Apr 30, 2020

Musings on the Triassic rocks in the garden

Part two from Sedgwick Museum Conservator, Sarah Wallace-Johnson, writing about her #MuseumFromHome in the Shropshire countryside
Category: 2020
Posted by: Sarah

My second blog about observing what can be seen in our now less mobile, more restricted, world of lockdown. I have travelled North from my home in Oxfordshire to be with my elderly Father who lives in Shropshire, a county that exudes geology from every pore. I grew up in Shropshire and was fascinated from an early age by the fossils of the Silurian Wenlock Limestone. This, luckily, led me to a career in caring for the geological collections of The Sedgwick Museum as Conservator.
My parents tolerated my requests to spend birthdays in the quarries of Wenlock Edge, hunting for treasures from the Silurian reef communities, beautifully preserved in the creamy white, highly fossiliferous limestone. How fortunate I am to be back in the green and rolling countryside of my childhood. However, as for all of us, more extensive countryside rambles will have to wait for more liberated times. Once again I find myself turning to our tiny back garden to quench my desire for objects of wonder from the natural world.

My parents have always collected rocks and interesting natural curiosities to decorate their garden; my Father was a biologist before he retired. Amongst their collection are two sandstone staddle stones and many large pieces of creamy white/buff sandstone, these all hail from a nearby hill called Grinshill, which has yielded good quality building stone for centuries. Many of the finest buildings in Shropshire’s county town of Shrewsbury have been constructed from Grinshill sandstone such as the library, railway station, Lord Hill’s column and Shrewsbury school. Further afield Chequers and No. 10 Downing Street both have Grinshill sandstone architectural details. The staddle stones show that it was used for more utilitarian purposes; staddle stones were used to protect haystacks or granaries from vermin. The building or hayrick would be constructed supported on several of them, the mushroom shaped top preventing rats and mice from gaining access to the stored crops above. Their use in the UK seems to have coincided with a rise in the population of the Brown rat during increased industrialisation in the 18th century.
(Image: Staddle stones)

The best Grinshill sandstone is a creamy white colour, unlike the usual reddish brown of most of the other sandstone in the area. The red colour derives from iron minerals in the rock. Grinshill’s sandstone has had the iron removed by super-heated mineral solutions, injected from a complex of volcanic fissures and dykes, this process stripped away the easily weathered iron resulting in a much harder wearing and resistant rock. Badly weathered red sandstone can be seen in many Shropshire buildings and was generally kept for less prestigious construction.

Image: Grinshill rocks in my family garden providing a valuable habitat for various animals and plants

Grinshill is one of several small hills that rise out of the glacial overburden of the North Shropshire plain. These hills are formed from Triassic New Red Sandstone, laid down when the climate of the super continent of Pangea was changing from the hot and dry environment of the Permian to more monsoonal conditions. Large rivers washed sediments down to the Shropshire plain from mountains that would have been left exposed during the Permian. Evidence of sand dunes and the rivers systems that laid down the sandstones of Grinshill can be seen in the walls of the old quarries. In 1838, vertebrate fossils were discovered in what is now called the Tarporley Siltstone Formation (Anisian Age, Middle Triassic, 247 million years ago), attracting the attention of Victorian scientists of the time. The specimens can still be seen today in Shrewsbury Museum and Art Gallery. The fossils are of an animal called a Rhynchosaur, a type of reptile. They were small, powerfully built and superficially pig-like, having large claws and a beak-like snout which they probably used to find and process their plant based diet. Trackways, those most evocative of fossils, probably of this animal have also been found at Grinshill. The fossils have been named Rhynchosaurus articeps. The Rhynchosaurs as a group all became extinct during the End -Triassic extinction event but were abundant and successful for many millennia.

My drawing of a Rhynchosaur

Wonderful to contemplate a world so very different from our own, represented in the restricted confines of a small garden, taking my mind to new horizons and other worlds.

Sarah Wallace-Johnson
Conservator, Sedgwick Museum