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.