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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?

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.

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.

Isles of Scilly location maps


These are interesting maps which will help to locate key localities and spatial relationships.  The upper map is from a NASA satellite image.  The lower map shows the relationships between land and sea in the Late Neolithic / Early Bronze Age, when sea level was still rising following the end of the Devensian glaciation.  Around 5,000 years ago all of the northern islands were connected, at least at low tide.

Monday, 18 April 2016

Something for the record book


This has to be the most southerly exposure of coherent glacial deposits on land in the British Isles.  This silty and sandy till with faceted erratics and incorporating granite breccia and also some raised beach materials, is exposed near Long Point on the west side of St Agnes Island in the Isles of Scilly.

The grid reference is SV875078.  Lat: 49 deg and 88' North. Long: 6 deg and 35' W

There are erratics on the coasts of Wingletang Down, a bit further south, but they are from disaggregated earlier glacial deposits and are now scattered through raised beach and solifluction materials.  

So there we are, folks.  You saw it here first........

For the record, this is the stratigraphy:

Top -- c 1 m of dark organic-rich silts and sands topped with modern soil.  Partly windblown?
1m of light brown stratified silts and sands -- colluvium
Sharp junction
50 cms of mixed till with faceted erratics of grits, sandstones, shales and unknown volcanics.  Some cobbles of raised beach origin?
No sharp junction -- layers grade imperceptibly one into the other
About 2m of rotten granite debris / breccia / lower head with large angular granite clasts.
Base not seen -- masked by modern beach.
 

The Devensian Ice Limit in the Isles of Scilly



This is my carefully considered ice edge map for the Devension glaciation of the Isles of Scilly.  The archipelago is a fantastic "geomorphology laboratory" in which it is possible to pick up obvious signs of glacial deposition in one locality before moving to another locality a hundred metres away to find that signs of glaciation are quite absent. The ice edge was quite sharp, but I agree with other authors that it was quite short-lived, and that it might well have been linked to a surge.  The morainic ridges described by Hiemstra et al (2006) seem to me to be very convincing glacial constructional features, but they are not terminal moraines that mark the ice's greatest extent.  So they might mark retreat stillstands of the ice edge, or else short-lived readvances.

In the map above the yellow line shows the southern limit of the Hell Bay Gravel, used by some as a proxy for the ice edge.  But it's not an accurate line, and neither is the Hell Bay Gravel a distinctive deposit -- so the yellow line is nothing more than an approximate guide.

The blue line shows the suggested ice limit of Mitchell and Orme (1967), as adopted by Scourse and other authors more recently with relatively minor changes in the shallows around Northwethel, St Helen's and Tean Islands. Scourse (1998), in the GCR volume on the Quaternary of SW England, straightens out the blue line as it is shown above, but it is uncertain whether this adjustment is based on detailed observations.

My main criticisms of the blue and yellow lines as shown above are as follows:

1.  There are fresh glacial deposits outside them, for example on Bryher, St Mary's and St Agnes.

2.  They do not take proper account of topography, and of the tendency of glacier ice to flow round obstacles and into troughs or depressions.  Given the shape of the sea floor in the archipelago, and given that Devensian ice has clearly overtopped the higher moors of Bryher, Tresco and St Martin's, it is unrealistic to draw straight ice edges that include hills over 40m high and also the straits between the present day islands.  The straits and sounds must have contained lobes or tongues of ice.

My red line on the map is drawn to enclose all of the fresh till exposures I was able to identify during the course of a week in the Scillies in April 2016. The red dots show the locations of the main exposures / outcrops of till which I noticed on my travels.  Ice lobes are now suggested in the sound between Tresco and St Martin's, in the strait between St Martin's and Tean, in the narrow channel between Tresco and Bryher, and in the wide sound between Samson and St Agnes.

I suggest that Samson and the Eastern Isles lay outside the glaciation limit, but that Annet might have been over-ridden.  I have not visited any of these, so observations from others are welcome.  To the west of Bryher, Gweal must also have been over-ridden.

When did this last ice incursion occur?   Scourse and others suggest a date of c 20,000 - 18,000 years BP, and this seems entirely plausible.

Finally a word about ice movement directions.  Scourse and others have suggested that the ice that affected the Isles of Scilly was moving from NE towards SW.  I disagree with this.  In relatively unconstrained situations, ice always moves perpendicular to the ice edge.  So it is entirely sensible to propose that the Devensian ice of the Irish Sea Glacier moved into the archipelago broadly from NW towards SE, except in the southern islands where it might well have moved through the Northwest Passage into St Mary's Road from west to east.  That makes perfectly good glaciological sense, and also explains the distribution of glacial sediments.

This might seem to be a radical redrawing of the glacial map of the Scilly Isles, but I contented that it is much more accurate than earlier attempts and that it is supported on the ground.  Much to my own surprise, the impact of Devensian ice on this remote archipelago off the tip of Cornwall was much greater than has previously been supposed.

Devensian till on the Isles of Scilly

 Clay-rich till with abundant erratics, near Gimble Point, north-west Tresco

 Erratic-rich Devensian till exposed on the Tresco coast to the north of Cromwell's Castle

Gravelly till exposure on Tregarthen Hill towards the northern tip of Tresco. There are many such exposures within the Devensian ice limit. This material is sometimes referred to as the "Hell Bay Gravel" -- but I prefer to refer to it as till.

Gravelly til exposed at the ground surface on the plateau near the eastern tip of St Martins.

Clay-rich Devensian till with silty bands in flow structures -- Bread and Cheese Cove, St Martins Island. 

Silty Devensian till incorporating faceted and broken erratic cobbles and raised beach debris, exposed in the cliff near Carn Morval on the west side of St Mary's Island. This lies well outside the supposed Devensian ice edge of other field workers.

Some of the literature suggests that the only deposit of coherent Devensian till on the Isles of Scilly is at Bread and Cheese Cove on the isle of St Martins.  This is incorrect.  There are exposures of the till in many different locations within the ice limit, generally at the coast in low cliffs which expose granite breccia and other deposits, but sometimes at the ground surface inland.  On St Martins we can see till in the core of the moraine that links White Island to the main island, and in the spectacular cleft called Chad Girt.  On Tresco till is exposed in the cliffs to the north of Cromwell’s Castle.  On Bryher there are many exposures of till, on the western and northern flanks of Gweal Hill, at the southern and northern ends of Popplestones Bay, and in the west-facing cliffs of Hell Bay.  On the high plateaux of Bryher, Tresco and St Martins there are clear indications of ice action over 40m above sea level, with a scatter of erratics in a gravelly and sandy matrix.  In the literature these high-level deposits are referred to as the Tregarthen gravels and the Hell Bay sandy silts, but I think there is no merit in seeking to differentiate them, since the matrix of the till is highly variable, depending on the nature of the terrain and the deposits which the ice has passed over.  There are some sandy silts on Tregarthen Hill, and some gravelly tills on the slopes of Hell Bay.

Contrary to what is stated in the literature, there is also thin and patchy Devensian till on the north and west coasts of St Agnes Isle, as far south as Long Point.  It is nowhere more than 50 cms thick, and it is difficult to interpret because it is not particularly clay-rich, but consists largely of reworked raised beach materials mixed with occasional faceted erratic cobbles.  As it moved in from the north or north-west, the ice must have overridden and incorporated large quantities of old storm beach and shingle beach materials.   At White Par the till lies on top of 2.5 m of “lower head” or grus with granite clasts in a gravelly granular matrix.  Elsewhere it lies above 1m of light brown silts showing some bedding structures, and beneath 1m of gravelly granite breccia capped by darker silts and sands at the ground surface. 

According to Scourse, Hiemstra and others the “Scilly Till Member” (the name given to this Devensian till) is overlain by the “Tregarthen Member” (sandy gravel with erratics) and then by the “Hell Bay Member” which consists of sandy silt with erratics incorporated.

I do not agree that the “Hell Bay Member” of the St Martins Formation is stratigraphically higher or younger than the “Tregarthen Member” -- life would be much simpler if we referred simply to till which has been subject to varying degrees of reworking and redistribution, depending upon slope and moisture conditions.  Some of the tills appear to be thin flow tills which have moved into depressions or gaps between granite outcrops. It appears to me that this “redistribution” was actually a part of the depositional process, and was not something that happened hundreds or thousands of years after the ending of the glacial incursion across the coastline.  Hiemstra et al (2006) refer to the sequence of events “glacitectonization” of older sediments pushed onshore during a short-lived surge, followed by “paraglacial redistribution” and then by “periglacial disaggregation”; this is all a bit of a mouthful, and I am not sure that there was much in the way of periglacial activity here anyway.  Of course there was, at the time, no such thing as a coastline here........

Clay-rich Devensian till exposed at Chad Girt, on the eastern side of White Island.  This till is very similar to that exposed in Bread and Cheese Cove on St Martin's Island.

 A patch of till exposed south of Carnew Point on the west side of St Agnes Island.  It rests on sandy reddish colluvium.  There are other similar exposures near Long Point, where the thin till rests on c 2.5 m of  rotten granite breccia.  This is the first time that Devensian till has been described from St Agnes Island.

Erratic rich Devensian till at little Popplestones, on the west side of Bryher Island.  This exposure is just outside a morainic ridge, and lies beyond the Devensian ice limit of other authors.

It appears from my observations that the Devensian ice certainly affected the western edges of the the Isles of Scilly as well as the north coast.  I was not able to visit Samson  and Annet during a one-week visit, but those islands, and the outer skerries, could well have been affected by ice coming in from the north and west, as St Agnes was.  The extreme southernmost extent of the Devensian ice must have been to the south of the islands.