Cauldron holes – holes that go by that name

Flanders Moss NNR

Coming out of an archive search by John Harrison, a local historian genius, was a map drawn up in 1797 but relating to features that would have been present 50 years previously. Below is a simplified version to make it easier to discern today. The map was fascinating as it gave details of what the moss was like over 200 years ago. One interesting feature was that individual trees were marked (note holly tree and birches) illustrating the fact that back then there were hardly any trees on the moss.

Another interesting feature were marked as cauldron holes. The only explanation given of a cauldron hole was that, unhelpfully, they were “holes on the moss known by that name”. More helpfully is that 250 years later the cauldron holes are still visible today and their mysterious presence and what they tell us can be explained.

Close up of the above map showing locations of the cauldron holes and featured trees.

If you visit the places marked on the map today you will find holes in the moss. A poke around with a stick can be quite alarming, they are deep, sometimes a couple of metres or more deep. Below is one that is at least 2m deep.

A closer look at their location and it can be seen that they are on the edge of the peat mass and usually lie on a flow line of water. A few years back we had a digger working close by out on the moss carrying out restoration work and I took the opportunity to divert it for half day and explore the nearby cauldron hole. The digger cleaned out the hole and confirmed my suspicions of what was going on.

The red dots mark the line of cauldron holes on the moss today, just west of the High Moss Pow.

About 2m down we found a hole about 30cm wide. This is what is called a peat pipe. It is where water flowing off the moss have found a crack in the peat under the surface and over the years has worn the crack into a larger tube. Sometimes the erosive effect of the water creates a big hole under the bog surface until the roof collapses and a sink hole appears. This looks like a crater in the middle of the peat and 200 years ago were described as cauldron holes.

What is interesting is that peat pipes 2m under the surface of the moss show that even 200 years ago Flanders Moss was drying out. The peat clearing, surface ditching and draining of the High Moss Pow was all having a damaging effect on the water table, drawing it below the surface.

So what did we do with the cauldron hole that we investigated? Well, using the digger we plugged up the peat pipe in several places. And then waited.

By the next day the cauldron hole had completely filled with water so creating a beautiful but very deep (2m plus) little dragonfly pool. But of possibly more benefit was the effect on the surrounding bog habitat. Where previously it was dry and heathery with the water table restored to the surface it has become very wet and sphagnum covered.

The restored cauldron hole

So the mystery of the cauldron holes have been revealed. Ancient sinks holes that related to the drying out of the moss from man’s activities 250 years ago. It is amazing what small features on ancient maps tell us about the history of the moss and the stories related to it.