Archaeology and Sea Level Change on the River Thames

Jane Sidell. Environmental archaeologist Recent Lecture

Jane Sidell began by pointing out that the Thames has been central to London for thousands of years. People have used the river not only for practical but also spiritual activities. Studies of sea level change can indicate not only past areas of occupation, but also what might happen in the future.

Over the 10,000 years since the end of the last ice age land now free of the weight of ice has been rising. Even 6000 years ago shorelines looked nothing like they do today. Scotland has risen significantly, with some former shorelines now 40 metres above present sea level. The south of the country has been sinking, but not as fast as was originally thought. Over the last 6,500 years the whole course of the Thames through the London area has shifted northward. Today, the Thames Barrier has to be raised with increasing frequency and trends are studied to try to predict future levels. Just a three metre flood would cover the Thames floodplain.

Work such as that by Bruce Watson and Gus Milne based on Roman quays has helped to fill the gaps in information left by 1970’s studies of sea levels. The height of quays indicates mean high water at Spring tides, and with the timber accurately dated with dendrochronology, river level can be tied to a date.

The river can also be studied laterally. Deposits on adjacent archaeological sites can reveal when they were covered by the river and when terrestrial. At Richmond half lock, the mouth of the Wandle and Battersea Power Station there are Mesolithic deposits, and perhaps some data will be obtained from the work at Syon Park. Boreholes like those sunk during the Jubilee Line construction can give information, but where there is deep sediment - 35 metres at Canvey Island – it is hard to obtain results. A study of small water creatures can indicate brackish or marine conditions.

Into the Neolithic the rate of sea level rise slowed down and there was massive expansion of wetlands which covered the Thames floodplain from Essex to Westminster to a width of four kilometres, creating semi-terrestrial conditions on which animals could be grazed in summer. Substantial wood peat developed. Buildings appear for the first time on the dry ground. In the Bronze Age there is evidence of the wetlands being exploited: timber trackways were perhaps used for moving livestock, and at Southwark ard marks indicate the sand islands there were being cultivated – the base from a wooden ard has been found; other sites all date to around 1500 BC. There can be little doubt that objects from the Bronze and Iron Ages found in the river, swords, rapiers, axes, were ritually deposited; expensive, some obviously never used, they may be an example of conspicuous consumption, showing the owners could afford to throw them away.

In the Iron Age evidence in the form of skulls suggests the river may have been used for burial. After this time river levels started rising again, the river swamped the floodplain and the field systems could no longer be cultivated. In the Roman period London was a very active port with substantial quays. The height of the quays indicates that the river level seems to have dropped in this period, but later on water levels started rising again. Unfortunately there is a gap in the data in the Saxon period. At present levels are going up 2 mm a year, but embanking of the river has an effect.

Present work on the river is throwing up many questions. Many new sites are becoming available but there is a need to synthesise results. The tidal reach is getting larger so is lower at low water, leading to erosion of sites but causing more to be revealed. There is a massive stretch of the river to be investigated. Jane urged us to get out our wellies and get down there.

Yvonne Masson