On comminution and giant erratics

Igneous geology of the west of Scotland (Wikipedia)

On a walk near the edge of the Nevern estuary (Parrog, Newport) yesterday, I found a couple of small pebbles which I think have come from Ailsa Craig.  They are microgranites with small bluish speckles and signs of some larger lighter coloured minerals, but the matrix is slightly pinkish -- rather than the pure white of genuine fresh Ailsa Crain riebeckite or micro granite. 

I'm not sure how unique the Ailsa Craig microgranite is, since related rocks also occur on the island of Arran -- no matter, since we can be reasonably sure that the pebbles have come from that general area of the Firth of Clyde.  There is nothing similar, as far as I know, on the Isle of Man or in North Wales.

The "curling stone granite" from Trefor Quarry in North Wales looks very different:

The blue, grey and pink granite used for curling stones -- from Trefor Quarry in North Wales.

Wikipedia:

Ailsa Craig is a spectacular, conical island in the Firth of Clyde about 20 km south of Arran (P914119). It is formed by a boss of peralkaline microgranite intruded into Triassic rocks. The microgranite is characterised by riebeckitic arfvedsonite and Zr-rich aegirine (Harding, 1983; Harrison et al., 1987); aenigmatite also occurs (Howie and Walsh, 1981). This distinctive rock-type is a widespread glacial marker southwards on either side of the Irish Sea (p. 160). It has traditionally been a favoured lithology for the manufacture of curling stones (p. 173).

Ailsa Craig curling stone quarry -- human being for scale

Anyway, the really interesting thing about these small pebbles with bluish spots is that they are all very small.  I have not seen one which is larger than a human fist.  Many thousands of tonnes of Ailsa Craig rock must have been removed  by overriding ice, and the original entrained blocks must have been of all shapes and sizes.  There is no reason to think that they were uniquely small before they started their journeys southwards towards Wales or westwards towards Donegal.

One of the assumptions in glacial geomorphology is that as large erratics travel within or under a glacier they are subjected to an assortment of processes which combine to cause comminution -- the gradual reduction in size as the rock mass is broken, broken again and then broken again until there is not much left apart from small pebbles.  These pebbles may be sub-angular, sub-rounded or rounded, and if water is involved towards the end of the journey they may even be well-rounded.

Different rules apply to supra-glacial transport because debris on a glacier surface is not subjected to abrasion or pressure-induced fracturing.  As Lionel Jackson and I explained many years ago in our article on the 930 km long Foothills Erratic Train in Alberta, Canada and the "Big Rocks" erratic cluster near Okatoks, huge rock masses that fall onto a glacier surface as a result of cliff collapse can be carried hundreds and even thousands of kilometres with relatively little modification.  The angularity of the giant erratics and their related debris may actually be increased as a result of frost (freeze-thaw) processes.

https://www.earthmagazine.org/article/stonehenges-mysterious-stones

The biggest erratic at Okatoks -- calculated to be 16,500 tonnes in weight.

Closer to home, we have other giant erratics on the shores of the Bristol Channel, including those at Limeslade, Lydstep, Freshwater Gut, Westonzoyland (now destroyed), Saunton and Shebbear, and on the tip of the South-West Peninsula at Porthleven.  

The famous Freshwater Gut (Baggy Point) erratic, made of granulite gneiss from Western Scotland (photo: Paul Berry).  It is reputed to weigh 50 tonnes.

So why is it that some clasts are comminuted down to pebble size over a glacial transport distance of 500 km, while other giant erratics survive?  Well, it has to be admitted that the great majority of clasts are broken down, while the giant erratic survivors are the great exceptions.  I have speculated before on this blog about the preferential survival of dolerite boulders in transport, and it seems that igneous boulders have a better chance of long-distance survival than sedimentary or metamorphic rocks.  If you look at a typical Pembrokeshire storm beach you will find that the great majority of pebbles and boulders have come from degraded or destroyed glacial and glaciofluvial deposits; maybe 90% of the clasts will not have travelled far, and maybe 10% will be from sources far away.

The clasts found in glacial deposits are typically of all shapes, sizes and lithologies, with variable surface characteristics as well.  Some will be polished and striated, and others will not be.  Some will have the "ideal" bullet shape, like the famous Newall Boulder found at Stonehenge, and others will be roughly rectilinear or even roughly rounded. 

 

So to answer the question raised above, I will have to say that we currently do not know why some giant erratics survive while others are broken down into small pebbles.  My best guess is that every clast undergoes a unique journey, related to its changing position in, or on, or under the ice; related to ice temperature and velocity and other glaciological conditions; related to rock type and internal structure; and related to distance travelled.  Is all of this random?  Well, not really -- the laws of physics apply, but as yet we do not fully understand them.  But "chance"factors come into play  -- for example when one clast in a vulnerable position is suddenly assaulted by something harder, sharper and heavier........

One final point.  Giant erratics are NOT restricted to the intertidal zone around British coasts.  That is a myth repeated over and again, even in learned publications.  So their distribution has nothing whatsoever to do with transport by floating ice.