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Tuesday, 21 September 2021

The Nordvestfjord threshold

Scan of the topographic map of the Hall Bredning area. Note that the fjord width is compressed to just 7 km between Pythagoras Bjerg and Kloftbjerge to the west.  The fjord then opens to about 10 kms width at its exit.   

This is from a somewhat complex paper about the hydrology and water characteristics of the Nordvestfjord - Scoresbysund fjord system.  Forget about the symbols and concentrate on the bottom profile!  The reverse slope at the threshold is truly spectacular.......
I'm revisiting the topic of thresholds at the exits of glacial troughs because the other day I needed to go into considerable detail when chatting to some visitors to my little Bluestone Museum on exactly this topic.  They had heard that ice does not travel uphill, from somebody arguing that the ice of the Irish Sea Glacier cannot possibly have surmounted the chalk escarpment at the western edge of Salisbury Plain and could not, therefore, have carried erratics from west Wales onto the chalk downs.

Well, the informant has got it all wrong, and if a group of senior glaciologists and glacial geomorphologists says that  the glacial transport thesis is perfectly feasible from a practical and theoretical standpoint, that's good enough for me.

'Dynamic cycles, ice streams and their impact on the extent, chronology and deglaciation of the British–Irish ice sheet.'
Alun Hubbard, Tom Bradwell, Nicholas Golledge, Adrian Hall, Henry Patton, David Sugden, Rhys Cooper, Martyn Stoker
Quaternary Science Reviews 28 (2009) 759–777

Back to Nordvestfjord and Scoresbysund:

The Nordvest Fjord - Scoresby Sund system has clearly been one of the major outlet routes for ice from the Greenland Ice Sheet, during the whole of the Pleistocene and maybe for much longer than that. Even today the Daugaard Jensens Gletscher, near the head of the fjord, is possibly the most productive glacier in the whole of Greenland. Because the ice here has been streaming so effectively in a narrow and constrained trough, the rate of downcutting has been impressive indeed. There are no proper bathymetric charts, but from the scattered soundings that have been made we see depths of 1372m, 1459m, 1372m, 1150m, 1237m, and 1290m between Eskimo Bugt and Syd Kap. The deepest sounding of all is 1508m (4,947 ft). These soundings show that the fjord is substantially deeper than Sognefjord in Norway (maximum known depth 1308m), which has just one short stretch deeper than 1200m. 

Sognefjord long profile, Norway.  Note that there is a 1000m high reverse slope at the trough exit, over a distance of c 20 km.  This is the threshold and the point at which diffluence or lateral spreading of the ice has reduced erosional capacity to a very low level.

But here on the flanks of Nordvest Fjord the plateau ice caps and mountain summits are almost all over 2000m (6561 ft), whereas there is little land over 1600m on the flanks of Sognefjord. So the full depth of Nordvest Fjord over a distance of about 80 miles is approx 3300m or 11,000 feet. I'll let somebody else work out how much material has been eroded and removed by ice from a trough of this size....... but it is indisputable that this is the deepest, longest and most dramatic fjord system on earth.

Hall Bredning, with the twin islands of Ingmikertaje right in the middle of the channel.  Ice has flowed around this blockage and over "sills"at a depth of c 300m -- but nonetheless the survival of these islands is striking. Here too the reverse slope at the fjord exit is about 1000m over a distance of only about 5 km.

Some very useful information is found in this publication, showing a series of connected basis separated by sills:

Long profile of the bed of Nordvestfjord from the snout of DJ Glacier to the vicinity of the trough exit.  The ridges between the troughs are a puzzle, and another puzzle is the lack of an obvious relationship between tributary glacier inputs and trough deepening.  However, the threshold backslope, about 1000m high (approx the altitude of Mount Snowdon) and just off the right end of the profile, is both sudden
 and very steep.  

The deepest section of the fjord begins about 20 km from the DJ Glacier snout, and continues for about 30 km with depths around 1400m before shallowing to around 1200m; this may reflect erosive power or capacity, but it may well be that sediments are much thicker in the middle and outer sections of the fjord, with the bedrock floor maybe several hundreds of metres below the "sediment floor".   Some troughs have beds which are divided up into a series of connected basins. According to Julian Dowdeswell and others the bed of Nordvestfjord is like this, with a series of deep basins (over 1200m deep) separated by sills between 600m and 900m deep. I have not seen the detailed long profiles, and so we can but speculate as to whether the sills coincide with outcrops of highly resistant rocks (on the basis of lithology or structure) and whether the basins coincide with pulses or additions to glacier discharge derived from tributary glaciers. Near the trough exit the bedrock floor rises very steeply indeed, but sediments in this exit zone can be very think as well.   In Hall Bredning the glacial sediments are at least 100m thick.  But the essential point, from the long profile soundings, is that the reverse slope is truly spectacular, rising from a depth of 1270m to the threshold shallows in only about 5 km. 

This is an extract from a geological map of the area, showing the bathymetry around the fjord exit.  Where the threshold occurs, the trough is about 10 km wide. The backslope is seen here to rise from a water depth of over 1000m to the two small islands over a distance of just 4 km.  The shallow sill stretches from the islands to the coast near Syd Kap  at a depth of c 300m.  But to the west of the islands the threshold is breached by a deeper channel in which the water is over 800m deep.  This channel swings round towards the SE between the islands of Ingmikertaje and the Bear Islands -- so this was clearly the main ice discharge route at some stage towards the end of each glacial episode. 

This is an Ernst Hofer image of the Bear Islands, looking NW from the southern tip of the archipelago.  Note the winding ridge of moraine, on the island called Sulugssut.  Is it linked to the Milne Land stage moraines in Kjove Land?  Some fieldwork is needed......

This is a good explanation from Wikipedia for what goes on in the deepest parts of closed troughs where overdeepening happens on a spectacular scale.

"Analytic glacial erosion models suggest that ice flows passing through constrained spaces such as mountain passes produced enhanced erosion beneath thicker, faster ice flows, which deepens the channel below areas both upstream and downstream. The underlying physical phenomena is that erosion increases with the rate of ice discharge. Although this simplifies complex relationships among time-varying climates, ice sheet behaviors and bed characteristics, it is based on the general recognition that enhanced ice discharges typically increase the erosion rate. This is because the basal sliding rate and the erosion rate are interrelated and driven by the same variables: the ice thickness, the underlying bed slope, the overlying glacial slope and the basal temperature. As a result, the modelled fjords are deepest through the narrowest channels (i.e., regions with the highest surrounding highest topography). This corresponds with actual physical observations of fjords.[16]"

Where was the ice surface when the Nordvestfjord Glacier was reaching its exit and starting to spread laterally?  Let's be conservative and pretend that the snout surface was around 200m in the vicinity of Syd Kap.  Suddenly a glacier that was 1.4 km thick was forced to become a glacier approx 400m thick, flowing over the threshold.  The ice volume in the threshold area was thus 1 x 7 x 5 kms = 35 cubic km of ice being forced like toothpaste out of a tube, up a reverse slope 1000m high, and then away into the unrestricted terrain of Hall Bredning.

Who was it that said that glacier ice could not possibly flow uphill?


Tony Hinchliffe said...

Have you given some consideration towards any landform which may survive as a relict of the Anglian glaciation e.g. along the escarpment of Salisbury Plain? (albeit affected by later erosion etc).

BRIAN JOHN said...

The scarp has to be pre-glacial in age --- the question is: how might it have been modified by overriding ice? The situation is very different from that of a fjord threshold, because here we had a much more open landscape in which the ice would have "fingered" into the lowlands and would only have overtopped escarpments when itv had sufficient thickness to maintain its profile -- ie a continuous gradient from source to outer edge. I would dearly like to see some medium-scale modelling on this, taking into account the details of local topography. (At the moment the modelling is all large-scale, covering the whole of western Britain....)

Tony Hinchliffe said...

Here's just a very amateurish, naive comment: around 8 or 10 miles ENE of Westbury is West Lavington. Leslie Grinsell the renowned archaeologist refers to the "Lavington Gap" on page 37 of "The Archaeology of Wessex", 1958. Last Thursday I drove along the Westbury B3098 road before turning SSE towarda Tilshead parish (within which Grinsell says is "the remarkable group of six long barrows). To make this SSE turn into the A360 I had to go through this " Lavington Gap" into the Salisbury Plain. Beyond these six barrows, to the SE, is Stonehenge itself.

BRIAN JOHN said...

I looked at the matter of the chalk scarp in 2010 and again in 2015. There are other relevant posts too. Maybe have a look at some of them by putting in "chalk escarpment" in the search box?

Philip Denwood said...

Kellaway says (1971 p.35) "the western part of the Marlborough Downs has some of the most characteristically glaciated profiles to be seen in this part of Britain. The moulded outlines, broad U-shaped dry valleys and melt water channels of the downlands are in some respects the most interesting of the glacial features …"

BRIAN JOHN said...

Well, the hills in and around the Somerset Levels certainly do look like roches moutonnees, and I have commented on this before. As for the dry valleys, periglacial processes explain them rather well, but of course we cannot discount meltwater from melting glacier ice as well.