“The Messenger” community magazine has just published this article, explaining how the soils of the local area come to be either sandy and full of pebbles or bogged down in clay.
“The Messenger” is enjoyed by the people of Northwick, Barbourne and Perdiswell, on the northern side of Worcester city, but many places in both Worcestershire and Herefordshire are built on a similar system of “river terraces”. The article appears on the Herefordshire and Worcestershire Earth Heritage Trust (H&WEHT) website with the Editor’s permission.
River terraces of sand and gravel flank the River Severn and other major rivers such as the Avon, Teme, Wye and Lugg. The terrace gravels are superficial deposits covering the bedrock, which in many parts of Worcestershire and much of eastern Herefordshire consists of reddish mudstones. The mudstones in Herefordshire, however, are about 100 million years older than those in Worcestershire, having formed in the Silurian and Devonian periods.
Slope up to St Stephen’s Church which lies on the the higher river terrace in this area.
What’s your soil like?
Ever wondered why some gardens in and around St Stephen’s parish have heavy, sticky, red clay, while others are light and sandy with plenty of pebbles? Well here’s why.
The sticky red clay comes from the bedrock that lies up to a kilometre deep under this part of Worcestershire, called Mercia Mudstone. It is about 220 million years old, and formed as sludge at the bottom of seasonal ‘playa lakes’, in a semi-desert environment. In the next 150 million years the land subsided, sea levels rose, and the whole of England was covered in seawater. During this time, more rock, including limestone (like on the Cotswold hills) and chalk (like the South Downs) was added on top of the mud. All the water was squeezed out of the mud and it turned to stone, deep underground.
So why isn’t it still buried?
Well, around 50 million years ago the land began to rise again and the sea retreated. The top layers of rock were broken up by the weather and carried out to sea by rivers. The River Severn does that today – especially in times of flood. Eventually, the mudstone itself was eroded away to the level it is now. Gardens with clay soil are sitting directly on this bedrock. This rock was quarried at Gregory’s Bank and used to make bricks. The houses on Kiln Crescent back onto the old quarry wall of red Mercia Mudstone, which is part covered and very vegetated.
Mercia Mudstone from across the Droitwich Canal (Feb 2021)
And what about the gardens with sandy pebbly soil?
These properties are on old river beds. Some of them are tens of metres above the present river bed, while others are just a few metres above the flood plain. There has been a river flowing through or close to Worcester since the time when a glacier gouged a shallow valley between the Cotswolds and the Malvern/Suckley Hills about 450 thousand years ago (note that is thousands, not millions). When it retreated it left glacial till (a mix of mud and stones) on the valley floor. A small pocket of till remains stranded at the top of Lyppard Hill which, at around 90m above sea level is the highest point in Worcester City.
Since that time, the river has continued to cut down through the old mudstone, but at the same time, our climate has fluctuated. Intermittent freezing conditions lasted for thousands of years. At times of deep freeze, rocks were rapidly broken up by glaciers to the north and by freeze/thaw action. When the ice melted, strong, fast flowing rivers washed the smaller rock fragments downstream, bringing sand and gravel to the Worcester basin, where it was deposited over the river bed.
At other times, there was no copious supply of sand and gravel, but the river continued to meander through the valley. It cut a course through the gravel then through the old mudstone. The remaining sand and gravel was left high and dry, forming terraces on the flanks of the river valley.
River terrace excavated approx. 1 metre deep by the side of Northwick Road. (Jan 2021)
Diagonal lines, climbing left to right, mark where a sandbank built up within the old river
This process repeated itself at least six times, so that river terraces of sand and gravel can be found at six different heights within 2 miles of our parish (our editor now lives on one of the higher ones).
Two of the most recent terraces are found within the parish. They are shown in the cross section below, which runs from the River Severn just north of Gheluvelt Park, past St Stephens church and across to the Barbourne Brook just upstream from Gregory’s Mill. Height is of course exaggerated in this simple diagram, but I think you will recognise where the main hills occur.
At the edge of a terrace, the sand and gravel tends to be washed downhill, covering the bedrock on the slope below. In some gardens there will be sand and gravel at the surface, but if you dig a little deeper you may encounter the underlying clay.
… and what about the pebbles?
But – you say – the gravel in my soil isn’t just any old bits of smashed up rock, I have lots of lovely smooth, rounded pebbles in various shades of white, pink and orange. Where did they come from?
That is a different tale, and it will have to wait…
Interested in local geology?
If you would like to know more about local geology, why not visit the website of the Herefordshire and Worcestershire Earth Heritage Trust. https://earthheritagetrust.org. You may even think of joining us or becoming a volunteer: either helping to clear interesting sites or taking part in one of our projects.
First published in “The Messenger” community magazine, March 2022.
Please note that some colour photos have been added in this version.