ysgrifennwyd gan Holly Addis, casglwyd gan Heather Crumpton
Four Ashes SSSI gravel pit has long been of geological interest. This site is five miles north of Wolverhampton in Staffordshire. In the 1960’s and 70’s it was a working quarry and supplied scientists with information concerning both the extent of the Devensian Irish Sea ice sheet in Britain, and palaeoecological and palaeoclimatological changes during the late Pleistocene. However, fifty years on, not only had most of the quarry been infilled with landfill, covering many of the exposures, little management had been done in reducing the impacts of vegetation cover and waterlogging. The site was assessed by Natural England as being in unfavourable condition and much of its scientific interest was thought to have been lost (Burek, 2012). Although only a small area of the original working face can still be viewed, clearance work and regeneration of the site began in autumn 2012 and the site has since recovered to favourable condition.
The original research on the Four Ashes site was from 1967 – 1970 (Morgan, A.V. 1973). On the basis of this work, the gravel pit was chosen as the type site for the Devensian glaciation in Britain and therefore it became an important reference site for the Quaternary study. In more recent years other examples of the Devensian period have been found elsewhere in Chelford (Bowen, 1999) and Dimlington (Rose, 1985) , however, Four Ashes still remains a key reference and stratotype site (Glasser, 2002). Although much of the site has been lost due to landfill, important till deposits can still be found at the former pit margin. The bedrock at Four Ashes is Triassic sandstone, though little of this can be seen today. The site is more important for the overlying gravel deposits and the organic content within and beneath them (Andrew and West, 1977). The sequence is capped by Irish Sea till. The lowest organic lenses are thought to date to the Ipswichian Interglacial and the base of the gravel unit was laid down by braided rivers in the early Devensian. Within these early-mid Devensian gravels there are further organic lenses which contain evidence of cooler climatical conditions and imply that whist the gravel was being deposited, Four Ashes had a deciduous woodland environment (Morgan, A. V. 1973). The insect fauna, particularly beetles, from these sediments was studied in detail by A Morgan (1973). Palaeoclimatic evidence from pollen and insect fauna indicate that summer temperatures only reached 10oC (Duff and Smith, 1992).
Radiocarbon dating from the Four Ashes site show that the last main glaciation in Britain peaked after 30,000 BP in the late Devensian between 26000-13000 BP (Bowen, 1973). Although till from the Four Ashes site indicates the Irish Sea ice extended as far as Wolverhampton, the site itself shows little evidence regarding the main glaciation other than these till deposits. Later periglacial conditions followed the deposition of the till, causing cryoturbation and freeze-thaw processes to occur (Bowen, 1999). This cold climate caused the formation of features such as ice-wedges which are seen penetrating through the profile. The ice-wedges found range from 71 to 215cm in depth and 6 to 51cm in width (Morgan, A. V. 1973). Unfortunately they are poorly preserved. Although results from the University of Liverpool are yet to be fully processed, they do indicate that, in the recently reviewed sections at Four Ashes, there are possible signs of cryoturbation. Although ice-wedge pseudomorphs do not provide evidence for the main glaciation itself, they do indicate the extent of permafrost and the climatic conditions of the later Devensian period (Bowen, 1973). Although Four Ashes has now lost much of its original profile studied in the 1960’s and 1970’s, it is an important geological site in providing insight in the extent of the Devensian ice sheet, and subsequent permafrost, and the climate variations during this time. More recent studies have utilised advanced technology, such as the University of Liverpool who may provide new dating of the Devonian ice sheet at Four Ashes. The University of Liverpool used optically stimulated luminescence (OSL) in the sands and gravels at the site in order to predict a more accurate age of the till. Results from this research are yet to be published.
Quaternary sediments are particularly vulnerable to plant root penetration and damage from soil processes (Bridgland, 2013) and this is evident at Four Ashes gravel pit. Not only has severe biological disruption damaged the site, infilling sections of the gravel pit with landfill, has meant that in the remaining areas only 2.2m of the original 4.6m profile can be seen (Morgan, A. V. 1973). Much of the scientific research, and consequent papers, was conducted during the 1970’s and since then very little research has been updated despite the advancements in technology. The site had not attracted recent scientific interest until the University of Liverpool decided that Four Ashes was important and relevant for research for the BRITICE-CHRONO project. The Four Ashes site is now part of a substantial five year project, funded and supported by the National Environment Research Council (NERC) consortium grant; BRITICE-CHRONO NE/J009768/1, in investigating the timing of the growth and demise of the last British and Irish Ice Sheet. From this investigation of the advancement and retreat of the Devensian ice sheet, the BRITICE-CHRONO project hopes to predict ice sheet change for current day ice masses; principally land ice retreat in Antarctica and Greenland, to help predict future global sea level change.
Richard Waller of Keele University visited the site in 2011 and wrote up a report about conservation suggestion measures of the site for Natural England. This led to the initial recent re-interest at Four Ashes and with aid of a Natural England CES (Conservation Enhancement Scheme) grant, the site landowner and the scientific interest of University of Liverpool, Four Ashes has been subject to management actions to return it to favourable condition for research. In the spring of 2014 members of the BRITICE-CHRONO project from the University of Liverpool conducted research using OSL, to date the sediments. The University is still processing the results from the site and the final data will be included within the BRITICE-CHRONO results. This return of research at the site initiated conservation action to take place by allowing access for University of Liverpool and potentially other scientists in the near future.
One of the main issues at Four Ashes was vegetation cover over the exposure and roots intruding and disturbing the upper part of the profile. This was overcome in a number of ways. Large trees had grown up around the site, covering the former quarry face and the un-dug reserve, and needed to be removed. This was done by cutting them down close to the base. However, caution was required due to the deep roots. In a few cases the remaining trees on the un-dug reserve have been uprooted, particularly in the 2013-2014 winter storms and have taken a lot of the topsoil upper part of the sequence with them. This damages the profile further and so stabilising the slope was also important, as well as opening it up from vegetation. A few trees have been left near the road side to reduce unauthorised access e.g. fly-tipping. Grassy vegetation was removed in certain areas; however, the development of herbaceous vegetation will help prevent further erosion. Waterlogging is also an issue at old quarry sites like Four Ashes as it hinders access to the former pit faces. A site inspection showed that the nearby drains had become blocked over time. The original drainage channel had become overgrown and this has been cleared to help lower water levels by around 4 feet. Since then brash from the vegetation clearance has been laid down in the waterlogged areas to help provide firm ground to allow vehicles (e.g. mechanical excavators) access to the former quarry floor. This clearance program meant that in spring 2014 members of the BRITICE-CHRONO project from the University of Liverpool were able to carry out their data collection. However, this solution was not without risks. Excavations using mechanical vehicles led to issues with root balls becoming unstable and the upper profile collapsing (Figure 1). This also potentially causes health and safety concerns for scientists wishing to research the site in the future. A recent site visit in July 2014 showed that the vegetation is already beginning to return and water levels are noticeably higher (Figure 2). Although the Four Ashes is a success in terms of site management, this will only be temporary unless further periodic management is put into place. The CES agreement is a five year project and so Four Ashes vegetation clearance will continue until the agreement expires. Annual clearance and localised monitoring are straightforward methods that will help maintain Four Ashes’ favourable condition for future research. The Quaternary Research Association (QRA) is marking 50 years by creating a list of the most significant 50 quaternary geological sites across Britain (Silva, 2014). Natural England has nominated Four Ashes to be included in this Top 50 list, reiterating Four Ashes importance and recovery.
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• Bowen, D.Q. (1999) A revised correlation of quaternary deposits in the British Isles. Geological society publishing house, Bath.
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• Waller, R. (2011) ‘Preliminary recommendations for the re-exposure of the four ashes SSSI.’ Report to Natural England.