Grateful thanks to James Scourse for publishing this information in his 1991 paper:
Scourse, J.D. (1991) Late Pleistocene Stratigraphy and Palaeobotany of the Isles of Scilly. Phil Trans Roy Soc B, December 1991, Volume: 334 Issue: 1271.
A long time ago, but geological information does not date! The hand sample identifications were made by JR Hawkes of the BGS Petrology Unit.
Below we reproduce info from the appendices of the paper, relating to the Scilly Till examined at Bread and Cheese Cove on St Martin's Island and to assorted samples collected from the related Hell Bay Gravel.
The great range of erratic materials is interesting. Particularly intriguing are the Lower Palaeozoic sandstones and the red / pink / purple / greenish sandstones and marls which Dr Hawkes speculates as possibly coming from Devonian outcrops in Pembrokeshire, Devon or Ireland. Brightly coloured Devonian sandstones are quite widespread, but brightly coloured Cambrian sandstones do not outcrop so frequently, and the most obvious source would be the southern coast of the St David's Peninsula around Porth Clais and Caerfai. How easy is it to differentiate these from the Devonian sandstones? As far as I know, nobody subsequently has attempted to work out whether these sandstones have come from Cambrian or Devonian outcrops -- there's an interesting project for somebody. The samples are apparently still held at the School of Ocean Sciences at Bangor University.
The heavy metal suite from the till is also interesting -- perhaps we could get a view from Myris on that?
Additional information: Dr Hawkes has also found a 10-tonne erratic block of olivine basalt on Great Crebawethan, one of the western rocks not far from the Bishop Rock Lighthouse, and more than 4 km west of St Agnes. This is reported in Scourse (1991). Who knows what other large erratics are still to be discovered?