“The peat on this bog is over 7 metres deep!” said my colleague Gill, our Ecologist. This was news to me. I measure peat depths on almost every peaty site visit, and I had previously recorded 2.6 metres on Steng Moss. “Yes, we have a journal article, about pollen core analysis done in the 1970s, where they found peat over 7 metres!”
Gill and I were visiting a contractor working on site – as part of a peatland conservation project. You can read more about that here. “That explains it,” nodded the contractor, “yes, the peat here is definitely more than 2 or 3 metres deep.”
Peat forms from sphagnum mosses – and there are at least 12 different types of sphagnum to look out for on British bogs. Some are more important peat-forming species than others – in particular Sphagnum capillifolium, Sphagnum papillosum and Sphagnum magellanicum. As a rule of thumb, peat grows at about 1 millimetre a year, so a peat depth of 1 metre represents 1,000 years.
Back at the office, the published article from the 1970s was duly dug out – a study which looked at 4 bogs in Northumberland, took a core at the deepest point, and analysed it for pollen grains. This can help to understand history – such as what crops were being grown in the area at certain times in history, or what tree species were growing in the area – which helps to understand how settled the population was.
The surveyors took 2 transects across Steng Moss and then took a core from the point where the transects crossed. Pollen analysis found that until the Bronze Age there would have been trees in the area, then there were periods of cereals and grasses being grown nearby, and periods when it was more wooded again. There was also a time, around 600 BC, when the climate was colder and wetter – ideal conditions for sphagnum, which started forming peat at a rate of 4mm a year, much faster than normal. What was also surprising was that even this far north of Hadrian’s Wall, pastoral farming and arable crops were in the area during the Roman occupation, and continued for a while after the Romans left. The types of crops people were growing in the area were barley, rye and wheat, and the types of trees in the area were birch and later, walnut.