When one thinks of elevations below sea level, England doesn’t normally come to mind. Maybe the Dead Sea or Death Valley or parts of the Netherlands would be considered natural choices for the list, but England? Indeed, a location in England that is believed to be its lowest elevation is actually below sea level by almost three metres. The spot can be found at Holme Fen, Cambridgeshire, specifically in the farmland adjacent to a 266 hectare National Nature Reserve.
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This was not always the lowest spot in England, and until modern times it used to be above sea level. A fen is a type of wetland similar to a bog or a marsh fed by surface or groundwater, and is part of the natural progression from open lake to firmly rooted woodland or grassland perched on solid ground. Holme Fen features a spongy, squishy layer of peat atop clay.
Farmers had long drained the surrounding countryside for agricultural purposes and over time the land began to sink. In 1852 they pounded a cast-iron post into the ground through the peat and into a layer of solid clay below, with the top of the post even with the surrounding terrain. In this manner they could measure receding elevation. The iron post became increasingly visible as the ground subsided over time. Later they added guide wires to keep if from toppling over. Today the post protrudes several metres from the surface. Another post was added nearby and they are now collectively known as the Holme Posts. Take a look at this photograph from the Great Fen Project and you can see exactly what happened. It’s rather startling. Land that was once above sea level is now well below.
Like a sponge, peat soil shrinks and contracts as it dries. Drain the area, lower the water table, dry the peat, and watch the surrounding landscape drop. But peat soil also has an additional and rather unusual property. Drained peat oxidizes. Similar to iron producing rust when exposed to oxygen, peat produces natural gases that float away into the atmosphere. Oxygen cannot reach peaty soil saturated by water but it begins to react immediately upon dried peat and the resulting wastage is nonrecoverable. Therefore, ground subsidence is is composed of two factors, consolidation due to drainage and wastage due to oxidization. Unlike a sponge, one cannot simply add water to dried peat and return it to its prior state. It has been changed unalterably. Greater technical information on this process is available through the East Midlands Geological Society.
Many European fens disappeared long before people ever understood their significance or their fragility and they were forever lost. They cannot be recreated in a single lifetime. With a dawning awareness, attention has been focused on preserving the last few remaining places where wild fens exist, such as the National Nature Reserve now surrounding Holme Posts. Even more ambitious efforts are underway including the Great Fen Project which seeks to link Holme Fen and Woodwalton Fen into a single green space and habitat for biodiversity.
The success of these preservation efforts will determine whether the lowest point of elevation in England shall remain at 3 metres below sea level or become something even lower.