Wookey Hole Chamber 20 SSSI audit

This report documents an audit of geological features of potential scientific and aesthetic importance in Wookey 20. This audit was created by the British Geological Survey (BGS) under commission by Wookey Hole Caves Ltd as part of the planning conditions for the extension of the show cave from Wookey 9 into Wookey 20

Subterranean glacial spillways: an example from the karst of South Wales, UK

Many karst areas in the UK have been glaciated one or more times during the last 0.5 Ma, yet there are few documented examples of caves in these regions being affected by glacial processes other than erosion. The karst of South Wales is one area where sub or pro-glacial modification of pre-existing caves is thought to occur. Evidence from the Ogof Draenen cave system suggests that caves can sometimes act as subterranean glacial ‘underspill’ channels for melt-water. This cave, one of the longest in Britain with a surveyed length of over 70 km, underlies the interfluve between two glaciated valleys. Sediment fills and speleo-morphological observations indicate that melt-water from a high level glacier in the Afon Lwyd valley (>340m asl) filled part of the cave and over-spilled into the neighbouring Usk valley, temporarily reversing non-glacial groundwater flow directions in the cave. It is suggested that this may have occurred during a Middle Pleistocene glaciation

British Cave Research Association field guide to the Bath Stone quarries, Box, Wiltshire

The venue for the 2016 British Cave Research Association Cave Science Symposium Field Trip is the underground ‘Bath Stone’ quarries around Box, near Corsham, Wiltshire (Figure 1). The aim of the field trip is to examine the cambering and gulling, gull caves and karstic features observed in the quarries. The underground quarries at Box lie at the southern end of the Cotswold Hills, on the southern side of the By Brook valley, a tributary if the River Avon. This valley has incised though the Great Oolite Group, a sequence of Middle Jurassic (Bathonian) limestones and mudstones. The underground quarries are developed within the Chalfield Oolite Formation. This is an excellent building stone, as it can be sawn by hand in any direction as a ‘freestone’ which then hardens on exposure to air, rather than having a distinct cleavage like slate

Groundwater in Cretaceous carbonates: KG@B field trip 21st June 2015

The Upper Cretaceous Chalk of southern England is the UK’s most important aquifer, providing more than 75% of the public supply for southeast England, including London. The aquifer also sustains rivers and wetlands, and their associated groundwater dependent ecosystems. However, the aquifer is facing a multitude of threats including over-abstraction, nitrate pollution, and climate change. The Chalk is a complex aquifer in which groundwater flow is through the matrix, fractures and karstic dissolutional voids. The Chalk matrix has a porosity of around 35% (Bloomfield et al., 1995). The matrix is thought to provide an important contribution to storage, although the size of the pore throats is very small, and therefore the permeability is very low (Price et al., 1993). The average permeability of 977 core samples was only 6.3 x 10-4 m/day (Allen et al., 1997). The matrix is particularly important in solute transport, because solutes move between the matrix and the more permeable parts of the aquifer via diffusion (Foster 1975). The unmodified fracture network provides an important contribution to storage and flow, and has a hydraulic conductivity of about 0.1 m/d, and a transmissivity of about 20 m2/day (Price, 1987). However, it is the dissolutionally enlarged fissures and conduits that make the Chalk such a good aquifer. The median transmissivity from 2100 pumping tests is 540 m2/day, and the 25th and 75th percentiles are 190 and 1500 m2/day respectively (MacDonald and Allen, 2001). Borehole packer testing, logging and imaging have shown that most of this transmissivity comes from a small number of dissolutional voids (e.g. Tate et al., 1970; Schurch and Buckley, 2002). Laterally extensive lithostratigraphical horizons including marl seams, bedding planes, sheet and tabular flint bands, and hard-grounds have an important influence on these groundwater flows. They are all horizons where downward percolation of water may be impeded. Dissolution often occurs where flow is concentrated along these horizons, creating conduits or fissures, especially where they are intersected by joint sets

Speleothem U-series constraints on scarp retreat rates and landscape evolution: an example from the Severn valley and Cotswold Hills gull-caves, UK

Modelling landscape evolution requires quantitative estimates of erosional processes. Dating erosional landscape features such as escarpments is usually difficult because of the lack of datable deposits. Some escarpments and valley margins are associated with the formation of mass-movement caves, sometimes known as ‘gull’ or ‘crevice’ caves, which are typically restricted to within 0.5 km of the valley margin or scarp edge. As in other caves, these mass-movement cavities may host speleothems. As gull-caves develop only after valley incision, uranium-series dating of speleothems within them can provide a minimum age for the timing of valley excavation and scarp formation. Here we present data from several gull-caves in the Cotswold Hills, which form the eastern flank of the Severn valley in southern England. U-series ages from these gull-caves yield estimates for both the minimum age of the Cotswold escarpment and the maximum scarp retreat rate. This is combined with data from geological modelling to propose a model for the evolution of the Severn valley and the Cotswold Hills. The data suggest that the location of the escarpment and regional topography is determined not by valley widening and scarp retreat, but by the in situ generation of relief by differential erosion

Karst hydrogeology of the Chalk and implications for groundwater protection

The Chalk is an unusual karst aquifer with limited cave development, but extensive networks of smaller solutional conduits and fissures enabling rapid groundwater flow. Small-scale karst features (stream sinks, dolines, dissolution pipes, and springs) are common, with hundreds of stream sinks recorded. Tracer velocities from 27 connections between stream sinks and springs have median and mean velocities of 4700 and 4600 m d−1. Tests to abstraction boreholes also demonstrate very rapid velocities of thousands of metres per day. Natural gradient tests from observation boreholes have rapid velocities of hundreds of metres per day. There is strong geological control on karst with dissolution focused on stratigraphical inception horizons. Surface karst features are concentrated near the Paleogene boundary, or where thin superficial cover occurs, but rapid groundwater flow is also common in other areas. The Chalk has higher storage and contaminant attenuation than classical karst, but recharge, storage and flow are influenced by karst. Point recharge through stream sinks, dolines, losing rivers, vertical solutional fissures, and soakaways enables rapid unsaturated zone flow. Saturated zone networks of solutional fissures and conduits create vulnerability to subsurface activities, and enable long distance transport of point source and diffuse pollutants, which may be derived from outside modelled catchment areas and source protection zones

The lithostratigraphical context of the English Chalk Rock (Turonian)

Correlations of borehole geophysical logs in the middle and upper Turonian Chalk Group are used to consider recent proposals for a revision in understanding of a unit of hardgrounds (Chalk Rock) and associated stratigraphy developed across parts of southern England. Along the northern edge of the London and Wessex basins, geophysical logs reveal a laterally continuous framework of correlatable inflection patterns in the New Pit Chalk, with the package of sediment immediately below the Chalk Rock showing a trend of lateral thinning and increasingly condensed sedimentation westwards into areas where the oldest of the Chalk Rock Hardgrounds (Ogbourne Hardground) is present. However, apart from local absence of the Glynde Marls Complex near the top of the New Pit Chalk, there is no evidence for the presence of a major erosion event. This questions recent interpretations of microcrinoid data, reportedly showing that the Ogbourne Hardground lithifies a stratigraphical level in the lower part of the New Pit Chalk, with the middle and upper parts of this unit corresponding with a hiatus and related short-lived globally significant sea level fall. Macrofossil biostratigraphy supports the geophysical log interpretations, with evidence of both younger and older parts of the New Pit Chalk below the Ogbourne Hardground. The data are consistent with the Ogbourne Hardground in Wiltshire and Berkshire representing a highly condensed equivalent of thickened nodular chalk fabrics at the base of the Lewes Nodular Chalk in the eastern Chilterns

Alluvial fan records from southeast Arabia reveal multiple windows for human dispersal

The dispersal of human populations out of Africa into Arabia was most likely linked to episodes of climatic amelioration, when increased monsoon rainfall led to the activation of drainage systems, improved freshwater availability, and the development of regional vegetation. Here we present the first dated terrestrial record from southeast Arabia that provides evidence for increased rainfall and the expansion of vegetation during both glacial and interglacial periods. Findings from extensive alluvial fan deposits indicate that drainage system activation occurred during Marine Isotope Stage (MIS) 6 (ca. 160–150 ka), MIS 5 (ca. 130–75 ka), and during early MIS 3 (ca. 55 ka). The development of active freshwater systems during these periods corresponds with monsoon intensity increases during insolation maxima, suggesting that humid periods in Arabia were not confined to eccentricity-paced deglaciations, and providing paleoenvironmental support for multiple windows of opportunity for dispersal out of Africa during the late Pleistocene

An early MIS 3 pluvial phase in Southeast Arabia: climatic and archaeological implications

Climatic changes in Arabia are of critical importance to our understanding of both monsoon variability and the dispersal of anatomically modern humans (AMH) out of Africa. The timing of dispersal is associated with the occurrence of pluvial periods during Marine Isotope Stage (MIS) 5 (ca. 130–74 ka), after which arid conditions between ca. 74 and 10.5 ka are thought to have restricted further migration and range expansion within the Arabian interior. Whilst a number of records indicate that this phase of aridity was punctuated by an increase in monsoon strength during MIS 3, uncertainties regarding the precision of terrestrial records and suitability of marine archives as records of precipitation, mean that the occurrence of this pluvial remains debated. Here we present evidence from a series of relict lake deposits within southeastern Arabia, which formed at the onset of MIS 3 (ca. 61–58 ka). At this time, the incursion of monsoon rainfall into the Arabian interior activated a network of channels associated with an alluvial fan system along the western flanks of the Hajar Mountains, leading to lake formation. Multiproxy evidence indicates that precipitation increases intermittently recharged fluvial systems within the region, leading to lake expansion in distal fan zones. Conversely, decreased precipitation led to reduced channel flow, lake contraction and a shift to saline conditions. These findings are in contrast to the many other palaeoclimatic records from Arabia, which suggest that during MIS 3, the latitudinal position of the monsoon was substantially further south and did not penetrate the peninsula. Additionally, the occurrence of increased rainfall at this time challenges the notion that the climate of Arabia following MIS 5 was too harsh to permit the further range expansion of indigenous communities