I have written endlessly on this blog about glacial entrainment and the transport of blocks and debris beneath, within and on top of a glacier -- but whenever I think about the processes involved I am reminded that we know hardly anything. All we can do is speculate, since direct observation is almost impossible, given the physical problems associated with working within or under a glacier, and the time scale involved. The time taken for an erratic to travel from its entrainment point to its "final destination" can be measured in centuries if not millennia........... or maybe in some cases, just a few years.
There is remarkably little in the standard glacial geomorphology text-books (including mine!) but some authors suggest that erratics are thrown together as in the photo above in a somewhat random fashion. But to call something "random" is the easy way out, and of course there are physical laws here which have determined why boulders from a dozen or so different locations have all ended up in the same place. Note that some boulders are more angular than others; this means that they are made of harder rock, or else that they have not been carried as far as those that are rounded. The size of the boulders is to some degree dependent upon the fracture pattern in the bedrock exposures from which they are derived; the more massive and unfractured a rock is, the bigger will be the entrained blocks, slabs or pillars.
The two most-cited articles about erratic entrainment, transport and deposition are by Geoff Boulton:
Boulder shapes and grain-size distributions of debris as indicators of transport paths through a glacier and till genesisG. S. BOULTON,
Sedimentology, December 1978, Volume 25, Issue 6
Pages 773-799
https://doi.org/10.1111/j.1365-3091.1978.tb00329
G.S. Boulton
Theory of glacial erosion, transport and deposition as a consequence of subglacial sediment deformation
J. Glaciol., 42 (1996), pp. 43-62
According to Darvill et al (2015): "No single model for the formation of erratic boulder trains exists, and it is possible that they can be formed in a variety of ways. This is not surprising given the reported variety in boulder size, train length, number of boulders, transport distance, and lithology. Two hypotheses prevail: (i) subglacial entrainment and (ii) supraglacial debris." Only two? Come along now, chaps. Things are much more complex than that...........
Geomorphology, Volume 228, 1 January 2015, Pages 382-397
Geomorphology and weathering characteristics of erratic boulder trains on Tierra del Fuego, southernmost South America: Implications for dating of glacial deposits
Christopher M. Darvill, Michael J. Bentley and Chris R. StokesAnyway, we already have three parameters to fit into our erratic transport algorithm: rock "hardness", fracture pattern and distance travelled. If we were to try and design a computer programme to model what happens during transport, I would hazard a guess that we need scores more parameters to be built into it.
We read a lot in the literature about erratic trails, or trains, or fans. There are many posts on these on this blog. Just search for them. We all like to look for lines or patterns in the landscape, and we like to think that landforms created by a single set of processes will have recognizeable characteristics; think of drumlin fields, or streamlined erosional landscapes, or Rogen moraine patterns, or anastomosing / braided patterns on fluvioglacial outwash plains. Looking back on it, David Sugden and myself, when we were writing our text called "Glaciers and Landscape", were looking for patterns all the time. But just to give one example, when we were looking at bedrock variables affecting the efficacy of glacial erosion (and hence the entrainment of lumps of bedrock) we identified (a) the type and quantity of basal debris in the glacier sole, (b) the susceptibility of the bed to erosion - because of hardness, fracturing etc, ( c) the roughness of the bed, and (d) the permeability of the bed.
During a single glacial episode it is quite possible for a single erratic block to be moved in all four compass directions, carried downwards and then upwards, and on a convoluted or zig-zag route. Maybe it will even be carried west to east in one phase and then east to west in another. In other words, a block can maybe travel twenty miles and then end up more or less where it started out from. If this sounds absurd, just bear in mind that in many glaciated terrains there have been early and late phases in each glaciation where local ice-caps dominated. In North Wales, it is quite possible for erratic blocks from Snowdonia to have been carried northwards towards the Irish Sea in small local glaciers, then southwards at the time of peak glaciation by the Irish Sea Glacier, and then northwards again by late-glacial expansions of the local ice caps after the retreat of the Irish Sea Glacier. There must have been similar erratic behaviour in the Brecon Beacons, in the Lake District, in some of the upland areas of Ireland, and even on Mynydd Preseli.
Then when we extend the story of erratic perambulations and zig-zag routes from one glaciation to three or four glaciations, all with different glaciological histories and time-spans, things start to get complicated.........
And it's also clear that erratics do not all travel along at the same rate when they are under, within or on top of a glacier or ice stream. Some erratics "leapfrog" other erratics, and it seems that while some occasionally get stuck or trapped, others may be carried past them. What exactly are the processes involved? Again the timescale may be measured in hundreds of thousands of years.
How many parameters do we have now? I am already beginning to lose count, and we haven't even started to talk glaciology. On a glacier bed, we know that as a glacier advances and as its snout retreats, and as it thickens and thins, over and again during a single glaciation, the transition between cold-based ice and warm-based ice migrates sometimes up-glacier and sometimes down-glacier. This means that the zone of optimal erosion and entrainment migrates too, with complex (but not random) consequences. An upstanding tor, which we might expect to be eroded by overriding ice, may be protected and may remain more or less intact, whereas an area or exposed rock in a "sheltered" position such as a lowland depression may be intensely eroded, providing innumerable blocks to be transported away by the moving ice.
I have tried to reconstruct possible scenarios on this basis to explain why bedrock blocks were entrained from the north flank or Mynydd Preseli but not apparently from the south flank. Sure, we are in the realms of theory, but it is a theory that makes sense, and which is capable of ground truthing.
Some day somebody will model all these parameters into a single coherent scheme, so that what appears right now to be an almost random picking up and dropping off of stones by overriding ice begins to make sense. We will understand perfectly why big boulders and little ones, black ones, white ones and red ones, rounded ones and angular ones, from a variety of unknown sources, all end up in one small patch about 3m x 3m in extent. Or maybe even in one small patch at a place we call Stonehenge.
For the time being, let us just be satisfied that glacial erratic transport is indeed a wondrous thing
3 comments:
that's really fascinating!
PeteG
It's so wondrous that most self - proclaimed Stonehenge archaeology "experts " are simple NAY - SAYERS, modern day flat - earth adherents......they just can't be bothered to properly pay due respect to the process of glacial erratic transport by considering the process and accepting that scientists are writing about it.
And that, children, is why they are called, “erratics.”
Fabulous complexity.
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