This month’s post comes from WallCAP’s Community Geologist, Dr Ian Kille.
Below Ian reveals all about the mystery rock picture he posted in the Hadrian’s Wall Community Archaeology Project March newsletter. If you’d like to receive our monthly newsletter and get involved with our Stone Sourcing activities, sign up as a volunteer here.
The First Mystery Rock
A few weeks ago, I posted a mystery rock picture in the Hadrian’s Wall Community Archaeology Project newsletter. Whilst the rock type is relatively straightforward to identify, the shape is entirely distracting. I 
love the shape(s) of this rock, and how it may have formed is a puzzle and the subject of this blog post.
When I went to do a recce of the area in preparation for a WallCAP fieldtrip I was not expecting to come across this feature. It was familiar though, in consequence of exploring an equally intriguing outcrop on the Northumberland Coast. What I would like to do in this blog post is to follow the journey which started with this Northumberland rocky feature, and how it relates to what I stumbled across on that recce. Understanding the nature of these outcrops requires an exploration of the sedimentary cycle, from deposition to rock formation, diagenesis, weathering and erosion. A useful journey.
The starting point is Cocklawburn Beach (a few miles south of Berwick-upon-Tweed) at a particularly low tide. On the seaward edge of the rock platform that forms the Middle Skerrs, the upper surface of the hard limestone is ribbed. The ribs look like large ripple marks, just like the sort you would find in sand after the tide has retreated. The rock surface slopes gently seawards, and the axis of the ripples is just a few degrees off perpendicular to the shoreline. How this was formed has started a debate which is ongoing and maybe fully explored in another post or even an academic paper.
In summary, the proposed explanations for the “ripple” formation are:
- Erosion of the limestone by the sea in its current modern environment
- Soft sediment ripples made at the same time as the limestone was deposited
- (another less likely explanation) modification of the limestone whilst buried, perhaps related to pressure and/or folding
Options one and two are the best contenders to explain this, and at Cocklawburn it seems likely to me that it can be explained by option 2 with ancient soft sediment ripples.
To further explore these processes a colleague, Alison Tymon, conducted a detailed survey looking for other outcrops as part of the Peregrini Lindisfarne Project. She was successful, finding more of these ripples in limestones along the Northumberland Coast. She also discovered similar grooves in sandstone just south of Spittal.
The grooves in the sandstone just south of Spittal are subtly different from those at Cocklawburn. They are flat topped, vary in width, meander more and the rock platform which contains them is wave cut rather than being on the bedding plane as at Cocklawburn. All of this points more conclusively towards the first of the explanations, where the grooves are caused by wave-erosion picking out lines of weakness (joints) in the sandstone.
Another colleague Elizabeth Devon aware of this work suggested a look at grooves to be found to the south coast. The winter before last I took a day trip to Birling Gap, in amongst the Seven Sisters, and Hope Gap a bit further to the west. I found lots of beautiful grooves cut into the chalk. As with the Spittal sandstone grooves they are formed in wave cut platforms. Here the chalk is quite homogenous, but particularly where there are layers of flints, the top surfaces of bedding planes undulate with peak to peak distances of approximately 0.5m. It is easy to imagine how waves interacting with this could turn those undulations into grooves. Waves erode with both swash and backwash. The percussive energy of the waves is enhanced by the erosive power of sand and pebbles entrained in the wave. The water tries to find the straightest line up and down the beach and so will preferentially erode dips in the undulations which permit this. Once grooves have started to form, this preference will be further exploited over time as it become even easier for the grit laden swash and backwash to use these channels.
Onwards to St Bees, where we can see exactly the same phenomenon. The ripples here are also cut into a wave cut platform. Joints in the sandstone and changes in bedding plane are picked out, but the platform is dominated by the formation of these beautiful grooves. Little channels of water can be seen flowing down them demonstrating that they are an easy route for water to find a pathway back to the sea.
The mystery rock from the Hadrian’s WallCAP newsletter is therefore part of the St Bees Formation. It was a tough mystery as none of the volunteers were shown this rock on our trip to St Bees (the tide was too far in). These were formed from sands that may have an aeolian origin (wind-blow desert) but which were ultimately laid down in a river. I will describe how the St Bees sandstones themselves were formed in another post. The current form of this rock as described is however a recent phenomenon brought about by the action of the waves to produce this pleasing design.
