England's Stonehenge may have been a "version 2.0" monument built with stones from an earlier version erected in Wales, a new study suggests.

The general location in Wales where rocks in Stonehenge were quarried has been known since the 1920s, but researchers say they've now identified two actual excavation sites.

A team of scientists from British universities and museums say they've pinpointed the quarry sites in the Preseli hills in Pembrokeshire in Wales.

Known as "bluestones," the rocks forming much of Stonehenge would likely have been broken off larger rock faces using wooden wedges, researchers say.

Holes cut into the rocky outcrops at the Welsh site match Stonehenge's bluestones in shape and size, they report.

Using carbon dating, the scientists determined the stones were quarried at least 500 years before Stonehenge was assembled, supporting the idea they may have been used in an earlier monument in Wales before being transported to England's Salisbury Plain.

It's unlikely they would have been quarried and then simply left to sit for half a millennium, researchers say.

"It could have taken those Neolithic stone-draggers nearly 500 years to get them to Stonehenge, but that's pretty improbable in my view," says Parker Pearson of the Institute of Archaeology at University College London. "It's more likely that the stones were first used in a local monument ... that was then dismantled."

Ancient communities moving eastward and eventually settling on Salisbury plain may have brought the bluestones with them as significant symbols of identity, the researchers suggest.

Teams of men or oxen could have transported the individual stones, which weighed around 2 tons, they say.

"We know from examples in India and elsewhere in Asia that single stones this size can even be carried on wooden lattices by groups of 60 — they didn't even have to drag them if they didn't want to," Pearson notes.

Researchers have been using geophysical surveys and aerial photography in a search for a possible site of a dismantled earlier monument in Wales.

"We think we have the most likely spot," says researcher Kate Welham of Bournemouth University. "The results are very promising. We may find something big in 2016."

Pearson says he's in no doubt about its existence, or that Stonehenge was built as a Welsh monument before it was an English one.

"If we can find the original monument in Wales from which it was built, we will finally be able to solve the mystery of why Stonehenge was built and why some of its stones were brought so far," he says.