Waves over Woodlands

If you were looking for evidence of former prehistoric woodlands, you’d be forgiven for not heading straight to the seaside…

…but dotted around our coastline are the remains of former land surfaces – peat deposits and submerged forests – sitting within the intertidal zone. Every so often, particularly after a storm, remnants can become exposed, such as those at Redcar, in Cleveland, for example, revealed by Storm Emma in 2018. Others, however, are always visible when the tide recedes.

In this article we describe these drowned landscapes, their significance and value, and, using a case study, illustrate how we are exploring the best ways to study, record and monitor them.

What are intertidal peats and why are they there?

Intertidal peats and submerged forests are the remains of land surfaces which formed when sea levels were lower, and which have been drowned by subsequent relative sea-level rises. Whilst the majority formed during the Holocene (the current warm period beginning at the end of the last ice age), some exposures, for example those along the Norfolk coast, are older.

Although they are sometimes referred to as ‘petrified’ forests, they are not actually fossilised.

Although they are sometimes referred to as ‘petrified’ forests (coming from the Greek word petros for rock or stone), they are not actually fossilised . Instead, they are preserved due to the wet site conditions; the waterlogging restricts decay processes by limiting the amount of oxygen available for decay organisms to function.

Why are intertidal peats important?

The organic remains that are preserved within the deposits vary from the micro - for example, pollen grains, diatoms (microscopic, single-celled algae), insects, and seeds - to the macro - the trunks and stumps of the trees themselves.

organic remains provide valuable information on past environments and landscapes as well as being suited for scientific dating

They provide valuable information on past environments and landscapes, as well as being suited for scientific dating (radiocarbon and dendrochronology). They can also preserve direct evidence of human activity and presence through associated archaeological remains and features such as artefacts and footprints.

Resource assessments and monitoring will help to identify research priorities by targeting those deposits most at risk.

The location of these deposits in the intertidal zone means that they can be at threat of loss through erosion, so it is important to record them to capture their evidence. Having ways of producing resource assessments and of monitoring deposits over time will help to identify research priorities for intertidal peat deposits by targeting those that are most at risk.

These deposits are also carbon rich, potentially releasing carbon dioxide and methane into the atmosphere when eroded.

How are we studying them? The Pett Level case study

Since 2014 Historic England has been involved with work investigating and recording part of an intertidal exposure at Pett Level, East Sussex.

Initially we funded research led by the University of the Highlands and Islands analysing the organic remains themselves. This was followed by further research led by Historic England which is comparing the effectiveness of a range of site-scale recording methods.

Given that the full length of the exposure extends for about 2.5 kilometres along the coastline (from Cliff End in the south west, to Winchelsea in the north east), a smaller, discrete area of exposure about six hundred metres long was chosen as the target for these studies.

Excavation, coring and analyses

With help from the local community more than two hundred exposed tree remains have been recorded and identified within this area.

They show that the woodland consisted mainly of wet-loving trees; alder and ash, together with oak, willow, birch, yew and hazel. The size of these trees is very impressive, with trunks of alder and ash up to eleven metres in length. This represents only the part of the tree that has been preserved, suggesting that some trees were substantially taller when alive.

Together with recording the trees, the peat itself was investigated through a series of auger transects along the peat shelf. This involved manually pushing a chambered auger down through the peat and then bringing it back up to record the sediments trapped in the chamber. This coring showed that the deposit consists of multiple peat layers which vary in number and thickness across the site; in places the uppermost peat layer is around two metres thick.

At two locations test pits 1x1x1 metres were dug into the peat in order to take samples for analysing the pollen, plant macrofossils and beetle remains. All of these techniques allow us to look at how the landscape at Pett Level has changed over time.

We have been able to date the woodland at Pett Level through radiocarbon dates on seeds and buds taken from the basal and upper layers of the peat, together with radiocarbon dates and dendrochronological (tree-ring) studies of the trees themselves. This work was co-ordinated by Dr Peter Marshall and Cathy Tyers (both of Historic England).

The dates have shown that woodland was present from at least 4,400 BC to 1,500 BC, from the Late Mesolithic to the Middle Bronze Age.

Putting all of the information together, we have worked with graphic artist Dr Alice Watterson of the University of Dundee to provide an interpretation of how this area of the Pett Level would have looked around 4,000 years ago.

Aerial recording and interpretation

The first phase of this aspect of recording work took place in 2016 using high-level (aeroplane) and low-level (drone) aerial recording techniques. Multiple types of imagery were acquired, including traditional RGB (red-green-blue) photographs, multi-spectral and thermal images, and video footage.

The photographs were processed to produce orthorectified images (that is, free from distortions) using Structure from Motion (SfM) methods.  A digital elevation model (DEM) was also produced. The newly-collected data, together with existing information from other sources (such as the Environment Agency), were then visually interpreted. They were used to map the inferred extent of the peat deposits, and other feature types, such as individual 'procumbent' tree trunks, (that grew along the ground without throwing out roots) were also mapped where they could be distinguished.

Soon after the aerial surveys, on-site recording was undertaken on foot, mapping the outermost limits and extent of the exposure as seen at ground-level. This produced a dataset which could be compared with the aerially-obtained datasets and the aerial image interpretations. Most recently, in 2018, small-scale sediment coring investigations were carried out to explore the deposits below the surface in order to test the thermal imaging results obtained in 2016.

Work to quantitatively compare all the datasets using a GIS (geographical information system) is in progress. Two ground-based survey outlines of the exposure currently exist, one produced in 2014 by Dr Timpany, the other in 2016.

The current plan is to revisit the site for recording in 2020. This will involve a further ground-based survey for comparison, as well as a repeat of the aerial-survey data acquisition.

The advantages of using rapid and remote recording techniques where time is short and access sometimes dangerous are clear.

If successful, the use of aerial recording will be hugely beneficial in identifying, recording and understanding such deposits and furthermore in recognising and monitoring any changes over time, be they vertical and/or horizontal. The advantages of using rapid and remote recording techniques in situations where the time available for recording is short, and where accessing intertidal zones on foot can be dangerous, are clear .

Continuing palaeoenvironmental work

Although Dr Timpany’s work funded by Historic England has drawn to a close, research continues at the site, assisted by CITiZAN (the Coastal and Intertidal Zone Archaeology Network). Regular site visits with community volunteers involve further recording and sampling. Plant macrofossils from the site (Test Pit 1) are currently being analysed as part of a dissertation project by an MSc student at the University of Leiden.

Warning!

Take care! The fact that these sites are located in the intertidal zone means that there are inherent dangers associated with visiting them.

Always consult tide times and seek local advice when planning a visit and bear in mind that access may require permission unless using public rights of way.

Acknowledgements

The aerial recording project was a collaboration between multiple Historic England teams within what is now the ‘Policy and Evidence: National Specialist Services’ division. In particular, thanks to Damian Grady (high-level aerial recording), Fiona Small (aerial investigation and mapping) and Vicky Crosby (ground survey). Skyeye (now Terra Drone) carried out the low-level recording.

About the authors

Further information

CITiZAN (Coastal and InterTidal Archaeology Network)

Coastal peat database

Timpany, S 2018 Archaeological and Palaeoecological Investigation of the Submerged Forest and Intertidal Peat at Pett Level, Sussex: Assessment report. Version 6. Unpublished report for Historic England, Project No. 6920.