Geology Corner

1st - 30th November

Geology Corner

Many thank to Dr Doug Robinson and Professor Chris King for this month’s instalment.

The oldest building stone in the Palace – Devonian sandstone (380 - 360 Ma) 

The image above is a photo of a single block of Devonian sandstone (arrowed). To find this block, stand in front of the Great Hall and it can be found under the third window to the right hand side of the entrance porch. You have to hunt for Devonian sandstone blocks, because they are one of the least used in the Palace buildings, They have an even purplish/red colour because they contain iron-bearing minerals, but they are formed mainly of sand grains of a fine and even grain size.

The sands were originally laid down on floodplains when large seasonal rivers flooded.  These ancient rivers flowed south-eastward from a mountainous region that lay across present-day western Wales. They flooded across the Mendip area, which was then a flat, arid floodplain before flowing into the nearby Rheic ocean, a short distance to the southeast. Rocks bearing iron minerals can record evidence of the former Earth’s magnetic field (palaeomagnetism) which shows that in Devonian times the area lay at a latitude of about 25° south of the equator – equivalent to the position of central Chile, where similar floodplain settings can be found today, as shown in photo below.  

The Devonian sandstones underlie the upper area of Pen Hill beneath the radio mast. Strangely, while the outcrop area is close to the city, this rock type is not a common building stone in the Palace or any other older buildings in the city. When you get your eye in though, you will find some of these stones in the older buildings of Wells, because they are the only ones with the characteristic purple/red colour and fine grain size.