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Thursday, 27 March 2014

Lennoxtown Erratic Train

I was digging into an old geomorphology text by Brian Sissons when I came across this splendid example of an erratic train.  It's called " the Lennoxtown Boulder train" and it is located in the central valley of Scotland, west of the Firth of Forth.  The discontinuous train of boulders comes from a very distinctive (and small) igneous outcrop near Lennoxtown -- in the early 1900's this rock was called "essexite" but is now probably called something quite different........

The train of boulders is at first very narrow -- just a couple of kilometres -- as it curves round the southern slopes of the Kilsyth hills.  Then it widens gradually to a width of c 6 km as it runs eastwards along the Firth of Forth.  The total length of the boulder train is about 60 km.  In the western part the boulders are quite frequent, but after that they are discontinuous, and the map is based upon many scattered occurrences logged by Peach and Horne in 1909.

If you look at the streamlines you can see that two ice streams converged in this neighbourhood and then flowed eastwards, with southern and northern ice streams in parallel -- and with the boulder train squeezed between the two.  This is exactly the mechanism proposed by Lionel Jackson and myself some years ago, in our article in EARTH magazine:

In discussing the Foothills Erratic Train and the bluestone transport hypothesis, we wrote:

To gain an appreciation for how the convergence of two ice sheets can create a virtual conveyor belt for the transport of erratics, we have to travel to the foothills of the Rocky Mountains in Alberta, Canada.

This amazing trail of pebbly quartzite erratics, called the Foothills Erratics Train, can be traced from the forested Macleod River region in Alberta to the United States-Canada border in western Montana 580 kilometers southward. Over most of its length, the trail is only a few kilometers wide, narrowing to less than one kilometer in some areas. Individual erratics range in size from less than a cubic meter to one rock that has the mass of 10 Stonehenges.

The source of the rocks is in the Great Divide in Jasper National Park. The rocks appear to have fallen onto valley glaciers, which carried them into the Foothills Erratics Train via glaciers in the Athabasca River Valley. Normally, mountain glaciers would spread into so-called piedmont lobes where they leave the mountains and spill out onto the plains, dispersing the rocks that they carry in a fan shape. Indeed, this occurred farther south in the American Rockies during the last glacial maximum about 20,000 years ago. However, in the case of glaciers flowing out of the Canadian Rockies, they encountered the western margin of the vast Laurentide Ice Sheet, which was diverted southeastward by the high topography of the range’s foothills. The Athabasca Valley Glacier carrying the erratics became a tributary to the Laurentide Ice Sheet and flowed southeastward with it.
This parallel flow of two ice streams, maintained by pressure from both sides, is quite analogous to the situation in Wales. As the two ice streams came together, they would have maintained a contact zone as the ice approached its easternmost limit in England. It is reasonable to believe that the contact zone of ice carrying bluestone erratics — and maybe some other stones from South Wales — would have resulted in an erratics train rather than a fan.

Unlike the blocks of the Foothills Erratics Train that fell onto the surface of the glacier from cliffs in the Rocky Mountains, the bluestone erratics train would have been plucked from outcrops and initially transported within the ice. However, once entrained, the blocks would have been transported relatively high within the body of the glacier (see sidebar, p. 39). By using the Canadian Rockies analogy, it suddenly becomes clear how the boulders of Stonehenge could have been deposited in a trail across southwestern England — and thus would have been easy pickings for Neolithic Britons.

1 comment:

Myris of Alexandria said...

No still called essexite.
Lovely. Distinctive in thin section. I used it for 30+ years to teach u/g how to recognise minerals.