The till exposure at Popplestones on the Isle of Bryher. Well-rounded raised beach cobbles are incorporated in the till.
Clifftop till near Flimston in Pembrokeshire. Here the till incorporates vast numbers of white quartz pebbles which are well rounded. They come in this case from ancient unconsolidated Oligocene clays and pebble beds that were once widespread on the Flimston coastal platform.
One of the interesting features of the glacial sequence in the Isles of Scilly is the occurrence of raised beach cobbles in Devensian till. This feature is especially notable on the island of St Agnes (which of course lies to the south of the Devensian ice limit according to other authors), where the till is thin and patchy and confined to the west coast. The till at Popplestones on Bryher also contains many rounded and striated cobbles that must have been incorporated into till by ice that rode over pre-existing raised beaches or storm beaches.
Is this so unusual as to cause a problem? Well, if you look into the older literature on the Quaternary deposits of Devon, Cornwall and the Scilly Islands there is much discussion about "redeposited till" and "till in a secondary position" -- and one gets a feeling that researchers have in some cases been reluctant to accept that ice picks up, rather indiscriminately, loose materials from any deposits that it happens to pass over. So till containing rounded pebbles is sometimes interpreted as very ancient, disaggregated and then reconstituted downslope after incorporating raised beach pebbles as a consequence of periglacial downslope movement under gravity. That line of reasoning can become very convoluted and very arid -- ignoring the fact that the simplest explanations for things are often the correct ones.
If you find a till containing raised beach cobbles the simplest explanation is that the raised beach was there first (somewhere upglacier) and that the till came later. In the case of the Isles of Scilly, the reasonable interpretation is that the raised beach is Ipswchian and that the till is Devensian.
https://www.researchgate.net/publication/328413421_Evidence_for_extensive_ice_cover_on_the_Isles_of_Scilly/figures?lo=1
In 1965 - 66 David Sugden and I were initially quite mystified by the occurrence of striated and rounded raised beach cobbles in till at multiple locations in the South Shetland Islands. We found fresh marine cobbles in till in 19 different locations, on six different islands in an area where there was still much lying snow across the landscape; so there must be many more undiscovered locations also. The till exposures were found at many different altitudes, up to an altitude of 275m. One of the interesting things about the pebbles and cobbles contained within the till was that a large proportion were "fresh" and unaffected by frost shattering -- unlike many of the pebbles in the higher raised beaches on the ice-free peninsulas.
Detail of residual raised beach materials found in an area of fresh till. The survival of these cobbles and pebbles suggests that the affected raised beach was not exposed to intense periglacial weathering (or frost shattering) conditions for very long before being overriden by glacier ice. Barton Peninsula, King George Island, at 150m above sea level.
On Barton Peninsula we observed this patch of more or less undisturbed raised beach at an altitude of c 150m, surrounded by fresh till and frost-shattered slope breccia.
In our interpretations of the cobbles in the till we had no problems at all with our basic interpretations of processes and relative ages -- but we were initially quite concerned about finding raised beach traces at 275m (902 ft) -- far higher than any other raised beaches ever found in Antarctica. At last our conclusion was that the extraordinary high altitude had something to do with the isostatic forebulge effect, with the sites around the South Shetlands lifted by a considerable amount as the ice mass over the Antarctic Peninsula and West Antarctica expanded and depressed the crust further south.
The lesson from all of this? If you find raised beach cobbles contained within a till, the standard rules of stratigraphy apply. If the till looks fresh and undisturbed, it is probably in its original condition and position. The raised beach that provided the cobbles to the overriding ice must be older, and must lie either in the immediate vicinity or an unknown distance up-glacier. That should not be a problem for anybody.
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