Sometimes you just want to go for a nice walk, nothing too taxing, a bit of a stroll, easily accessible, dry underfoot, intermittently up and down, World Heritage Site, spans both hemispheres, occasional telescopes, geology-focused, nowhere near enough to make a day of it but a nice walk all the same.
You've likely walked round Greenwich Park before, perhaps up to the Observatory and back, but you probably haven't walked round paying special attention to the rocks under your feet. I certainly hadn't. So I was pleased to discover that the London Geodiversity Partnership have produced a self-guided walk around the area accompanied by 19 pages of maps, diagrams, photos and associated subterranean analysis. If you prefer a walk where you learn something and get great scenery into the bargain, maybe give it a try. [pdf]
The walk starts at the Greenwich Foot Tunnel where it delights in pointing out that the information board by the entrance is geologically incorrect. The tunnel was not dug through chalk as one paragraph claims, but through the sands and clays of the Lambeth Group, a muddled layer laid down 55 million years ago. The underlying chalk is apparently 14m below the base of the tunnel, deeper even than the DLR. The strata at the surface are instead river-related, either alluvium or Kempton Park Gravel laid down between 100,000 and 50,000 years ago as the last Ice Age came to an end. The waterfront by the Cutty Sark is also part of a river terrace, the remains of a former floodplain left behind after sea level slightly lowered. Meanwhile the council estate a few steps away is built on the current floodplain, a couple of metres lower... which yes is suddenly obvious if you look at the slope down to the flats by the river.
The Old Royal Naval College, through which the walk kicks off, also lies atop the gravels. But things change as you approach the park's edge, specifically along a boundary line through the National Maritime Museum and Queen's House. This is where the sands and clays begin, in this case a layer of Head formed by mixed materials eroded from the nearby slopes. It's also the alignment of the Greenwich Fault, one of the larger ruptures in the bedrock of the Thames Basin, but I think that's a geological coincidence. You won't see anything change because the bedrock's covered by soil, buildings and grass, but if you're ever wandering around the museum or settled with a coffee in the cafe, a lot is going on beneath your feet.
The next stop is at the foot of the slope on the western edge of the park where a lone symmetrical brick building sits amid the trees. This is Conduit House, part of a 300 year-old water supply system which fed the Royal Hospital and a separate conduit outside the park in Hyde Vale. It was fed by springs emerging from underground, specifically at the boundary between the upper sands and lower clays a few metres up the slope, as water flowed down from the plateau at the top of Greenwich Park and percolated through the pebble bed. It still does but I failed to see any groundwater emerging as I yomped up the muddy incline behind the Conduit House, only the sudden appearance of rounded flinty stones in the soil.
The third stop is supposed to showcase these Blackheath pebbles which are plentifully found at these upper levels all the way from here to Erith. It's time to enter a small depression in the southernmost corner of the park called The Dell which was originally a gravel pit, likely built primarily to source materials for road-mending. According to the notes accompanying the walk the earth here is bare so black rounded pebbles are still readily found, and photographic evidence from 2017 concurs. However The Dell now doubles up as an ericaceous garden and the earthen path has recently been liberally scattered with bark chippings so I saw none. It's a gorgeous spot though, planted with shrubs and spring flowering bulbs, with the first of the camellias already in bloom and the rhododendrons still to come.
The Blackheath pebbles are the uppermost stratum in the park but the trail documentation mentions "a natural exposure of Chalk slightly further west" so I went half a mile off piste to try to track that down. Almost all the exposed chalk in London is south of Croydon but a small patch was exposed by the River Ravensbourne carving through the plateau, roughly either side of Elverson Road DLR. The larger lump at the foot of Morden Hill used to be the site of the Heathside and Lethbridge estates, a sprawl of 400 unloved concrete flats which Lewisham council have almost finished replacing. Here now are 1200 very modern apartments, appropriately including one block called Chalkhill House, and an intriguing-looking set of steps to a viewing platform at the very back. Might be worth visiting later, but currently padlocked so don't come now.
Back in the park, specifically in the Flower Garden, another gravel pit has been transformed into a lake. It's a lovely spot, especially at present with a roped-off carpet of rather a lot of snowdrops blessing the waterside. Other unlandscaped gravel pits can be found out on Blackheath, the largest an overgrown depression at the top of Lewisham Hill, but the finest geologically speaking is over in Charlton where a unique cliff of exposed strata has made Gilbert's Pit a Site of Special Scientific Interest. The London Geodiversity Partnership have a separate Geotrail covering that, if you're interested.
Most of Greenwich Observatory is behind a paid-for cordon but another geological oddity can be found in the back garden just before the ticket office. It's a 100-foot well dug at the request of the first Astronomer Royal in the 1670s, since called Flamsteed's Well, into which a just-under-100 foot telescope was lowered. The hope was that making observations vertically from the bottom of a dark hole would be an excellent way of observing stars passing directly overhead, and they hoped digging down as far as the underlying chalk would be far enough. Alas conditions at the bottom proved oppressively damp so only a few observations were made before John Flamsteed gave up, and doubly alas nobody quite knows where it was dug so the ring of bricks in the flowerbed is a reconstruction.
Next the trail visits the other great viewpoint in the park which is One Tree Hill. The summit boasts a fabulouspanorama across the City and Docklands, which looked very different in 1809 when Turner painted it, and is a lot less well known than the tourist perch by General Wolfe's statue. Two years ago the viewpoint was enhanced by a lovely loop of wooden seating and a long swooshing guardrail, while a gentler accessible path was added, which helps explain why my ascent was held up behind a slow-moving pushchair and a well-dressed whippet. On a clear day you can see right across the Thames Basin towards the Bagshot Sands atop Hampstead Heath.
And finally it's steeply down to lower park level, where another Conduit Head mops up the groundwater. In just a couple of minutes you pass from the upper Blackheath pebble beds to the Lambeth Group clays to a layer of eroded Head atop a tongue of Thanet Sands, and in another couple of minutes you could be out of the park striding across estuarine Kempton Park Gravels. What's more the dry valley behind One Tree Hill is called East Coombe and was formed by glacial meltwater released by the Anglian ice sheet at its greatest extent. Greenwich Park is nothing if not a geology field trip par excellence.
Whether you end up in the museum cafe or scuttling to the library to learn more about sedimentary rocks and the Greenwich Anticline, you'll hopefully give thanks to the London Geodiversity Partnership for revealing the secrets of a familiar landscape hidden in plain sight.
