Some of the ideas discussed in this blog are published in my new book called "The Stonehenge Bluestones" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
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Monday, 18 August 2014

More on thrusting, shearing and glacier tectonics

Thrust planes outcropping at glacier surfaces.  The top photo is from a glacier in Alaska, and the bottom photo is from the Swiss Alps.  In both cases the angle of the thrust is quite steep, and large amounts of debris have been carried to the glacier surface. In the top photo, right in the centre of the picture, we can see that a very large sub-rounded boulder has been brought to the surface. Click to enlarge.

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.

A classic roche moutonnee -- Lambert Dome, Sierra Nevada, California.  The ice has flowed from right to left.  On the right, a gently sloping smoothed and striated face, and on the left the cliffed face where plucking or quarrying has occurred.

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.

 Triple thrust planes outcropping at the surface of the Chistochina Glacier in Alaska.  These features appeared after an earthquake in 2002, illustrating how brittle ice can behave in response to stress.

Thrusts on the surface of Variegated Glacier, Alaska.

Let's apply this to the Preseli situation.  In the lee of a substantial obstacle like a mountain range there may be a vast expanse of "dead" or stagnant ice which may have very little effect upon the landscape. It could well be frozen to its bed, because the ice overriding mountain crests tends to be thin.  Later on, as a glacier thickens during the waxing phase of a glacial episode, the stagnant ice "plug" can be overridden by more active glacier ice flowing up over the mountain ridge and then away in whichever direction the ice stream dictates.  So in the Preseli Hills case, the most active ice, capable of erosion and the entrainment of blocks of bedrock, may have had no contact with the tors and other
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.

 This extraordinary photo shows part of the Thompson Glacier push moraine on Axel Heiberg Island.  The gently-sloping "shelves" are capping patches of detached dead ice, and they are remnants of the thrust planes within the glacier snout along which debris has been transported.

A "blocking ridge" near the edge of the Greenland ice sheet, SW Greenland, inland of Nuuk.  The ice from the ice sheet is flowing broadly from NE to SW here, and the ridge is more or less transverse to this direction of flow -- and is thus acting as a barrier.  The pattern of crevasses is highly complex, but it is almost certain that the moraine on the northern side of the ridge is supplied by thrusting within the ice and the upward carriage of debris.  If you look at the western end of the ridge, and then at the ice surface to the north, you can see a series of long linear crevasses transverse to the direction of ice flow -- almost inevitably these will be the surface expression of thrust planes within the ice.

In some cases, for example at the North Norfolk coast, it can be shown that overriding ice frozen to its bed is capable of dragging away and incorporating into the basal ice layers huge slabs of bedrock which may be more than a kilometre in length.  Such features are very spectacular when exposed in present-day cliffs; they are referred to as "chalk rafts."  The incorporation of large slabs of bedrock could have happened in North Pembrokeshire as well, so long as the "freezing bond" between glacier bed and bedrock surface was very strong, and so long as there were pre-existing weaknesses (rock fractures or maybe a contact between permafrost and unfrozen ground) in the ground being over-ridden by ice.  This is something worthy of greater investigation in Pembrokeshire.  For example the geologists might address this question: was the dense fracture pattern at Craig Rhosyfelin adequate for the bulk extraction of large masses of bedrock by this mechanism?  The idea is an attractive one.......

A famous "chalk raft" incorporated into glacial sediments at Overstrand, Norfolk.

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.


Jon Morris said...

Nice to have that summary (at the end) on record, but would have been better if it had been done pre-emptively?

BRIAN JOHN said...

Thanks Jon. I see your point, but it's not one of my skills to predict the misunderstandings of others -- and I suppose that naively I assumed that people will by now have read my book or at least some relevant parts of this blog. The diagram referring to compressive flow, entrainment and shearing has been on p 128 of my book for six years now! So of course I have long since pre-empted any points people might make about entrainment mechanisms. That having been said, we are still in "best guess" territory here -- a lot more work must be done before any of this speculation can be confirmed. All we can say if that there are well understood mechanisms that COULD have applied in the case of the bluestones.

But I have to admit that I never thought anybody would come up with the argument that glaciers would preferentially pick up erratics from the SOUTHERN flanks of the Preseli Hills, after all my efforts at education!