Shingle Ridge Geomorphology

Slapton sands could be considered a deceptive name as the beach is actually made from shingle, small pieces of rock eroded by aerial and sub-aerial processes, such as wave action.These particles are then transported around by the waves and deposited adding to beachmaterial.  One key process affecting beach size and shape is longshore drift, this happens when waves hit the shore at an angle and will move sediment in a certain direction along a stretch of coastline.

Illustration of longshore drift along Start Bay

Slate is the bedrock of Start Bay and so is found in the cliffs bordering the beach, it is a metamorphic rock, changed from clays by heat and pressure, but is relatively soft and so erodes easily.  You may find large pieces of slate near the cliffs that have recently been eroded but because it breaks down quickly it is hard to spot much slate on the centre of the ridge. 

Schist is a very resistant metamorphic rock, and forms Start Point headland jutting out to sea at the south end of Start Bay.  It was formed in the Devonian around 395 million years ago.  The schist and some of the slate has seams of quartz running through it, quartz is the most abundant mineral in the Earth’s crust and you will be able to find plenty of it on the ridge.  It is a milky white.  You can find some igneous rock on the beach, most transported by rivers that have their source on Dartmoor.  Quartzite is pinkish and forms a very smooth pebble.  It can be found just offshore and was transported up onto the beach by rising sea levels.

South Devon geology - click thumbnail for larger image

Most of the shingle making up the ridge is flint; it makes up about 80% of the material on the beach.  Flint is only found about 30-40km offshore from the present day coastline which suggests that Slapton Sands originates from this area.

During the last ice age sea levels dropped dramatically as water was stored in ice sheets on the land, with the coastline much further out than present day a flint beach was formed - with flint being eroded from the sea floor by wave action.  As the ice age ended, around 10,000 years ago, sea levels began to rise as melt water from the ice sheets returned to the oceans.  The rising sea levels the flint beach landward through a process called roll-over, with beach material being moved from the front of the beach, over the crest and to the rear of the ridge and thus over time the whole beach is moved backward.  Around 3,000 years ago the ridge reached the headlands at Strete Gate and Torcross, its current position, but even now Slapton Sands is moving slowly landward.

Animation showing how the last Ice-Age has affected our coastline

Along the top of the ridge runs the A379 which is an important social and economic transport link for the area.  This helps to stabilize the ridge but is in a vulnerable position with waves reaching the road on several occasions throughout the last few years.  In January 2001 part of the road was completely undermined by a storm and so this important transport link was shut for three months whilst a new section of road is built 21 metres inland from its previous position – a coastal management strategy called managed retreat. 

Damage to the road after the storms - click on the image for a video of a news report	Photo showing the early stages of construction of the new section of road

As well as moving the road itself, several tonnes of shingle was transferred from Strete Gate, the north end of Slapton Sands, and placed in front of the realigned section of road to provide added protection.  These mounds of shingle are called bastions and are a type of soft engineering, a form of coastal management that tries to work with natural processes occurring along the coastline.  Today, there are five bastions present on the beach – three more were added in 2005 to protect the Memorial car park and more of the road.  They have to be regularly maintained as the shingle is allowed to move from the action of the waves and in November 2008 diggers were on the beach reshaping the bastions so that they remain an effective sea defence.

Diggers maintaining and rebuilding the bastions - click image for video	The newly refurbished bastions protecting the Memorial Car Park

After the events of 2001 the Slapton Line Partnership was created to consider the future of the area and provide support for the communities that will be affected if the A379 is lost.  It will not be possible for the road to be maintained indefinitely, if sections are realigned when damaged and soft engineering continues it is predicted that the road could last only 30-50 years.