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Wednesday, 11 January 2017

Brutalised glacial discharge dendritic patterns

For many years I have been fascinated by the difference between dendritic drainage patterns developed by rivers and those developed by glaciers.  Here is a past post:

The illustration above is a satellite image from the northern coast of Iceland, in the uplands west of Akureyri.  (David Sugden and I used a similar image, with more snow, and false colours, on the front cover of our geomorphology text.)

On the main troughs in these images, there has been so much ice to be discharged that all of the smaller and more delicate fluvial valleys that existed in pre-glacial times have been "brutalised" out of existence, and many of the larger interfluves have also been removed.  So the troughs are wide and deep, with relatively few feeder valleys -- and even these latter features are very wide, with prominent trough heads.

Contrast this sort of landscape with that of Sognefjord:

This is one of the biggest fjords in the world (see recent posts), and yet it has retained much more of a delicate dendritic pattern which looks as if it has not changed all that much since the time when there was a fluvial landscape of hills and valleys on the western slope of the Scandinavian mountain range.   Some people use the term "fractals" when looking at an image like this.  Why the difference?  It's a bit of a puzzle.........

Now let's look at two segments of the Putorana Plateau, in central Siberia.  This one is from the NE segment of the plateau.  It was heavily glaciated at least twice, but there are no cirques in this landscape, and the trap plateauaway from the valleys appears almost unmodified by glacial processes.  The dendritic valley pattern is delicate -- and reminiscent of that of Sognefjord.

The plateau edge in the above image is just off the top edge of the photo.  So it appears that close to the point at which the glacier in question was decanting onto the adjacent lowland, the trough actually narrowed.  Was that because ice was spilling out of the trough and flowing across the slopes on either side?  Very intriguing.  My assumption is that in this part of the Putorana ice sheet the ice was cold-based and frozen toits bed -- except in the deep valleys where bottom melting occurred, and where glacial processes could operate.  Let s callthis a classic example of a "glacially protected" landscape......

If we then look at the NW segment of the plateau, only about 200 km away, we see a completely different type of glaciated landscape:

There is almost a rectilinear pattern here (maybe controlled to some degree by fractures in the plateau basalts) but the pattern is much more akin to that of the landscape west of Akureyri in Iceland.  The old river valley pattern has been simplified and brutalised, with smaller valleys and interfluves obliterated.  The ice that operated here is much more likely to have been warm-based, and presumably it flowed rapidly in these deep, wide troughs.

And if we zoom in we see a landscape dominated by glacial erosional features.  There are literally thousands of cirques, the majority of them opening northwards -- these must date from glacial phases before and after the episodes of fully-fledged ice sheet cover.  We are looking at more or less the centre of the photo above.

I must seek some glaciological opinion on all of this.........

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