diamond geezer

 Thursday, June 03, 2021

This is a post I'd been planning to write in March 2020, but circumstances conspired against.

Hornchurch is an impressively nondescript East London suburb, with little more to show historically than a medieval church and a former WW2 airfield. But geologically speaking it's extraordinary - the very furthest south that glaciers came in Britain - and we only know this because of the Overground's least used railway line.

The Romford to Upminster branch line, constructed in 1892, included a short cutting across fields to the north of St Andrew's Church. Hornchurch Cutting was only eight metres deep but it exposed an unexpected seam of boulder clay overlaid by sand and gravel. Local geologist T.V. Holmes, then the vice-president of the Essex Field Club, undertook a field trip to the excavations and discovered several Jurassic fossils that could only have been carried from the Midlands by an ice sheet. No such glacial deposition has been found further south than Hornchurch, confirming this as the ultimate limit of ice penetration during the last two million years.

Every time you ride a train through Emerson Park, which admittedly isn't very often, you are in fact passing through a point of considerable geological significance. With TfL in increasing need of a moneyspinner to boost their finances, they could do worse than rebrand the Romford to Upminster line with an Ice Age theme, perhaps with Pixar characters on the rolling stock and cut-out dinosaurs lining the sides of the track, to encourage repeat journeys.

It's possible to go down into the historic cutting because this brief section of track is one of a handful of places in London with a pedestrian-only level crossing. Find your way to Maywin Drive or Woodhall Crescent, follow the alleyway and make your way down the zigzag ramp to track level. Plainly it's not somewhere you should hang around, nor is there any access to the trackside, plus most of the exposed pebbly soil is covered with green sheeting, but there is a geological frisson from descending into an artificial trench that changed the way we look at Ice Age Britain.

There's more. What excited Thomas Holmes wasn't so much the glacial deposits as the gravel on top of them, because normally that's found underneath. This was evidence that the Thames was younger than the fossils, confirming that the Thames was diverted to its present course after the arrival of the ice sheet. Hornchurch is the only place this layering of Thames gravel and boulder clay has been seen, hence the only location that confirms precisely when the big shift took place.

Half a million years ago the River Thames flowed across Hertfordshire via what's now the Colne valley and the Vale of St Albans, then eastward across Essex towards the coast at Clacton. 450,000 years ago the Anglian ice sheet intruded, blocking the route to the sea and creating a series of lakes which eventually overtopped and sent the Thames on a more familiar southerly course. The ice is known to have reached Bricket Wood and Finchley, with the lobe that pushed down towards Hornchurch one of the last obstructions.
I've massively oversimplified this, sorry, so for more than three sentences you'll need to explore further.

There's more. In the 1970s the Electricity Board built a new substation in a former gravel pit round the back of St Andrew's Church, quarter of a mile south of the railway cutting. They found more boulder clay while they were digging the foundations, again unexpectedly, which encouraged more geologists to descend and check the soil. This time they failed to find further glacial deposits at the far end of the pit, confirming that the substation was actually the further south the ice sheet ever came.

In Hornchurch the old gravel pit is known as ‘The Dell’ and is thought to have been used as an amphitheatre for sporting events in the 18th century. These days it's securely fenced at one end and massively overgrown at the other, then surrounded by a cemetery, so not somewhere anyone could easily clamber down into. I eventually found a vantage point just off the main path and peered over the rim, with trees and vegetation erupting from below, and essentially saw absolutely nothing. But how brilliant to have uncovered the spot where the ice stopped 450,000 years ago, and to know that Hornchurch is by no means as impressively nondescript as it first appears.

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