From Ian West's stimulating site:
http://www.soton.ac.uk/~imw/Sarsens-Erratics.htm
An exfoliating mass of pink granitoid rock measuring 1.8m by 1.5m by 0.9m was found west of the hardway on the north side of Chichester Channel at Itchenor Ferry. A block of speckled grey rock, approximately 3.1m by 1.7m by 1.4m was seen on the foreshore of bare London Clay 0.4m west of Longmore Point (White, 1915). Incidently this boulder was apparently not visible in 1891. In May 1914 it was 22.9m distant (seaward presumably) from the face of the cliff (White, 1915). This was the largest erratic in the area covered by the Lymington and Portsmouth Geological Survey Memoir. It shows sign of rapid distintegration along gaping fissures, and at its eastern end lie four detached masses (ranging up to 1.37m in diameter), which when in position probably added another 0.45m to 0.6m to its total length (thus it was originally about 2.3m in length).
White (1915) submitted the rock for petrographic study. Dr H.H. Thomas contributed the following note on the basis of a petrographic examination:
"E 11220. The specimen submitted has been sliced and proves to be a much crushed Quartz diorite. The rock has no similarity with anything in the south of England and Wales, except perhaps with some of the Pre-Cambrian Quartz diorites of Pembrokeshire. Quartz-diorites are common in the Channel Island and the North of France, and these localities would seem to furnish a more likely source than Pembrokeshire, especially as other boulders have been found on the South Coast, which, with good reason, have been considered to be of extra-British origin" - H.H.T.
It is just appropriate to note at this point that the Stonehenge Bluestones, although not diorites, are dolerites supposedly from Mynydd Preseli in Pembrokeshire.
See also: Belderson, Kenyon and Wilson, 1973
White 1915
Reid 1892
Kellaway et al 1975
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