I have fallen down the rabbit hole of London Geology.
For this I am grateful to the London Geodiversity Partnership, an organisation formed in 2008 with the aim of promoting, protecting and interpreting geodiversity in London. If there's a cracking rock formation somewhere, best we preserve it, protect it and know all we can about it.
Not only do the LGP undertake research and organise events, they also have a brilliant website rammed with resources that exceed all reasonable expectations. For an entry level fix, try this map that shows the location of London's SSIs, RIGs, LIGs and SGIs. That's 7 Sites of Scientific Interest, 34 Regionally Important Geological Sites, 27 Locally Important Geological Sites and 27 Sites of Geological Interest. There's likely one near you even if you live in a lowly London suburb. And every site has its own pdf-ed geological summary to peruse, or you could download a full set of 79 summaries with maps and everything here and then you might end up down the rabbit hole too.
I've been out to visit a couple of Locally Important Geological Sites. (I might throw in a Site of Scientific Interest and a Regionally Important Geological Site tomorrow too)
GLA 58:Coldfall Wood(Muswell Hill) Current geological designation: LIGS
Coldfall Wood is a decent-sized patch of ancient woodland on the western edge of the borough of Haringey. Slip in from one of the adjacent avenues and you enter a shady expanse of oak and hornbeam with a minimum of ground cover, a truly atmospheric landscape ripe for playful roaming. It slopes conspicously from south to north, so if you bring your dog for a runaround you're going to get a workout too. And three tiny streams trickle down through the muddy soil, each too insignificant to explain the incised gullies they've carved, which is generally a good sign that something geomorphologically intriguing has taken place. [pdf]
In 1835 local geologist Nathaniel Wetherell unearthed an unlikely mix of rocks and fossils here that could only have come from further north in the British Isles. His discovery was the first to suggest that glaciation had stretched as far south as London, indeed it turns out Coldfall Wood marks the edge of the Anglian ice sheet at greatestextent. As the ice melted and the subsequent permafrost thawed, the outwash of gushing water would have helped to carve those anachronistic notches we see in the hillside. And the slope itself is topped with glacial till, that's material deposited directly by the ice, on top of a layer of gravel on top of a layer of clay. One wood, three strata.
The till is confined to the southwestern corner of the wood adjacent to the allotments. All I saw with my eyes fixed to the ground was heavy damp soil, but that's because the till doesn't break the surface and is apparently best seen in freshly cut graves in the neighbouring cemetery. Below the till, again in a southwestern stripe, comes a layer of stones known as Dollis Hill Gravel. This was laid down before the last ice age by a river flowing north towards the Thames, which back then flowed through the Vale of St Albans before glaciation pushed it south. And below that comes the London Clay, the capital's ubiquitous bedrock, deposited as mud 50 million years ago.
London Clay covers the majority of the wood and is apparently best seen in the marshy riverbank down by the bridge. Alternatively go hunting for differential drainage because, assuming it's wet enough, the springline can be found where the permeable gravels make way for the impermeable clay. I took most of this on trust as I walked around, even with the aid of the LGP's excellent 4-page explanatory pdf, increasingly convinced that Coldfall Wood must be a brilliant natural resource to have on your doorstep. And then I walked back to East Finchley station, that's from just under the snout of a glacier to just beyond it, because a geological safari is a proper learning experience.
GLA 76:Rockingham Anomaly(Elephant & Castle) Current geological designation: Recommended as a LIGS
The only geological site in the borough of Southwark is a bit of a dip just round the corner from Elephant & Castle. And they've built flats on it... not the modern upthrust of Elephant Park but the 1930s Rockingham Estate on the other side of the road. It's very council, a mix of long balconied blocks and irregular patches of tarmac and grass, with far more parking spaces than you'd ever get away with in a modern development. But if you step back and look, or keep a close eye as you walk in, you'll see that the land drops gently away on all sides to a depth of about five feet. It's the Rockingham Anomaly, a circular depression approximately 300m across and central London's only patch of peat. [pdf]
Pretty much everyone else nearby lives on a gravel terrace, and nobody's quite sure how this came to have a small island of peat in the middle. One theory is it's thanks to a geological fault letting water trickle thrugh, another is that it could be a pingo, a dome of periglacial permafrost that melted causing the surface to subside. Whatever the source we know it's a mighty chunk of peat because in 2017 they sent down a borehole and still hadn't reached any gravel after 16m of digging. According to the research (pdf) they also found pollen from alder trees suggesting this was once boggy fenland, as well as prehistoric seeds, fragments of insects and husks of mollusc. And nobody knew it was here until the Southwark and Bermondsey Storm Relief Sewer pushed through in 1906 and the engineers suddenly thought what the hell is this?
The most famous former resident of the Rockingham Anomaly is gay rights activist Peter Tatchell, or at least he's the only person to have been gifted the honour of one of Southwark's blue plaque. I doubt he realised he was living on a peat island, and I doubt that many of the current residents of Bath Terrace and Rockingham Street realise either as they walk back to their flats, lug a pushchair downstairs or shove more boxes into a scrappy lock-up. It's important too because should the Thames ever flood this side of Southwark is already in big trouble, and anyone living in a 1.5m depression would be at more risk than most. Geology's not just fascinating, it can be critical.
