On the preferential survivability of dolerite erratics

Erratics in the vicinity of St Bride's Haven.  One of them is a Devonian quartz conglomerate -- 

the others are dolerite.

Yesterday my wife and I took a walk along the southern shore of St Bride's Bay in beautiful calm and sunny winter weather.  We were on a stretch of coast near St Bride's Haven where the ice came ashore from the NW after crossing the bay. On the clifftops and in the embayments there are erratics everywhere -- and the great majority of them are made of dolerite.  That's interesting, because there are no dolerite outcrops in the vicinity, and the bedrock is for the most part reddish Old Red Sandstone. 

The same general rule applies around all of the coasts of Pembrokeshire.  I haven't done any statistical analysis, but wherever we are, and  whatever the local geology may be, the great majority of big erratics (over 1m in diameter) are made of dolerite -- mostly unspotted, but sometimes spotted. The smaller erratics are much more varied -- sometimes they are made of local rock, and sometimes of far-travelled rocks including some from North Wales or further afield.  Very rarely we find large and very exotic erratics from Scotland or the Lake District -- inevitably these were the ones that attracted most attention from the field geologists of the Geological Survey simply because they were "prominent" and "different."  And they were hugely important too, because they were the erratics that allowed geologists like Hicks, Jehu, OT Jones, HH Thomas and others to determine where the ice that covered Pembrokeshire had come from.

The general rule of erratic transport is that erratics are comminuted or reduced in size with distance from source, and over time.  To appreciate this, we simply have to look at the average pebble beach in Pembrokeshire -  made up largely of cobbles and pebbles derived from glacial deposits.  The smaller the size fraction, the greater the range of rock types represented.  At Abermawr, for example, if you look at pebbles under 5 cm diameter, there is an extraordinary variety of rock types, some local and some far-travelled.  On Flat Holm in the Severn Estuary, the majority of pebbles in the beaches are of limestone and calcareous shale, but some are very far-travelled.  In contrast, large erratics are very rare on the island, as we have noted in previous posts.

Beach pebbles collected from Freshwater West.  Many of these are of igneous origin.  They have to be searched for!  These are mostly the comminuted remains of larger cobbles and boulders.

Comminution happens because of the stresses to which contained rock fragments are subjected during glacial transport. There is high-pressure assault from adjacent rock fragments that are in effect "destructive tools".  Compression is generally more effective than tension when we consider chunks of rock in transport. Gentle pressure results in abrasion or striations;  greater pressure results in crescentic and other gouges; even greater pressure results in the shearing off of rock flakes or chunks, leaving facets on the parent stone; and ever greater pressure results in a rock being splintered, crushed or destroyed.  Where erratics with two different rock types are in contact under or within a glacier, the harder or more resistant of the two will be the tool, and the softer rock will suffer the most damage.  "Hardness" is difficult to measure or define, as all geologists know; rock texture comes into play, as does the mix of mineral crystals, as does the presence of foliations, fractures and bedding planes.  Some hard rocks, if they are heavily fractured,  may be destroyed in transport more easily than "soft" rocks like chalk or shale.  All that having been said, there are always exceptions to the rule, and there are many famous examples of "soft" giant erratics that have been transported more or less intact for tens or hundreds of kilometres when, according to glacial theory, they should have been destroyed.   As described many times on this blog, big erratics carried on an ice surface, or within the body of a glacier, have a much greater chance of survival than equivalent erratics dragged along on a glacier bed.

Now let's move to Stonehenge.  I have done many posts on the bluestone erratic assemblage at Stonehenge, emphasising that the majority of the 43 surviving monoliths are made of dolerite.  They look like heavily abraded and weathered erratics, because that is what they are.

https://brian-mountainman.blogspot.com/2016/08/the-stonehenge-boulders.html

https://brian-mountainman.blogspot.com/2015/02/understanding-bluestone-circle.html

https://brian-mountainman.blogspot.com/2015/12/the-bluestone-circle-erratic-assemblage.html

So where are we headed with all of this?  Well, having pondered long and hard, I am increasingly convinced that there is something about dolerite that makes it uniquely resistant to the forces that operate during glacial transport.  This might have something to do with the internal structure of dolerite -- its mineral composition and the manner in which crystals are arranged or interlocked. I need some geological advice on this! Another factor may be the relatively low frequency of intersecting joints, which causes dolerite to react by breaking down into massive rectangular blocks, slabs or pillars which are then internally very resistant to further breakdown. This is a matter of rock mechanics....... 

Carn Goedog -- a dolerite tor broken up by glacial and periglacial forces and with block shapes determined above all else by the preexisting fracture pattern.

Here is something I found on the web about the "erodability" of diabase or dolerite:

Consider the two other chemical equivalent of diabase: gabbro (coarse-grained) and basalt (glassy and fine-grained), which should potentially be similarly erosion resistant. This is not the case. Both gabbro and basalt erode very easily.
Coarse-grained rocks, in general, erode easily. The large grain size means it is relatively easy to dislodge individual grains out of the rock. You don't need a lot of water, for instance, to percolate between the not-many grain boundaries and release them. To make it worse, gabbros are full of minerals that are easily erodable such as plagioclase and olivine.
On the other hand, basalts also tend to weather easily. Having cooled fast, they are usually glassy which tends to crack during cooling. Glass is general is less resistant to chemical alteration than minerals, under similar conditions. Basalts also contain gas bubbles, which cause weakness spots in which the rock can break, leading to preferred weathering.
Diabase is fine grained, and contains no glass. It does not suffer from these two conditions. It is composed of very fine interlocking crystals that are very hard to dislodge, and is not penetrable by fluids.
That said, remember that it is all relative. A diabase will still weather faster than even the worst type of granite.

I don't agree with that last statement.  Granite erodes quite easily, in my experience, and in the majority of the outcrops I know, the bedrock granite is very rotten indeed, producing this mysterious material called "grus" -- but don't let's get diverted.......

Conclusion?  As I have explained many times before, the bulk of the bluestones at Stonehenge are heavily abraded boulders with rounded edges and facets and with shapes best described as boulders or slabs, with some ( especially the modified ones) worthy of being called pillars.  They look like the glacial erratics of Pembrokeshire, and they look as if they could have been collected from any ice edge anywhere in the world.  There is no way that they are "quarried monoliths" -- that is sheer fantasy.  The bulk of the 43 bluestones are dolerites.  This is not because dolerites were particularly valued or revered by our Neolithic ancestors; indeed, nobody has ever shown that dolerites were especially valued or preferentially used in stone settings in Pembrokeshire.  No -- the answer is simply that dolerites survived glacial transport better than any of the other rock types represented in the erratic assemblage of the Irish Sea Glacier.  In other words, there was a "preferential survivability" which makes good geological sense -- and which is confirmed from observations elsewhere.

Stone 39 at Stonehenge -- made of spotted dolerite.  Acknowledgement to Simon Banton.

Simon's full photographic record of the Stonehenge bluestones and sarsens is here:

http://www.stonesofstonehenge.org.uk/search/label/Bluestone