The Formation of the fens.
Ancient geology.
Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age and it is the actions of the ice sheets that have formed the landscape we see today.
The starting point in geological time for the fens was over eight thousand years ago The English channel and the North sea did not exist and the river Rhine flowed where the North sea is now. The east coast English rivers were tributaries of the Rhine, and the ancient fen area was clay dating back to the Jurassic period.
Since then, there have been repeated ice ages. Sea levels rose and fell in response to the freezing and melting of the water. At times when ice was starting to melt the rivers could not empty into the sea because of the ice further north and so silt built up in the fens and formed fresh water lakes, called meres. Then when the polar ice melted, the sea water rose and flooded the fens, sumerging plants and animals that had been on the fen.
The effect of this has been to build up layers of clay, silt and peat. Peat was first layered on the Jurassic clay between the ice ages and new clay was laid on the previous layer of peat as the ice melted after each ice age.
At one stage in the fens history the area was covered in deciduous woodland. The remains of the trees are still being dug up from the peat to this day. The fen people call them bog oaks but they are mostly pine not oak. Radiocarbon dates of 4300-4600 years ago have been reported for the remains of these bog oaks
Recent geology of the fens
The Fens has been called "the sink of thirteen counties," meaning that rivers drained most of Middle England into these low, flat lands.
In the spring, these rivers would run in high floods, heavy with sediment. When they hit the flat Fens, they would slow down and drop the heaviest of their sediment load. These sand and clay bars would obstruct the channels and send the rivers into wide meandering patterns, perhaps doubling their length before they hit the North Sea in the Wash, a large shallow bay. Of course, the longer a river took to fall to the sea, the slower its water, and the more sediment it dropped. This left the Fens with a shifting landscape of sluggish channels choked by sediment banks both new and ancient. In between standing water would foster rich marsh vegetation. This in turn caused the formation of peat, nearly pure plant material partially rotted to a brownish black mass, with the rotting halted by lack of oxygen in the standing water. Peat, unlike normal vegetation, will never rot as long as it stays in standing water; over eons, it will turn into coal instead.
The effects of the peat and clay layers underneath the surface affect the surface in a number of ways. The clay prevents natural drainage so water is retained at the fen surface.
So to increase areas that could be farmed pumps were deployed to drain the surface water off the peat.
The problem.
The planners had forgotten about peat. Peat shrinks when dry.
Between 1630 and 1655, the Dutch engineer Sir Cornelius Vermuyden was employed by the Earl of Bedford and others to carry out a comprehensive scheme to reclaim the Great Level of the Fens. The scheme involved the cutting of the Old and New Bedford Rivers to bypass the meandering course of the River Great Ouse and create a huge wash land from Earith to Salters Lode to store flood water.
Within the Middle Level he cut a number of new straight drainage channels such as the Sixteen Foot, Forty Foot and Twenty Foot drains with the bulk of the Middle Level area draining into the Great Ouse at Salters Lode. The improved drainage caused a rapid shrinkage of the peat fen and land levels dropped.
By the early 18th Century, lowering land levels had required the Middle Level rivers to be embanked and many wind pumps to be built to lift the water from the field dikes into the rivers. By the early 19th Century many of the wind engines had been replaced by steam to allow the water to be lifted through greater heights, to take account of the even greater peat shrinkage resulting from the improved drainage.
By this time it was clear that improvements to the drainage systems would be a never ending process and, in 1844, an Act of Parliament was passed to allow the cutting of the Middle Level Main Drain, which moved the main outfall sluice from Salters Lode to St Germans, some 9 miles further down the Great Ouse, where low tide levels were 7 feet and lower.
At Mullicourt Aqueduct, the old drainage channel, which is still a statutory navigation, is carried over the Main Drain, providing a graphic illustration of the effects of land shrinkage.
To claim this earthcache you should :-
1. Email me your estimate of the land shrinkage of the fen at this point, using the difference in height of the 2 rivers at the aqueduct.
2. Research the area to find and then email me 3 items that the villages could get from the Fen Commons for subsistence.
3. And email me how it got its name of “Mullicourt”
4. Take a photo at the co-ordinates of anything, but please dont show the information boards.