Holme Fen pole now stands 4 meters above ground

The Holme Fen cast iron pole in Cambridgeshire now sticks out of the ground by about 4 meters. In 1851, the top of the pole was level with the surrounding ground, but over time, the land beneath it has sunk significantly.

Drainage of a lake caused the subsidence

The subsidence was not caused by a flood or an earthquake, but by the draining of a lake, which led to the peat around the pole compacting and oxidizing. A lake in Cambridgeshire was drained between 1850 and 1851, resulting in the loss of approximately 4 meters of ground over a century.

Local landowners led the drainage project

Between 1850 and 1851, a group of local landowners, led by William Wells of Holmewood Hall, drained Whittlesey Mere. According to the Holme Fen page on Wikipedia, steam pumps were used for the operation, and each landowner received a share of the dried land in exchange for funding the work.

Whittlesey Mere was once a significant lake

Prior to drainage, Whittlesey Mere was already significantly reduced in size, covering less than 1,000 acres with a depth of about 90 cm. It was formed around 500 BC when silts from the Nene and Welland rivers caused water to recede, allowing dead plants to accumulate into peat.

Drainage had ecological consequences

The lake was used for boating, fishing, and ice skating in the winter and was home to the original population of the machaon butterfly in Great Britain. The Great Fen website notes that several species, including the large copper butterfly, disappeared after the lake was drained and converted into agricultural land.

William Wells anticipated subsidence

William Wells understood that draining the lake would cause the peat to behave like a squeezed sponge, and he wanted a gauge placed at one of the lowest points in the area. His report on the drainage was published in 1860 in the Journal of the Royal Agricultural Society.

Holme Fen poles measured land subsidence

The principle involved driving a chestnut pole through the peat down to the clay, cutting its top at ground level, and then observing the ground descend along the wood. Wikipedia attributes the operation to engineer John Lawrence in 1851.

Cast iron column replaced wooden pole

The Great Fen dates the first wooden pole to 1848 and places the replacement by a cast iron column in 1851. At the time, the pole was located about half a mile from the edge of the lake, with nearly 22 feet of peat between the surface and the clay.

Subsidence continued over time

Following the drainage, the ground lost about 9 inches per year for the first thirty years, and by 1955, the ground had already lost 12 feet, about 3.7 meters. Two mechanisms contribute to the subsidence: the peat compacts by losing its water and then oxidizes upon contact with air and crops.

Additional measures were taken to stabilize the pole

Steel guy wires were added in 1957 to stabilize the column as it became unstable upon emerging from the ground. A second column was planted the same year, 6 meters northeast of the original. Its metal plates bear ground levels and corresponding dates.

Holme Fen is now a protected site

Both columns are still standing, and the Historic England file classified them as Grade II on August 11, 1988. The visitor standing at the base of the pole is located 2.75 meters below sea level, which the Great Fen presents as the lowest point of land in Great Britain.

Great Fen is a nature reserve

The reserve, managed by Natural England, covers 657 acres, about 266 hectares, and is part of the Great Fen project, which aims to create a wetland area of 3,700 hectares. The two columns are visible from the north road that crosses the site.