Strandflats, skerries, archipelagos and areal scouring

Breidafjordur, west Iceland.  Here there is an extensive archipelago of 2,700 islands.  Click to see this image at higher definition.

Lochmaddy, North Uist, Outer Hebrides, Scotland.  A classic example of a "skerries landscape" linked to a local strandflat

In recent posts I have examined some of the places where areal scouring, dead-ice conditions and maybe cold-based glaciers have combined to create undulating terrain and occasional areas of "knock and lochan" terrain.  The main feature is always sluggish and diffuse ice flow without any concentration of flow into streams, troughs or channels.  These rather beautiful wilderness areas are usually at high altitude, on plateau surfaces where ice cap generation takes place and where -- after ice cap growth -- the highest and most remote ice domes or ice-shed axes are located.  The Glama and Dranga plateaux in Iceland are examples, as are the Teifi Pools area in mid Wales and the Hardangervidda in Norway.  So these are essentially "ice source" areas where very specific glaciological conditions have applied, over and again during the Quaternary as one glaciation has followed another.

There are obvious similarities between these plateau landscapes and the coastal landscapes that fringe some areas of intensive glaciation where fjords have been created.  These are some of the most spectacular landscapes on earth -- for example the fjord landscapes of western Norway, East Greenland and parts of Arctic Canada.  The general principle that seems to apply is that when ice-flow is concentrated within outlet troughs on the fringes of an ice sheet or an ice cap, erosion will make each trough deeper and deeper as long as there are supplements to discharge -- but as soon as the possibility of diffluence occurs (ie when the glaciers reach a pre-existing mountain front) ice-flow will spread sideways and erosive capacity will be suddenly diminished. Then instead of troughs and channels created by streaming ice, we will see the development of wide open plains of undulating bedrock under the influence of areal scouring processes.  This is one of the most spectacular "process transformations" in nature, and when David Sugden and I were writing "Glaciers and Landscape" back in the stone age, we were very fascinated by it!  I have done a number of related posts on this blog......

The Sognefjord long profile, showing how the trough has been deepened steadily so long as there have been supplements to glacier discharge.  But as soon as diffluence became possible, close to the outer coast of Norway, erosive power was lost, and the fjord bed rises to the trough "threshold".  The skerries occur off the left edge of the diagram, where concentrated ice flow and erosion were replaced by areal scouring.

At the end of each glacial episode, when sea-level returns eustatically to more or less its interglacial level, these low-relief undulating rock platforms are flooded by the sea and are transformed into skerries and archipalagos with a  myriad of small islands, straits and channels with rocks and shoals all over the place.  Because there are no big glacial exit routes across these areas, the water is nowhere very deep, and fractures and other structural controls can generally be picked out in aerial photographs and satellite imagery.  There's a big debate about whether these extensive undulating platforms located at or around present sea-level are really cut by ice, or by other processes including marine erosion or by the cutting of ancient platforms by river erosion in pre-Quaternary times.  Some authors suggest that the Norwegian strandflat is a very ancient feature that has been covered by sediments which were stripped away in pre-glacial times -- suggesting that glacier ice has simply "occupied" an ancient feature without necessarily altering its appearance very much at all.  There is a good discussion here:

https://en.wikipedia.org/wiki/Strandflat

The archipelagos around the coasts of the Baltic, in Sweden and Finland, are different again, since here we have one of the world's great Pre-Cambrian peneplains.  There are abundant traces of ice scouring on a large scale across that part of the Stockholm Archipelago which I am familiar with, but there is no doubt that the essentials of the landscape are pre-glacial.  Some archipelagos, like the Bear Islands in Hall Bredning, just beyond the threshold of Nordvestfjord in East Greenland, might be referred to as the products of local rather than regional glaciological conditions, and the same may be said of Lochmaddy in North Uist -- see the photo at the head of this post.

This is an intriguing topic, worthy of greatly extended treatment -- and of course this is another question worth asking:  "Why is it that some glacier troughs that carried vast volumes of ice during the Quaternary glaciations have extensive skerries beyond their exit thresholds, and others do not?"

Another photo of the skerries in Breidafjordur, western Iceland.

Bing satellite image of part of the Stockholm Archipelago -- an area of modest surface relief underlain by the rocks of the Scandinavian PreCambrian Shield.  The area was covered by thick ice flowing directly southwards, but there are no clear troughs or other discharge routes.  Areal scouring affected this whole area. The details of coastal configuration are determined above all else by fracture patterns and lithologoical variations in the PreCambrian basement rocks.

Belcher Islands in Hudson Bay.  One of the strangest archipelagos in the world..........  made of PreCambrian sedimentary rocks which have been heavily eroded by overriding ice across several glaciations of North America.  The rocks are tightly folded and deformed -- if we look carefully we can see several pitching synclines and anticlines.

Archipelago of heavily abraded Shield rocks near Nain, on the east coast of Labrador.

These prominent peaks on one of the Bear Islands, in Hall Bredning, Scoresby Sund, East Greenland, lie within an unusual small archipelago around the outlets of Nordvestfjord and Ofjord.  This area has been affected by laterally spreading ice from these two huge outlet glacier routes, but strictly the Bear Islands should be interpreted as the seriously damaged right flank of the Ofjord glacier trough outlet.    Substantial glacial erosion has damaged and even whittled away most of the trough wall, leaving a few spectacular ridge remnants.