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Wednesday, 20 April 2016

Pre-Devensian slope deposits on St Agnes Isle

In most of the literature about the glaciation and geomorpholohgy of the Isles of Scilly, the raised beach is referred to as "the basal deposit" -- in other words, the lowest deposit in the Quaternary stratigraphic sequence which is assumed to rest directly on bedrock.  So if the beach is of last interglacial age (Eemian / Ipswichian / Marine Isotope stage 5e) it must have formed at a time of relatively high sea-level in the Scillies, maybe a little above present MSL, between 130,000 and 115,000 years ago.  But are there really no older deposits on the islands?

Well, we know that there must have been older glacial deposits, because faceted erratics found in the raised beach and in the head outside the last glaciation limit must have come from somewhere.  So have these old deposits been completely destroyed and disaggregated?  My bet is that one day they will be found......

But for the moment, the oldest known deposits on the islands are these, found near Browarth Point on St Agnes beneath the raised beach:


In both cases the cemented raised beach lies on deeply rotted granite and grus which may be in situ; this gravelly debris may be very old indeed -- ie pre-Quaternary.

Somewhat younger slope deposits are found under the raised beach near Dutchman's Carn on the west side of Peninnis Head, St Mary's:




At this location we see around 2m (base not seen) of cemented coarse granite breccia made mostly of grus but with much silt and sand also.  There are signs of crude stratification, and bands of iron oxide precipitate.  No obvious erratics were seen in this material -- the angular fragments seen in the photos are all of granite or related igneous rocks found in the vicinity.

This material must be more than 130,000 years old, and it probably dates from the Wolstonian cold phase which pre-dated the Ipswichian interglacial. Conventionally it is dated to 350,000 - 130,000 years BP.   Remarkably little is known about this cold phase, and we do not know whether the pre-Devensian ice that affected the Isles of Scilly arrived during the Wolstonian or the Anglian glacial episode c 450,000 years ago......

For the moment, let us assume that the deposits beneath the raised beach on St Mary's Island are periglacial slope deposits and rockfall debris accumulated immediately prior to the advent of the Ipswichian interglacial.

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