Southstone – the first in a series about top Earth Heritage sites in Herefordshire and Worcestershire.

Ian Fairchild  writes: Do visit this atmospheric wooded site where a spectacular tufa deposit has formed from a perched spring. 

If you go down to the woods today…and you have never been to Southstone Rock…..then you’re sure of a big surprise! It lies in one of the steep dingles that flank the south-west side of the Teme Valley. The dingles have names such as “Hellhole” and “Devil’s Dingle” and their stream beds are choked with boulders, woody debris and intertwining trees.

Southstone is an extraordinary cliff of limestone only a few thousand years old, perched on the hillside.  This cliff lies hidden in the wooded landscape and there is the thrill of it coming into view as you get close. The limestone is a tufa, a precipitate of calcium carbonate downstream from a spring and associated with growth of aquatic plants and photosynthetic bacteria. Mollusc fossils in the limestone show a similar wooded habitat when it formed as today.

North margin of Southstone showing a freshly growing tufa surface, mostly covered in moss.

In 2015, I was asked to review this Local Geological Site for the Geological Conservation Review, which is a process to evaluate potential Sites of Special Scientific Interest. In this case the target was sites of actively growing tufa. Through doing this review I first had contact with the Trust – and I haven’t looked back!  Sadly though, Natural England has not had the resources to progress this active tufa part of the Review, but we hope it will be published in due course. There are two potential Sites (A and B on the map below), both marked as tufa on the British Geological Survey maps.

Topography from Lidar survey with superimposed superficial deposits. Calcareous tufa is mapped by the British Geological Survey at A (Shelsley Walsh and B (Southstone Rock). Parking places are shown by P. Contains British Geological Survey data, Crown Copyright. LIDAR image processed from raw data provided by Edina Digimap.

Tufa forms in streams because of the effects of degassing of carbon dioxide. This is like the effect of opening a bottle of fizzy pop. However, in streams the degassing is smooth: there are no gas bubbles. This is because there is only 10 to 50 times excess carbon dioxide, whereas in the fizzy pop the excess is more like 1500 times! The degassing makes the water supersaturated for calcium carbonate and the result is a modern limestone.  The process is the same as that forming stalagmites and stalactites in caves.

The bedrock geological map (below) explains more of the context. Intriguingly, both the tufa deposits that have been mapped lie just downslope of a limestone bed in the bedrock Old Red Sandstone (which here is mostly mudstone with some sandstones). This Bishops Frome limestone is a calcrete that formed by evaporation as a fossil soil on an ancient river floodplain. It represents a break in deposition within the Old Red Sandstone that coincides with the Silurian-Devonian boundary as defined by fossil fish.  Does having a limestone bed here control the location of the tufa deposits? Perhaps this limestone represents a convenient source of calcium carbonate that dissolves and then is reincarnated as tufa?

Solid geology of the Southstone and Shelsley Walsh area. The substrate is Old Red Sandstone divided into Silurian Raglan Mustone and Devonian St. Maughans Formation, with the Bishops Frome Limestone inbetween.

But things are not as they seem. As in the best detective stories, the obvious suspect may be innocent!

And another thing, if the woods have been around for thousands of years, how come the conditions have changed?  Although there is still tufa growth continuing, it is only covering a small part of the rock. The most obvious feature is its steep east face – clearly eroding now. More widely in the UK, tufa growth appears to be more limited than in the early Holocene after the end of the Ice Age – this is referred to as the ‘tufa decline’, sometimes referred to the effects of pollution or land management, but no general solution for the decline has been established.

Coming back to the Teddy Bear’s picnic, those who do know this site and have never seen the photograph below will also have a big surprise! It is clearly not long since the hillside was largely unvegetated!  How often we need to be reminded how drastically and rapidly environments can change, and particularly so when human action is involved.

Historic postcard of Southstone showing an unvegetated hillside and a cottage on top of the rock. Previously it had been the site of a hermitage for centuries.

The next photograph gives us some clues.  It shows us a small waterfall in the bed of one of the dingles.  The waterfall is there because of the presence of a hard bed of sandstone.  You can see the sandstone on the right with softer mudstone underneath it. The mudstone is a floodplain deposit and the sandstone formed in a river channel 400 million years ago. To the left though, tufa is growing on the steep surface locally covered in moss and with a slightly textured growth surface.

Sandstone on mudstone (right)of Old Red Sandstone at waterfall in the with tufa cascade to left.

It is now well-known in the tufa literature that degassing is enhanced over convex sections in the stream bed and hence tufa growth is concentrated around cascades and waterfalls. Here we see the beginnings of a cascade tufa.  Its growth is limited because of flood events that erode the stream bed and bring down trees. My measurements of water chemistry of streams in the dingle show that the dingle streams all become supersaturated for calcium carbonate within 100 metres of so of their source springs.  The groundwater that feeds the springs is rich in carbonate because there there are small amounts of calcium carbonate throughout the Old Red Sandstone aquifer.

The Bishops Frome Limestone is hard, like the sandstone layers, and forms an escarpment that you can see on the Lidar images. This is the main reason for extensive tufa development at this level in the main stream as seen at Shelsley Walsh (A). Southstone differs because the tufa it arises from a spring on the hillside. It grew upwards to form what is called a perched springline tufa in the classification of Pedley (1990).  Diagrams of its evolution are shown below.

There are two physical reasons why the Southstone tufa grew so large.  One is that it was not as prone to erosion, even perched on a hillside, as the extremely dynamic environment on the floor of the dingle. In the dingle bed, after rain the streamflow is greatly augmented by surface water flowing from the catchment above the head of the dingles. This leads to big floods and erosion. A second reason is that the normal discharge of the spring that fed Southstone Rock must have been much higher in the past. A simple explanation is that the flow diminished because the spring became choked by calcium carbonate precipitates over a period of thousands of years. It is not quite the same, but the famous Roman aqueduct, the Pont du Gard, in southern France, likewise lost its carrying capacity for water because of precipitates in its channel.

Southstone and Shelsley are worthy of SSSI designation because they are the most extensive tufa deposits in the UK forming on a pre-Carboniferous substrate. This substrate is siliciclastic (made of particles of silicate minerals), not limestone as would be usual. The dynamic landscape beautifully illustrates the role that a convex bed, such as a waterfall has in controlling outgassing and tufa formation. In this setting, the late Holocene tufa decline can be understood as resulting from a choking of the Southstone spring by calcareous deposits.

You can download a leaflet here about Southstone. It was written by Cheryl Jones of the Trust for the Abberley and Malvern Hills Geopark around 2005. It gives details of what can be seen as you walk around the rock and also refers to St. Andrews Church in Shelsley Walsh which is built of tufa. A thorough technical article on the nature, ecology and history of Southstone was published by Pentecost et al. in 2000 in the Transactions of the Woolhope Club and can be downloaded here. An interesting account of its reputation in 1851 by the famous local historian John Noake can be found here: The rambler in Worcestershire 1851 Southstone Rock.

Tufa block, St. Andrews church Shelsley Walsh. The tufa is laminated and there are many moulds of vegetation.

Click here for directions to the sites. Do wear stout footwear and bring a stick after a period rain or in winter as the approaches become slippery and there are some steep slopes.  Please do not attempt any climbing or damage the rock surfaces – it will be dangerous both for the rock and you!