The Jennycliff Wall Project

Summary

Jennycliff Bay from the North (SHIPS Project)

On the seabed at the southern end of Jennycliff Bay on the east side of Plymouth Sound is an unusual rock feature. This wall of boulders is 570m long, lying between 160 and 200m from the eastern shore and roughly parallel to it. The origins of the feature are unknown; initial investigations by divers show that the visible parts of this feature have a structure similar to that of a dry stone wall, which suggests that the wall may possibly be man-made.

The aim of this project is to determine the extents, nature, construction and purpose of the rock built feature in Jennycliff Bay.

The site has been visited by local sports divers for many years but the wall itself has not ever been investigated as far as we are aware.  The wall was first seen by one of the SHIPS team in 2006 during an ROV survey in the area and was initially thought to be the remains of a paleo-river channel from the Tamar/Plym which formed thousands of years ago when the sea level was considerably lower.

The Jennycliff wall is sited on and partly buried within the seabed.  The wall appears as a ‘step’ in the seabed with the seabed on the western side lower than the seabed on the eastern side. The wall ‘step’ has an average height of 1.5 metres over most of its length but it disappears into the seabed at both the north and south end of the structure.  At the northern end of the wall the lower, western seabed is 7.4m below datum and the shallower eastern seabed is 6.4m.  At the southern end the deeper western seabed is 10.0m below datum and the shallower side is at 8.5m so the wall slopes slightly downwards along its length.

Location of the Jennycliff Wall (SHIPS Project)

The visible part of the wall is made of natural limestone and siltstone boulders and small stones that are readily available in the area.  It is likely that the wall extends below the seabed on the western side as sediment has accumulated in Jennycliff Bay since the Breakwater was built. How much of the structure is buried beneath the seabed is as yet unknown but probing the seabed at the base of the wall suggests that some of it does lie buried below the existing seabed level.

The seabed on the shore or eastern side of the wall is shallower than the western side with the sediment on the shore side reaching the top of the wall.  The sonar surveys show the top edge of the wall is slightly higher than the seabed on the shallow side forming a small lip, suggesting that the eastern side seabed has in-filled behind the wall, rather than this rock structure having been carved out from the existing seabed.

There is no evidence that suggests the rock that forms the wall was quarried based on the samples recovered so far. The visible seabed on the eastern side of the wall is made up of clay and sand with small boulders while the seabed on the western side is fine sand and shell.

Multibeam sonar bathymetry data is available for the whole of Plymouth Sound and this wall appears to be a unique feature in the local area. Researchnhas been done to try and find similar features in other places but nothing of this particular design has been found.

A mooring buoy for large naval vessels called Anchorage F lies to the west of the wall and a circular area around the mooring is kept at a maintained minimum depth of 8.6m by dredging.  The dredged area runs right up to the wall on its western side so it is possible that some other features associated with the wall have been removed by dredging and what remains is only part of the original structure.

The wall may be a very unusual, natural feature but it is not obvious how this could have been formed. If the wall is man made then we do not yet know what the wall was for or why it was built, it could be the remains of a large fish trap, a harbour, a shore defence or something else.

3D View of the wall from the south-west (Swathe Services)

Detail view of the Jennycliff Wall (Swathe Services)

Investigations in 2012

One of the angular rocks that form the wall (SHIPS Project)

The SHIPS Project team undertook reconnaisance dives on the wall in the summer of 2012.  General survey observations and rock samples recovered from the wall showed that most of the blocks are angular and not rounded in appearance as would be expected from rocks subject to erosion by a river.

The wall is also extremely uniform in appearance with the same slope and width along most of its length.  The blocks appear to be of a limited range of sizes with very few small rocks and none of the larger blocks appear to be bigger than could be carried by one or two men.  This led the team to conclude that the wall may be man-made rather than being the creation of natural fluvial or marine processes. 

In September 2012 a high resolution multibeam sonar survey of the wall was completed by Swathe Services Ltd. for the SHIPS Project using an R2Sonic 2024 sonar.  This survey produced a 3D model of the wall that could be viewed using Fledermaus software and also produced high resolution GeoTIFF images that could be incorporated into a Geographic Information System (GIS).

Also in September 2012 a very high resolution side scan sonar survey of the southern end of the wall was done for the SHIPS Project by Sonardyne International Ltd. using their new Solstice side scan sonar.  This survey produced an extremely high quality sonar image of the wall and surrounding seabed.

In November 2012 a sub-bottom profiler survey was undertaken on the site using a GeoPulse Plus SBP loaned to the project by Kongsberg GeoAcoustics.  Multiple runs were completed across the wall and were tied in to longer survey runs across the Sound to identify the buried paleo-river channels.

Project Aims and Objectives

The aim of the project is to determine the extents, nature, construction and purpose of the rock built wall feature in Jennycliff Bay.

The SHIPS team propose to excavate a number of small trenches around the wall to:

  1. Confirm the depth of burial below the current seabed surface on the deeper, western side of the wall
  2. Define the shape of the wall on the shallower eastern side
  3. Investigate the northern and southern extents of the wall where they disappear into the seabed
  4. Recover sediment samples from the trenches in an attempt to date the construction of the wall
  5. Run a training course in underwater excavation for the SHIPS Project team and as a Nautical Archaeology Society Part III Advanced course
  6. Use the project as a means to promote awareness of the submerged cultural heritage of Plymouth within the community, nationally and internationally

Project Partners

Our thanks go to the partners and supporters of the project:

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