Although not a henge or built initially anywhere near the coast, Seahenge is the name given to the timber circle and oak tree found at a tidal estuary in Holme, Norfolk. 

Its excavation by archaeologists was challenging. But much is now known about the time and method of this unusual henge monuments’ construction. However, the mystery of Seahenge’s exact function remains.

The Discovery of Seahenge

Seahenge was periodically revealed by the tide several times over the years. However, it was not until 1998 that it became fully exposed when the tips of its upright timber posts became clearly visible above the silt of the Norfolk estuary it had occupied for 4000 years. 

Initially, the feature appeared to be a random collection of roots and branches. Closer examination revealed an unbroken timber circle of 6.6 metres in diameter. Some of the timbers still had bark attached. Most unusual was the feature at the centre: a whole oak tree. Initially considered to have grown in situ, it was later discovered to be inverted, with the roots protruding skywards.

Excavating Seahenge

Excavation began in May 1999. Various factors made it an unusually challenging dig:

• Twice-daily tides complicated the digging processes. A cofferdam was not practical as the soft sandy soil of the estuary would not support the weight of a structure large enough to keep the tides at bay. So water and silt needed to be cleared before work could begin for the four hours or so before the sea reclaimed the site.
• The location of Seahenge is an ecologically protected area of special scientific interest. The vast numbers of people attracted to the phenomenon of Seahenge could prove threatening to the rare species of birds, flora and fauna that called the site home.
• Seahenge was already decaying. Its timbers were already showing damage from the sea. Covering the monument over was not a practical solution, nor was the erection of a concrete barrier because this would further endanger the area’s ecology. Despite controversy from some groups who believed the monument should not be removed from its context, it was decided that the only way to preserve Seahenge was to fully excavate it and then conserve it away from the estuary site.

Seahenge’s History

The excavation established various facts about the monument:

• The Landscape of Seahenge. The site of Seahenge was not originally on the coast. The climate changed over the course of the Bronze Age, altering its original environment from a freshwater marsh to a peat bed, which gradually enveloped and ultimately preserved the timbers. The sea then moved inland, progressively eroding the peat bed until the monument was exposed once more.
• The overall structure of Seahenge. Seahenge was composed of 55 oak posts forming an elliptical barrier around the central oak tree. The tree itself was 150 years old before being cut down a year before the monuments’ construction. Each timber post of the surrounding palisade was one half of a complete tree split down the middle, arranged with bark sides outwards, giving the impression of one huge trunk or barrier of bark.
• How Seahenge was built. From wood chippings and shavings found in the backfill surrounding the palisade’s foundations, archaeologists established that Seahenge’s timbers were brought to the site from elsewhere but shaped on-site to fit into the ‘U’ shaped trench specially dug to receive them. The posts were lowered in position down a small ramp built at the top of the trench before being pulled into an upright position using ropes. One such rope was found around the oak tree’s trunk. It is unique in the history of prehistoric rope making as it is made of honeysuckle, a plant never previously identified with rope making.
• Dating. Dendrochronology could not provide a conclusive date for the site because the exposure of the timbers had damaged the tree rings used in the count. Radiocarbon dating suggested a date somewhere in the Bronze Age, which matched the tool marks found in the palisade timbers and the fact that a bronze age axe was found nearby. A conclusive and narrow final date was established when botanical analysis of finds on the site was combined with the carbon dating using a 250-year-old mathematical formula. It seems that the central oak tree was cut down between April and June 2050BC and that the timber posts were cut in the spring of the following year. Seahenge was constructed in the same year.
• Seahenge’s function is thought to be ceremonial, although its exact function has not been established.

Resources

British Archaeology Issue no 40, December 1998 —News

Maeve Kennedy, The Guardian, Thursday, December 2, 1999

British Archaeology, Issue no 51, February 2000 — News