Late Pleistocene Stratigraphy and Palaeobotany of the Isles of ScillyJ. D. Scourse
Phil Trans Roy Soc B, December 1991
Volume: 334 Issue: 1271
Published 30 December 1991.
A re-evaluation of the Pleistocene stratigraphy of the Isles of Scilly has enabled the formal definition of eight lithostratigraphic units of member status grouped into two formations. A chronology of events has been provided by radiocarbon determinations, optical and thermoluminescene (TL) dates. Intersite correlations have been strengthened by palynology, which has aided palaeoenvironmental reconstruction. The defined units have been incorporated into two lithostratigraphic models, one for the `northern' (glacial) Scillies and one for the `southern' (extra-glacial) Scillies. Raised beach sediments of the Watermill Sands and Gravel in the southern Scillies are overlain by the Porthloo Breccia, a unit of soliflucted material derived exclusively from the weathering of local granite. Organic sequences at Carn Morval, Watermill Cove, Porth Askin, Porth Seal and Bread and Cheese Cove occur within the Porthloo Breccia, and are interpreted as the infillings of ponds associated with active solifluction. Radiocarbon determinations from these organic sediments are critical because they pre-date units associated with a glacial event. The C14 age determinations indicate deposition of the organic material between 34500 and 21500 years BP and provide a maximum age for the glacial event and the first radiometric dates for the coastal `head' sediments of southwest England. The pollen assemblages from these organic sites all record open grassland vegetation, and represent the earliest vegetational record for the Scillies. High Pinus values are interpreted as evidence of climatic deterioration. In the southern Scillies, the Porthloo Breccia is overlain by the Old Man Sandloess, a coarse aeolian silt with subdominant fine sand, TL-dated to 18600 years (Wintle 1981) and optically dated to 20000 and 26000 years (two samples; Smith et al. 1990). This material occurs in a variety of facies related to different modes of reworking. In the northern Scillies, the Porthloo Breccia is overlain by three units that are all related to a single glacial event. The Scilly Till, a massive, poorly sorted, clay-rich pale brown diamicton containing abundant striated and faceted erratics of northern derivation, occurs at Bread and Cheese Cove, and Pernagie and White Island Bars. This sediment is of uncertain depositional facies, although the available data suggest that it may be a lodgement till. At Bread and Cheese Cove the Scilly Till occurs in association with a matrix-supported sandy gravel, the Tregarthen Gravel, which has an erratic assemblage consistent with the underlying Till. The distributional relation between the glacially derived sediments, marine bars and morphological varieties of granite tors suggest that some of the bars may be remnant moraines, and that the glacier was erosive in the northern Scillies. Aeolian loessic processes in association with the glacial advance resulted in the deposition of the Old Man Sandloess in the southern Scillies. The relative coarseness of this material is interpreted as a function of its proximity to glacially derived source material. The mineralogy of the Scilly Till is sufficiently similar to the Old Man Sandloess to suggest a genetic link between the two units. Overlying the Scilly Till and Tregarthen Gravel in the northern Scillies is the Hell Bay Gravel, an extremely widespread matrix-supported gravel containing a similar assemblage of striated and faceted erratics to the underlying Till, but alongside a considerable proportion of locally derived granitic material. The matrix of the Hell Bay Gravel is identical to colluvially reworked facies of the Old Man Sandloess. This material represents an initial phase of solifluction, post-dating the glacial event in which the Scilly Till, Tregarthen Gravel and Old Man Sandloess were mixed and transported downslope. In situations where these sediments were stripped from the land surface, weathered granite once again became the dominant raw material for solifluction, this subsequent phase being represented by the Bread and Cheese Breccia in the northern Scillies and the upper Porthloo Breccia in the southern Scillies. The evidence therefore suggests that ice advanced at least as far as the northern Isles of Scilly during the Dimlington Stadial of the late Devensian Substage. This conflicts with previous interpretations which place the glacial deposits within the Wolstonian Stage. However, the late Devensian event was probably not the first glacial event to have influenced the Islands because erratics are widespread in some exposures of the Watermill Sands and Gravel; the age of this earlier event remains uncertain.