Speleogenetic evidence from Ogof Draenen for a pre-Devensian glaciation in the Brecon Beacons, South Wales, UK
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References
A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records
Karst Hydrogeology and Geomorphology
Karst Hydrogeology and Geomorphology: Ford/Karst Hydrogeology and Geomorphology
Origin and morphology of limestone caves
River terrace systems in north-west Europe: an archive of environmental change, uplift and early human occupation
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Frequently Asked Questions (17)
Q2. What was the reason for the vadose cave development?
The subsequent period of vadose cave development was doubtless a result of Anglian glacial incision altering base-levels, allowing resurgences to develop at lower elevations.
Q3. What is the composition of the gravel terraces?
gravel terraces of similar composition occur up to 0.5 m above the present stream level, representing former channel stages.
Q4. What is the way to preserve cave passages?
Away from active drainage networks, relict cave passages can be preserved untouched with little or no evidence of sub-glacial modification.
Q5. What was the evidence for the influx of sediment?
The influx of sediment led to ponding and localized paragenesis; blocking some passages, reactivating others and, in some cases, facilitating the development of new conduits (Farrant and Smart, 2011).
Q6. Why is the current autogenic catchment small?
The present autogenic catchment is very small because the limestone forms only a relatively narrow outcrop along the steep scarp of the Usk valley.
Q7. What is the only unequivocal Anglian age deposit in South Wales?
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).
Q8. What did the evolution of the drainage pattern in Ogof Draenen mean?
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.
Q9. What is the recent evidence of glaciation in the UK?
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.
Q10. What is the significance of the influx of sediment in the Llangattock caves?
Simms and Hunt (2007) provide evidence of sediment influx, glacial flooding and impoundment in Agen Allwedd and suggest that glacial damming and recharge from meltwater might have been a significant factor in the development of the Llangattock caves.
Q11. What evidence indicates that the Afon Lwyd valley was already incised?
Speleothem U-series evidence presented here indicates that the Afon Lwyd valley was already incised sufficiently deeply to allow groundwater to flow south towards Pontypool prior to MIS 9.
Q12. How did the Megadrive conduit drain south-east?
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).
Q13. How high are the plaques of cross bedded sands?
Further upstream, plaques of cross bedded sands (Figure 5) can be seen high up on the passage walls, at least 4-5 m above the present passage floor and extending to within a couple of metres of the roof, here around 8-10 m high.
Q14. Why is the accuracy of the U–Th dating technique so low?
Due to the limitations of the U–Th dating technique, the absolute precision on isotopic age’s decreases as samples approach the line of secular equilibrium.
Q15. What is the plausible explanation for the sediments in Ogof Draenen?
The most plausible explanation is that the sediments were emplaced during glacial or pro-glacial conditions when glacial meltwater was able to transport significant amounts of sediment into the cave.
Q16. What is the time required to capture the drainage?
Given the time needed to initiate, develop and incise the present streamway to sufficient depth to allow speleothem growth, the authors suggest that the incision of the Afon Lwyd valley required to capture the drainage occurred mostly during or shortly after the Anglian glaciation.
Q17. What is the relationship between the cave and the surface landscape?
The relationship of the cave to the surface landscape indicates the eastern Brecon Beacons attained much of its present morphology during or prior to the Anglian glaciation, with relatively little modification in subsequent glacial advances.