Once boulders, slabs or monoliths of rock are trapped on, within or under a glacier they will be moved at varying rates, depending upon the mechanics of glacier flow. These mechanics vary in intensity and relative importance from one glacier to another, about which more in a moment. But first, we can say that the simplest mode of transport occurs when rockfalls or avalanches from mountainsides above a flowing glacier introduce either continuous or intermittent (pulsed) supplies of debris onto the ice surface. The Foothills Erratic Train, which I have described several times in this blog, started life as a big avalanche of rock onto the surface of a dynamic valley glacier in the Rockies; but there must have been many more avalanches from more or less the same place, because the erratic train is strung out over a great distance and formed over six centuries. We do not know whether the avalanche debris spread out as an apron over much of the glacier surface, or whether it was restricted to the valley side. In the latter case, we can envisage a classic lateral moraine situation such as we see on hundreds of valley glaciers in all of the high mountain areas of the mid and high latitudes. In most cases, however, the supply of debris is seasonal if not continuous from all parts of the valley side, with frost shattering and gravity playing key roles.
Down-valley, where valley glaciers converge, a lateral moraine will often be joined to another at the tip of a spur to become a medial moraine. Complex glaciers, which have several tributary glaciers flowing into them, will sometimes take on a striped appearance with a number of medial moraines moving along in parallel with one another, as on the Barnard Glacier in Alaska; but if one or more of the glacier tributaries exhibits surging behaviour, the moraines will be displaced or pushed out of position in a manner which is instantly recognizable by glaciologists. These strange surface patterns are far more frequent than was once supposed -- and surges can move erratic trains many miles to the right or left of their predicted courses.
As long as this lateral / medial morainic material continues to move along on the glacier surface beneath the firn line, where the glacier surface is melting, it will remain at the surface and will not be greatly modified by the processes of abrasion. In other words, the edges of the boulders, slabs and pillars will remain sharp and angular, and will not be rounded off. When we see medial and lateral moraines emerging at a calving glacier snout, we are often amazed to see that they are superficial, sitting on the ice surface and with hardly any debris beneath them, deeper within the ice. Medial moraines can sometimes penetrate deeper, because when two glaciers come together there will be some disruption and redistribution of surface materials as two ice streams are blended into one.
If a rockfall introduces debris onto a glacier surface above the firn line, it will quickly be buried by accumulating snow, and will effectively travel downwards as well as forwards, into the body of the glacier. In this case the debris might be scattered or dispersed within the ice, and might also be subjected to the processes of abrasion as smaller particles move past larger ones and leave them polished, scratched or even gouged. They can also be broken in circumstances where pressure variations occur within the ice. In very rare circumstances (where there is a great deal of bottom melting beneath a glacier) large blocks may even pass down from the surface of a glacier all the way to the bed........ and the chances of such blocks remaining "fresh"and sharp edged in such circumstances would be very low indeed.
We can say with some confidence that the Darwin Boulders of Tierra del Fuego (which are found in two erratic trains of limited length, and do not extend back to the place of origin) were probably transported supraglacially (on the surface of the ice) following short-lived pulses of rock supply associated with avalanches of valley-side collapses. We can also say that the Foothills Erratic Train was a result of intermittent but ongoing avalanches or collapses on a high mountainside, with massive piles of rock debris transported initially along the edge of a valley glacier and then along the contact zone between the Cordilleran Ice Sheet (in the west) and the Laurentide Ice Sheet (in the east). From the "debris input point" to the eventual ice edge hundreds of kilometres to the south, the ice surface must always have been melting rather than accumulating, since the erratics that were dumped in a long string (as shown in a previous post) show remarkably few signs of glacial erosion.
Erratic fans are in fact best interpreted as series of diverging erratic trains of differing ages.
Where fans of erratics are found in the landscape the explanation may be that entrainment from a particular rock source continued over a long period of time during which the direction of ice movement changed -- sometimes by more than 45 degrees. Sometimes the erratic train created during the early phase of a glaciation is buried by later sediments so that the erratics are not displaced or moved sideways as later erratic trains are let down either to right or left. But more commonly, I suspect, the erratics left in a landscape may be of many different ages, having been entrained originally in one glacial episode and then again (maybe more than once) in later glaciations. If we had been able to track their movements, I suspect that we would have seen something akin to a zig-zag pattern, ie something that appears to be truly erratic or eccentric. However, nature is actually rather well organized, so the pattern is not chaotic or random; it is subject to analysis like everything else, and capable of explanation. Each point in the zig-zag would represent a change in the direction of ice flow or a temporary pause (maybe lasting for a whole interglacial!) in the movement of the boulder; each segment of the line would represent a transport phase when ice movement was more or less stable or consistent.
Let's move on to consider erratics carried on the bed of a glacier. Here it is quite possible for movement to be more erratic still, with abundant stops and starts and changes of direction. If a large boulder remains on the glacier bed while it is transported over a great distance, that implies that the glacier is warm-based and that it is melting on its bed. This melting provides water for lubrication, and facilitates the faster movement of the glacier, the erosion of the bed, and the abrasion and even the splitting or crushing of the boulder itself -- so it might reduce in size, and it would almost certainly have most of its rough edges knocked off. If the rate of melting is very fast, then a lodgement till might be plastered onto the glacier bed, and large boulders might be effectively buried and incorporated into this thickening till layer. if that were to happen, the onward transport of the erratic would stop. But if a large boulder does keep moving, it might be carried on a sinuous course by the plastic deformation of basal ice around and over obstacles on the bed; it might "stick" for a while and then be released and moved on; and it might eventually come to a complete and final halt on the bed if it is wedged up against a large and immovable rock outcrop or hill, so that the ice then simply moves over it as it moves over a drumlin or any other streamlined form (of which there are many).
Finally, to erratics which are carried within the body of a glacier. Now things become much more complicated, if they are not complicated enough already. If big erratics are carried up into a glacier by ice subject to compressive flow (in other words, if the ice is decelerating or slowing down because it has encountered an obstacle), there may be internal deformation of the ice if it is close to the pressure melting point. However, if the ice is cold the stresses will result in shearing or faulting, with shear planes carrying relatively fast-moving ice up and over masses of ice that are slowed by friction at the bed. If an obstacle to the glacier's progress is small, a single shear-plane (which may be many hectares or square kilometres in extent) may suffice to enable the glacier to maintain its forward motion, but if an obstacle is substantial (ie a subglacial range of hills or mountains) then there may be multiple or "stacked" shear planes, with each slice of the glacier moving faster than the slice immediately below it. A cross section through such a glacier would look like a multiple-layer cake, with sponge slices (the ice layers) separated by thin cream layers (the shear planes). Other analogies can be found in the field of geology.
What happens to the erratic boulders that are trapped in the ice in this sort of scenario? It's difficult to be quite sure, since these environments are incredibly difficult to observe. We can obtain some clues from the snouts of glaciers like the Thomson Glacier and White Glacier on Axel Heiberg Island, which I visited many years ago. Here there are massive thrusts at the glacier snout. A huge moraine at the snout acts as a barrier to the forward movement of the ice, and so the advancing glacier has pushed new ice up and over the barrier via a series of immensely complex thrusts or shear planes. Some of these are short-lived and operate only for a while, before being intersected and sliced up by later episodes of shearing. Debris which is in contact with these shear planes has to be substantially modified by abrasion, gouging and crushing processes, because the environment is a very dynamic one. So boulders will show much evidence of smoothing and scratching by the tools which have been fellow travellers. But what happens to boulders which escape the attentions of these shear planes and which travel within the "slices" of glacier ice? In theory, they could remain almost unmodified, transported for maybe hundreds of kilometres within the glacier without ever coming into contact with other debris.
If you think this is all too complicated, rest assured that what I have written above is a simple summary of what goes on during glacial transport -- to get a more realistic picture you need to read a few of the thousands of detailed articles about transport processes in the glaciological and geomorphological literature........
So let's apply some of the above to the situation in South Wales and Southern England, in the domain of the Irish Sea Glacier and the glaciers coming southwards from the Brecon Beacons and the South Wales Coalfield. On this blog I have published many maps showing the idealised directions of ice movement as far as we can work them out from striae on rock surfaces and from the distribution of erratics on the ground surface and in sediments. One such map (based on the work of Kellaway) is shown below, together with a map of known or assumed erratic trails in South Wales. Both maps are composite, blending evidence from a number of different glacial episodes, collected by a number of different researchers. The erratic trails which appear smooth were in all probability much more complex, with Pembrokeshire erratics (for example) moved SE, SSE or ESE at various stages. Those erratics that were carried across Carmarthen Bay towards Gower and the Vale of Glamorgan would also have been moved S, SE or even SW at various times by large outlet glaciers moving down towards the Bristol Channel Lowlands from the Welsh Ice cap. These erratics would have followed zig-zag courses.
One interesting fact to have emerged from both field research and glacier modelling exercises in recent years is that the big ice masses of the British Isles showed a sort of "pulsing" behaviour during the Devensian, with advances and retreats coinciding with "binges" (growing or accumulation episodes) and with "purges" (rapid evacuations or advances) respectively. These binge and purge episodes were generally out of phase in various sectors of the British-Irish ice sheet. The amazing "animated" models created by glaciologists in Aberystwyth and other UK universities illustrate this perfectly. The scenario during the Devensian is that of expanding glaciers flowing down towards the Bristol Channel from the South Wales uplands in an early glacial phase; the arrival and expansion of the Irish Sea Glacier (or a Celtic Sea Piedmont Glacier) during the peak glacial phase, around 20,000 years ago; and finally one or more phases of valley glacier advance during a late glacial phase. We have to assume that this scenario, or something like it, was followed during each of the big glacial episodes to have affected western Britain..........
Back to the erratics. The coastal lowlands of South Wales must have been affected during the Ice Age by a number of episodes involving the Irish Sea Glacier and a number of episodes involving Welsh ice, pouring out of the South Wales valleys and perhaps forming piedmont glaciers, as shown by Prof David Bowen on many of his maps. In such circumstances, what chance might there have been for glacial erratics from Pembrokeshire or Ireland to have survived in situ either on the Gower Peninsula or in the Vale of Glamorgan? Not a lot. It would, in my view, have been comparatively rare for Pembrokeshire erratics to have been dumped inland of the present coastline; most will have been dumped in the area now inundated by the sea, in Carmarthen Bay and the Bristol Channel. Those that were dumped on the area that is now dry land would, in all probability, have been "flushed out" towards the south by the South Wales Valley glaciers in the later phases of each glaciation.
In my view, it is extraordinary that there are ANY Pembrokeshire erratics left on the Gower or the Vale of Glamorgan; but against the odds there are indeed a few -- in particular igneous erratics from the St David's Peninsula found in western Gower. Other erratics from Pembrokeshire have also been found at Pencoed and near Cardiff; some of these are in the "Storrie Collection" held at the National Museum of Wales.
Has this got us anywhere? I hope it has. Maybe the foregoing will help to explain why I roll my eyes and sigh when people say to me "If the bluestones at Stonehenge really are glacial erratics, why isn't there a continuous trail of them all the way from Preseli to Salisbury Plain?" If only the theory of glacial transport was that simple.......... but it isn't, as I keep on trying to explain to the archaeologists.