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Saturday, 2 February 2019

When is a till not a till?

A typical ice-margin environment.  In the words of the great sage:  "In such a place, anything can happen, and it usually does......"  Here, lodgement tills, flowtills, meltout tills and glaciofluvial deposits are all mixed up.  Multiple variations on a theme......

Following my 2016 visit to the Isles of Scilly, I described some of the variations that exist within the Devensian till exposures on the islands.  Some photos are published here:

I was quite convinced at the time that there are many different lithologies within this till, which we can label as "Scilly till", using the terminology of Prof James Scourse and others.  In my QN article, published last year, I devoted some space to discussing the possible mechanisms by which the till deposits had been formed and laid down.  Some appeared to be basal lodgement till with shear structures and other signs of "glacitectonic" stresses, suggesting deposition on the glacier bed; others seemed to be flowtills, subject to rapid movement and redeposition; and others seemed to be "normal" meltout tills let down from a glacier surface by ablation processes.  In some cases it is difficult to home in on the precise depositional mechanics without very detailed fabric analyses. To make matters more complex, there is sometimes an interdigitation with glaciofluvial deposits or periglacial deposits including frost-shattered slope breccia.  To make matters even more complex, we see signs of wind-blown materials as well, incorporated into till, and where the ice has advanced across old beach sediments rounded pebbles, sand and gravels will be incorporated as well.  In other words, wholesale chaos -- familiar to anybody who has spent time in an active ice-wastage environment on a glacier margin.
The classic locality for the lodgement till referred to as "Scilly Till" is Bread and Cheese Cove, on St Martins island.  This has been studied by a number of researchers, and described in detail.  It has been referred to as the "type locality" -- but the section is rather messy, and I am not sure why the till from this one locality should be thought of as "authentic" Scilly Till while others are somehow not authentic!  As I said in another post:

I had problems with the "Scilly Till" type locality at Bread and Cheese Cove since the till there is associated with glacigenic structures which are not all that widespread and since other till exposures elsewhere are perhaps more typical of the northern fringes of the islands. ............  I had problems with the "Tregarthen Gravel" and the "Hell Bay Gravel" labels since they did not seem to have consistent characteristics and since some exposures were not at all gravelly. The labels seemed to me to be surplus to requirements.

I was reminded of all this when I was looking through my field notes from the South Shetlands Project of 1965-66.  Here is a field sketch from Three Brothers Hill on King George Island:

The sketch was made in an area of hummocky dead-ice terrain near an ice-cap edge, showing how a till cover resting on dead ice is transformed in some places (not everywhere) by melting processes where scallops or scars are created and where slumping and flowage occurs.  Beneath all of this is lodgement till.  So when the ice has finally melted away we will see a chaotic mixture of lodgement till, flowtill and meltout till, with multiple lithological variations.  It would be an arid exercise indeed if we should seek to claim that one exposure in this sequence should be referred to as "authentic" and as a "type locality" whereas the others (which are deemed to have been "redeposited") are somehow non-typical!   That's why I have problems on the Scilly Islands with the deposits referred to as "Scilly Till, Tregarthen Gravel and Hell Bay Gravel" -- since they are all parts of a continuum, all the same age, and all indicators of the same glacial episode.........

For more information, see Chapter 11 on "The Processes of Glacial deposition" in "Glaciers and Landscape" by David Sugden and myself, published in 1976 and reprinted at least thirteen times. There have of course been many other books since then which deal with till and glacial depositional processes.

Here is another nice diagram from Prof Geoff Boulton, in which he describes some of the relationships in the real world:

If found in a Quaternary stratigraphic context, with other deposits above and below, these glacial deposits with differing characteristics will all be taken as indicators of the same glacial "event", and there really is no point in arguing about which of them is the most important or the most "authentic".

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