The quest to fully unlock Stonehenge's origins is an ongoing one, and in a lengthy piece for the New Yorker, Simon Akam recaps some of the current research. His piece focuses largely on the bluestones that make up the monument. The sarsens are the behemoths; the bluestones are the smaller ones that stand among them. They're also the more interesting ones, in terms of their petrography—the science that involves tracking down the origins of rocks. While the sarsens are pretty similar to sarsens all over southern England, and have largely been traced to a spot about 15 miles away, the bluestones are much more challenging to place—and have long been thought to come from a staggering distance away, perhaps as far as 140 miles.
For the last 99 years, geologists have been trying "in earnest" to do just that, with Herbert Henry Thomas in 1923 suggesting the bluestones hailed from the Preseli Hills in west Wales, flagging Carn Alw in particular. Where we've gotten after nearly a century of research: "Their work had the effect of moving the posited site of origins two miles across one Welsh hillside," writes Akam. "Their work" would be, in large part, that of Richard Bevins and Rob Ixer, who connected on the subject in 2008. Akam gets pretty technical on what transpired, but it involves Ixer suggesting to Bevins that he had come up with a more precise way to establish the bluestones' origins, a shoebox of bluestone fragments from 1947, others collected in the '70s from the Preseli Hills, and new chemical analyses. There's a lot more to it, but they have flagged Carn Goedog as the likely source of a lot of the bluestones. (Read the full piece, which also talks about the theory that Stonehenge was a "moving monument.")