This months post comes from WallCAP’s Community Geologist, Dr Ian Kille.

Ian trained as a research geologist at Oxford and Imperial College and provides expertise on the geology of Hadrian’s Wall.  Since moving to Northumberland in 2006 Ian has been actively involved with geological work, running walks for the Northumberland Coast AONB, running talks and walks for local groups and setting up community training events as part of Northumbrian Earth.

Below Ian discusses some of activities that we’ve been undertaking at Thirlwall Castle part of the Stone Sourcing and Dispersal strand of the project. If you’d like to get involved and sign up to some of our future Stone Sourcing activities, sign up as a volunteer here.

From the north-facing ditch of Hadrian’s Wall as it leads due east up the hill towards Walltown, the jagged remains of Thirlwall Castle are veiled by the russets and yellows of the autumn beech leaves that line the course of the Tipalt Burn. The castle is set on a crag of sandstone on the opposite bank of the burn with chunky blocks of sandstone exposed by the burn-edge where its spate keeps the geology exposed. Volunteers have been busily measuring and examining the castle stones in detail to really understand them by dwelling on their size and shape, their surface markings and most particularly what the stones are made of. Viewing the castle from this perspective is a good starting place to find out more about the raw geology onto which the Wall and the Castle have been built.

Keep heading east towards Walltown and the Whin Sill emerges into the landscape with north-facing crags and south-facing dip-slopes. These form the natural ramparts onto which the Wall was built for the next 10 miles between Walltown and Sewingshields Crags. This 300-million-year-old igneous intrusion cuts the older Carboniferous sedimentary rocks which underlie the rest of the landscape hereabouts; its crystalline hardness and resistance to erosion by the recent advance (and retreat) of ice make it such a dominant landscape feature. As geologically exciting as all this is, it doesn’t bear directly on the geology and landscape directly under Thirlwall, but it will be interesting to see if the proximity of the sill influences some of the material used in constructing either the Wall or the Castle.

Back at the Castle, the underlying rocks are from the older Carboniferous series of sedimentary rocks. The particular succession of rocks are known as the Alston Formation – geologists describe groups of rocks by dividing them first into Periods, then subdividing them into Groups and then into Formations. In this case the rocks are in the Alston Formation, which is part of the Yoredale Group which in turn is part of the Carboniferous Period. The Alston Formation was laid down over a period of approximately 3 million years around 330 million years ago. It consists of cycles of deltaic sediments (sandstones, siltstones, shales and coals) repeatedly interleaved with marine limestones and shales. Between them these provide the materials required to build the Wall, both sandstones for the block work and limestones to manufacture the lime to bond them.

The sandstones in the Alston formation (and in the Yoredale group as a whole) tend to be localised, so that if you trace a sandstone outcrop over any distance it will disappear. For this section of the wall there are several sandstone beds, one directly under the Castle and others nearby. I went in search of these to see if I could find fresh sandstone to compare to those found in the wall. The nearby spoil heap from a now disused and overgrown quarry and the low cliff line under the castle by the Tipalt Burn provided the samples I was looking for.

In the meantime, the volunteers were ever so carefully looking at the stones in the castle wall. The data they have collected now needs to be put into a database and analysed back at the lab. However, the volunteers made some immediate excellent observations. The first is that the larger blocks of stone used to make the window surrounds and the quoins are made of the same stone.This pale slightly green tinged medium-grained sandstone quite commonly showed bedding planes and not infrequently cross-bedding.

The rest of the stones which seem to have the size and dimension of the Wall-stones are a more mixed bunch with what look like two, possibly more, distinct types of sandstone. There is one group of pale grey coloured sandstones, occasionally bedded and another group of finer grained yellow-brown sandstone (see below). This latter sandstone commonly has the rust coloured and beautifully delicate parallel lines and curves, cross cutting the sedimentary bedding and caused by precipitation of iron from water flowing through the sandstone after it was laid down. This diagenetic iron is on occasion so developed that the iron bands become dense and harder than the surrounding sandstone with the latter eroding more quickly leaving the iron bands upstanding.

These are exactly the sort of observations needed to categorise the stones from a geological point of view. This is where the comparison with the fresh samples might prove interesting… I will explore how this went in the next blog entry. In the meantime, do have a look at the blog post Nicky Garland wrote on Thirlwall Castle last month.

@Northumbrianman

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Thirwall Castle looking to the north-east

Thirlwall Castle, located near the village of Greenhead, Northumberland, was built in the 13th century and likely incorporated large quantities of stone robbed from the line of Hadrian’s Wall. Today the castle survives only as a ruin, looked after by Northumberland National Park and protected as both a Scheduled Monument and Grade I listed building by Historic England. Evidence for the reuse of stone from the curtain wall is visible in the size and shape of the stone, which appears very similar to in situ stone found elsewhere along Hadrian’s Wall. The line of Hadrian’s Wall passes the castle to the south and through the field to the east. Although no evidence for the wall is visible above ground, there is a slight mound running through the field suggesting that perhaps some of the foundations of the wall survive under the turf of the field. It is thought that the curtain wall does not survive in this area as opportunistic medieval builders removed what was left to build Thirlwall Castle.

As part of the Heritage at Risk strand of the project WallCAP staff with the help of volunteers have been undertaking geophysical survey of the field to the west of the Castle. A number of volunteers came out to the site in July and were trained how to use the geophysical survey equipment (both Gradiometer and Resitivity) to try and trace archaeological features beneath the ground surface and the Global Positioning System (GPS) and laser scanning to map the earthworks in the field (Note: if you want to learn more about these techniques see Ray Purvis’ previous blog post here). Although there is still some survey to finish off, what we have done appears to have picked up the line of the Wall going through the field from east to west (see below).

WallCAP volunteers undertaking a Resistivity and Gradiometer survey

The results (so far) of our Resistivity survey (the wall and wall ditch is shown as two parallel dark bands running from left to right in the middle of the image)

As part of the Stone Sourcing and Dispersal strand of the project, WallCAP has also been examining the castle itself. The first stage of this work was to undertake a laser scan survey of the castle to get a highly accurate 3D model. A laser scanner fires millions of laser beams, which hit an object and are reflected back. Using these readings, the equipment creates an accurate three-dimensional representation of the structure. In the field we place the scanner in strategic positions around the castle so that it records all surfaces of the walls and windows. The laser scanner also takes a series of photos, which can be overlaid on to the 3D image and provide colour. A number of spheres (pictured) are placed in key positions so that laser scans can be combined into one 3d model of the castle. Nicky and Alex from the WallCAP team went out to undertake the survey over a couple of sunny days in June. Once processed this information was then used by WallCAP volunteers to record the stone itself.

The laser scanner and target spheres

Two weeks ago, Rob and Ian from the WallCAP team, alongside ten volunteers took a trip to Thirwall Castle to examine the fabric of the building in more detail. Project volunteers undertook a detailed investigation of the remains of Thirlwall Castle to assess how much of the stone used in its construction is re-purposed material from Hadrian’s Wall and how much may have come other sources. They examined the geology of the masonry and any possible toolmarks that may give us clues to the origin of the stonework. Using the data recorded during the laser scan, the volunteers systematically recorded the dimensions of each of the stones and made notes about any possible markings on the stone themselves. By examining the stonework in this way we will start to quantify the amount of Wall-stone contained in the castle and calculate how much of the Wall would have been reused in the construction of the Castle. This in turn will help verify (or otherwise) the repurposing of the stone, give more information on the work involved in building the Castle and indicate how much other lost Wall material is still to be accounted for.

WallCAP volunteers recording the dimensions of probable reused Roman masonry

An orthographic image of the northern elevation of the castle created using the laser scan

This is the beginning of a number of other WallCAP events where volunteers will examine structures built in the post-Roman period and identify stone that may have been recycled from Hadrian’s Wall. If you’re interested in the activities we undertook at Thirwall, be sure to sign up to our volunteer portal for more opportunities to help the WallCAP team record these ‘recycled’ buildings! As always, keep an eye on our social media (Facebook, Twitter, Instagram) for more details of projects as they are posted on our portal!

Also keep your eyes peeled for a future blog post by Ian Kille, WallCAP’s Community Geologist, who’s going to be writing about the geology along Hadrian’s Wall and what the WallCAP team have been doing as part of the Stone Sourcing and Dispersal Strand of the project. Watch this space!

@Nicky Garland

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