Pitch Coppice Quarry – the fifth in a series about top Earth Heritage sites in Herefordshire and Worcestershire.
Deep in the forest, on the very northern edge of Herefordshire, there lies a small, but clear and clean exposure. If it were in China, it would be celebrated with pathways and signage, and perhaps even a visitor centre. But this is Britain, and we take our geological heritage more for granted here.
Just above the top of the 30 cm ruler is the junction between a limestone below and a more friable calcareous shale above. An exposure like hundreds of others you might think except that geologists have declared this to be the prime place in the world to mark a particular instant in geological time: the end of the Wenlock Epoch and the beginning of the Ludlow Epoch within the Silurian Period.
For a few decades now, geologists have separated the idea of dividing up time from subdividing the rock record. For example, during the Silurian Period a set of rocks were formed called the Silurian System, which originally was Roderick Murchison’s title for his great monograph of 1839 which you can read online at https://library.si.edu/digital-library/book/siluriansystemfimurc .
In the preface he states: Having discovered that the region formerly inhabited by the Silures, celebrated in our annals for the defence of the great Caractacus, contained a vast and regular succession of undescribed deposits of a remote age, I have named them the ” Silurian System”. In his original scope this included what are now assigned to the Cambrian and Ordovician Systems and was bounded above by the Old Red System. Nowadays the oldest Old Red rocks are assigned to the Silurian! Despite Murchison offering chapters on the Silurian rocks of the Black Country, Malvern, Woolhope and May Hill, the references to places in the Mortimer Forest area are scant, although clearly he had traced the main units there. Rather, to examine the boundary between Wenlock limestone and overlying Ludlow shale, the reader was referred to Presthope on Wenlock Edge.
So, the fame of the Mortimer Forest area is a 20th century development and one linked with a loose association of geologists known as the Ludlow Research Group, and in particular one of their founders Charles Holland (1923-2019 [ https://www.geolsoc.org.uk/About/History/Obituaries-2001-onwards/Obituaries-2019/Charles-Hepworth-Holland ].
From 1954 the Group met annually in the field and grappled with understanding the stratigraphy in Wales and the Welsh Borderland, expanding their scope from Ludlow Series rocks to the whole Silurian in 1962.
The key Mortimer Forest work was carried out by Charles Holland with J.D. Lawson and V.G. Walmsley and published in 1963. From 1966 Holland led the geology department at Trinity College Dublin, but continued to be active in the Geological Society of London (and was President 1984-1986). In the late 60s, he was commissioned to lead a group compiling the first volume in the Geological Society of London’s series of stratigraphic charts, presenting the Silurian System results in 1970.
He became an important international figure, chairing the International Union of Geological Sciences (IUGS) Subcommission on the Silurian from 1976 to 1984. The key achievement of those years was the formal definition of Silurian boundaries in actual rock sections, two of which were in the Mortimer Forest.
There are a number of small quarries in the Mortimer Forest area and collectively they have established the overall understanding of geological events. Exposure is clearest at the Pitch Coppice Quarry itself and it was designated as the GSSP (the global stratotype section and point) – the section with which all others should be compared. The Wenlock-Ludlow boundary corresponds to the lithological change from limestone to calcareous shale and various methods have been used to show how this relates in time to sections elsewhere in the UK and internationally. The quarry is one of eight units which collectively make up the Mortimer Forest Site of Special Scientific Importance. This means there must be no disturbance of the site or collecting material. In 2020 the Earth Heritage Trust also designated it as a Local Geological Site to emphasize its educational importance within the region.
At Pitch Coppice itself the beds can be traced laterally for around 20 metres. The Wenlock limestone (now formally known as the Much Wenlock Limestone Formation) here was deposited further offshore than at the Black Country Global Geopark at Dudley but patterns of sea level change and carbon isotope composition of limestones have been traced across the region. Many shallowing-upwards cycles a few metres in thickness occur in the Wenlock; the boundary with the Ludlow represents a relatively sudden deepening after the last of these.
Notable in the Wenlock strata at Pitch Coppice are three altered volcanic ashes (bentonites) which weather into the rock face – John Payne’s hands are pointing to two of them in the photograph below. Although not characterized at Pitch Coppice specifically, trace elements from bentonites have been used to fingerprint individual volcanic eruptions and hence correlate various West Midlands sections. Absolute ages can be obtained from measurements of isotopes in the uranium decay series found in the mineral zircon (zirconium silicate) in bentonites. As a result of such studies the Wenlock-Ludlow boundary has been estimated by the IUGS to represent an age of 427.4 million years ago with an uncertainty of just half a million years.
In the field, the Ludlow strata have some interesting macroscopic features. There are discrete sand-textured storm beds with rippled tops, for example at the 50 mm mark on the ruler in the photograph below.
There are also small heads of coral about 30 cm across in the rock face. In 2020, one had conveniently fallen to the base of the exposure revealing lovely surfaces of the colonial tabulate chain coral Halysites.
In recent decades, studies of Much Wenlock Limestone lithostratigraphy and palaeoenvironments have been published by Ken Ratcliffe and Alan Thomas. David Ray has combined much new fieldwork with the application of modern techniques – including sequence stratigraphy and carbon isotope studies, as well as the radiometric dating mentioned above – partly in collaboration with Alan Thomas.
Unfortunately, the geological trail to Mortimer Forest is no longer in print so there is no locally available information about these sites, but we hope you are encouraged to visit by this article. Details of how to get there with some links for more background information are at: https://www.earthheritagetrust.org/sites-to-visit/ .
The links include a useful summary of the site prepared for the visit of an IUGS Working Group in 2011, hosted by Alan Thomas and David Ray. Parking is at SO 474371 which is at the top right of the map (above). Download what you need before you go as the phone signal is patchy on-site.