Geology Site Account

Deeply buried geological feature. Of educational interest only.

-----------------------------------------------------------------

Site description

When the first Blackwall Tunnel was completed in 1897 it was the longest underwater tunnel in the world and a remarkable engineering achievement, not least because of the unusual geology of the site. Beneath the drift deposits of sand and gravel dating from the Ice Age the surface of the bedrock in this part of London is usually fairly level with no variations more than about 5 metres (15 feet) in height. However, the survey for the tunnel revealed a deep drift-filled depression at Blackwall extending through the bedrock of London Clay, Woolwich Beds and Thanet Sand, right down at least 60 metres (180 feet) into the Chalk. This presented real difficulties to the tunnellers and the tunnel took six years to build. The second Blackwall Tunnel, constructed in the 1960s downstream of the first, encountered similar difficulties.

The two tunnels had passed through a buried, elongated, basin-like feature with almost vertical cliff-like walls and completely filled with sand and gravel. It was one of many such depressions that have since been found to be present beneath central London between Battersea and Canning Town. These depressions were originally thought to have been formed by the scouring action of glacial meltwater but this does not explain their great depth and the fact that there is usually bulging of the strata at their base. Research published in 1979 finally came up with a most unexpected theory for their formation which is today accepted as the answer to the puzzle.

The depressions are now considered to be the scars left behind by the melting of pingos at the end of the last glacial stage. Pingos are massive bodies of ground-ice that grow by drawing water from unfrozen ground beneath the permafrost, the influx of water from below accounting for the bulging of the underlying strata. It is thought that at the end of the glacial stage, melting of the pingo resulted in the collapse of overlying deposits into the hole left by the melting ice. The sudden release of water previously confined in the Chalk aquifer below would have deepened the hollow and accounts for the presence of large chalk blocks that were found in the gravel some 15 metres (45 feet) above the Chalk strata.

Pingos exist in Arctic areas today such as north-west Canada, giving us a vivid reminder of conditions that existed along the Thames valley during the most recent glacial stage, one of the coldest periods of the Ice Age.

A section through the bedrock beneath the Thames at the site of the Blackwall Tunnel. The deep depression, now filled with gravel and sand, is thought to be the remains of a pingo. Illustration: British Geological Survey.