UK glacial limits must now be revised

The interesting discussions on social media regarding West Kennet and the rotten granite boulder has been quite entertaining and good humoured. It follows the press release put out by Rob Ixer, Richard Bevins, Duncan Pirrie, Mark Gillings and Josh Pollard (below) and the two articles in "British Archaeology" which I have already commented on.

All of a sudden, everybody is wondering what GRUS is, and where it comes from.  Well, it is simply the weathering product of coarse crystalline rocks such as granite, in which the rock loses its coherence and is reduced to a gravelly mess with crumbly larger fragments scattered through its mass.  It is thought to be associated with "deep weathering" which is largely chemical in nature, and it can run very deep -- sometimes to hundreds of metres beneath the ground surface.

Rotten granite or grus in a fissure on St Agnes, Isles of Scilly

Mobilised grus on St Marys, Isles of Scilly -- containing a rather nice little glacial erratic......

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Press Release

STONES IN AVEBURY WORLD HERITAGE FROM NORTH OF ENGLAND

· Lumps of granite at West Kennet, Avebury could have been carried south for twice as far as bluestones’ journey to Stonehenge
· Weathered granite known as grus found in remains of large timber structure and in grave, dating from time of Stonehenge
· Purpose of stones not known
· Research continues to identify exact source

It's long been known that some of the Stonehenge megaliths, known as bluestones, came from Wales, 150 miles away in a straight line (250 km). Archaeologists have now found stones in the Avebury World Heritage Site that could have come from as much as twice that distance (300 miles/450 km) – not from the west, but the north.

The stones consist of over 70 pieces of decayed granite, known as grus and in total weighing around 50 lbs (22 kg) – about the same as a bag of cement. They were found in deep post-holes of a large timber structure at West Kennet, which dates from around 2500BC, contemporary with nearby Silbury Hill and Stonehenge to the south. Some of the stones were arranged in a ring around a grave holding the remains of two people.

Archaeologists struggle to explain why the stones are there. They have no obvious use – they are broken lumps, not megaliths. It is also early days in determining exactly where they came from.

After sophisticated geological analysis, it is thought their ultimate source was the Cheviot Hills in Northumberland, near the border with Scotland. People could have collected the stones from there. Some of the granite was carried south by glaciers in the Ice Age, so people could also have found fragments along the east coast of England down to Norfolk. Further research is planned.

There were already signs that Neolithic people in Wessex had links with people in the the north and east, among them high quality flint used in arrowheads possibly from Norfolk, and distinctive Grooved Ware pottery of identical designs at West Kennett and Rudston in Yorkshire – close to where the newly announced carved chalk drum was found at Burton Agnes.

Rob Ixer, UCL Institute of Archaeology
Richard Bevins, Aberystwyth University/Amgueddfa Cymru – National Museum Wales
Duncan Pirrie, University of South Wales
Mark Gillings, Bournemouth University
Joshua Pollard, University of Southampton

The find is described in British Archaeology magazine, out on February 16th

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It seems to me that the idea of human beings collecting lumps or rotten granite and carrying them 450 km from Cheviot to West Kennet is so "far out" that even the usual suspects are reluctant to accept it as reasonable.  So we are left with an interpretation that involves glaciers and the Quaternary.  Glaciers do not dump little piles or lumps of grus -- so there must have been a coherent granite erratic here (like the erratics on Flatholm and Saunton) and that must have been so long ago that it has subsequently almost rotted away -- ie transformed into grus.  We do not know how long the process of "boulder transformation" takes, but we can probably think in terms of hundreds of thousands of years, if not millions of years.

Pink granite erratics at Saunton (above) and Flat Holm (below). These have been kept fresh by constant pounding by the waves, but nobody knows when they were dumped by glacier ice.

Our problem is that West Kennet lies well to the south of the line accepted by many as the southern limit of Pleistocene glaciation in the British Isles.  As we all know, most experts think that Salisbury Plain and adjacent territories were never glaciated -- although I have always argued that glaciation was perfectly possible from the west.  Now we need to think of glaciation from the north as well -- and we must seriously reconsider the placing of the "Anglian" ice limit and the edge of the GBG or Greatest British Glaciation.  The great difficulty in determining vary ancient ice edges (whether they be of Wolstonian, Anglian, or Cromerian age) is that over vast lengths of time weathering and erosional processes transform if not actually remove almost all traces of glacial action.

 The glacial and interglacial sequence over the past 2 million years or so.  Intensive episodes of glaciation are left clear, and lightest green shade is used for episodes of moderate glacial activity.

So if glacier ice extended further south than the line shown on the map below, where was the ice edge?  That is something we have to work on........

The Anglian ice edge, after Gibbard, Clarke, Williams-Thorpe and many others.  The extension from the west, incorporating the Somerset Levels and the Mendips, is mine -- but it is based on glaciological principles and field evidence.

Finally, if you think that it is easy to establish an ice edge on the ground, think again. The map below shows some of the ice limits suggested in the literature for the Devensian or LGM in South Wales  -- all based on abundant stratigraphic and landform evidence which is less than 30,000 years old. Highly experienced geomorphologists and geologists have managed to agree on virtually nothing!  By comparison, in the area often referred to as the southern England proglacial or periglacial area (including Salisbury Plain and the downs), the evidence is rare and very subtle indeed.