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Thursday, 14 June 2012

The Dartmoor Ice Cap

One of the photos from the new paper.  This is one of the most spectacular glacial landforms on Dartmoor -- a terminal moraine ridge (right centre of the photo)

Grateful thanks to Dr Stephan Harrison for giving me sight of the PDF of this important article.  This is the full reference:

The glaciation of Dartmoor: the southernmost independent Pleistocene ice cap in the British Isles
David J.A. Evans, Stephan Harrison, Andreas Vieli, Ed Anderson
Quaternary Science Reviews 45 (2012) 31-53



This is a long and detailed paper which provides very convincing evidence for glaciation on Dartmoor during the Devensian or Last Glacial Maximum.  The four authors are also convinced that there was some ice remaining in the Younger Dryas, and that this ice was quite active.


In fieldwork and from photographic evidence, the authors identify till and fluvio-glacial deposits, lateral and terminal moraines, hummocky moraine features, meltwater channels, ice-smoothed bedrock slabs, roches moutonnees, eroded tors, streamlined drift forms and other features -- adding up to a very convincing picture of a small and relatively thin ice cap which was at one time a coherent single feature and at other times broken into several segments.  At its greatest Devensian extent, it was about 10 km in diamater, and almost circular.  Here are some of the illustrations from the paper:

 
 A substantial lateral morainic ridge (arrowed)

 Ice-smoothed granite slabs

In addition to the presentation and analysis of the field and mapping evidence, the authors seek to understand the dynamics and dimensions of the Dartmoor Ice Cap that was responsible for most of the landforms described.  They create a number of models which match very closely the evidence on the ground.  Here are two of them:


Suggested ice thicknesses for the Dartmoor Ice Cap after 1400 years of development.  Note that in places the ice is more than 140m thick, with a maximum thickness of c 180m where ice was flowing rather than stagnant.
Suggested flow rates for the ice of the Dartmoor Ice Cap.  Much of the ice was almost stagnant, but in places, in the main outlet valleys where glaciers have been located, ice movement was up to 15m per year.  That is still very slow compared with many outlet glaciers from ice caps in the polar regions today.

The authors attribute most of the described features to the Devensian glacial episode, but they do admit that some of them might be composite, having been modified during a number of phases of glacial action.  I would have liked a little more consideration of the extent of "fossil" or "inherited" glacial features from earlier glacial phases such as the Anglian --  but perhaps that work will come later.  For the moment, this is a very significant paper which deserves attention.  I'll return with further comments on the implicatioins for the glacial history of Southern England.

In the meantime, here are some key extracts:


ABSTRACT

The granite uplands of Dartmoor have traditionally been considered to be relict permafrost and peri-
glacial landscapes that lay beyond the limits of Quaternary glaciations but a variety of landform evidence indicates that a plateau icefield existed on the northern part of the moor, constituting the southernmost independent ice cap in the British Isles. Overdeepened or weakly U-shaped valley segments fringing north Dartmoor document an early, extensive phase of glaciation but the most convincing landform evidence relates to more recent, valley-based glacier occupancy. A moraine ridge on the Slipper Stones represents the most unequivocal palaeo-glacier on north Dartmoor with a palaeo-ELA of c.460 m above sea level (asl), although this relates to the youngest and most restricted phase of glaciation. A longer term ELA is likely to be represented by the Corn Ridge proto-cirque at 370-410 m asl. More extensive valley glaciers are recorded in each of the major drainage basins of north Dartmoor by arcuate and linear bouldery ridges and hummocky valley floor drift, which are interpreted as latero-frontal moraines deposited by outlet lobes of a plateau icefield. Recession of these lobes is marked by inset sequences of such ridges and occasional meltwater channels. Plateau ice was predominantly thin and protective, and snowblow and preferential accumulation in valley heads facilitated the modest glacial erosion and debris transport recorded in the landforms and sediments. It is proposed that the highest plateaux have been occupied by ice for the longest cumulative period of time throughout the Quaternary (“average glacial conditions”), explaining the distribution of different tor types on northern Dartmoor. This also explains the lack of tors on the most expansive of the highest plateau terrain (ice dispersal centres) as the product of: a) average glacial conditions preferentially removing tors or dampening their production rates; b) the
survival of high relief (Type 1) tors during glaciation if they occupy summits too narrow to develop
significant plateau icefields and/or ridges that are bypassed by faster moving ice in adjacent deep valleys; and c) the survival of subdued (Type 2) tors in areas glaciated less regularly during the Quaternary.  Simple ice flow modelling indicates that a plateau icefield type glaciation is required for significant ice flow to occur and confirms thin ice cover, in particular on narrow summits, thereby supporting the explanation of tor class distribution. The modelling allows us to spatially correlate the geomorphological evidence of margin positions into two major stages and further indicates a strong altitude-mass balance feedback leading to an ice cap that is not in balance with its climate and with an extent that is limited by the length of the cold phases rather than their severity.

EXTRACT

7. Age of glaciation

While at present there are no dates on the glaciations described
here, on morphostratigraphic grounds we can speculate on the
likely ages of the glacial phases. We hypothesise that the devel-
opment of the plateau ice cap and associated valley glaciers
occurred during the LGM at a time when the Late Devensian ice
sheet reached the northern Scilly Isles to the west and south of
Dartmoor. The modelling suggests that such more extensive ice-
fields with significant ice flow require several hundreds to thou-
sand of years to build up. We further suggest that the restricted
glacial advance producing small niche and cirque glaciers in the
West Okement Valley occurred during the Younger Dryas. In this
scheme the long term ELA at Corn Ridge of 370-410 is slightly
higher than that reconstructed by Harrison et al. (1998) for the
glaciation of the Punchbowl on Exmoor which had an ELA of 334 m
asl but remains similarly undated. Similar low values are recorded
in parts of South Wales such as the Black Mountains with Younger
Dryas ELAs around 290-350 m asl (Barclay, 1989). Recent unpub-
lished work (Hägg, 2009) reports the use of cosmogenic nuclides to
assess the age of tors and long term erosion rates on northern
Dartmoor. He used the dates to derive higher long term catchment
denudation rates than present values. Although he recognized the
possibility that Dartmoor may have been glaciated especially in the
central plateau region where our modelling predicts the existence
of plateau icefields, most of his sample sites were on the fringes of
this region and therefore do not help test this assertion. However,
high rates of catchment denudation may result from glaciation
given that burial of surfaces under ice would provide shielding from
cosmic radiation and thus lead to underestimation of surface age or
overestimation of erosion rates (Hägg, 2009, p. 233). Clearly,
cosmogenic nuclide dating on selected surfaces will be crucial to
provide future information on the age of the Dartmoor glaciations.

EXTRACT

8. Conclusions
The extensive glaciation of Dartmoor is proposed in this paper
based upon the interpretations of several geomorphic criteria and
simple numerical flow modelling which strongly indicate the
development of a plateau icefield landsystem. The glacial evidence
is subtle; glaciers flowing at low gradients produce low driving
stresses. Nevertheless, we argue here that the glacial evidence
comprising moraines, drift limits meltwater channels, ice-scoured
bedrock and glacial sediment is compelling. The ice cap repre-
sents the southernmost independent Pleistocene ice mass in the
British Isles and is hypothesized to have last developed during the
LGM and Younger Dryas. The palaeo-glacier reconstruction with
which we have the greatest confidence, the Slipper Stones niche
glacier, represents a palaeo-ELA of around 460 m OD based upon
the MELM method. When combined with the altitudes and
breadths of the main plateau surfaces of north Dartmoor on
a “Manley curve”, this palaeo-ELA indicates that upland ice would
have been thin and protective and outlet glaciers would have
received significant nourishment from snowblow into valley heads,
although caution must be exercised when using only one glacieret
to determine a regional ELA. If the weakly developed bedrock
amphitheatre on Corn Ridge is accepted as a cirque, its floor altitude
of 370-410 m OD could be regarded as a long term ELA, thereby
making plateau icefield development more viable on north Dart-
moor. The flow modelling allows a 3-dimensional reconstruction of
a potential ice cap which spatially verifies the mapped marginal
moraine features. Due to the low surface gradients of the Dartmoor
plateau, the numerical modelling produces a plateau-style rather
than a valley glacier style of glaciation in order to initiate significant
ice flow. As a further consequence of the low surface gradient, such
a potential ice cap is exposed to a strong feedback between surface
mass balance and altitude which suggests that the reconstructed
glaciation extents do not relate to steady state conditions and that
these extents are limited by the length of cold phases. Furthermore,
the modelled ice extent, thickness and flow speed are in general
consistent with the development and preservation of the observed
tor style distribution. Our findings further support the earlier work
on glacial deposits and landforms on Exmoor and suggest that
other high moorland in the region (such as Bodmin Moor to the
west) may also have hosted ice masses during the Quaternary in
response to the sub-Milankovitch or millennia-scale excursions to
cold climate which seem typical of the last glacial stage and which
could have driven the repeated buildup of ice caps that were
limited by the length rather than severity of cold phases.

20 comments:

Anonymous said...

Splendid.
The morraine is immediate.
What really lovely work although the technicalities are beyond me the care and correctness beam through.
A non-specialist summary of the whole paper10 votteek in time would be very useful Brian.

What a week- this and the publication of MPP's book with its photograph of the Rhos 'quarry'and proto-orthostat.

Truely we are the playthings of the Gods.
M

Anonymous said...

"While at present there are no dates on the glaciations described
here, on morphostratigraphic grounds we can speculate on the
likely ages of the glacial phases"

Enough said, my dear Brian.

Sherlock

Anonymous said...

“Truely we are the playthings of the Gods.”

More like we are the playthings of the Archeologists! But then again, many Archeologists think they are Gods and can create Prehistory in their own imagination!

thinker

Tony H said...

I see in the summary of MPP's "Stonehenge" book (found on Google) that his self-proclaimed 'Bluestonehenge' consisted of 25 bluestones, even though he unearthed only about one - third of the stone holes, minus the stones. Some old chestnuts, it seems, will never go away! So it will again be Open Season for both the proponents of human transportation of said so-called "blue" stones, and the protaganists of said Old Chestnut. Some of the air will turn blue.

Tony H said...

Think I said 'proponent' when I meant 'opponent' on previous comment.

But it's clearly Open Season for all manner of quarries (double entendre).

BRIAN JOHN said...

Sherlock, what are you on about? The four authors of this piece are serious academics who will not put firm dates on anything until radiocarbon or other dating evidence is available. Perfectly commendable. But geomorphologists use stratigraphy just as geologists do, and look also at the relationships between landforms. Then they use comparisons with dated features elsewhere to make their "best guesses" as to age. You may have a problem with that. I don't.

Anonymous said...

The initial bluestones found at Bluestonehenge are not spotted dolerites.
There was not much non-sst.
M

Tony H said...

Aaah, but the Hampshire River Avon is close at hand, Myris, and myths of megalith- transportation, by river and sea, fuel the imagination (of some), regardless of the absence of solid geological evidence from so-called "Bluestonehenge".

Anonymous said...

Brian,

In light of this new compelling scientific evidence, lets review some of the facts. We have,

1) The glaciation of Dartmoor just south and southwest of Salisbury Plain
2) The encroachment of the Irish Sea Glacier along the coast on the West and into the Bristol Channel
3) Glacier Lake Harrison forming during the melting of the glaciers North of Salisbury Plain with River Avon drainage.
4) River Avon running through Salisbury Plain and draining into Bristol Channel.
5) The Younger Dryas Ice Age lasting some 1400 years around 8500 BC

What can we reasonably conclude from this? In my opinion, it is reasonable to conclude that the Irish Sea Glacier to the West and the Dartmoor Ice Cap to the south of Salisbury Plain formed a glacier dam of meltwater drainage from Lake Harrison to the North. This glacier dammed lake would of course freeze solid during the Younger Dryas, forming my hypothesized 'local ice cover' of Salisbury Plain.

Perhaps this hypothesis will now be scientifically investigated by the same team of scientists that have now determined Dartmoor was glaciated.

Kostas

Anonymous said...

"The glaciation of Dartmoor just south and south west of Salisbury Plain"

"...the Dartmoor Ice Cap to the south of Salisbury Plain"

Which Atlas are you consulting, Kostas? Dartmoor is well over a hundred miles west, and not in any way south, of Salisbury Plain

A humble Geographer

BRIAN JOHN said...

Humble Geographer -- Kostas (who is supposed to be in voluntary exile but who pops up in disguise every now and then) can't be bothered with such mundane things as geographical relationships and topography. If he would just look at a map on Google -- not exactly difficult to get at -- he would save us all a great deal of bother. No more about this idiotic frozen lake please, Kostas. I will delete further posts since we have been over all of this far too many times already.

BRIAN JOHN said...

Myris -- relating to Bluestonehenge, presumably you mean there was not much that wasn't sarsen?

Geo Cur said...

Dartmoor is south of Salisbury Plain .

Anonymous said...

"Dartmoor is south of Salisbury Plain ."
Actually it's a lot more West than it is South.
PeteG

Anonymous said...

Brian,

The scientific evidence of the Dartmoor Ice Cap fits well my past descriptions of a 'local ice cover':

1) forms and melts in place
2) there is no advance and retreat of glaciers flowing into the area
3) it provides a protective thin ice cover of the ground, and
4) when the ice cover completely disappears it leaves behind no typical glaciation evidence.

What evidence is there on the ground for a 'local ice cover' is subtle and requires advanced scientific methods to discover. Except if you have eyes like an eagle and can think from higher elevations.

With this new study showing the existence of the Dartmoor Ice Cap, we are entering a new era of scientific research of the geomorphology of southwest UK.

Time to do some rethinking of your 'glacier transport' theory. Advancing glaciers from Preseli to Salisbury conveniently carrying stones for opportunistic prehistoric people to use cannot explain the emerging scientific evidence now being uncovered!

Kostas

Geo Cur said...

It is still south of the plain . "not in any way south, of Salisbury Plain ", is false .

Anonymous said...

Ah good question Brian not I did not mean that.

Sorry I do not look at sarsen-if it crosses my desk it is because it has been mis-identified in the field/initial triage.
I look at bluestones/potential bluestones only.
So I really have no idea how much sarsen there was -good question.

The data are not mine to release here and will appear in the series of Riverside monographs.
BUT
the bluestones were
a) exotic (used in the sense they do not belong to the local/regional geology)
b) did not belong to any of the standard SH bluestone classes.
c) there were (and I hate to say this knowing your fondness for huge varieties of rocks at SH) a number of non-Palaeozoic/non-Altar stone sandstones. That was my throw-away line.
But but
Like Saul of Tarsus/Micah I say Bluestonehenge is not least amongst SH locations and might yield a rod for someones back.
The trouble is the locality has been the dumping ground for millenia.
GCU In two minds. (M having summer break but I hope will return as even avatars need avatars)

Tony H said...

GCU In Two Minds
"The data.....will appear in the series of Riverside monographs".

Pray,when?

Anonymous said...

Two Minds, you write

“The trouble is the locality has been the dumping ground for millenia.”


Would this be a human dumping ground or dumping ground by Nature?

thinker

Tony H said...

GCU in two minds

"I say Bluestonehenge is not least among SH locations and might yield a rod for someone's back"

Would you agree that the interpretation of "Bluestonehenge" as a work in progress (rather as it was the season before it was partially excavated) and further excavation may yet yield yet another interpretation, not least as regards the "Blue-" element in its nomenclature? And we await the relevant issue(s) of the Riverside monographs with bated breath.