Rock and Role of the Geoarchaeologist

Dr Stephen Mandal of the Irish Archaeology Field School, describes the role of the geologist in archaeological research at the Blackfriary, a C13th Dominican Friary site in Trim, Co. Meath, Ireland.

As a geoarchaeologist one of my main research interests is in the use of stone in the archaeological record as a source for making tools and other material objects, and as a building material. The Black friary was made from rock and understanding the building materials used – where they were sourced, how they were used, and why they were chosen – is an important part of the story of the friary.

The main building stone used in the friary was limestone, which is not surprising given it is the underlying bedrock of the area. Individual beds were laid down over 300 million years ago, during the Carboniferous Period, at a time when time Ireland was submerged under a shallow tropical sea. Each horizontal layer (bed) is roughly 10-30cm thick and much later folding and faulting of this bedrock has given vertical joints and fractures. This combination of depositional layering and post-depositional fracturing provided the medieval architects and builders with perfect, locally abundant, blocks to build the friary and the other medieval structures in the town, such as the castle and the yellow steeple.

The curtain wall of Trim castle; the wall was built on top of the limestone bedrock, from which the stone was sourced for the building of the castle.

During the first season of excavations at the Blackfriary in 2010, it was discovered that limestone was not the only building material used. There were at least three other types of stone used; slate as roof tiles, red / yellow sandstone in decorative architectural pieces and an unusual and highly distinctive limestone used in the cloister columns and arches. It is the latter that is the focus of this post.

The main building stones of the Blackfriary: 1. Limestone; 2. Sandstone; 3. Slate; 4. Purbeck

When the friary was systematically dismantled in the 1700s to reuse the stone to service a building boom in the town, the builders clearly encountered, but appear to have not valued, the decorative stone of the cloister arcade. Beautifully ground and polished architectural fragments of an obviously different limestone was either ignored or in places used to provide a flat surface for the carting away of the ‘useful’ limestone building blocks.

Visiting the site early in that first season, Kevin O’Brien (architectural heritage expert with the Office of Public Works, Ireland) suggested that these architectural fragments were made from an imported stone, Purbeck Limestone, from quarries in Dorset on the south coast of England.

My role in this rock story was to verify this identification, and the purpose of this blog post is to describe that process. Whilst this stone is visually distinctive – it is a variety of colours from green to red and comprises 90-95% small shelly fossils in a very fine grained matrix, consistent with Purbeck Limestone – to confirm a source requires a more detailed examination. This required the taking of samples to make thin slivers of the rock to view under a microscope.

To take a sample of an architectural fragment clearly requires damaging it, and to do this requires permission from the National Museum of Ireland. A representative sample of small broken pieces of the material were chosen and a ‘Licence to Alter’ was applied for. The application included details of the methodology to be used to take the sample and examine it, the research objective, and a justification for the damage caused. The license was issued (Licence no. 5811), and the samples were taken to the Geological Laboratories in Trinity College Dublin for preparation. A diamond tipped saw was used to take a small portion from each of the samples and these were fixed to a glass slide and ground down to a specified exact thickness of 30 µm, producing what geologists call ‘thin-sections’.
One of the nice by-products of the process was that the unused portions of the samples were left with highly polished surfaces, allowing us to envisage how magnificent the stone would have looked when the friary was built.

The thin-sections were viewed under a special type of microscope – a polarising light microscope; one of the most important tools of a petrologist (a type of geologist who specialises in the identification, interpretation and origin of stone). This microscope differs from a standard microscope in that the light source is below the thin-section, so the petrologist views the sliver of rock with the light shining through it, and the viewing plate rotates. Importantly, the microscope allows the user to polarise the light waves; an invaluable aid in identifying the minerals that make up the rock, as different minerals behave in different ways when they are rotated through polarised and crossed polarised light.

Example: Polarized Light Microscope, showing the different parts

Under the microscope, it was immediately apparent that the shell types, their alteration, and the fine crystalline matrix in which they sit are all consistent with this being Purbeck Limestone. I am working with Dr Patrick Wyse Jackson of the School of Geology, Trinity College Dublin to record the thin-sections and compare with Purbeck source materials. The results of this work will be published in due course, but what is now clear is that the use of important Purbeck Limestone provides an interesting piece of evidence as to how wealthy this friary was (or more accurately, its patrons were).

Image of Purbeck limestone in thin section, showing the abundance of shelly fossils – the width of the image is 3cm.


Originally posted on 11/04/2017