My thanks to the London Geodiversity Partnership whose website includes a list of 79 key sites across the capital from Harefield to Hornchurch and Chingford to Coulsdon, all of them mapped and accompanied by multiple pdfsworth of in-depth background information. Yesterday I blogged about two Locally Important Geological Sites (LIGS) and today I'll top that with a Site of Special Scientific Interest (SSSI) and a Regionally Important Geological Site (RIGS).
GLA 14:Gilbert's Pit(Charlton) Current geological designation: SSSI
One of London's big 7 geological SSSIs can be found just south of the Thames Barrier where the valley takes a sudden turn upwards. This high ground once hosted a Roman camp, being ideal as a lookout, and from the late 18th century was worked extensively for sand. So extensively in fact that two pits were gouged out of either side leaving just a thin mohawk-like strip sticking up in the middle. The biggest was Gilbert's Pit, named after one of its managers, which when exhausted in the 1930s was purchased by London County Council to help enlarge Maryon Park. What makes it special isn't any of the layers of rock present, it's the fact that the sequence of strata can be so clearly seen up the side of the pit. [pdf]
The top layer is pebbly flints in sand - the Blackheath Beds - as deposited at the bottom of the sea around 50 million years ago. Then come shelly deposits and clays, this time from tropical lagoons, and below that a thick wodge of Thanet Sand. That's the layer the early miners were most keen to extract, mainly for the production of glass and to supply sand to the Woolwich Arsenal. Below that is a heck of a lot of Late Cretaceous White Chalk, although you can't see that now because 10m of wartime rubble on top has been dumped on top. This is such a good set of strata that they bring engineers here to show them "look, that's what you might be tunnelling through or laying your foundations into." And that'll be why they built these steps.
This staircase was erected up the eastern side of the pit in 2016 to provide a sturdy multi-layered platform, because that's better than scrambling up a steep slumped slope and avoids users damaging it. The big catch is that the gate at the bottom of the steps is always locked unless a group is visiting, indeed all the geologically interesting bits of Gilbert's Pit are sealed off by spiky fences unless you're specially invited to see within. Free access is provided to some of the ancient woodland and the grassy clearing at the pit bottom, plus anyone can climb a different set of steps as far as another locked gate but won't be let in unless they're some kind of student of geology.
These earthy steps are good if you like a viewpoint because the top of the flight is high enough to be able to look down on the Dome and Docklands. But the view was much better 10 yearsago before the surrounding trees grew up, and was also better in winter before fresh leaves blocked out pretty much everything except the Thames Barrier. It's galling not to be able to get through the gate because this leads to a path along the pit's central spur, but again that's geologists only. Instead you'll have to console yourself with a decent look at the uppermost layer of the Early Eocene Lambeth Group, namely the Blackheath Beds, here exhibited as a lot of smooth black flinty pebbles scattered across the path.
Here's a brief diversion via the 380 bus to see the Blackheath Beds in simpler situ. Blackheath is well known as an open common of longstanding, but what's less well known is that it was named after its pebbles. These are unusually round and unusually black and generally only found by digging down slightly beneath the surface. Hence a number of old gravel pits litter the heath, specifically in the southwest and northeast corners, mainly from the late 17th and early 18th centuries. [pdf][pdf]
Eliot Pit at the foot of Mounts Pond Road is an overgrown dent offering sufficient privacy for much unseen drinking, although it was thankfully empty when I wandered in. The Vanbrugh Pits are multiple and easier to explore, but still generally ignored by the good folk of the neighbouring villas nipping inbetween to the shops. Pick the right slope and the dips won't be too steep to drop down. And there across the soil will be more of those lovely smooth pebbles, their rinds removed by millennia of underwater rolling, and this time you are allowed in to have a look.
GLA 36:Pinner Chalk Mines(Pinner) Current geological designation: RIGS
And now for something completely different on the other side of town. Most of north and west London is built on low clay hills perched on a layer of chalk, and in Pinner that underlying chalk comes closest to the surface. This means that not only does Pinner have chalk mines, a rare enough human intervention, it also has the deepest such shafts anywhere in the country. Chalk was mined here for centuries, generally in the summer months to feed lime kilns for construction purposes, so chains of white caverns exist at more than one location beneath Pinner Hill.
One such complex caused chaos at Pinner Wood primary in 2017 when geotechnical surveys following unexpected subsidence forced the school to close, indeed there were fears it might never reopen. Subsequent laser scans revealed the existence of 30 further voids, a situation eventually fixed via 5000 tonnes of compaction grouting, and children were thankfully back at their desks after nine months. A short distance away at Pinner Hill Farm the shaft is known to be 35m deep with two main galleries 25m in length, which is quite something for suburban Middlesex.
But the London Geodiversity Partnership are particularly interested in the mine at the rear of Montesole Playing Fields at Pinner Green because that's borderline accessible. Yesterday its playing fields were busy with a lively match organised by Pinner Cricket Club and a number of families monopolising the play area by the main road. But the far end was entirely empty bar one jogger doing a full circuit, its gorgeous shimmer of buttercups entirely overlooked, and beyond that is the wood where the chalk mine lurks. [pdf]
Dingles mine opened in 1830 and was owned by a local brickmaker who used the chalk to prevent his finished products from cracking. The pit has subsequently become steep-sided deciduous woodland with a narrow overgrown path around the rim of the bowl. Evidence from tyre tracks suggests it's quite the spot for BMX, but the sandy loops and leaps were empty on my visit and all I got to enjoy was birdsong. Some of the weirder earthworks are now inhabited by badgers, the entrances to multiple setts laid out in sequence like some Biblical street. I would never have stumbled across this peculiar hollow without direction.
And amid the vegetation in one corner is the entrance to chalk mineshaft number 3, now securely fenced off for health and safety reasons. Somewhere under that locked lid is a pitch black vertical drop, descent of which would originally have been by candlelight and requires a particularly long rope ladder. About 17m down is a layer of puddingstone, Hertfordshire's signature flint conglomerate, which provides protective strength for the roofs of the caverns below. The mine itself consists of 6m-high caverns worked by the ‘pillar and stall’ method, which these days we can alas only see in photos of former field trips (or illicit urbex adventuring, such as this report from November 2022).
The woods in Montesole Playing Fields hide one further secret which is a section of Grim's Dyke. This linear earthwork once stretched to Stanmore and is thought to have been built for defence or as a boundary marker some 2000 years ago. You won't see much in the trees, just a long bump and a plaque... but this isn't geology it's archaeology and that's a whole different rabbit hole to fall into.
