In the "FOCUS" article from which I quoted the other day, Mike Pitts says that the idea of glacial transport is now discredited because the new geological work on the provenances of some of the bluestones shows that they have come from the northern slopes of the Preseli Hills in Pembrokeshire, and not from the southern slopes. He suggests that if ice had entrained and transported the stones, they should have come from the south-facing or down-glacier slopes, given that the Irish Sea Glacier
overrode Pembrokeshire from the NW and flowed onwards to the S and SE. I am not sure where Mike got this idea from, but it demonstrates an unfortunate misunderstanding of the complexities of glaciological theory.
Let's try to explain. When glacier ice flows across a landscape and inundates everything, and where there is a basal-sliding regime with ice more or less at its pressure melting point, there is indeed a tendency for plucking or "quarrying" to occur on the down-glacier flanks of obstacles. What we may well find, in areas of hilly terrain, are (a) smoothed and striated up-glacier flanks (which are often gently-sloping) on whaleback forms or roches moutonnees; and (b) broken or fractured down-glacier flanks which can be transformed into steep cliffs because of the process of block removal. This is explained in glaciology by reference to bed pressure and the flow law, with basal melting occurring up-glacier where rock surfaces are under compression, and freezing and thawing -- and the "dragging" away of blocks -- occurring down-glacier where rock surfaces are subject to pressure release and tension. It is probably that theory to which Mike is referring when he speculates about the origins of bluestones and their mode of transport.
Then things get more complex. In certain situations (for example where there is a major ridge or mountain range aligned transverse to the direction of ice flow) other processes come into play. This is well covered in glacial geomorphology text books such as that by Benn and Evans (2010). Thrusting can occur in relatively thin ice or in thick ice where there is a frozen-bed regime, with faults or shear planes rising from the glacier bed sometimes inclined at angles as steep as 45 degrees. Sometimes it seems that these thrusts occur at the junction between sliding ice and "static" ice which is frozen to its bed. (It's never actually static, because ice is always deforming, but it can certainly be very sluggish.) A requirement for thrusting is that there should be large horizontal compressive strain rates in the ice (Moore, Iverson and Cohen, in Geophysical Research, 2010). Thrust planes are of course capable of carrying eroded blocks and debris upwards towards the glacier surface -- and this process is seen in glaciers today in Svalbard, Antarctica, Sweden and Arctic Canada. There is an abundant literature. To make things even more complicated, folding can also occur in the ice, through the process of internal deformation. Benn and Evans show that some debris is carried in overthrust ice slabs, and other debris moves along on the thrust plane itself, sometimes assisted by meltwater lubrication. In the former situation the blocks and other debris may be "encased" in ice and effectively protected from further abrasion, while in the latter case clasts and larger blocks may be broken, striated or abraded. Neil Glasser, Mike Hambrey and others have shown this thrusting and debris transport going on in non-surging polythermal Svalbard glaciers.
landforms of the southern slopes. On the northern slopes, in contrast, if some of the ice was cold and subject to large horizontal compression strain rates, shearing or thrusting (analogous to faulting in solid rock) could well have occurred, as described above. There would also be a mechanism also for the partial destruction of tors and the entrainment of large slabs of bedrock. We see evidence for this type of thrusting -- and the incorporation of vast quantities of bedrock debris -- close to the snouts of certain present-day glaciers, especially in association with the formation of push moraines. On this blog we have already looked at the spectacular landscape at the snout of the Thompson Glacier on Axel Heiberg Island. We also see signs of thrusting and debris incorporation well inland from the edges of the Greenland ice sheet.
Then things begin to get even more interesting. Recent modelling work on the British and Irish Ice Sheet by glaciologist Alun Hubbard and various colleagues has shown that it exhibited a sort of "pulsing" behaviour, with alternating surges and ice surface collapses especially in the ice sheet's western sector. James Scourse and colleagues have shown that one such surge carried glacier ice as far south as the Scilly Isles only about 20,000 years ago. One of the features associated with surges is
thrusting, as fast-moving ice encounters older and more sluggish ice that happens to block its path. It remains to be seen what relevance this observation has for the entrainment of erratics on Preseli and in the area between Preseli and the north coast.
So let's put this on the record: the preferred locations for the deep glacial quarrying and entrainment of bedrock slabs, monoliths and other debris when the Preseli Hills were deeply inundated by ice would have been the NORTHERN SLOPES, and not the south-facing ones.