Thread by @Tweetisaurus

After a small op this week I can't run or cycle, so I'm strapping on the trail running shoes for a short walk in the South Downs to test the body. Join me for The Geology of my Short Walk..a 🧵

The South Downs are an escarpment of hills extending along the south coast of England. They reach a max of about 200m asl. To the north lies the Weald Basin

The South and North Downs are a fold, an anticline, formed by the same earth movements that formed the Alps. The core of the anticline has eroded, exposing the older, underlying Wealden Group, which is softer than the chalk of the Downs, and thus is lower lying

The chalk is Upper Cretaceous and was deposited in a warm, tropical sea, while the Wealden Group is Lower Cretaceous, and was deposited by deltas and in lagoons. The Wealden Group is famous because it is where the first dinosaurs recognised as such were found.

Those first dinosaur fossils were found by Gideon Mantell, a country doctor who lived in Lewes. That house would set you back £2mill today I reckon

Anyway, back to the chalk. The chalk of the Downs is made almost entirely of coccoliths, skeletal plates from microscopic algae. It's porous, but not very permeable, because the pore throats are tiny. This makes it a great aquifer

The folded chalk extends under London, and it's said that when they drilled into it for the fountains in Trafalgar Square, the aquifer pressure was such that the fountains erupted naturally and didn't need to be pumped. Of course they do today, because the aquifer has depleted.

You can see from my walk that the grass here is dry and dead, but the trees look pretty okay. I guess that's because our groundwater levels are okay here. We've not had any hosepipe ban

We also had a very dry winter though. In fact, until this week, when we had some big thunderstorms, I don't recall any significant rain since March. This is a dew pond, a clay-lined pond dug to water livestock on the Downs. Mud cracks in the bottom, but bullrushes!

The Winterbourne Stream is an ephemeral chalk stream. Bourne in old Sussex means spring: literally the winterspring. It arises west of Lewes and flows through the town, sometimes under ground, like here under the school field, where its course is marked by the dead grass

And sometimes it's channelized, like here in the gardens of Southover Grange. As its name suggests, it flows in the winter. Except this winter it didn't flow at all.

We need a wet winter, or the rare chalk grassland habitats and chalk streams of the South Downs National Park @sdnpa will begin to disappear..