When one considers elevations below sea level, England doesn’t normally come to mind. Maybe someone thinks of the Dead Sea or Death Valley or parts of the Netherlands as natural choices. But England? Indeed, England’s lowest elevation actually sits below sea level by almost three metres. This spot exists at Holme Fen (map) in Cambridgeshire. Specifically it’s farmland adjacent to a 266 hectare National Nature Reserve.
Previously Above Sea Level
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. It’s part of the natural progression from open lake to firmly rooted woodland or grassland perched on solid ground. So 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, researchers 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 intended to measure receding elevation. The iron post became increasingly visible as the ground subsided over time. Later they added guide wires to keep it from toppling over.
Today the post protrudes several metres above the surface. Later researchers added another post nearby and collectively called them 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 once above sea level is now well below.
How it Happened
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. However, it begins to react immediately with dried peat and the resulting wastage is nonrecoverable.
Therefore, ground subsidence 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. The change is permanent and irreversible. The East Midlands Geological Society provides greater technical information on this process.
Many European fens disappeared long before people ever understood their significance or their fragility. Nobody can recreate them in a single lifetime and they are lost forever. With a dawning awareness, scientists now focus attention on preserving the last few remaining places where wild fens exist. This includes the National Nature Reserve now surrounding Holme Posts. Even more ambitious efforts are underway including the Great Fen Project. That one 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.
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