Why is Glastonbury so muddy? ... by Rachel Dearden

Muddy Glastonbury, courtesy of Amanda Borrhamm
The Glastonbury festival is famous for turning into a quagmire seemingly every year. It’s almost an expected highlight of the event!

But why is it so muddy?

The geology underlying the festival site near Pilton comprises the Blue Lias and Charmouth Mudstone Formation. The key is in the name ‘Mudstone’. This Sedimentary Bedrock was formed approximately 183 to 204 million years ago in the Jurassic and Triassic Periods when the local environment was dominated by shallow lime-mud seas.
The bedrock of an area directly influences the type of soil present at the surface and thus at the festival site; the soil is very clay rich (around a third of the soil is clay) and it forms a deep mud when it is churned up.
Glastonbury Festival bedrock and superficial geology map

Soils like those at Glastonbury are densely packed mixtures of fine clay- and silt-sized particles, with only very small amounts of sand and organic matter. David Entwisle, Engineering geologist, says that it’s the plasticity of the soil that really matters; as clay absorbs water, its consistency and behaviour changes. It’s volume increases (it starts to swell) and it becomes a malleable, or in less technical terms - a squidgy mess.
Muddy Glastonbury, courtesy of Amanda Borrhamm

At the Glastonbury festival site, the plasticity is medium to high, so when it rains, the ground will quickly become very wet and malleable, and it won’t drain away because the underlying rocks have low permeability too (so the water cannot soak away through them) and the site lies within a valley (a lovely bowl of mud).  Vehicles, wellies, shoes and feet remould the surface, mixing the water and clay together, reducing the flow of water into the ground even more and increasing the depth of mud.

If you want to find out more about the geology of the UK go to our Geology of Britain viewer  and if you’re particularly interested in soils explore our UK Soil Observatory Map viewer or our mySoil app (we’d love a soil description from a festival go-er).