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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