Glaciation on the Isles of Scilly

 Striated stones from glacial deposits on the north coast of St Martin's (credit:  Peter Knight)

Here we are on the Isles of Scilly, and already I have found glacial deposits outside the ice margin as shown on the maps of previous field workers.  More of that anon.  In the meantime, this is a nice summary from Peter Knight.

From Peter Knight's web site:
http://www.petergknight.com/scilly/glaciers.html

If you know Scilly at all, you will be familiar with the gravelly orange cliffs of "ram" or "head" that overlie the bedrock all around the islands' coasts. This is NOT glacial sediment, so don't get excited. Actually it is still quite exciting, as it is what we call a "periglacial" deposit, created by mass movements (soil creep and landslides) when the ground was going through periods of thawing and freezing during the ice age when glaciers were nearby but not actually covering the islands. The glaciers only seem to have covered a tiny little bit of the north of Scilly, and that is where you can find the glacial sediments: the "till" that contains the erratics. The glacial sediment generally has more clay and silt in it compared with the dominantly sandy gravel of the ram or head around the rest of the islands, but both the till and the ram have lots of stones and boulders in, so it is not always easy at all to tell the difference! In section of cliff on St Martins, there is a layer of till sandwiched between layers of ram above and below it. What geologists call the "type site" for the Scilly till, in other words the location where they had a good look at it and were able to describe and define it properly, is Bread & Cheese Cove on the north east side of St Martins. In fact, this location features in a guide to the UK's most important Geological Sites relating to the history of the last 2 million years or so that was published in 2015 (see reading list below). Over the years a number of professional (and amateur) glacial geologists have explored the northern coastlines of Scilly looking for glacial deposits, and it seems that you might strike lucky anywhere along that thin strip of coastline across the northern perimeter of Scilly from the eastern end of St Martins around to the northern coasts of Tresco and possibly even Bryher. It has been suggested that the White Island bar might have been pushed up as a ridge of rocks by the bulldozer effect of the front of the glacier, and I think (although I have not seen this published anywhere) that the ridge, a few metres high and a few hundred metres long, that runs parallel to the coast around the north west corner of St Martins overlooking White Island looks very much like a glacial moraine (a ridge of debris deposited or pushed up by the ice). There are is also a layer of gravels on Tresco (called the Tregarthen gravels!) that are supposed to have been deposited in streams running off the glacier, and there is a line of boulders at Shipman Head on Bryher that is supposed to be a glacial moraine: a ridge of debris pushup up by the edge of the ice. Some people have argued that the top of Round Island shows signs of glacial erosion, indicating that the glacier actually ran over the top of it.


Key references on the glaciation of the Isles of Scilly:

• Scourse, J.D. (1998) The Quaternary History of the Isles of Scilly (Chapter 8 in "Quaternary of South-West England", edited by S. Campbell et al.) Joint Nature Conservation Committee; Springer Science.
 • McCarroll, D. and others (2010) Exposure-age constraints on the extent, timing and rate of retreat of the last Irish Sea ice stream. Quaternary Science Reviews, Volume 29, Pages 1844-1852.
 • Hiemstra, J.F. and others (2006) New evidence for a grounded Irish Sea glaciation of the Isles of Scilly, UK. Quaternary Science Reviews, Volume 25, Pages 299-309.

Isles of Scilly on the agenda

 Illustration from Hiemstra et al, 2006

Tomorrow I'm off for a short holiday on the Isles of Scilly with my wife and four old friends.  Should be fun, if it's not too cold and wet.......

I have mentioned the islands many times on this blog, but have never been there before -- and I hope I can escape from the madding crowds now and then in order to do a bit of geomorfin'........

There are some rather interesting questions still to be answered with respect to this archipelago.

1.  Was the glacier ice that reached the islands in the Late Devensian (the dating seems to be pretty secure) really in the form of a "surge lobe" which pushes southwards from the St George's Channell to the Scillies and beyond?  As I have said many times before, this defies some of the laws of glacier physics, and I have real problems in accepting that a lobe like this could have had the form shown in the top left diagram. There must have been a calving bay with a concave shape at its outer end, and it must have spread laterally rather than flowing as a narrow lobe.  (Ice only flows in the manner shown when it is constrained within a clearly defined channel.  Such a channel does not exist in the Celtic Sea, and there is no evidence that it ever did.)

2.  To what extent are the curving depositional features on the northern islands related to actual moraines that once marked ice edge positions?  The evidence presented in various papers looks pretty convincing....

3.  Does the limit of the Hell Bay Gravel actually delimit the maximum ice extent during the Devensian?  Ice limits are often more difficult to define than one might imagine!

This map is from the Isles of Scilly chapter by James Scourse, published in the GCR review for SW England (1998).  Many other authors have suggested similar limits, and it will be good to see some of the evidence on the ground.

4.  In most of the recent literature on the glaciation of the Isles of Scilly it is claimed that the Devensian Irish Sea Glacier had a greater extent (at least in the Celtic Sea arena) than any of the earlier glaciations of the current ice age.  Scourse claims this, as do others with whom he has worked -- and yet there appear to be "widespread" erratics in some exposures of the "Watermill Sands and Gravels" which  underlie and pre-date any of the Devensian periglacial and glacial deposits on the islands.  Was there an older and more extensive glaciation, or was there not?  It will be interesting to check out the evidence on the ground. (Watermill Cove, the type locality for these early deposits, is on the isle of St Mary's, where we are staying -- so I shall certainly check it out.  It's well to the south of the Devensian glacial limit as currently defined.)

5.  How many raised beaches are there?  Some writers say one, and others say two.......

6.  What are the similarities between the Isles of Scilly and Pembrokeshire?  The Devensian stratigraphy does appear, from the descriptions, to be closely correlated.

The Pwllderi cliffs

When I was over at Pwllderi, on the west side of the Pencaer Peninsula, the other day, I was suddenly struck by the extraordinary degree of ice moulding on the west-facing cliffs.  Features like these would not be out of place on the coasts of Greenland or Antarctica, and they indicate just how powerful glacier ice can be when it is moving directly onto an old coastline (here from the NW) and is being forced to move upslope.  So there must have been powerful abrasion, a great deal of entrainment of loose material,  and a lot of shearing within the ice mass under compression.

Much of the coastline between Abermawr and St David's Head looks like this, and hillocks on the edge of the coastal plateau have also been smoothed off and rounded.  I must do some more work on this, when I can find the time...... and one interesting question (one of many) is this:  to what extent are these Devensian features, and to what extent are they inherited from earlier glacial episodes?

Rainbow ice

This is rather freaky -- saw it on a web site with no explanation of the phenomenon, or location.  I suspect it is somewhere in Antarctica.  Quite wonderful!!

Martha Morgan Country now launched

 In the footsteps of Mistress Martha.  Model: Rhiannon James.  Photographer: Steve Mallett

Excuse my absence from the blog in recent days -- it's been all hands on deck for the launch of my new "literary tourism" project centering on the books of the Angel Mountain Saga.  That was yesterday, at the Boat Club in Newport, and it was a great success -- a packed room and support from all over the place for our attempts to bring more visitors to the area, focussing on the "heritage" features of the landscape.  The stars of the show were model Rhiannon James and photographer Steve Mallett, who are jointly responsible for a fantastic portfolio of photos which can now be seen here:

http://www.marthamorgan.co.uk/photo-gallery.html

Actually, Rhiannon is not a model at all, but a very attractive and very smart young lady who is studying law in Cardiff University at the moment.  She is currently Miss Pembrokeshire, and she's also a very close match for the heroine Mistress Martha Morgan in the imaginations of many readers.......

Here is one of the "heritage" pages from the new web site:

Here is some of the press coverage:

http://www.countyecho.co.uk/article.cfm?id=103884&headline=Author%27s%20eight-novel%20saga%20at%20heart%20of%20new%20tourism%20project&sectionIs=news&searchyear=2016

Who needs quarries anyway?

Following on from my recent post, I came across these two photos in my collection.  They both show the extent to stone litter on uncleared land; I suspect that there are hundreds of acres of this sort of terrain on Preseli and Carningli, except that over large tracts the stone litter or erratic assemblage has been buried beneath peat, windblown sediments, colluvium and stratified slope deposits.

The top photo is from the western flank of Carningli, and the lower one is from a morainic mound near Gernos Fawr -- the surface has been exposed following foraging by pigs.   The stone litter in the foreground of the top photo has been described by Roger Worsley as a "chevaux de frise"  similar to the features at Carn Alw and Castell Henllys.  (The feature is a defensive one consisting large numbers of sharp stones set into the ground at an angle, to deter Iron Age tribal groups from mounting cavalry charges towards the entrance of a fortified settlement.)   In this case, I beg to differ -- I can see no trace here of any "arrangements" of the stones, and no evidence that pointed stones have been deliberately placed in one particular direction in order to protect the settlement's main entrance.  There archaeologists have very vivid imaginations.......but we knew that already.........)

Why would anybody be stupid enough to quarry bluestones?

 Natural stone litter, mostly dolerite, on the common near Gernos Fach, Preseli

 Natural stone litter on the surface of  a Devensian moraine, near Tafarn y Bwlch, Preseli

 Stone walls built of smaller boulders from field clearance work, near Ffordd Bedd Morris, Newport.  The larger elongated stones have generally been used as gateposts.

On several occasions on this blog I have asked the question: "Why would anybody be stupid enough to quarry bluestones?" 

Whether we are talking about the Neolithic, Bronze Age, Iron Age or historic times, the North Pembrokeshire landscape was and is so littered with large slabs and boulders, many of them elongated, that nobody with any sense would ever have considered that actually going to a rock face in a deep valley and quarrying stones there, from the living rock, would have been an option.  If you wanted "orthostats" or monoliths in the past, you would just go and pick them up from wherever was most convenient, and if it were not for our wonderful planning rules, the same principle would apply today.

The only grounds on which that principle might be overturned might be for a bedrock outcrop site to be so revered that stones taken from it might be invested with great significance -- and indeed that is why Darvill and Wainwright have argued that Carn Meini was exploited as a source of "healing stones" which had to be taken from one place, and one place only.  But as we know, that theory has fallen flat because the geologists now think that the spotted dolerites did not come from there at all.  And as for Carn Goedog and Rhosyfelin, there is absolutely no evidence that the stones from those sites were revered in any way.  Spotted dolerites are used in stone settings, but as I have pointed out, we do not know of a single instance in which Rhosyfelin foliated rhyolite was used in a megalithic setting, either in Pembrokeshire or anywhere else.

This all came into mind again recently as I have been wandering about looking at the Preseli landscape, and wondering at the sheer abundance of stones available at the surface.  Then I took a look at  ET Lewis's old book called "North of the Hills" and discovered these words on p 22, relating to the area of origin of the bluestones:   "..... in the triangle formed by Foel Drygarn, Garn Alw and Cerrig y Marchogion.  Those who took them could have chosen their stock from here, without undue quarrying, for the supply of stone is practically endless."

He also says:  "It should be emphasised at this point that igneous boulders are strewn over an extensive area south of the ridge.  Almost every field in many parishes was brought into cultivation after the partial removal of these boulders.  The process of clearance continues to this day; in fact, it has accelerated immensely in recent years.  Northwards of the ridge, hundreds of boulders can be observed, but invariably these have been selected for particular purposes, notably as gateposts, over a period of many centuries."

Thus spake the prophet.

ET Lewis was a pretty astute prehistorian, and he knew the territory intimately, and I think he was on the ball as far as stone collection was concerned. 

So why all the fuss about bluestone quarries?  It would help if some people just lifted their eyes unto the hills, and looked at the landscape......