THE BOOK
Some of the ideas discussed in this blog are published in my new book called "The Stonehenge Bluestones" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click
HERE

Friday, 21 February 2020

Ice sheets and frontal ice lobes

This map shows just how complex the LGM ice edges were to the south and east of the Baltic coasts -- there were lobes and embayments everywhere.  The article mentioned below relates to the area to the south of the Gulf of Riga.  The line marked "15" shows the approx ice edge 15,000 years ago.

I have been thinking about the behaviour of the front edges of ice sheets during advance, stagnation / stability and then deglaciation phases. This is of considerable importance to our understanding of what happened near the edge of the British and Irish Ice Sheet (or Celtic Ice Sheet) at the glacial maxima of however many glaciations there might have been.  How did the ice edge behave in the Bristol Channel area and in the Celtic Sea?  Did it actually reach Salisbury Plain, and if so, what was the shape of the ice front?  Was it a lobe or a straight ice edge?  (I have always argued for an Anglian Glaciation lobe of ice from the Welsh Ice Cap and from the Irish Sea Glacier, pushing into the Somerset Lowlands and bounded by hill masses to north and south.....)

A great deal is now known of the lobes at the southern extremity of the Laurentide Ice Sheet in North America -- more on that in due course.  But in the meantime, I have come across an interesting article on the Devensian / Weichselian lobe which extended southwards from the Scandinavian Ice Sheet around 15,000 years ago -- in the midst of multiple retreat stages and readvances over many thousands of years.

Reference:
Lamsters, K., Zelcs, V., Subglacial bedforms of the Zemgale Ice Lobe, south-eastern Baltic, Quaternary International (2014), 13 pp.

http://dx.doi.org/10.1016/j.quaint.2014.10.006


The topographic context -- the Zemgale Ice Lobe pressed into the lowland to the south of the Gulf of Riga, bounded and constrained by the uplands shown here quite clearly.



Ab s t r a c t

Presented in this paper are the results of the mapping of ~6600 subglacial bedforms of the Zemgale IceLobe (ZIL), their composition and internal structure in the south-eastern Baltic region. Topographic maps at scale 1:10,000 were mainly used to map bedforms in Latvia, while a digital elevation model with cell size of 5 m was used for North Central Lithuania. The ZIL operated during the deglaciation of the Late Weichselian Scandinavian Ice Sheet (SIS), at least in the Middle Lithuanian and North Lithuanian glacial phases, and created a subglacial landform assemblage consisting of glacial lineations such as drumlins, mega-flutings, Mega Scale Glacial Lineations (MSGLs) and transversal morphologies, such as ribbed moraines. It is observed that lineations of different scales co-exist, suggesting subglacial bedform transition and possibly a continuum of bedforms. Highly elongated lineations with a length of up to 24 km and an elongation ratio of up to 1:50, interpreted as MSGLs, are observed in the NE part of the Middle Lithuanian Plain. In the so-called Zemgale drumlin field, 20% of lineations have elongation ratios >10:1, indicating the fastest ice flow in the central part of the main body of the lobe. Based on test surveys and investigated outcrops, the drumlin cores consist of sorted sediments with different levels and depths of glaciotectonic deformation overlain by subglacial till. The main drumlin-forming ice stress was mostly parallel to the crests of drumlins, while some ice stress from inter-drumlin depressions acted during the final episodes of their formation. The observed sediment structures indicate multiple episodes of the basal ice/bed coupling and decoupling, which can best be explained by the mosaic ice-bed deformation model. Ribbed moraines are characterized by a complicated structure of multiple till units interbedded with sorted sediments that have been formed by repeated subglacial overthrusting.

https://www.academia.edu/9256806/Subglacial_bedforms_of_the_Zemgale_Ice_Lobe_south-eastern_Baltic?email_work_card=thumbnail


Drumins and other bedforms showing ice flow directions around 15,000 years ago.  Most of the observed "glacial lineations" are drumlins or elongated flutes, but here and there the patterns are confusing because Rogen moraines and other transverse ridges have also been formed in response to changing bed conditions.

The interesting thing about the Gulbene Glacial Phase is that the ice has responded in a very subtle fashion to topographic controls (uplands on 100m height were quite sufficient to divert ice which was quite thin). Also, by and large, the streamlined bedforms are perpendicular to the ice edge, as they should be.  In the later Linkuva Glacial Phase, the lobe was of more limited extent.  What is also intriguing is that the ice has swung round through about 160 degrees in the Vadakste area, almost flowing in the opposite direction to the original flow across the Baltic, from north to south.

All other things being equal, ice will fill depressions and spread laterally in response to quite subtle topographic controls.  I am more convinced than ever that when the Irish Sea Glacier was at or near its southernmost position during the LGM, most of the Bristol Channel and the outer reaches of the Severn estuary MUST have been covered with glacier ice.  And during the Anglian Glaciation, when ice was almost certainly more extensive, was there glacier ice on the Somerset Levels, at least as far east as the chalk escarpment?  Almost certainly.


If Anglian ice reached the shelf edge in the Celtic Sea, it is inevitable that the Somerset Lowlands were inundated by glacier ice.


Suggested Devensian (LGM) situation, with Celtic Sea ice pressing eastwards at least as far east as Gower, and morec restricted ice lobes in South Wales.

BRITICE 2016 reconstruction of the extent of ice at the LGM in the Celtic Sea arena.  I do not believe that the streamlines shown are feasible.  Ice MUST have reached the cliff ramparts of Devon and Cornwall, and MUST have extended further up the Bristol Channel.



Lobes and separating hilly ridges on the edge of the ice sheet in West Greenland.  Nature abhors straight lines, and glaciers love lobes.........

Wednesday, 19 February 2020

Fildes Peninsula geomorphology


This is an attractive new geomorphological map of the Fildes Peninsula on King George Island -- the place that I called home for several weeks in 1966.  A fascinating place, but now sadly destroyed by assorted scientific bases, and too many tourists..........


Flat Top Peninsula, on the west coast.  I suppose one could climb up with the proper gear -- but when we were doing our work on the peninsula we landed on the top, courtesy a Royal Naval helicopter.  We found nothing much of interest.........



On Fildes Peninsula west coast the raised marine platform is the most prominent landscape feature. Note the prominent residual hillock on the far horizon.


Residual peaks and areal scouring


Two views from the little Norwegian archipelago of Traena, where these isolated residual peaks can be seen from a great distance away, across the gently undulating surface of the strandflat.

I have been intrigued to see the use of the word "Rauk" used in various contexts -- meaning the steep residual peaks that are left isolated on the Norwegian strandflat or on other extensive areas of areal scouring.  I'm unhappy about the use of this word because in Swedish and Norwegian it is also used for small sea stacks along the coastline.  Clearly these are vastly different in terms of scale, process and surface characteristics -- although, that having been said, there are in some cases old sea caves at the break of slope at the base of the steep rock faces.

In Norway, according to Hans Holtedahl, the word "nyker" is used for these large features which dominate their local landscapes -- but that's a difficult word to translate since "ny" means "new" and "ker" can be used for skerries or low undulating shoals and islets close to sea level -- as in "skärgård" in Swedish..........

Another classic landscape dominated by residual peaks and ridges is that of the Lofoten Islands, to the north of Traena:













There are similarities, but the mountains and ridges are much larger and more spectacular in the Lofotens, and the landscape is best interpreted not as a landscape of areal scouring but as a landscape of intense glacial activity which has been partly flooded by the postglacial rise of sea level.  Look at all those cirque basins, all the way down to sea level.  Shades of the Faroe Islands.  There are traces of the strandflat here, but they are not very extensive.  If we look at the Lofoten landscape as a whole, there are more similarities with the fjord landscape of western Norway than with the outer skerries and strandflat.


As we can see from the above satellite image, the Lofoten Archipelago is really a part of the Norwegian mainland, where the mountains just happen to reach the sea.  In the main islands we are looking at the northern side of a major glacier outlet trough called Ofotfjorden -- clearly visible in the image.  There are other (rather complex) outlet troughs to the north.  The straits between the islands are very narrow, created by ice scouring along bedrock fractures.  The spectacular ridges and isolated peaks are all of glacial origin.

 

Here is another set of rather spectacular peaks -- the Seven Sisters in Hordaland, again very close to the Norwegian coast.  These are thought to have a geological / structural origin, since they are made of hard rocks flanked by strips of softer rocks which have been heavily eroded by ice streaming out towards the coast.  If we want, we can call the summits residuals.........

Fildes Strait, King George Island, South Shetlands, Antarctica.  A skerries landscape (strandflat?) with prominent residuals (probably volcanic plugs) along the present shoreline.

North Spit, South Shetland Islands.  A raised marine platform (strandflat?) with a prominent isolated residual on the right.


And the point of all this?  Well, residuals or isolated peaks with very steep flanks can be formed in a number of different ways.  Like the "monadnocks" on St Davids Peninsula in Pembrokeshire, which almost always coincide withy Ordovician igneous rock outcrops, they may at one stage have been islands in a shallow sea, but glacial, periglacial and even desert processes may at some stages have played a part in their formation.



Carnllidi, near St David's -- seen from Ramsey Island


Coastal platform and residuals (normally referred to as "monadnocks") at the western end of St David's Peninsula. (Photo: Deborah Tilley)






Monday, 17 February 2020

Tröllaskagi, Iceland -- a classic glacial trough pattern

There are trolls in them thar mountains..........


If you shift your focus from the delicate snow-covered uplands in this image to the trough pattern, you can see how glacier outlet troughs evolve, via greater and greater concentrations of ice in the main troughs and the gradual elimination of smaller valleys or feeder troughs.  Eyjafjordur is the long fjord to the right of centre.

Again, click to enlarge.

David Sugden and I were so impressed by a false-colour image of this same area that we used it for the cover of "Glaciers and Landscape".  Our text that was reprinted many times over more than 20 years of use as a key glacial geomorphology textbook for university students.  The cover designer took some liberties with the orientation........



See also this post, from three years ago:

https://brian-mountainman.blogspot.com/2017/01/brutalised-glacial-discharge-dendritic.html

In our book, David and I discussed the relationships between selective linear erosion, glacier trough development, and divide elimination.  This was a favourite topic for Prof David Linton, one of the great thinkers on glacial processes and landscape evolution.  in the 1960s he turned his attention, in a number of influential articles, to LANDSCAPES of glacial erosion, expanding his field of vision from that of earlier researchers who had concentrated on glacial processes and landforms.  These are two of his articles:

Linton, D.L. (1963), The forms of glacial erosion. Trans. IBG 33, 1–28.
Linton, D.L. (1967), Divide elimination by glacial erosion. In: Arctic and Alpine environments (Wright and Osburn Eds), 241-248.

Even more interesting was his seminal chapter on "Landscape Evolution" in the 1964 volume entitled "Antarctic Research" edited by Priestley and others (pp 85 - 99).

He noted how, in the Antarctic Peninsula region, the efficacy of glacial sculpture varied from place to place depending upon aspect, ice cover, altitude and other factors.  He noted how in some areas there were rounded or gently convex mountain tops emerging from the ice sheet edge or else capped by local ice-caps, while towards the coast, at lower altitudes, there were sharp-crested ridges with jagged or pointed peaks.  Some of these ridges were heavily scalloped, especially at lower levels.  These scallops were interpreted as the remnants of cirques, cut by small independent cirque glaciers and in the catchments of small independent valley glaciers.  So the bigger and higher features were the result of erosion by outlet glaciers evacuating ice from large ice masses, while the smaller and lower peripheral features were seen as the result of "local glaciation" at times of less extensive ice cover, for example during interglacial episodes.  The plateau surfaces were interpreted as relatively old remnants of pre-glacial land surfaces, while the scalloped and sharp-crested ridges, aretes and horns (pinnacled peaks) were thought of as being relatively new. 

Given that the landscapes of West Antarctica and the Antarctic Peninsula have evolved across many glacial and interglacial cycles, Linton  developed a model of glacial landscape evolution in which, close to the ice centres, large outlet glaciers get larger and larger, and dismember the pre-existing drainage divides.  Those first-order divides that do survive become longer and longer, but also narrower and narrower as the scalloping effect of intermittent cirque glaciation runs its course.  That's the mpodel, but in the real world, of course, the irregularity of glacial / interglacial pulses has a major effect on how the landscape will ultimately appear..........


The Linton model of trough expansion and drainage divide elimination as a result of long-continued glaciation.  On the left, the assumed original fluvial (preglacial) fluvial dendritic pattern.  On the right, an expanded outlet trough with truncated interfluves or spurs separating a number of contributing glaciers.  The interfluves are narrowed and scalloped by intermittent cirque glaciation and frost processes to create a landscape of aretes and horns.


I'm not sure how often the Linton model would apply in the real world -- for example in Greenland, Iceland and Antarctica.  The model assumes that the whole of the ice supply feeding into the trough comes from within the original drainage cachment area.  In reality, as a glacial episode runs its course, and an upland area "grows" its own ice cap or ice sheet,  the ice supply into the trough comes from OUTSIDE the original catchment, spilling over and into a trough head.  So initially there is a vastly greater ice supply than the trough can cope with -- so it is greatly enlarged, to a disproportionate extent, chopping off many of the interfluves shown in the right-hand diagram above. That is the situation that applies when properly brutalised dendritic patterns develop -- with a huge main trough and very small flanking feeder troughs with their bases "hanging" well above the floor of the main trough.  This is exactly what is seen in Sognefjord and many other big fjords when bathymetric surveys are undertaken. 


Iceland at its best


This is one of the most beautiful satellite images of Iceland that I have ever seen.  The clarity is astounding, and the colours are amazing..........

The image is dated 2020, but I suspect it was captured during the autumn, following the early snowfalls in the northern part of the country.

Click to emlarge.

Vestfirdir, NW Iceland -- more brutalised dendritic patterns


This is a fabulous and brand new satellite image of the Vestfirdir Peninsula ("the western fjords") in NW Iceland, showing once again how streaming ice flowing within outlet glacier troughs tends to eliminate interfluves and spurs, leading to a simplified dendritic pattern dominated by the major ice routeways...........  This pattern has probably evolved over many different glacial episodes.

Sunday, 16 February 2020

Faroe Islands -- and a miniature brutalised dendritic drainage pattern


I'm intrigued by the "brutalised" dendritic drainage patterns that we find in glaciated areas which have been subjected to the intense streamflow of large outlet glaciers during some episodes of the Ice Age. These patterns exist where so much flow is concentrated in the main channels that the delicate bits and pieces of the original dendritic river drainage pattern (in other words the smaller tributaries) are simply chopped off. Interfluves or spurs are shortened, and the pattern is greatly simplified.    We can see this in the annotated satellite image above.

In the Faroes almost all of the remaining drainage routes are directed southwards and south-eastwards.  So the main watershed runs very close to, or actually at, the position of this outer coastline.  This is where there are many cliffs over a thousand feet high.   There are a few northward-flowing trough or valley remnants, as we can see on the image, but by and large there has been so much coastal erosion from the west and north that the original landmass of "trap" or horizontal lava flows has been chopped in half.

So there has been intense glacial activity here during pre-Devensian glacial episodes.  These have not, as far as I am aware, been carefully studied in the archipelago.

The other very intriguing thing about these islands is that these "brutalised" patterns and streamlined troughs are really rather small.  The fjords are seldom more than 4 km wide, and more than 20 km long.  The flanking uplands (on the interfluves) are seldom over 400m high.  (The highest peaks lie over 800m asl.)  So these are really miniature fjords or troughs when set alongside those of Norway, Greenland and Arctic Canada.


It is also interesting that if the above map of Devensian glaciation and iceflow directions is correct (I think it comes from a Norwegian research group) then the old trough pattern seems to have had relatively little influence on glacier behaviour around 20,000 years ago.  Indeed, in some places the ice appears to have flowed ACROSS the old troughs rather than along them.  More work is needed on this.

The most spectacular feature of the glaciated landscape as we see it today is the superabundance of cirques and cirque remnants, some of them with basin floors less than 200m above se-level.  How many generations of cirques are there?  Quite possibly several, although there is a preferential orientation of north and north-east -- as we would expect with winds (and snowdrift) from the W and SW, and heavier shading on the N, E and NE flanks of the upland ridges.






Ridges and cirques in the NE part of the archipelago. Norte the old trough pattern.




Cape Enniberg, at the northernmost tip of the archipelago. At 754m high, this is the highest sea cliff in Europe and maybe in the world, depending on how one interprets the mix of natural processes involved in cliff creation! Anyway, to the left there was once a substantial landmass,

now removed by the sea........


Just to get some sort of scale here, the top edge of the Enniberg cliff lies at a far greater altitude than the highest point on Mynydd Preseli (536m).


================================

More on the glaciation of the islands:

Jørgensen, G., Rasmussen, J., 1986. Glacial striae, roches moutonne´es andice movements in the Faroe Islands. Geological Survey of Denmark; DGU Series.


Pleistocene glacial history of the NW European continental margin
Hans Petter Sejrup, Berit Oline Hjelstuen, K.I. Torbjørn Dahlgren, Haflidi Haflidason, Antoon Kuijpers, Atle Nygard, Daniel Praeg, Martyn S. Stoker, O. Vorren.
Marine and Petroleum Geology 22 (2005) pp 1111–1129



Extract:
Glaciation of the Faroe Islands

Few details are known from the glacial history of theFaroe Islands (Fig. 2a). Nevertheless, pioneering work by Helland (1879) demonstrated that the Faroes once were glaciated. During the Weichselian glaciation the ice cap covered the islands up to at least 700 m above sea level, and extended onto the shelf well beyond the present coastline (Jørgensen and Rasmussen, 1986). The latter authors also concluded that during the Quaternary the Faroe Islands were repeatedly covered by a local ice cap. No foreign erratics have been found, and glacial striations show a radiating pattern from the larger islands. Radiocarbon dating and pollen stratigraphy indicate that glaciers may have been present in the larger valleys until the beginning of the Holocene (Johansen, 1975; Johansen, 1985).  Marine geological studies suggest offshore grounding of the Faroe Islands ice cap down to present water depths of at least 400 m (Waagstein and Rasmussen, 1975). Around the Faroe Islands evidence of glacial reworking of sea bed sediments has been found down to water depths of around 750 m (Kuijpers et al., 1998), whereas seismic sections fromthe northern slope of the Faroe Platform display evidence of glacial reworking of sediments in water depths of up to 900 m (Nielsen and van Weering, 1998). In addition, on the Iceland-Faroe Ridge (Fig. 2a) isolated iceberg ploughmarks are observed down to 960 m water depth (Kuijpers et al.,1998).