Let's see if we can encourage a debate on this.
Beneath the ice
It stands to reason that when a glacier is moving across a landscape, eroding material and entraining it into the body of the ice, the process of entrainment is not continuous but intermittent. I have talked a lot in previous posts about the processes that operate on the glacier bed, on its flanks and above its flanks on mountainsides and rocky peaks. When abrasion us under way under a fairly steady basal thermal regime, small fragments of gravel, sand and even cobbles might be entrained on a small scale every day or even every hour. But where larger chunks of rock are involved, fractures and entrainment events may only occur two or three times a year, when the relationships between ice and underlying bedrock (we are talking glaciology and rock mechanics) are just right. The process referred to as "quarrying" or "plucking" kick in, and on the downstream side of rock fractures massive slabs of rock can be dragged away from their places of origin. There may be a debate about how rapidly this "extraction" process takes place; we can probably assume that rocks on the glacier bed will not be moved away from their source positions at anything like the ice velocity as measured at the surface. So entrained erratic blocks or slabs may only be moved at a rate of 50 - 100m per year, even if the ice is surging or flowing at a rate of 1km / yr.
Here's that quote again, from Prof Dave Evans of Durham University:
Quarrying (Prof David Evans)
Progress in Phys Geog 2004
Quarrying involves two separate processes: (1) the fracturing or crushing of bedrock
beneath the glacier; and (2) the entrainment of this fractured or crushed rock. Fracturing
of bedrock may take place where a glacier flowing over bedrock creates pressure
differences in the underlying rock, causing stress fields that may be sufficient to induce
rock fracture (Morland and Boulton, 1975; Morland and Morris, 1977). Fluctuations in
basal water pressure may also help to propagate bedrock fractures beneath a glacier
(Röthlisberger and Iken, 1981; Walder and Hallet, 1985; Iverson,1991a). Brepson(1979)
has successfully simulated the sliding of temperate ice over an obstacle in the
laboratory, and noted that large cavities form in the lee of obstacles, aiding quarrying.
Evacuation of rock fragments along joints in the bed is possible where localized basal
freezing occurs, for example as the result of the heat-pump effect proposed by Robin
(1976). Although Holmes (1944) originally argued that quarrying could occur beneath
both thick and thin ice, and outlined a theory based on pressure-controlled freezing of
meltwater in joints in bedrock, there is now general agreement that quarrying is
favoured beneath thin, fast-flowing ice (Hallet, 1996). Modelling studies indicate that
low effective basal pressures (0.1–1MPa) and high sliding velocities are the dominant
glaciological conditions required for quarrying because these conditions favour
extensive ice/bed separation (subglacial cavity formation)and also concentrate stresses
at points, such as the corners of bedrock ledges, where ice is in contact with the bed
(Iverson, 1991a; Hallet, 1996).
The Sleek Stone super-erratics, for example, appear to have been entrained by ice flowing over Ramsey Island and intermittently incorporating large blocks of bedrock initially made available by fracturing on the glacier bed, possibly in association with shearing within the ice mass.
In one of the classic examples in the literature, the roche moutonnee on the island of Rodloga Storskar in the Stockholm Archipelago, we can actually see how a series of massive slabs have been sheared off the down-glacier side of a whaleback feature and dragged away by the overriding ice. First, there comes the fracturing, and then the extraction -- and these events are clearly intermittent.
See also this post about a huge fracture observed on the same island:
In this case, the glaciation came to an end before the down-glacier mass of rock could be broken up or dragged away.
On the ice surface
The intermittency of input or entrainment events is much easier to understand here, because the processes operating on glacier surfaces are much easier to observe. In the case of the Foothills Erratic Train in North America, the material dumped onto the glacier surface has come from intermittent landslides or rockfalls high in the mountains, in one rather small area with a recogniseable rock type. Once dumped onto the glacier surface, the glacier has simply carried the debris away, acting more or less like a conveyor belt:
We are talking here not about continuous small rockfalls every spring (which might well have been happening too, of course) but about intermittent and widely-spaced catastrophic events, each one of which dumped thousands of tonnes of broken rock onto the glacier surface below. The landslide that dumped the vast group of erratics later emplaced at Big Rocks was not the first to occur, but it must have been one of the biggest.
These rockwall collapses might not have been as spectacular as the earthquake-induced landslides that have affected the Lamplugh Glacier and the Sherman Glacier, but they must have been pretty impressive nonetheless.
Something similar has happened in Tierra del Fuego, in the case of Darwin's Boulders. They appear to have been dumped onto a glacier surface during a series of "pulses" or events which have given rise not to a long and continuous erratic train but to elongated clusters of erratics in various locations which cannot be traced all the way back to their places of origin. So each cluster is discrete, and each one owes its origin to either a single landslide event of a series of events within a limited time frame.
When we refer to mountain earthquakes and major landslides in the mountains, we must bear in mind that these might be directly glacier-induced. The vast ice load of a large glacier, ice cap or ice sheet on a small part of the earth's crust can in itself induce earth tremors; and rockwalls adjacent to glaciers are especially vulnerable when the ice begins to melt at the end of a glacial episode. A process called "pressure release" occurs, as the crust adjusts to a reduction in its surface load -- and catastrophic rockfalls or landslides can occur as a consequence. This process of isostatic readjustment and unloading can continue for thousands of years after complete deglaciation. But that's another story.........
The lessons to be derived from all of this? Glaciers do not continuously pick up erratics from the bed during a glacial episode. Sometimes they entrain bog blocks, and sometimes they do not. You should never assume that there "should be" a continuous erratic train leading from A to B. And you should not necessarily be surprised if there was and is a cluster of old and very weathered erratics from Pembrokeshire in the vicinity of Stonehenge, with not very much to be found between the source area and the final destination.
I keep on trying to explain all of this to the archaeologists, but they are not very good at listening.