The evidence of massive tree-sized foundations cut across an Early Bronze Age 'burnt mound' in Monmouth continues to make archaeological headlines.

Monmouth archaeologists, who are excavating the site which lies on the shores of a prehistoric lake, believe they have made another extraordinary discovery – the remains of a second, almost identical but unrelated set of foundations – this time on what was once the bottom of the lost lake.

The mound, which consists of stones and pebbles burned in a fire and used to boil water, has produced a radiocarbon date of 3,630 years before 'the present' (before 1950) and the archaeologists have also found Early Bronze Age pottery – sherds probably representing different pots – which had been decorated with diamond and herringbone stabbing.

The flints found include a Bronze Age 'thumb scraper' made from a New Stone Age polished flint axe, which was thousands of years older.

The wooden structures themselves had rotted away thousands of years ago but the foundations were preserved when anaerobic clay filled the voids left by what are believed to be huge timbers.

There were three exactly parallel round-bottomed cuts running across the mound; these resemble the sides of trees – one of them is a metre wide – and all extend over the mound for at least 50 feet before continuing into the western section of the excavation.

The envisaged timbers were not set very deeply into the ground – presumably the shallow cuts were sufficient to prevent any sideways movement of the structure.

Although there is radiocarbon dating from charcoal taken from the burnt mound, the foundation slots themselves have only produced struck flints which were imported from outside Wales during prehistoric times.

Therefore, the foundations themselves have no date except that they must have been dug into the mound at some time after 1,680BC – the approximate date of the burnt mound.

The archaeologists say it need not have been a great deal later.

There is no sign of a rise in the ground level between the creation of the mound and the digging of the slots, whereas, after the timbers had rotted, a metre of clay accumulated up to the present topsoil.

Also, there is no sign in the sections that the slots have been cut from a higher level – which would be obvious if they had been, and if the slots had been truncated by ploughing, the burnt stones and the clay in the slots would have been mixed.

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They are both undisturbed, with clean sharp edges, and appear to have remained unchanged since the site was abandoned. So, although it is speculative, the foundations may not be all that younger than the burnt mound.

The latest excitement has followed excavations to remove the peat which was laid down after the ancient lake had begun to silt up.

All lakes silt up and change from lake to swamp to marsh to bog and eventually to dry land.

It now seems that a second massive foundation had existed before the lake formed or before it had risen to inundate the area.

This may have been during the great decline of the weather towards the end of the Bronze Age and into the Iron Age.

The archaeologists' interpretation of the evidence is that two massive parallel timbers, like those on the burnt mound, had been shallowly set in the ground before they were covered by the lake.

While under the lake, Potamogeton pondweeds produced white lime marl which gathered in a thick layer on each side of the round timbers – retaining the rounded profile of trees.

It appears that by the time the timber had decayed, the lake had reached the second or third stage of silting up and this meant that without the pondweed the production of the lime marl had been replaced by the production of peat.

The peat, while preserving the shape of the tree, had filled the void and then covered all the remains – reaching a metre in thickness nearby.

Scientific estimates for the time taken for the peat to attain that thickness (at c0.5mm per year) is around 2,000 years.

Wetland specialists and other archaeologists in Britain and on the Continent have been expressing great interest in the discoveries, mostly admitting that they do not know what the foundations represent and that they have never seen any so large.

However, the original suggestion that they are the foundations of a narrow longhouse is not excluded.

Members of Monmouth Archaeology are excavating the site for Charles Church Wales, the developers.