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Monday, 19 December 2011

Impossible? Apparently not..... (modified)


On one of the discussion threads recently somebody said that it was impossible for morainic features to cross one another.  Well, it's not impossible.  I have seen morainic features that intersect, in response to shifting ice edge positions -- for example around the snout of a small glacier.  Here is another interesting example from Bruarjokull in Iceland.  What we have here is a fluted (ground moraine) feature running from the foreground and to the left of the girl, and crossing it at right angles is a crevasse fill -- typical of something that forms in a heavily crevassed and dirty glacier that is wasting rapidly.  Neither of these features on the ground has anything to do with a previous ice edge position.

Also,  notice the scatter of erratics in the background -- the definition on this pic is very good -- just click on it for enlargement.

Postscript

I'm not going to get sucked into one of those interminable exchanges with Kostas here, but as an aid to universal enlightenment here is a map of Wisconsin moraine positions in Illinois.  Note that there are many intersections of moraines.

 The reason for these intersections is that when an ice sheet edge is retreating across a landscape it is very rare for the whole ice edge to retreat at the same rate.  Indeed, there may be short-lived advances during the course of an overall retreat.  These small advances will of course have different dynamics in them -- and the ice streams may come from different directions from those of the earlier ice sheet.  So moraines get truncated and overridden. Hence intersections.  Perfectly simple.

6 comments:

Anonymous said...

Brian,

Quoting from your post:“On one of the discussion threads recently somebody said that it was impossible for morainic features to cross one another.“

Quoting from 'the discussion thread': “... the tracing in time of a receding ice edge – whether circular or linear. It is impossible for such traces to intersect”

A more careful reading will convince you the quote from the 'thread' is true.

BRIAN JOHN said...

Wrong. Linear moraines -- representing retreating ice edge conditions -- frequently intersect.

Anonymous said...

Wrong! “ representing retreating ice edge conditions” is not the same as retreating ice edge!

You are being a little sophistic here! Play on words may fool some of the people some of the time, but does not fool me!

BRIAN JOHN said...

Come off it, Kostas! All right -- I'll say "retreating ice edge." Now -- just go off and do some research, and you'll see what I mean. Google will help you, I'm sure.

Anonymous said...

Brian,

With Christmas just days away, I like to thank you for all that you do and wish you a Joyous Christmas and a Happy New Year to you and all your family.

Your blog on Stonehenge is special just because good discussions can take place. Your generous spirit makes this happen. I know because I know how 'impossible' I can be. Yet here I am, time after time!

Kostas

Anonymous said...

Brian,

One of this intersecting 'glacier moraines' looks like meltwater channel deposits. So straight and so well confined to a very even narrow band. While the other which crosses it does appear to be glacier deposits along the ice edge. If so, its not surprising that these should intersect.

But of course, none of this has anything to do with what I am arguing re: stone alignments marking the ice edge of a contemporaneously retreating glacier locally.

Kostas