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.