How Does Wood Survive Underground For Thousands of Years?
Flag Fen - Bronze Age Post Alignment and Timber Platform
Flag Fen, Peterborough, Cambridgeshire
Scheduled: 2012
NHLE entry: Listing details for Flag Fen
Organic materials, such as wood, were used by our ancestors from the earliest times. People understood the properties of wood and chose different types according to their building purpose. However, wood rarely survives archaeologically, so our understanding of its use in the past is limited.
Wood can be preserved in wet or waterlogged sites. This includes coastal waters, rivers, lakes and marshes, and archaeological features that reach below the permanent water table, such as wells.
The Bronze Age causeway at Flag Fen in Cambridgeshire - a post alignment and timber platform - is an excellent example of a site where waterlogged conditions have preserved not only wooden artefacts but also wooden construction features.
What happens in the ground?
The survival of timbers and artefacts within Flag Fen - dating from about 1000 BC - is remarkable. Visitors can view a preserved section of the causeway as well as many of the artefacts found during excavation. However, much of this exceptional site still survives below ground. It is therefore crucial that the water levels and water quality are maintained so that the structures and artefacts in the ground remain preserved.
Normally, when wood is buried it decays rapidly. However, if the burial environment is very wet it can be preserved for centuries. Bacteria and fungi will still degrade the wood, but when the oxygen supply is limited - under wet or waterlogged conditions - this process is much slower than in the air or in a well aerated soil.
The organisms which live on the wood and digest it leave cavities and tunnels behind and these voids within the wood cell structure then become filled with water from the surrounding environment. Hence the term 'waterlogged wood'. The water therefore helps to preserve not only the overall shape of the wooden object but also fine details, such as tool marks and carvings.
What happens after excavation?
Waterlogged wood is sensitive to rapid changes in environmental conditions which, if not carefully controlled, can lead to deterioration during excavation. Uncontrolled drying of waterlogged wood will result in evaporation, and when water evaporates it exerts surface tension.
This can cause the timber to split, twist and even shrink and, if left unchecked, the wood will develop cracks and can totally disintegrate - a loss of rare information and even whole artefacts. To avoid this happening, it is important that waterlogged wood is kept wet until conservation can take place.
Did you know …
In contrast to wood, archaeological iron will grow after it has been excavated from the ground. Now you've learnt about the properties of wood and why it shrinks, perhaps you'd like to do your own research into the remarkable properties of expanding iron…
