Bluestone of Stonehenge
The term "bluestone" in Britain is used in a loose sense to cover all of the "foreign" stones at Stonehenge. It is a "convenience" label rather than a geological term, since at least 20 different rock types are represented. One of the most common rocks in the assemblage is known as Preseli Spotted Dolerite—a chemically altered igneous rock containing spots or clusters of plagioclase feldspar. It is medium grained dark and heavy rock, harder than granite. Preseli bluestone tools, such as axes, have been discovered all over the British Isles. Many of them appear to have been made in or near Stonehenge, since there are petrographic similarities with some of the spotted dolerites there.
The bluestones at Stonehenge were placed there during the third phase of construction at Stonehenge around 2300 BC. It is assumed that there were about 80 of them originally, but this has never been proven since only 43 remain. The stones weigh between 2 and 4 tons each. The majority of them are believed to have been brought from the Preseli Hills, about 250 miles away in Wales, either through glaciation (glacial erratic theory) or through humans organizing their transportation. If a glacier transported the stones, then it must have been the Irish Sea Glacier.
Recently the archaeological find of the Boscombe Bowmen has been cited in support of the human transport theory, while new glacier modelling supports the erratic theory. Preseli Bluestone dolerite axe heads have been found around the Preseli Hills as well, indicating that there was a population who knew how to work with the stones.
A summary which outlines the major aspects of the Stonehenge "bluestone conundrum" has recently been published. A new book devoted specifically to the problem of bluestone provenance and transport concludes that the Stonehenge bluestones are essentially an ill-sorted assemblage of glacial erratics. Research into the origin of the bluestones is ongoing as of 2012.
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