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. Professor Bowen and colleagues have obtained a date of c.
I'll put up another post with a You Tube link on it, in case anybody is interested.
It is in my view entirely useless -- we do not know where the sample came from either at Stonehenge or at the place of its origin. Age and geomorphic history of Meteor Crater, Arizona, from cosmogenic  and  in rock varnish, Geochimica et Cosmochimica Acta 55: 2695-8.
Unless you know precisely where a sample came has come from, ie what its exposure to cosmic radiation and weathering might have been over many thousands of years, you can say NOTHING reliable or scientific about the age of the stone surface you are purportedly dating.
Looks like a bit of Chilmark stone to me -- please correct me if I'm wrong. The date apparently came out as showing that the stone was first exposed to the atmosphere around 40,000 years ago.
On that basis the good Professor says -- on the film -- that "there is no way that this rock could have been transported by glacier ice from Preseli to Stonehenge." He says that the last ice sheet that might have affected this area was in existence about 650,000 years ago -- goodness knows where he got that date from........