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Thursday, 21 April 2016

Glacial landforms on the Isles of Scilly

One might think that we are lucky enough to find glacial sediments on the Isles of Scilly, given the archipelago's extreme southerly location -- but we can do even better than that, since there are some clear glacial landforms as well.  They are fairly subtle, since there are no real uplands on the islands (there is hardly any land more than 40m above sea level), but I am pretty convinced about their origins.

1.  Ice moulded forms and possible roche moutonnee -- Round Island and St Helen's Island in the far north.

 Round Island (with the lighthouse) and St Helen's Isle, seen from the northern end of Tresco.  Round Island is deemed to be scoured almost clean of sediments by over-riding ice, and St Helen's does appear to have a roche moutonnee form...... but whether ice action in this southerly location would have been prolonged enough and intense enough for glacial erosion on a grand scale is another matter......

2.  Ice scoured rock surfaces near Shipman Head, Bryher

 Ice scoured surface on Badplace Hill, near Shipman Head, Bryher. 

3.  Morainic ridges on Bryher
Hiemstra et al refer to a morainic ridge on the eastern side of Shipman Head; I was unable to examine that for lack of time.  But there are others not far away:

 Hell Bay Moraine on the west side of Shipman Head, Bryher.  It runs straight downslope, and is made of a jumble of erratic boulders and slabs mixed with angular blocks of local origin.  This feature is shown on the maps as "castle ramparts" -- or as an Iron Age or Bronze Age defensive feature.  However, I am convinced it is natural, and that the archaeologists have got it wrong..... some of the boulders and slabs are enormous, and this looks quite unlike the defensive Iron Age ramparts I know from Wales.

Morainic ridge at Little Popplestones, on the western side of Bryher.  It is littered with erratics of many sizes and lithologies, and is aligned NE-SW -- ie perpendicular to the direction of ice movement.  It does not mark the position of greatest ice extent during the Devensian -- till is exposed in the drift cliffs of the bay outside this moraine. Gweal Hill is seen across the bay.

Little Popplestones moraine location, at the northern end of Popplestones Bay, west Bryher

4.  Morainic ridges -- White Island Bar and Pernagie Bar.  There is also a curvilinear offshoot on the east side of the latter bar. These ridges look at first sight like tombolos, formed by longshore drift and beach processes to link inshore islands to the mainland.  But they are much more complex than that, and the visitor is immediately struck, on examining them, by the multitude of erratic pebbles and boulders which litter their surfaces and buy the core of clay till which is sometimes exposed at low water.  I agree with Hiemstra et al (2006) that these ridges are remnants of morainic ridges left by a lobe of ice that pressed into the shallows between White Island and Pernagie Island from the north-west.  They are not necessarily parts of a terminal moraine, because the Devensian ice certainly extended further to the south at the peak of the glaciation.  They may represent a retreat stage (or stages) or short-lived readvance of the ice edge.

 The red lines show the locations of probable morainic ridges at the northern tip of St Martin's Island.  Also shown is the location of Chad Girt, where there is a significant exposure of Quaternary deposits in a narrow chasm under attack from storm waves.

 White Island Bar, seen from the St Martin's end.  It is littered with erratics of all shapes and sizes and many lithologies, and is cored with clay-rich till.  It is interpreted as a constructional feature.

In the middle distance, Pernagie Bar -- another morainic feature.  Seen from St Martin's Isle.  At high tide the bar is covered.

5.  Golden Ball Brow, on the west side of St Helen's, is also interpreted as a morainic feature, marking a former ice edge. In fact, Hiemstra et al speculate that there is a double ridge here, formed by two ice edge stillstands.  I cannot comment on these features because I have not examined them.

6.  Elongated moraine ridge at the northern extremity of St Martin's, near the pebble maze and on the south side of the footpath.  Above the ridge there is a slope leading to the Rabbit Rocks.  This ridge is quite prominent, and I find it convincing as an ice edge feature probably associated with the White Island Bar and Pernagie Bar moraines.   Hiemstra et al (2006) refer to a number of smoothly crested symmetrical ridges 2-3 m high and up to tens of metres long.  I did not observe these, but wonder if they might be storm beach features in what is a very exposed location?

7.   On the gentle slopes facing Great Bay, Hiemstra et al (2006) refer to breaks of slope and asymmetrical ledges associated with small kettle-like depressions.  They interpret these as "remnants of ice-contact slopes" linked to an old ice edge position.  I did not have time to look at these features so I cannot comment on that interpretation.

8.  Ridge like expressions are also referred to by the same authors on St Martin's towards Bread and Cheese Cove.  I looked quite carefully at this area, but found nothing that might be interpreted as glacial landforms.  But I do agree about the presence of clay till in the exposures at the head of the bay!

9.  Ice-moulded tors
Scourse and other authors have argued that one way of distinguishing the glaciated from the unglaciated parts of the Isles of Scilly is to look at the shapes of the granite tors that are found on all of the islands.  They argue that delicate pinnacles and balanced rocks could not have survived gklaciation, and so where they exists they must have lain beyond the Devensian ice edge.  On the other hand,  tors with subdued forms might indicate submergence beneath Devensian ice, and might signal a certain amount of damage or "cleaning up."   I am not too sure about this, having oberved teetering tors inside the ice limit and subdued forms outside it!  Many other factors are at play in determining the forms and fragility of tors, so on this matter I shall reserve judgment.......

 Granite pinnacles on the western tip of St Agnes, which does seem to have been affected by Devensian ice......

Apparently denuded tors on the eastern coast of St Mary's, in an area supposedly not affected by Devensian glaciation......

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Interestingly enough, the subtle and subdued depositional features on the Scillies are reminiscent of those on Gower and in Pembrokeshire............ but where are the fluvioglacial deposits and landforms?  They must have existed.  Could it be that they were all in the lower areas which are now submerged and destroyed beneath the sea?

5 comments:

TonyH said...

You'd think that people would have had enough of Scilly landforms
I look around me and I see it isn't so, oh no......

PAUL McCartney 1976 [More or less]

BRIAN JOHN said...

Sorry if people have got a bit bored with all this Scilly stuff. I think I have said more or less what I wanted to say now, after a flood of posts! For better or worse, I wanted to get the info on the record -- including the photos, which I think might be important for our better understanding of the Quaternary in the southern British Isles.

TonyH said...

Original post - Beatles [the band originally known as The Quarrymen, Myris and MPP, in the '50's!!] Paul McCartney song, "Silly Love Songs.

One line is:-

It isn't silly, it isn't silly at all......

Let's hope that Brian's recent Scillies field trip, which has produced this wealth of Quaternary glacial observation, will indeed lead to a better understanding of the Quaternary in the southern British Isles, and consequently more wisdom in thinking regarding the movement of glacial erratics, etc, from SW Wales towards what we term Wessex (i.e, in particular, Somerset, Gloucestershire and Wiltshire).

Myris of Alexandria said...

Excellent post on the Scilly Isles you should write it all up for the icemen mag. Reminded me of Lower Nubia but more clement.

Steve Marshall's paean to Avebury is published. "Exploring Avebury the Essential Guide. BUY IT. Stupid price, less than £15.(what is that, a couple of bottles of White Lightning and a kebab, lots of pictures for the slower reader, so no whinging comments please that it is not free).

This book should be on the list of best Archy books 2016. It shows what a non-professional archaeologist(Steve cannot be called an amateur in any respect)can do.

It has the very great attribute of restraint, with a sparse well-pitched text and photos, photos, photos. Imagine being taken around the sites on a warm June evening with an urbane, knowledgeable, favourite grand uncle with the promise of a drink at the end.

BRAVO.

Myris

BRIAN JOHN said...

Lower Nubia?!!

Thanks Rob -- yes,I am being encouraged to submit a small paper about my observations to one of the geomorphology mags. Probably will do, when I can find a moment. I have two other articles about Rhosyfelin to submit first.......