solstice was overcast or foggy. The "druids" and neo-pagans look miserable enough on our tellies when they get just one. After a century or two of the disappointment of no-shows from the sun, you'd go off and worship in some druidic groves and leave the stones to the bat and the hoopoe. "Coming to the Solstice, Myfanwy?" "Nah, Eileen - I reckon I'll go off to them groves and learn to be Welsh."