Ludlow, Murchison and the Limestone Conundrum

October 17, 20151:26 pmJune 23, 2016 1:33 pm
Tectonic plate reconstruction of the mid Silurian (~425Myrs ago) during continental collision. Directions of convergence indicated by yellow arrows. (Base image © Ron Blakey, Colorado Plateau Geosystems, Inc.)

Tectonic plate reconstruction of Europe during the mid Silurian ~425Myrs ago during continental collision between Avalonia, Laurentra and Baltica. Directions of convergence indicated by yellow arrows. Base image © Ron Blakey, Colorado Plateau Geosystems, Inc.

In the mid Silurian, about 430 million years ago, the present day area of England lay at the north eastern margin of a continent called Avalonia. To the north lay the Iapetus Ocean and beyond that the continent of Laurentia; made up of parts of North America, Canada, Greenland and what would become Scotland. Continental drift caused the gradual closure of the Iapetus and a collision between Avalonia and Laurentia which brought Scotland and England together as one landmass.

The seas on the continental margins of Avalonia at the edge of Iapetus during its final years provided the environment in which the famous fossiliferous limestones of the Silurian were deposited. Sea level rose and fell through the Silurian, in response to global variations in climate and volumes of ice at the poles. During warmer periods, large transgressions of the shelf occurred and during these times a shallow sea stretched from approximately the Welsh border to well east of the Malvern Hills across the Welsh Borderland. Two such periods correspond to the best known units in the succession; the Wenlock and Aymestry limestones.

Hand coloured generalised cross-section of the Silurian stratigraphy in the Welsh Borderland, from Murhison's The Silurian System (1839) [image © H&W EHT]

Hand coloured generalised cross-section of the Silurian stratigraphy in the Welsh Borderland, from Murhison’s The Silurian System (1839) [image © H&W EHT]

One of the first geologists to really understand the stratigraphical order of these limestones and to map them right across the area was the famous Sir Roderick Impey Murchison. Born the son of a wealthy landowner, Murchison was as arrogant as he was talented, famously arguing bitterly for many years with his onetime friend, Adam Sedgwick, over the correct categorisation of the strata between his Silurian and Sedgewick’s Cambrian System, as if over some disputed territory. In the Silurian System, published 1839 he set out the stratigraphy for the Silurian, divided into four parts and founded on palaeontological evidence, which largely survives to this day. He recognised that a similar sequence of rocks could be found in diverse locations including the Mortimer Forest, Wenlock Edge and the Woolhope Dome. If we look at his beautifully hand coloured cross sections we see the succession up through Wenlock Shale, Wenlock Limestone, Lower Ludlow Shales, Aymestry Limestone and Upper Ludlow Shale that geologists recognise today (despite changes in the number of units and their names over the years). This process of pattern recognition to correlate the sequence of rocks in one area with that of another is the enormously powerful basic method at the heart of most geological mapping.

However when we come to look at building stones and try to trace their past movements from quarry to building we have a problem. The quarrymen, labourers and builders of the past, (unsurprisingly) not thinking about stratigraphy, have removed the stone from its geological order and pattern creating a jumbled jigsaw of blocks. We are thus left with the lithology alone to go on.

Welsh Basin paleogeography during Wenlock times, adapted from Bassett (1974)

Welsh Basin paleogeography during Wenlock times, adapted from Bassett (1974)

This would be fine if the Wenlock limestone or any other unit of rock was the same everywhere but it is not. A brief consideration of the geography of the area during the Silurian is enough to convince us that conditions of deposition likely varied between one location and another. There are numerous variants of palaeogeographic reconstruction but all agree broadly that there was a shoreline somewhere east of the Malverns and that the shelf edge – the break between the shallow seas of the continental shelf and the deep ocean – began at about Ludlow, with water depths rapidly increasing to the west. Within this environment the “typical” Wenlock limestone, rich in fossils, which has attracted the attention of so many palaeontologists, formed a barrier reef stretching from the eponymous Wenlock Edge to Ludlow and running at an oblique angle to the shelf edge. As it approached Ludlow, being nearer to the shelf edge, and therefore less protected from large storms sweeping in off the open ocean, the reef-building corals and shells found it harder to survive. As a result the number of fossils and the amount of lime (a direct by-product of their biological activity) in the rock decreases. Consequently the Wenlock Limestone changes greatly in character across the area. The same is true of the Aymestry Limestone.

A further problem has emerged more recently due to the work of the Ludlow Research Group, a collective of geologists who sought to reinterpret the Silurian stratigraphy of the area from the 1950s onwards. One of their findings, based on very detailed analysis of the fossils in each bed, was that the top of the Aymestry limestone was not the same age everywhere. They proposed a new name for most of the formation – the Bringewood Beds – based chiefly on the occurrence of fossils. This means that in some places (including the type section [pdf]) what would have been called Aymestry Limestone in the older terminology is now placed in the overlying Lower Leintwardine Beds. This gives us a problem when we come to classify building stones. A stone which looks like Aymestry Limestone might be one of three different formations. In a quarry or an outcrop we might be able to establish its age by comparison with type sections but, as previously discussed, in a building such extra information has been lost. Because of this we will probably have to keep using the term Aymestry Limestone in the context of buildings until such a time as the exact formation of the stone can be properly determined.

Kirkidium Knightii the zone fossil for the Upper Bringewood Beds (from Holland, Lawson and Walmsley, 1963)

Kirkidium Knightii the zone fossil for the Upper Bringewood Beds (from Holland, Lawson and Walmsley, 1963)

How, then, do we overcome this conundrum to correctly identify the sources of building stones and properly name them? Given the stratigraphic units are defined by their fossil content, fossils are an obvious place to start. Most units lack a single diagnostic zone fossil, however, positive identification of any fossils to species level will help to limit the possibilities of what it might be. The Aymestry Limestone (or more properly the Upper Bringewood Beds) does have a defining fossil, Kirkidium knightii (previously known as Pentamerus knightii and Conchidium knightii), which is only found within the formation. These are strong shelled, ribbed brachiopod molluscs well adapted to life in the comparatively rough waters of the marginal continental shelf. Finding these in a building gives us an unequivocal identification of the formation. We have recorded examples in buildings including Gatley Park Folly (where we have the additional surety of a personal memory of the stone having been picked from the Aymestry Limestone in Leinthall Earls Quarry) and the tomb of Thomas Andrew Knight, a friend of Murchison’s for whom the fossil was originally named.

Other stones used in the area include Downton Castle Sandstone which is an important source of dimension stone for quoins and lintels, Grinshill Sandstone from Shropshire, which is common in Victorian and later repairs, and even Coal Measures ironstone from Clee hill, used for the copings of Pipe Aston Church. The latter of these is likely material that fell off a cart bringing ore to the forges that existed around the area from the 16th century, particularly in Bringewood. There the coincidence of ready supplies of timber for firewood, and the strong flowing Teme to power the bellows made an ideal location for iron smelting, upon which the Knight family fortune was made.

In order to further pin down the sources and identities of limestones we are now planning to undertake a programme of measurement of geochemical data by Portable X-Ray Fluoroscopy (PXRF). It is hoped that this may give us a more objective way in which to fingerprint different formations or individual quarries and tie them to the historic buildings they have supplied with stone.

This article is based on a lecture given at a Geologists’ Association regional conference, The Geology of the Marches Murchison to the Modern Era, in Ludlow, Shropshire, 2nd-4th October 2015. We are indebted to project volunteer Michael Rosenbaum for much of the research.

 

Written by Elliot Carter - Modified by Ella Young

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