Blackfriars Thameslink evaluation
Archaeological investigation by MOLA on the site of the Thameslink Programme at Blackfriars Station has revealed some new and exciting findings. Network Rail is investing £5.5 billion in the Thameslink Programme to improve rail infrastructure and rolling stock. New stations are being built at Blackfriars and London Bridge; at Blackfriars, the station will span the Thames. It will be the first new station to be built in this area for over 120 years.
Traditional archaeological trench investigation was impossible in such a heavily built-up area with soft alluvial clays buried under several metres of modern and historic rubble. Therefore, the best way to evaluate the archaeological potential of the site was with a series of deep boreholes. The boreholes were drilled down through the river bed to London Clay bedrock, retrieving the sequence of sediments. From these sediments, the past environment and topography has begun to be reconstructed.
The evaluation was carried out by MOLA’s geoarchaeology department. Drilling was monitored and the sediment cores were brought back to the lab where they were opened up. The sediments were then described, interpreted and sampled. Microscopic faunal remains, pottery and building material were processed and assessed by specialists to identify the age of the deposits and their characteristics.
The building work at Blackfriars provides a rare opportunity to investigate the movement of the river and its tributaries over time in this important area. On the north bank, the Fleet joins the Thames. On the south, another tributary exits between two low islands. Because it spans the river, the construction project enables comparison of the evidence for past landscape change in the City and in Southwark. Information from the latest and previous boreholes has been used to form plans of the lay of the land in early prehistoric times. This is the template upon which sediment subsequently built up.
Results so far confirm that the north and south banks differ significantly in character. On the north bank, bedrock and gravel rise steeply to form Ludgate Hill, with a deep depression in the base of the river bed. This hollow was probably gouged or scoured out at the end of the last Ice Age and further eroded in prehistoric and Roman times, perhaps by the eastward migration of the Fleet. The hollow filled up with gravel during the Roman period, perhaps as a result of lower river levels at this time. Similar features on the north-bank river bed have been dated to very early prehistoric times (early Mesolithic), so the late date of the Blackfriars scour was quite a surprise. Roman sediments are tentatively identified on both sides of the river. They are thought to have been deposited within channels at the mouth of the tributaries of the Fleet on the north side, and possibly the Neckinger on the south.
In the past, the river was much wider than in the present day. However, the north-bank boreholes must lie on the river side of the medieval and early post-medieval waterfronts (of the 14th and 16th centuries), as the artefacts retrieved during archaeological investigation were dominated by material dating to the 17th century or later. It is possible that the Tudor city wall, waterfront and Roman shoreline may survive in the northern part of the site. The upper layers within the sediment cores comprised dumps of post-medieval refuse and construction debris interspersed with episodes of floodplain alluviation. These naturally deposited silts appear to have been laid down within a body of perhaps slow-flowing freshwater, with the river occasionally rising above the waterfront during floods or high tides.
In contrast to the north bank, the landscape on the south side is low-lying and characterised by a series of sandy islands. The south arch of Blackfriars Bridge appears to lie within a Thames tributary channel that runs between two islands now buried beneath alluvium and modern debris. This channel could be part of the now extinct Neckinger River. No datable material was recovered from the south-bank boreholes, but the sediments retrieved may correspond to prehistoric activity known to have taken place on the island to the east. This island (known as the Hopton Street ‘eyot’) would have been attractive, fertile, fairly dry land within the floodplain that in prehistoric times (several thousand years ago in the Neolithic and Bronze Ages) was ploughed using an implement called an ard.
MOLA’s geoarchaeological work at Blackfriars has helped to increase understanding of local environmental conditions and to refine interpretations of the character of the deposits. This will lead to greater understanding of the age, depth and likely state of preservation of archaeology on the site and contribute new knowledge about the evolution of the prehistoric Thames floodplain in central London.