Boulder trains in Tierra del Fuego

I did some posts a few years ago on erratic entrainment and transport, and on the Darwin Boulder Train in Patagonia. The key text at the time was this one:

Evenson, E.B., Burkhart, P.A., Gosse, J.C., Baker, G.S., Jackofsky, D., Meglioli, A., Dalziel, I., Kraus, S., Alley, R.B., Berti, C., 2009. Enigmatic boulder trains, supraglacial rock avalanches, and the origin of “Darwin's boulders,” Tierra del Fuego. GSA Today 19, 4–10. http://dx.doi.org/10.1130/GSATG72A.1.
If you put "Darwin Boulder Train" into the search box you will be able to see the relevant posts.

I omitted at the time to mention this very interesting article by Darvill, Bentley and Stokes:

Christopher M Darvill, Michael Bentley, and Chris R. Stokes, 2014. Geomorphology and weathering characteristics of erratic boulder trains on Tierra del Fuego, southernmost South America: Implications for dating of glacial deposits

Geomorphology 228, pp 382-397

DOI:
10.1016/j.geomorph.2014.09.017

https://www.researchgate.net/publication/266321104_Geomorphology_and_weathering_characteristics_of_erratic_boulder_trains_on_Tierra_del_Fuego_southernmost_South_America_Implications_for_dating_of_glacial_deposits[accessed Jul 20 2020].

Abstract

Erratic boulder trains (EBTs) are a useful glacial geomorphological feature because they reveal former ice flow trajectories and can be targeted for cosmogenic nuclide exposure dating. However, understanding how they are transported and deposited is important because this has implications for palaeoglaciological reconstructions and the pre-exposure and/or erosion of the boulders. In this study, we review previous work on EBTs, which indicates that they may form subglacially or supraglacially but that large angular boulders transported long distances generally reflect supraglacial transport. We then report detailed observations of EBTs from Tierra del Fuego, southernmost South America, where their characteristics provide a useful framework for the interpretation of previously published cosmogenic nuclide exposure dates. We present the first comprehensive map of the EBTs and analyse their spatial distribution, size, and physical appearance. Results suggest that they were produced by one or more supraglacial rock avalanches in the Cordillera Darwin and were then transported supraglacially for 100 s of kilometres before being deposited. Rock surface weathering analysis shows no significant difference in the weathering characteristics of a sequence of EBTs, previously hypothesized to be of significantly different age (i.e., different glacial cycles). We interpret this to indicate that the EBTs are much closer in age than previous work has implied. This emphasises the importance of understanding EBT formation when using them for cosmogenic nuclide exposure dating.

The main emphasis in the article is cosmogenic dating, but it is also suggested that erratic transport distances are quite modest in Patagonia, and that the main erratic trains are made of rockfall debris and boulders dumped onto glacier ice in the catchment areas and then transported supraglacially.  The cosmogenic dates support the idea that the erratic boulder trains of the studied ice lobe are relatively young, and this is supported by the appearance of the erratics themselves -- they are often very large, but relatively angular, with limited abrasion traces.  (If they had been transported subglacially, a higher degree of abrasion and rounding off of edges would be expected.)  However, there were exceptions, and the authors conclude that some of the erratics with anomalous ages have probably been recycled from the erratic trains and glacial deposits laid down in earlier glaciations.

There is an interesting but brief discussion on boulder size versus distance travelled, since the assumption is that large supraglacial erratics can travel for tens of kilometres without being dramatically modified, but that that scenario does not apply to erratics carried on a glacier bed.  Glacier modelling suggests that erratic breakage and comminution are key processes on the glacier bed, and the authors test this by citing ten other studies of erratic boulder trains from across the world.  Their graph (Figure 2) is not particularly helpful, partly because it is seriously lacking in data.  But all credit to the authors for trying to sort this out.  They say:

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"..........our synthesis of previously published data suggests an apparent trend between transport distance, boulder size, and the proposed transport pathway (Fig. 2), with those moved greater distances (e.g. N~10 km) more likely to have been transported supraglacially. The relationship with boulder size is unsurprising given the association between transport pathway and boulder erosion (Boulton, 1978), but is important in the context of er- ratic dispersal more generally. For example, the principles of ‘half-dis- tance’ transport (Salonen, 1986) and concentration peaks (DiLabio, 1981, 1990; Boulton, 1996) may better relate to subglacial EBTs, where- as supraglacial EBTs are also controlled by the maximum transport distance and the preservation potential of the boulders."

Clearly this work has considerable implications for our considerations of the "bluestone erratic train" stretching from Preseli to Salisbury Plain.  Most of the bluestones at Stonehenge are abraded and weathered boulders -- as one might expect of material carried within or beneath a glacier.  But the monoliths that are shaped more like pillars are more difficult to explain.  Did they travel as pillars without being broken, or are they remnants of larger slabs or pillars? Most of the pillar bluestones have been shaped in the Stonehenge landscape by the people who found them and used them -- was that fashioning cosmetic, or quite dramatic in scale?  There is certainly enough bluestone "debitage" to suggest quite extensive working.   I have already done many posts on this topic.

This paper by Darvill et al is an interesting contribution, but there are abundant erratic boulder trains all over the world that they have not considered and which might allow interesting insights.  Also, the authors do not consider englacial transport mechanisms.

Ice does sometimes move by shearing, and debris does get lifted from the glacier bed towards the ice surface when it flows across basal ice that is blocked by transverse obstacles. Even without shearing, in areas of compressive flow within a glacier, erratic blocks and smaller debris must be lifted off the glacier bed and carried up into the body of the glacier for transport downstream.  Abrasion on a glacier bed is favoured by basal melting, which renews the "tool load" on the bed.  But the process of entrainment is a complex one, and even without regelation (the addition of ice layers on a glacier bed) if large plucked blocks of bedrock are not entrained of lifted off the bed, the basal debris load would become so thick that further erosion would become impossible.  And in every glaciated area we find deep bedrock hollows which are "clean" in the sense that the rock that once occupied them has been removed and quite literally lifted away.

This is an idealised diagram showing how ice flow in a glacier may carry ice (and debris) upwards towards the surface (compressing flow) or downwards towards the bed (extending flow).  We do not need to invoke actual shearing -- but it may occur.  This is for an idealised small glacier with a steep gradient.  Stretch it out to an ice stream more than 200 km long with a very gradual gradient or a flattish surface long profile (as in the case of the Irish Sea Ice Stream) and things become more complex and more subtle, with topography and the nature of pre-existing bottom sediments becoming more important.  But the same principles apply -- where there are obstacles or upslopes, there will be compressive flow, carrying erratic boulders and finer debris up into the body of the glacier and maybe even up onto the ice surface.  But if the area concerned happens to be above the equilibrium line (ie in the accumulation zone) the debris will immediately be buried by accumulating snow and firn, and incorporated into the body of the glacier.  

Ice that flows in constrained troughs (think Sognefjord and Nordvestfjord) flows UP reverse slopes as it approaches downglacier thresholds or if erosive power is lost because of diffluence.  As studies of intensively-glaciated terrain continue in Antarctica and Greenland, it is clear that ice erodes deeply and transports or evacuates erratics and debris UPSLOPE for considerable distances in closed troughs and in many other situations.  It is unrealistic to assume that the millions of tonnes of debris removed are all transported on the glacier bed.

Long profiles of glacier troughs, showing the depth of the trough floor beneath msl.  Above, Sognefjord, and below Hardangerfjord -- both in Norway.  Note that the floor of Hardangerfhord rises irregularly, because diffluence occurred across many cols and through distributive troughs in relatively low-lying coastal terrain  In Sognefojord supplemental inputs of ice from tributary glaciers enabled the Sognefjord Glacier to keep on "digging" until it was near the edge of the uplands -- at which point there was large-scale diffluence, and the glacier bed rose by 

about 1000m in just a few km.

https://brian-mountainman.blogspot.com/2017/01/on-uphill-transport-of-debris-by.html

https://brian-mountainman.blogspot.com/2020/01/strandflats-skerries-archipelagos-and.html

https://brian-mountainman.blogspot.com/2017/01/more-on-closed-subglacial-troughs.html

The debate continues........