Peat Depths

“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.

sphagnum capillifolium

sphagnum capillifolium

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.

Domed lens

What is also interesting is that the sketch of the transects shows how domed the bog is. This is something you notice when you are on a bog.  A bog can be a bit like a domed lens, raised in the middle, at which point it is only getting its water from rainwater and not from any side streams.

Illutsration of domed bog

The surveyors must have had a theodolite with them.  The bog is about 4 metres higher at its dome than at the sides.  We never normally measure this when I take peat depths today – but perhaps I should!  The surveyors made a distinction between cotton grass type peat, woody peat, and phragmites peat – also something I don’t record these days, but perhaps should.


Geographical Information System

As a lover of data and maps, I wanted to digitise these peat depths however, there is only a sketch map in the journal article, no grid references!  And using my knowledge of the site, I just could not digitise the peat depths in what I felt were the right places.  Undeterred, I georeferenced the sketch map – this means working with the sketch inside our Geographical Information System (GIS) software, and “pinning” it to earth in at least 4 places, so the computer knows where it goes.  Then there was a lightbulb moment as soon as the sketch displayed within the GIS when I realised that the surveyors had used what is the end of an old wall or boundary as the start of transect A – this would make perfect sense as being one of the only features there.

Interestingly, the sketch map included the 305m contour line, which is no longer shown on OS maps.  And other sketch maps in the article illustrated a 381m contour and a 213m contour.  At first I could not understand why the surveyors also mapped a contour line in such detail!  Then, Gill pointed out, that these correspond to 1,000 ft, 1,250 ft and 700 ft!  And a volunteer corroborated this, adding that during the 1970s the OS were moving from illustrating contours in feet and starting to use metres – but until the new contour lines were mapped they simply labelled the old contour line in metres.  Which is why the odd numbers!

Still inside the GIS, we used a tool called “points along a line” to illustrate regular 50m intervals along the transects, so that we could digitise the peat depths taken in the 1970s.  These points now automatically upload overnight to an app on our mobile phones, so I can see all the peat depth data when out in the field – in 6 years we have collected nearly 2,000 data points.

Practical Day

Then, during a practical day with Newcastle University students, we decided to try to replicate these depths and transects.  Working in 2 teams with a mobile phone and a tablet to record on, it is possible to navigate to the digitised point and take a real peat depth there.  We use a set of drain rods – as we are not taking a core.  Each stick is 90cm long and so a set of 10 sticks (they all screw together and unscrew apart) would make a depth of 9 metres (which would represent 7,000 BC).

school children on practical day at Steng Moss

Year 7s from Dr Tomlinson’s Middle School Rothbury adopt a bog at Steng Moss and discover the importance of peat for carbon storage and clean, safe water at Steng Moss near Elsdon, Northumberland

We did find a peat depth of 7.1m, and without at least 2 people we would not have got the peat sticks out again!  At another event in the future, we could take other peat depths close by to see if we can get to 7.35m.  But of course an interesting fact to note is that since the late 1970s when the surveyors took their readings, the peat should have grown a further 5cm!

We plan on a future field trip to take peat depths on a grid of 25m or 50m, working all the way across the site.  This would then allow us to calculate the entire volume of stored carbon in the bog.  We have done this once before close to Chew Green, using an equation.  There we estimated that the bog stored 11,233 tonnes of carbon, where the deepest peat was 2.39m.  I would guess that Steng Moss stores more carbon than that.

One of our volunteers on this year’s practical tasks removing sitka saplings, Chris Wright, is studying for a degree at the Open University.  He decided for one of his modules to study how much carbon Steng Moss can store.  Once back in good condition, it will store between 100 – 200 tonnes of CO2 per year.  And he estimated it already contains about 400,000 tonnes (which is the carbon footprint for about 50,000 households).  Not bad for a 46-hectare bog!

Another idea for student research and fieldwork would be to see if there is a correlation between the deepest peat on a bog, and the presence or density of the most important peat-forming sphagnum species in quadrats nearby.

What is the deepest peat bog?

The deepest bogs in this part of Northumberland are up to 15m deep in places – these are on the Border Mires. This goes back as far as the end of the last Ice Age and the retreat of the glaciers, there will be no British peat bogs older than these.

Why do we take peat depths?

We take peat depths for a number of reasons.  The most common one is when assessing a site for peat restoration.  If there are old drains dug on a peat bog it causes the land to dry out, whereas sphagnum prefers to be wet for most of the year.  Normally this would be restored with a digger and a skilled contractor building peat blocks every 10m along each drain.  But for this technique to work, the base of each grip has to have at least 10cm of peat, so that the block will be keyed in watertight.

Another reason is to assess whether a site is suitable for tree planting or not.  New tree planting schemes, if they are to be compliant with Forest Standards, should not be on peat deeper than about 40cm or 50cm.  So just one peat stick pushed in just over halfway represents 50cm.  You can explore our peat depth data here.

A third reason is to show people the incredible depth of the peat – it is fairly easy to push a peat stick into the ground, and this illustrates that without the covering of bog grasses and mosses on the surface, the peat is actually a bit like butter or porridge – quite fragile.  This is why exposed peat is vulnerable to erosion.  You may feel the peat stick hit bedrock, which is the bottom of the underlying ‘bowl’.  Then, when the peat sticks are out of the ground, it can become a bit like a history timeline:

  • A bog that is about 8m deep would have started forming around the time that Britain became an island
  • A bog that is about 5.5m deep would have started forming when the wheel was invented
  • A bog that is about 4m deep would have started forming when Stonehenge was completed
  • A bog that is about 2.1m deep would have started forming when paper was invented

It is amazing when you stand on a bog, all that history is under your feet!  As well as, through pollen analysis, a record of how settled farming was or how wooded the landscape was.