The Stockholm Archipelago and areal scouring

 

This is a fantastic image posted on Facebook, showing a large chunk of the Stockholm Archipelago in March 2024.  The spring thaw is under way, but we can see a lot of sea ice still choking the straits, sounds and bays of the inner archipelago.

Although in detail we can see multiple traces in the landscape of glacial erosional processes, glacial debris transport and dumped glacial deposits, this is a classic landscape of "areal scouring" in which ice flow has been more or less uniform over the whole landscape, with only minor diversions into streaming flows in shallow troughs or valleys.  Ice flow here was almost exactly north to south -- away from the camera.

In previous posts I have described 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. These rather beautiful wilderness areas can occur 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.

But the Stockholm Archipelago was not an ice source area -- it's an ancient PreCambrian Shield area close to sea level, with water that is not very deep and hills that are not very high.  The ice that originated far to the north extended far to the south.

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......

If you put "Stockholm Archipelago" into the search box you can see some of my earlier posts.

By the way, our summer cottage is very close to Granören, on the left edge of the photo -- many of the glacial features on the eastern edge of Blidö are described in other posts on this blog.