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Thursday, 11 October 2018

Abermawr, Flimston, Caldey, Carnew and Popplestones -- five of a kind

When I was at Abermawr the other day, I was struck by the similarity between the deposits near the outer end of the northern "drift cliff" and the deposits found at Flimston and Caldey in South Pembrokeshire,  at Popplestones on Bryher (Scilly Isles) and at Carnew Point, St Agnes (Scilly Isles).    Many other sites too -- but that list is enough to be going on with.  Some of these sites are inside the currently accepted Devensian glacial limit, and others are outside. That limit is going to have to be modified, as I have suggested many times on this blog.  

So what are the characteristics that these deposits have in common?  Here are some images:


Ballums Bay, Caldey

Flimston, South Pembrokeshire

Popplestones, Bryher, Isles of Scilly

Near Carnew Point, St Agnes, Isles of Scilly

We need to get a bit technical.  First the deposits are matrix-supported diamictons -- where cobbles and pebbles of many different lithologies, shapes and sizes are supported in a matrix of finer material.  Second, the matrix does not have very much clay in it -- unlike, for example, the Irish Sea till at Abermawr and the massive "Celtic Sea till" at Chad Girt on St Martin's in the Scillies.  Third, many of the contained stones are faceted, fractured and even striated -- good indicators of a glacial origin.  Fourth, there is a reddish or rich brown colour, indicative of oxidation and iron-staining.  As many have noticed, that characteristic is often associated with colluvium or hillwash, and with windblown deposits -- and these deposits are associated particularly with relatively vegetation-free environments in proximity to glacier fronts.

There are local variations of course.  At Ballum's Bay the deposit has the reddish colour associated with the Old Red Sandstone, across which the incoming ice passed before reaching the Carboniferous Limestone at the eastern end of the island.  At Carnew Point on St Agnes, the matrix comprises a mixture of windblown silt and colluvium and "grus" which is the product of granite weathering.  At Popplestones the reddish colluvium is ubiquitous.  At Flimston there are many beautifully rounded quartz pebbles derived from patches of Oligocene "gravels" and pebble beds which have been destroyed by overriding ice and incorporated into till.  At Abermawr there is a real mess, with the same mix of stones as in the underlying Irish Sea till and with patches of flowtill, glaciofluvial sands and gravels, colluvium and periglacial slope deposits all scrambled together.

My conclusion is that all these deposits are clear indicators of glacial activity in the neighbourhood.  But they are probably not primary tills in the positions where they were originally emplaced.  They have been mobilized in very wet (ice wastage) conditions -- moved downslope either on dead ice surfaces or on ground slopes, and redeposited a little distance away.  How far?  My estimate, in all cases, is somewhere between 10m and 50m. From what I know of ice wastage environments, transport of a deposit that still contains many of the characteristics of a primary till is unlikely to have been maintained for a greater distance except in exceptional circumstances -- for example in a catastrophic liquidised debris blow in a long gully.

So I think we are looking here at "paraglacially redeposited tills" in the modern phraseology.  

I'll do another post on the current exposure at Abermawr, which has led me to this conclusion.


Here is a definition from Olav Slaymaker: 

"..........It is argued that the term ‘paraglacial’ defined as ‘non-glacial processes conditioned by glaciation’ describes landscapes that are adjusted neither to Last Glacial Maximum nor to contemporary geomorphic processes. Where a landscape is paraglacial it can be characterized in terms of rate of change and trajectory of that change. It cannot be defined in relation to glaciers (as in proglacial) or by cold-climate processes (as in periglacial). Almost all paraglacial landforms and all paraglacial landscapes are transient and transitional. An interesting challenge of paraglacial landscapes is then to determine their rates of change; how far they have advanced along the trajectory from glacial to non-glacial; and how to recognize empirically the temporal and spatial relationships between proglacial, periglacial, paraglacial and fluvial landscapes.

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