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Tuesday, 19 April 2016

The Isles of Scilly -- one raised beach, or two?

There is much debate in the literature about the raised beaches that are ubiquitous on the Isles of Scilly.   They are almost everywhere -- it is almost impossible to take a half-day walk along the coastline of any one of the islands without seeing a raised beach exposure somewhere.  The classic raised beach location -- at Watermill Cove, on St Mary's Island -- is in fact one of the least spectacular, and its importance to geomorphology lies not in the beach characteristics but in the characteristics and dating of the overlying sediments.  By far the most spectacular examples of the raised beach are to be seen in the bay of Porth Killier, at the northern tip of St Agnes Island.  A few photos of it are shown at the head of this post.

The cobbles in the photos above are for the most part well-rounded,  and in some parts of the beach they are well packed together, and in others held in a sandy and silty matrix.  The beach here is partly or solidly cemented with iron oxide and manganese oxide cement -- this is a common feature, but there are some places where the beach is uncemented, and friable enough to extract cobbles with ease from exposures.  Note that in all of the examples shown above, there are rounded erratics incorporated in the beach, suggestive of the incorporation of foreign materials from earlier glacial episodes.

At Porth Killier, there are not many sediments above the raised beach, because there are no adjacent steep slopes which could supply abundant slope materials over a prolonged period of time; although in the lower photo we can see an overlying layer of rich brown blown sand (with signs of layering) and a modern organic-rich soil horizon.  In some exposures at the northern end of St Agnes, thin glacial deposits lie directly on the raised beach or on the blown sand layer.

Most of the raised beach deposits are found at around extreme high-water mark, but some are found down to mid-tide mark, beneath the cobbles and boulders of the present-day beach.  No raised beaches were found more than 5m above HWM -- so in all respects these raised beaches can be matched with those of Pembrokeshire.

Mitchell and Orme (1967) considered that there are TWO raised beaches on the Isles of Scilly, dating from different interglacials.  They may have been influenced in this hypothesis by the fact that some beaches are cemented and stained with a black manganese oxide crust, while others are "fresh" in appearance.  They also cited localities where an apparent "lower" raised beach was separated from an apparent "upper" raised beach by an intervening sandy layer.  Scourse and other later researchers have disagreed with this assessment, and have argued forcefully that there is but ONE raised beach on the Isles of Scilly, dating from the Last Interglacial, which shows considerable variation in its altitude above OD and in its internal characteristics. I agree with this latter interpretation; I have seen no evidence to suggest that the raised beaches in the Scillies are of different ages.

Solidly cemented raised beach cobbles resting on bedrock at Watermill Cove, St Mary's Island.  Above the beach there are bedded gravels and rockfall debris.  Above the gravels we see about 3m of rough granite breccia, interpreted as a Devensian periglacial slope deposit.

Raised beach platform about 2 m above HWM, near the southern end of Gugh Island.  There are similar remnants of rock platforms cut across the local granite all around the island coasts, particularly in exposed locations.

Raised beach platform just above the reach of storm waves, east side of Gugh Island, near Old Man standing stone.

 Cemented raised beach of local granite cobbles near the southern end of Gugh Island.  The beach grades upwards into a sandy and gravelly deposit made up of grus and broken and rotten granite clasts derived from nearby rock outcrops.

 Raised beach exposed in the drift cliff near Carn Mahael, on the west side of Peninnis Headland, St Mary's Isle.  Some large clasts of local granite are incorporated into the beach, and it is overlain by a thick breccia of broken and rotted granite clasts in a sandy and gravelly matrix.  Note the small erratic pebble of grey shale towards top right.


sciencebod said...

Does anyone know where the iron and, especially, the manganese came from in those cemented layers? Are they merely part of the mineral geology at unexceptional concentrations that have simply migrated and become more concentrated? The reason I ask is partly because there's a literature on iron and manganese "hard pans" in an entirely different context (Silbury Hill). One wonders if those two transition metal elements might in fact be markers for original plant or animal remnants, maybe thousands of years old. Manganese is of special interest, given it's a component of several enzymes. In fact, the presence of a manganese-activated superoxide dismutatse ("SOD")in animal mitochondria, distinct from the copper/zinc one in the cytosol, is strong evidence for the endosymbiotic theory of mitochondria - that they were originally free-living bacteria that somehow got incorporated into eukaryotic cells, since bacterial SOD is the manganaese variety.

BRIAN JOHN said...

Intresting question! I'm afraid the chemistry is beyond me -- my simple assumption is that wherever groundwater is in circulation, it sometimes dissolves things and accumulates iron and manganese oxides (and iother things too) and sometimes precipitates them. Maybe others have a view on whether there may also be biological processes involved? Many old deposits are cemented with black and reddish cement, and I have often thought that the more solid the cement, the older the deposit. I now think that it's not quite so simple -- I think the degree of cementation has more to do with temperature, amount of groundwater movement and other environmental factors.

sciencebod said...

Thanks. Yes, it may be simple translocation. It's presumably pH that predisposes to iron precipitating as the hydroxide or hydrated oxide, and presumably the same applies to the manganese too. Yet one reads of the iron and manganese oxide/hydroxide bands being seen in calcareous rock where the pH is already alkaline, where it's inconceivable that it's ever sufficiently acidic to allow iron and manganese to exist in the dissolved state. That's what makes me think that those coloured bands when found may be a marker for a previous presence of organic material - plant or animal. As it rots down, especially under anaerobic conditions, one can expect acidic conditions to develop (weak organic acids, e.g. acetic, propionic etc being well known fermentation products) which help to solubilise the iron/manganese co-factors from the organic material (haemoproteins, mangano-enzymes etc) allowing them to percolate down through the soil layer for a short distance. Finally the alkaline conditions prevail, and the briefly mobile iron and manganese salts then promptly precipitate in a relatively narrow band to create the coloured strata - red for iron, black for manganese.

I now need to do some image searches to see whether the red and black are separate but immediately adjacent. That might give extra credibility to the notion that the two minerals have migrated from a common source enriched in both relative to the surrounding rock, but were only able to make it a short distance before co-precipitating almost but not quite at the same depth below the presumed organic layer.

I'll spare you my theory on Silbury Hill, your site as you remind us being geomorphological rather than archaeological, but was intrigued to see the reference here to iron/manganese cementation of particulate rock just one day after encountering the reference to iron/manganese "hard pans" when leafing through a weighty tome on the enigmatic man-made mound in the British Library! I refer to Leary, Field and Campbell's 350 or so page 2013 monograph, which the first of those candidly referred to in a recent email as "eye-wateringly" expensive (approx 5 times my return rail fare to Kings Cross!).

Colin Berry

sciencebod said...

PS: here's a link that might interest you BrianJ:

It's a bit of HTML code from an independent site that instals a "Recent Comments" on one's Blogger site. I've just trialled it on my sciencebuzz site, a 2 minute job, and hey presto it's working fine (so far at any rate).

Were you to instal it, I and maybe others - yourself included - could usefully, i.e. visibly, respond, e.g. to one of your commenters who way back in 2011 said (as discovered by googling) that there's "no evidence for excarnation" at Stonehenge. No direct evidence maybe (how can there be?) but I would suggest there's a growing body of indirect evidence for excarnation, indeed sky burial, and the bluestones can be accommodated within the dossier of supporting evidence as I've said recently on my own recent postings.

Nope, it ain't geomorphology, the raison d'etre for this site, but Stonehenge IS in the blog title, and maybe I'm not alone in thinking that a multidisciplinary approach is required if one's to find solutions to long-standing enigmas where generations before one have failed to do so.

Colin Berry

BRIAN JOHN said...

Thanks Colin -- I will have a look at that....