Chlorine-36 dating has important potential for archaeology, but recent Chlorine-36 dates on `bluestones' of Stonehenge have been misinterpreted. Professor D.Q. Bowen of the University of Wales, Cardiff and colleagues have dated a fragment of igneous rock reported as having been found at Stonehenge (exact type unknown, but not a spotted dolerite) at 14,000[+ or -]1900 years, and surfaces of outcrops at Carn Menyn in Preseli at 5400[+ or -]400 and 4900[+ or -]400 years (Bowen et al. 1994; Bowen 1994: 211). This information has been interpreted as indicating that the bluestones of Stonehenge could not have been transported to the site of Stonehenge by ice, because the ice sheets were extensive enough only at c. 400,000 years or earlier, when the fragment and outcrop were apparently still buried or covered, and not exposed for ice transport (Bowen 1994: 211; Hawkes 1994; British Archaeology News 1995).
Chlorine-36 dating gives an estimate of the length of time that a rock surface has been exposed to the atmosphere, by measuring the amount of Chlorine-36 produced by exposure of the rock to cosmic radiation. If the rock or surface has been covered or buried, the date obtained will reflect the reduced time of exposure to air. Thus a Chlorine-36 date may reflect either recent exposure of a surface due to processes such as frost shattering, or an original exposure date. This difficulty of interpretation is why Chlorine-36 dating is normally done on boulders or lava surfaces whose erosional history is known (e.g. Phillips et al. 1991).
Professor Bowen and colleagues have obtained a date of c. 14,000 years exposure time for the fragment from Stonehenge. However, it is not possible to tell if this is an original exposure date, or if the fragment was brought to Salisbury Plain by ice 400,000 years ago or earlier, and was subsequently buried within superficial deposits on Salisbury Plain for part of its history. Or it could have been broken off a larger erratic lying on Salisbury Plain, by natural processes such as frost shattering. The rock type of this fragment is unknown, and the sample now completely destroyed (Professor D.Q. Bowen, in discussion at the meeting of the Lithic Studies Group, Cardiff, 28 January 1995), so it may be nothing to do with the bluestone monoliths.
Carn Menyn loses material from outcrop surfaces every year through frost shattering. This will reduce the date obtained on an outcrop. A date of 5000 …