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Thursday, 26 May 2011

The Foothills Erratic Train

Two new maps of the Foothills Erratic Train, which is over 600 km long.  Note how narrow it is, and note how there are a number of changes in direction, some of them quite sharp.  In some areas the trail is discontinuous, and in some areas there are "clusters" of erratics.

The famous Foothills Erratic Train in Canada is the most famous example of a "train" or trail of erratics -- as distinct from an erratic "fan" where boulders from one source are spread widely across an arc of countryside.  Both types are well documented -- but it should be borne in mind that a fan is nothing more than a collection of erratic trains which are quite close together, arising from changes in ice directions during the course of a glacial episode.  There is no mechanism for erratics to "spread sideways" in a glacier of ice sheet -- they have to move in the direction of ice movement, up or down, but always forward and never laterally.

The interesting thing about the Foothills erratic Train, as pointed out by Lionel Jackson and many others, is that the string of erratics was created by the parallel streaming of two very large ice masses -- the Cordilleran Ice Sheet to the west, and the Laurentide Ice Sheet to the east.  With very good reason, Lionel and I proposed that this was a close parallel to the situation than occurred in the Bristol Channel area at the peak of the (Anglian?) glacial episode:
http://www.earthmagazine.org/earth/article/1a1-7d8-c-1f

6 comments:

Constantinos Ragazas said...

Brian,

What is the mechanism that results in an 'erratics train' of stones placed on the surface of the ice, as compared to stones being transported within the ice mass? Is it that in one case the glacier ice completely overruns the source outcrops of these stones, while in the other case the source outcrops remain above the glacier surface?

Another question: Is it fair to say that the two main methods of the melting of ice sheets is 'from below' (through an increase of soil/water temperature) or 'from above' (through an increase of air temperature)? Am I correct in thinking so? And are there distinctive characteristics to these two modes of glacier melt that can be discerned in the field and in the geological records?

Kostas

BRIAN JOHN said...

Yes -- where there is supraglacial debris (moraine and erratics) on a glacier, it is usually derived from rockfalls from high crags and valley sides in the glacier source areas. But sometimes englacial or subglacial material can be carried up to the surface as a result of shearing and thrusting -- as in some of the glaciers of Arctic Canada -- or simply because of ablation in the zone beneath the firn line.

Mostly melting of ice sheets is from above, because of relatively high air temperatures. Where there is basal melting you sometimes get the creation of lodgement till -- material is "let down" onto the glacier bed, and the glacier then slides over it.

Constantinos Ragazas said...

Thanks, Brian. That is helpful.

If the melting of an ice sheet is from 'above' (by increase in air temperatures) does it follow then that when a solidly frozen lake begins to melt (with good melt water drainage) it will happen from 'above down' ?

Kostas

BRIAN JOHN said...

You are very keen on your solidly frozen lakes, Kostas! in my experience such things are extremely rare, even in Greenland and the Antarctic. There is generally water under the ice, and that having been said, the floating ice will melt from both above and below. This is often a seasonal process, with refreezing in the winter and melting in the summer.

Constantinos Ragazas said...

Brian,

Don't intent to hide my intentions from you Brian! Why am I so persistent with this 'local ice cover' ? It's the only theory that makes sense to me, Brian. It explains everything!

I believe what happened geologically at the UK at the time when Stonehenge was made can explain the making of Stonehenge -- and all other prehistoric monuments.

A solidly frozen lake or sea over a chalk bedrock would have unique and different melting characteristics. Chalk being so porous, the bedrock would fuse with the ice so that the melting of the ice sheet from above would not allow water at the bottom. The ice sheet would be fixed in place and would melt in place with the melt water draining into the sea.

This, along with other geological conditions (like geothermal hot spots, volcanic activity, the 'great melt' followed by the 'big freeze' and isostatic rebound) may have created the 'perfect storm' for the geomorphology we now see.

Indulge me on this Brian! I am looking for answers, just as you. Though our ideas may differ sometimes, our goal is the same. Finding the Truth about Stonehenge!

Kostas

BRIAN JOHN said...

You are very persistent, Kostas! No more on this please, until you can show me some analogies, from real world situations, for the highly unlikely mechanisms you are proposing.