Sand and ice 2: sorting out a glacial mess

Those folk who have had the pleasure (dubious, at times) of wandering around the front of a glacier know what a mess the place is – hardly an ideal campsite, but then necessity, as I can attest, can be a mother.

Glaciers are mighty combinations of bulldozers and conveyor belts on a massive scale, scouring the landscape and transporting huge volumes of carelessly jumbled debris: boulders, pebbles, gravel, mud, and, of course, sand. Only when the ice melts do the streams of water begin to sort things out; but they can do this quite efficiently, and significant sources of high quality aggregates, sand and gravel, owe their origin to the processes going on as glaciers retreat (I’ve written earlier of how the glacial sands of Long Island built New York). But those processes are complex and varied, and leave behind a chaotic landscape, a topography filled with arcane terms – eskers, kames, drumlins, and kettles. But hidden within that chaos are the geo-clues that, if examined carefully, provide the forensics for reconstructing the retreat of the glaciers. And it is this kind of detective work that is the subject of the second in my thematic miniseries, “Sand and ice.” As I described in episode 1 last week, a random selection of two journals from the stratigraphic pile of reading-to-catch-up-on provided this theme, the second one being from The Proceedings of the Geologists’ Association. In the latest issue, Stephen Livingstone and his colleagues at Durham University tell a meticulously documented story of how an area of Cumbria, known to the cognoscenti as the Brampton kame belt, records the last retreat of the British-Irish ice sheet ten to twelve thousand years ago.

 
The ice sheet in this region retreated westwards, away from the uplands of the Pennines and the Lake District, eventually leaving behind, as sea level rose, the landscapes of the Solway Firth and the surrounding lowlands. As the ice retreated, all the geological rubbish that was carried on top of the glaciers, trapped within them, contained in meltwater channels below them, and transported by rivers in front of them, was left behind, dumped. The landforms that result can be referred to as glacial karst. This was not a term that I had encountered before, thinking of karst only as the dramatic limestone scenery that results from the collapse above dissolved cavities and large cave systems. But that process of solution and collapse occurs as a glacier melts – except that the solution is the melting and disappearance of the ice. Among the strange topography that results, are kames - “steep-sided, variously shaped mounds composed chiefly of sand and gravel, formed by supraglacial or ice-contact glacifluvial deposition.” These landscapes often show topographic inversion – in other words, what was once a low point is now high, and vice-versa. Think of a lake forming in a depression on top of a glacier, and visualise that lake being filled with sediment carried into it by meltwater and flowing from collapsing moraines; imagine then what happens when the glacier disappears. The pile of lake sediments are undermined and will subside and be deposited on what was the land surface below the ice and form a hill, a kame – a topographic high. It’s this kind of process that formed the jumbled topography of the Brampton kame belt.  

The approach that Livingstone and his colleagues have taken is simply a classic, elegant, piece of geological work, putting together and meticulously documenting, the pieces of the puzzle. The land today is covered in farms, villages, and vegetation, but high-resolution radar imagery provides an uncluttered view of the landscape for detailed mapping. Boreholes provide data from the subsurface, but, most importantly, there remain a number of sand and gravel pits in the area, some of them still active. These provide three-dimensional views of glacial rubbish dumps:

  The paper describes how the authors defined a series of lithofacies associations (LFA) from these spectacular exposures. A “facies” is the collection of characteristics of a sediment that are diagnostic of the environment in which that sediment was deposited. In broad terms, the sands of a beach will be different from those of a river bar, but the diagnostic distinctions of different facies are far more subtle and detailed than that. “Litho” simply means that the facies are defined by sediment character (grain size, structures, and so on), rather than including biological or fossil evidence. And associations means that two  or more facies are commonly found together and jointly contribute to the environmental diagnosis. So, having got the jargon out of the way, here’s the kind of detailed depiction of what can be seen in two faces of the aggregate pit shown above (best to open up the full-resolution image in a separate tab in order to see the detail).

 
All this data is collected from observations over just 15m vertically, and perhaps 30m horizontally; the illustration shows the spatial distribution of the different sequences of sediments, the grain size and internal structures (ripples, for example), and the relationships between underlying and overlying beds – all of which lead to the definition of three distinct LFAs. These sediments are interpreted as having been deposited in complex river (fluvial) systems flowing across the debris-strewn “dead ice” of the melting glacier. The paper references modern-day analogues from Iceland for the lithofacies associations – in the middle photograph below, this kind of “dead ice” can be seen in the banks of a lake set in the midst of the dynamic sedimentary chaos on top of a melting glacier.  

 
In the walls of the Brampton sand pits, there is further dramatic evidence of subsidence as the ice beneath piles of sediment melts: faults. In the lower right photo of the exposures, above, there are beautiful miniature faults cutting across the beds, formed as the pile of sediments collapsed.

This kind of approach is documented from boreholes and other sand pits, and the paper concludes with a description of the sequence of events that leads to the development of these complex landscapes of glacial karst and topographic inversion (ah, one other piece of terminology – diamicton refers to any sediment that contains a wide range of grain sizes, from boulders to mud, generally glacial in origin and otherwise known as till) :

I enjoyed this work as the second episode of my miniseries on sand and ice, and I hope that I have conveyed its classic and meticulous approach. It’s a great illustration of how careful examination of the sedimentary record (and, of course, the testaments of sand in particular) help us understand our surroundings and the landscapes of our planet. I’ll leave you with a couple of stunning photographs of glacial sediments from the Open Geoscience site of the British Geological Survey:

 

[The full reference for the Livingstone et al. paper is: The Brampton kame belt and Pennine escarpment meltwater channel system (Cumbria, UK): Morphology, sedimentology and formation, Stephen J. Livingstone, David J.A. Evans, Colm Ó Cofaigh and Jonathan Hopkins, Proceedings of the Geologists’ Association, 121 (2010), 423-443. This was first published online in 2009, but unfortunately is not open access. However, Livingstone’s doctoral thesis, “Reconstructing ice dynamics in the central sector of the last British-Irish Ice Sheet” is available in its entirety (all 119 mb!) online at http://etheses.dur.ac.uk/195/. The image at the head of this post of kames in the proglacial area of Virkisjokull, Iceland is from the BGS image collection.]

 

 

Comments

[F]

I love glacial geology. And it connects back to Solway Firth? I'll be re-reading those posts again. :)

[Sandglass]

Yes, it's strange how things keep linking up - but, then again, maybe not so strange....

[sand pits]

I can remember taking a Geography trip to the Alps when studying for my A-levels. I visited a glacier (can't remember where exactly in the Alps, it was a long time ago) as part of the trip and I have to agree that once you get over the size of the glacier there really isn't too much to look at apart from the debris and destruction it has caused. I visited in the summer on a particularly hot day, so one thing I did appreciate was the cool air that seemed to emanate from the glacier when you stood near it.

[Richard Bready]

Not much to look at? Artists disagree. See, for intriguing references, http://www.tate.org.uk/tateetc/issue21/watercolourmacfarlane.htm
Not much to look at? Tell that to the moraines.

[Sandglass]

Yes, glaciated landscapes have long been an inspiration for artists fascinated by "the sublime" - Towne was one, as was Ruskin, and, along those lines, see my post on Viollet-le-Duc and Mont Blanc - http://throughthesandglass.typepad.com/through_the_sandglass/2010/02/architecture-mont-blanc-and-eventually-sand.html

And interesting to see that the piece you link to from the Tate is written by Robert Macarlane, author of several compelling books, including my favourite, "Mountains of the Mind" - a fascinating reflection on the relationship between mountains and us. He describes the grip of the sublime on our imaginations.