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Monday, 21 March 2011

On ancient shorelines



When I was a young researcher, I spent a lot of time hunting for shorelines -- in Iceland, Greenland, Antarctica and Norway -- and in the UK as well.  If one can find them, they can tell one a lot about events related to glaciation in general, and glacier retreat in particular.

Raised shorelines can either be related to  sea-level (if there is access to the open coast) or to pro-glacial lakes, which tend to form in "closed" situations where water levels can oscillate rapidly and where catastophic drainage can sometimes occur.  Quite literally, one year an ice-dammed lake  maybe 200m deep may be there, and the next year it's gone.  Some lakes can fill and empty over and again, causing some confusion in the interpretation of shorelines or "washing limits."  But the general rule is that the highest shorelines are the oldest, and the lowest ones are the youngest.  Another rule is that where there is isostatic readjustment going on, the highest shorelines (if you can trace them along many miles of a shore) will have the highest tilts, and the lowest ones will be increasingly closer to horizontal. Where you find sets of shorelines, they can intersect.  In general, if you find a shoreline you assume that it represents some sort of stillstand which coincides with a stabilisation of ice volumes over maybe decades or centuries.  But the speed of development of a shoreline will depend on the amount of wave action on a shore, the annual length of the ice-free season (when the surface of the water is free of ice), the fetch, and the nature of the shore itself.  A sediment shore will leave traces of a stillstand far better than s steeply-sloping clean bedrock shore.  You have to think in all dimensions -- and interpreting shorelines involves a highly complex sort of multivariate analysis.

The pictures above show the 3 shorelines of the Parallel Roads of Glen Roy, in Scotland, a set of shorelines alongside a pro-glacial lake in South Georgia, and a shoreline (yes, that's me in 1966!) in the South Shetland Islands, Antarctica.

This info is all relevant when it comes to trying to work out where the Devensian ice limit was in SW Britain, since there is much talk of lakes particularly on the southern coasts of Cardigan Bay. (See the earlier posts relating to the South Wales End Moraine, Glacial Lake Teifi and "Lake Brynberian"........)

In spite of years of hunting, I have never found anything that I would call with any certainty a glacial lake shoreline, either in the Teifi Valley or the Nevern Valley.  I'm pretty sure that this means the glacial lake inundations were VERY short-lived, with no water level static enough, over a long enough period of time, for the formation of shorelines.  As I have suggested earlier, the answer may lie in the varves.  But I'm still putting my money on the ice remaining at its maximum position for no longer than a few decades.

Another complication, with Lake Teifi and maybe the other North Pembrokeshire lakes, was that maybe the oldest shorelines were lower than the youngest ones -- with the suite of shorelines representing a gradually rising water-level as the ice advanced across valley mouths and meltwater accumulated to higher and higher levels as the ice dam became stronger.

3 comments:

The Stonehenge Enigma said...

Nice article Brian.

You suggest that flood areas must be short lived as you have failed to find the evidence and you judge this to be just a 'few decades'.

Unfortunately, that is not very scientific! There must be sufficient volumes of this evidence (whatever it is) you are searching for with the estimated length of time it took to produce from the inundation to calculate the time necessary for minimum volumes to be seen.

As example in rivers alluvium sediments are sought to confirm river height and flow. But these sediments take 1,000's of years of river flow to produce sufficient quantities (an inch or so!)) that can survive the 10's of thousands of years between origin and our search.

So what exactly are you looking for? and in what quantities?

RJL

BRIAN JOHN said...

Not sure what your point is here, Robert. It is perfectly scientific to look for evidence, fail to find it, and then say so! Scientists do that sort of thing all the time. One reason why shorelines might be missing is that 20,000 years or so have elapsed since the ice melted away and these lakes drained -- and on steep slopes in a moist environment like West wales, solifluxion and root disturbance might have had a considerable effect in wiping away small shoreline traces.

When I get the time I'll try to find out more about the varves in Lake Teifi........

Alluvium in rivers need not take thousands of years to accumulate an inch or so. You can get an inch of alluvium formed during the course of a single storm or flood event. Whether that will survive and get incorporated into a thicker sequence of sediments is in the lap of the gods -- ie it is down to exactly what sequence of erosion / deposition "events" may occur at a particular spot on a river bed.

Constantinos Ragazas said...

Brian,

I am always delighted whenever you discuss lakes, ice and glaciers! Thanks for your post! I found it very informative.

Kostas