SPELEOGENETIC EVIDENCE FROM OGOF DRAENEN FOR
A PRE-DEVENSIAN GLACIATION IN THE BRECON
BEACONS, SOUTH WALES, UK
Andrew R. Farrant
1
, Christopher J. M. Smith
2, 3
, Stephen R. Noble
4
, Michael J. Simms
5
,
David A. Richards
2, 3
1. British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK. E-mail: arf@bgs.ac.uk
2. Bristol Isotope Group (BIG), Wills Memorial Building, University of Bristol, Queen’s Road, Bristol,
BS8 1RJ, UK.
3. School of Geographical Sciences, University of Bristol, University Road, Bristol, BS8 1SS, UK.
4. NERC Isotope Geosciences Laboratory (NIGL), British Geological Survey, Keyworth, Nottingham,
NG12 5GG, UK.
5. Department of Geology, National Museums Northern Ireland, Cultra, Holywood, Co. Down, BT18
0EU, Northern Ireland.
Abstract
The British Isles have been affected by as many as 30 glaciations during the Quaternary.
However, the evidence for pre-Devensian glaciations in upland regions is scarce.
Understanding the extent and timing of earlier upland glaciations is essential for modelling
the long term evolution and sensitivity of the British Ice Sheet (BIS). Caves, being protected
from surface erosion and weathering, can preserve evidence of earlier glaciations in the form
of speleothem and sediment archives. The ~70 km long Ogof Draenen cave system in South
Wales, UK, contains multiple cave levels related to changes in the surface topography and
drainage during the past 0.5 Ma. The cave contains evidence of massive influxes of sediment
that were sufficient to choke the cave and alter the underground drainage. Analysis of the
cave sediments, passage morphology and geometry suggests the cave once acted as a
subterranean glacial spill-way before being overridden by ice. Speleothem U-series data
demonstrates that this sediment influx occurred before Marine Isotope State (MIS) 9,
probably during the Anglian glaciation (MIS 12). Evidence from Ogof Draenen indicates the
impact of subsequent glaciations on the landscape evolution of the region was minimal and
that much of the surface topography dates from the Anglian.
Keywords: speleothem, glaciation, Wales, U-series dating, U–Th, landscape evolution, Ogof
Draenen
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(Note: Welsh terms used in this paper: Ogof = Cave, Afon = River, Cwm = Valley, Mynydd
= Mountain)
1. Introduction
Most of the upland karst areas in the north and west of the UK have been glaciated multiple
times during the past million years, with the greatest advances during Marine Isotope Stage
(MIS) 12 (Anglian) and MIS 2 (Devensian) glaciations. Until recently there was evidence for
only a small number of glaciations in the UK (Bowen, 1999; Bowen et al., 1986; Clark et al.,
2004). Now perhaps as many as 30 glaciations are known (Böse et al., 2012; Lee et al.,
2012; Lee et al., 2011; Thierens et al., 2012; Toucanne et al., 2009), dating back about 2.6
Ma, although the timing of many remains equivocal. Equally, recent work has shown that the
climatic thresholds required to build glaciers in Britain were much lower than previously
considered with glaciers existing throughout the Little Ice Age (LIA), from the mid-16
th
to
mid-19
th
centuries (Harrison et al., 2014; Kirkbride et al., 2014). Collectively, they indicate
the British Ice Sheet (BIS) was as dynamic and responsive as other Northern Hemisphere ice
sheets, and highly responsive to even subtle changes in climate.
Frequently, the evidence for pre-Devensian glacial activity in many upland areas is often
lacking, and is often inferred only from exotic clasts in river terrace deposits (Whiteman and
Rose, 1992). Typically this absence is attributed to the erosional effect of Devensian ice
sheets removing any evidence of former glaciations, particularly during the Late Glacial
Maximum (LGM). Bias in the glacial record is particularly evident in South Wales, where
evidence for pre-Devensian glaciations is scarce and limited to lowland areas. The
Llanddewi Glacigenic Formation on the Gower Peninsula is the only unequivocal Anglian
age deposit in South Wales, and represents the margins of the Welsh ice sheet at this time
(Gibbard and Clark, 2011).
Based on geomorphological analysis and dating of cave sediments and speleothems, it is clear
that cave systems in upland areas of the UK often pre-date the last glaciation (Waltham et al.,
1997) and, in some cases, extend back to the early Pleistocene (Lundberg et al., 2010; Rowe
et al., 1988; Waltham and Lowe, 2013). These caves can preserve evidence of surface
processes, including glacial activity over long timescales. Glaciations can have profound and
complex effects upon karst landforms and their underlying aquifers, and may destroy, inhibit,
preserve, or stimulate karst development (Ford, 1987; Ford et al., 1983; Ford and Williams,
2007). Glacially-induced valley incision can instigate major changes to underground
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drainage systems as the conduits adjust to new, lower base levels. These modifications are
recorded within cave systems by changes in passage morphology and geometry, and are
analogous to fluvial terraces as recorders of base-level change (Palmer, 1987). Some caves,
depending on local circumstances are affected by glacial meltwater, a modern example being
Castleguard Cave in Canada (Ford, 1983). Sub-glacial water flow can be considerable,
especially in active, wet-based ice streams, and at the margins of glaciers and ice sheets.
Where these are in contact with karstified aquifers, there is scope for significant input of
allogenic meltwater into pre-existing cave systems (Lauritzen, 1984, 1986), injecting fluvio-
glacial sediment deep underground. These caves act as sediment repositories, protected from
subsequent weathering and surface erosion processes on timescales up to 10
6
years. Away
from active drainage networks, relict cave passages can be preserved untouched with little or
no evidence of sub-glacial modification.
Crucially, caves also host speleothem deposits, which can be accurately dated using uranium-
series (U-series) methods (Meyer et al., 2009; Richards and Dorale, 2003). These are often
interbedded with or overlie cave sediments, thus allowing both the timing of cave formation
and sediment deposition to be constrained over the last 500 ka, and with suitable samples,
beyond 500 ka using U–Pb methods (Richards et al., 1998). Given the lack of preserved and
datable surface material in glaciated upland areas, cave systems offer some of the best
prospects for preserving evidence for pre-Devensian landscape evolution. In this study, we
present evidence from speleothem U-series dating, cave sediment analysis and speleo-
morphological data for pre-Devensian glacial activity in upland areas of South Wales, an area
where the preservation of evidence for earlier glaciations is limited.
2. The study area
The Brecon Beacons in southern Wales is a large upland area (900 km
2
) situated on the
northern edge of the South Wales coalfield (Figure 1), which occupies a large elongate east-
west orientated synclinal structure 90 km long and 25 km wide. The Brecon Beacons are
composed predominantly of Devonian sandstone (the ‘Old Red Sandstone’), which dips
gently (between 5° and 20°) to the south. These are overlain by Lower Carboniferous
limestones and a thick sequence of Upper Carboniferous siliciclastics, including the Twrch
Sandstone Formation ('Millstone Grit') and the ‘Coal Measures’, a cyclical sequence of
sandstones and mudstones with some coal seams (Barclay, 1989). The Lower Carboniferous
limestones outcrop around the coalfield, locally forming a relatively narrow but well
developed escarpment, especially along the north-eastern edge of the syncline.
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The limestones are well-karstified, particularly on the northern edge of the coalfield. Many
sinkholes, stream sinks and cave systems are known, with more than 230 km of cave passage
discovered and surveyed. Eight of these cave systems each contain over 8 km of passage
(Table 1). Together they represent some of the best examples of interstratal cave systems in
the UK (Waltham et al., 1997). All are characterised by extensive high-level relict passages
perched above more recent active streamways. Most of them contain copious amounts of
silty or sandy sediment preserved in the higher level relict passages long abandoned by active
streams. This is true of Ogof Draenen, the caves beneath the adjacent Mynydd Llangattock
(Agen Allwedd, Daren Cilau and Craig yr Ffynnon; Smart and Gardner, 1989) and Ogof
Ffynnon Ddu, 40 km further west (Smart and Christopher, 1989). This study is focused on
Ogof Draenen, where a detailed examination of the cave geomorphology (Farrant and Simms,
2011; Farrant and Smart, 2011) coupled with U-series dating of speleothems from the cave,
has enabled a detailed chronology of the cave’s formation and sedimentary history to be
constructed.
3. Ogof Draenen
Ogof Draenen [51.79966ºN, 3.09439ºW] is a complex multiphase intrastratal cave system
located near Blaenavon, 6 km south-west of Abergavenny, South Wales (Figure 1). It
currently stands as one of the longest cave systems in the UK, with ~70 km of surveyed
passages spanning a vertical range of >150 m (Stevens, 1997; Waltham et al., 1997). The
cave underlies Gilwern Hill, The Blorenge and Mynydd y Garn-fawr, which together form
the interfluve between the deeply-incised Usk valley and the smaller Afon Lwyd valley. The
cave has a long and complex history (Simms et al., 1996; Waltham et al., 1997) which is
discussed in detail in Farrant and Simms, (2011). Speleogenesis combined with valley
incision and base-level lowering has left a vertically-stacked series of relict passages
preserved in the limestone beneath the Twrch Formation cap-rock. The highest, and
therefore the oldest cave levels are preserved up to 150 m above the present cave stream with
progressively younger, lower passages developed sequentially down dip to the west. Tracer
tests show the cave stream resurges 6 km beyond the present southern limit of the cave in
Pontypool (Maurice and Guilford, 2011). A relative chronology of cave evolution has been
constructed from speleo-morphological observations throughout the cave, including passage
geometry, dimensions and morphology, and the analysis of palaeoflow directions from
dissolutional scallops, stratified cave deposits, cross bedding and ripple marks. Other
observations such as the transition from vadose to phreatic passage morphologies have
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enabled palaeo-watertable elevations to be fixed. Analysing the relationship between aquifer
geometry, surface topography and the various active and relict conduits in Ogof Draenen has
enabled us to relate these palaeo-watertable elevations and cave levels to changes in the
surface landscape (Simms and Farrant, 2011).
Ogof Draenen comprises four vertically stacked, genetically-separate cave systems linked by
phreatic under-captures (passages developed in the phreatic zone by water draining from an
existing conduit into a newer conduit), shaft drains, chance passage intersections and invasive
vadose inlets. Only the lowest level is hydrologically active today although some relict
passages contain misfit streams. The present autogenic catchment is very small because the
limestone forms only a relatively narrow outcrop along the steep scarp of the Usk valley.
Consequently, recharge throughout the cave’s history has been predominantly allogenic,
derived principally from numerous small streams draining the Upper Carboniferous
siliciclastics that overlie the cave. Streams draining the sandstone feed into a series of
conduits that drain initially down dip and then trend approximately along strike to resurge at
springs in the surrounding valleys. The oldest relict underground drainage system is
represented by the Megadrive conduit and the associated War of the Worlds conduit (Figure
2a). This conduit system drained south-east, roughly along strike to former resurgences at c.
360 m above sea-level (asl) in the Usk valley (Farrant and Simms, 2011). This was
abandoned when the drainage was captured southward to a suite of progressively lower
resurgences at 360-320 m asl following incision in the Afon Lwyd valley. Continued
landscape evolution led to a second major change in the underground drainage pattern, this
time in response to valley incision in the Clydach Gorge to the north, effectively reversing the
hydraulic gradient. This allowed the development of a new, lower level series of passages,
the ‘The Score-Gilwern Passage’ conduit to develop down dip to the west. This drained
northwest to a former resurgence in the Clydach Gorge at 320-300 m asl (Figure 2b).
Renewed incision in the Afon Lwyd valley caused a second reversal in flow direction, this
time to the south. Ultimately, new springs developed 10 km to the south near Pontypool at
120 m asl (Figure 2c) to which the ‘Beyond a Choke’ streamway presently drains. Ogof
Draenen thus represents a hydrological see-saw, with successive conduits at progressively
lower elevations each draining to different resurgences in response to incision in three
separate valleys. This sequence of events is thought to span much of the Middle to Late
Pleistocene, possibly extending back over a million years into the Early Pleistocene (Simms
and Farrant, 2011).
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