CIRAM recently dated two pieces of fossilized wood: a "morta" oak and a marsh yew. Both were found buried in the same location, in the Bessin and Cotentin marshes in Normandy (Calvados and Manche). The state of preservation of these two fragments is remarkable. Radiocarbon analyses date the oak to between 4,344 and 4,066 BCE (Before Common Era), and the yew to between 2,027 and 1,778 BCE. Despite being 6,000 and 4,000 years old, respectively, these woods retain an almost intact structure and visual appearance.
How can such preservation be explained? And what do these ancient woods tell us about the history of the landscape? Here’s a brief overview of a material that is as fascinating as it is precious.
Extraction of a morta tree by Atelier JHP.
(source: Le morta – Atelier JHP)
Morta : wood from submerged forests
Morta is a particularly ancient oak species found buried in the peat of marshes. Hidden there for millennia, it now constitutes as an exceptional material at the crossroads of geoarchaeology, wood science, and fine craftsmanship.
Archaeological assessments carried out as part of the dredging work on the Brière canals, undertaken by the Brivet Watershed Syndicate (SBVB), show that morta trees constitute veritable environmental archives. These ancient trees allow us to document, in particular:
- Variations in water levels in the marshes,
- The phases of burial of ancient forests,
- Traces of ancient human activity (charcoal, detrital deposits),
- Landscapes evolution since the Neolithic period.
The woods are thus integrated into a comprehensive understanding of the territory, encompassing both archaeological and paleo-environmental aspects. Radiocarbon dating places the origin of the bog oak between the late Neolithic and early Bronze Age; that is, having grown approximately 5,000 to 7,000 years ago in a forest environment that has since disappeared. This period, marked by significant hydrological changes in the marshes, led to the gradual burial of the trees under peat deposits. The wood was thus deprived of oxygen, which prevented its normal decomposition.
The processing and marketing of morta
The commercial harvesting of bog oak is now strictly regulated: it takes place over a short period each year, in areas defined by local authorities. In the Brière marsh, for example, harvesting is only permitted for six weeks a year, from September 15th to October 30th, when the marsh is not too flooded, and the extraction site is located in conjunction with the Brière Regional Natural Park (PNRB). The ground is probed, and any wood found is harvested without mechanical assistance to preserve the integrity of the soil; the site is then filled in after extraction. An application for Protected Geographical Indication (PGI) status for the Brière marsh holly (morta) is currently under consideration to safeguard and regulate the harvesting of this unique natural heritage as much as possible.
And if this wood is so prized, it's partly due to its technical and aesthetic properties. It was once used for making sturdy frames or as firewood. Today, it's not sold in its raw state but transformed into knives, pens, pipes, sculptures, or other small turned or carved objects.
Silence, it’s growing! Video of the harvesting and processing of a morta by Atelier JHP.
(source: Le morta – Atelier JHP and Le morta – Silence, ça pousse ! – France Télévisions)
Analyses conducted by CIRAM
However, let us return to the analysis of the samples : in these anoxic conditions, our two wood species from Calvados underwent a long fossilization process, rendering them rot-resistant under natural conditions. This anaerobic environment is the source of the exceptional preservation and slow transformation into a dense, dark, and stabilized material. As previously mentioned, carbon-14 dating reveals two periods of burial, even though our oak and yew were found in the same location. We can reasonably deduce the existence of periods of rising and falling water levels, responsible for their decline and subsequent entrapment in peat deposits.
For oak, the black color is obviously not its original color, as it is a wood with naturally light brown hues. It is this particular coloration, evidence of chemical interactions that occurred over millennia in a humid environment, that gives it its aesthetic charm, particularly prized for works of art. Under the same conditions, yew (Taxus baccata) wood has darkened slightly, whereas it is usually a brownish-orange wood.
Wood species analyzed by CIRAM laboratories: "morta" oak (black) and swamp yew (brown).
Anatomical Analysis of Wood Species
Our oak sample was compared to a contemporary oak. The cross-section of this thousand-year-old oak reveals all the characteristic features of the deciduous oak genus, namely:
- A porous initial zone,
- Very wide wood rays, clearly visible to the naked eye, and others that are very thin,
- A tangential alignment of large vessels filled with tyloses in the earlywood and small vessels forming flame-like structures in the latewood.
The curvature of the growth rings is slight: the tree was therefore of large diameter and its growth slow. It was an old specimen. This observation attests to the incredible preservation of the wood from its death to the present day!
Cross-section of a Morta sample (left) and a contemporary oak (right) after fine-grit sanding (x1200).
Technological Analysis of Wood Species
Finally, the morta oak exhibits particular mechanical characteristics. The mineralization of the wood during its burial was accompanied by a significant change in its density; the average measured on several samples of our "morta" oak gives us an average value of 974 kg/m³. However, according to the Tropix technical data sheet published by CIRAD, the density for a sessile oak or a pedunculate oak (Quercus petraea or Quercus robur) is around 740 kg/m³, representing an increase of 31% (source: Quercus robur / petraea technical data sheet).
Our yew tree underwent equally dramatic mineralization: measurements taken on 6 samples yielded an average of around 750 kg/m³, whereas the usual density is between 480 and 600 kg/m³, representing a densification of 24 to 56%! This significant increase in density reflects the physico-chemical transformations of the wood in a water-saturated and oxygen-poor environment, which permanently alter its mechanical properties.
This high density makes them particularly resistant materials but also demanding to work with. Working with bog oak requires specific tools and expertise, as it is very dull (carbide-bladed tools, negative cutting angles, reduced passes to limit vibrations, slow and controlled work, etc.).
Interactive experience: find the bog oak!
What if you put theory into practice? As part of a Scientific Game Jam (March 10–12, 2023), an online video game was designed in just 48 hours around bog oak. The premise: explore a marsh, probe the ground… and try to locate buried logs, just like artisans do in the field.
This serious game offers a unique immersion in prospecting methods, translating scientific data into an interactive experience accessible to everyone. We bet you'll be digging! 😉
Screenshot from Planet Bog, a serious game dedicated to the search for morta.
(source: Planet Bog and Planet Bog Tutorial)
Acknowledgments and Sources
All the data used in this article comes from archaeological work carried out by the Brivet Watershed Syndicate, the Grande Brière geoarchaeology research program (team led by Yann Le Jeune, Loire-Atlantique Archaeology Service), various journalistic articles, as well as field observations and technological analyses of the material. We also thank Jean-Henri Pagnon, the creator of Morta knives, for his invaluable advice and permission to use his photographic resources.
- SBVB (2023), Final Report on the Archaeological Survey – Brivet Watershed
- Le Jeune, Y. (2025), Geoarchaeology of the Grande Brière Marsh – PCR, UMR 6566 CReAAH
- Ouest-France (2024), Le Morta, a Brière marsh oak, is a candidate for PGI status
- L’écho (2025), Brière. Morta: 10 Things to Know About This Wood That Has Been Buried in the Marshes for 5,000 Years
- "Quiet, it's growing!" (2022), Video – Le morta
- CIRAD Tropix, Technical Data Sheet: Quercus robur / petraea
- Technical data and field observations (basic article – wood density and anatomy)
- Scientific Game Jam (2023), Serious game Morta – 48-hour development (March 10–12, 2023)
- Morta knives: photos, documentation, and video
