1- How have geomorphological processes shaped the coastal landforms at Porlock Bay?

shingle ridge at Porlock

The five kilometre long shingle and pebble barrier ridge at Porlock Bay which borders the Bristol Channel in the county of Somerset in the south west of the United Kingdom (see location maps Resource 1, Resource 2, Resource 3 and Resource 4 ) is the longest continuous shingle and pebble barrier system on the western coast of Britain. The area of land known as Porlock Marsh extends inland from the shingle and pebble ridge, which separates it from the sea, as far as the five metre contour north of Porlock village.  Porlock shingle and pebble ridge was formed as sea levels rose as a result of a warming climate and associated melting ice sheets approximately 6000 years ago, following the end of the last major glacial epoch about 12000 years before present.  The shingle and pebbles were eroded from head deposits, composed of a clay matrix with a wide range of sizes of angular rocks, which covered the sea cliffs to the west of Porlock Bay such as those at Porlock Ford cliff shown in Resource 5.   The eroded pebbles and shingle were then transported eastwards and deposited through the process of longshore drift, first as a drift aligned spit which gradually extended out across Porlock Bay and finally as a complete barrier with enclosed freshwater marsh and meadows behind, fed by a number of streams and small rivers.  Orford (2003 p 4) describes how at Porlock Bay there is ‘a net easterly beach drift system powered by Atlantic swell waves and depression-generated storm waves running from the north west to west into the Severn estuary’. Most of the coarse shingle and pebbles which form the barrier are today in the 5 cm – 20 cm diameter size range.

Consolidate Your thinking 

  • Look carefully at the two photographs of the pebble and shingle barrier at Porlock Marsh in Resource 6 and Resource 7 and align them with the Ordnance Survey map in Resource 4.  What would you estimate the height of the coastal barrier to be?
  • On a copy of the picture of Porlock Bay in Resource 8 and referring to the Ordnance Survey map in Resource 4 annotate the picture with the following labels:
  • The settlements of Porlock Weir, West Porlock, Porlock and Bossington
  • Mark on the route of the South West Coast Path
  • Bossington Farm
  • Bossington Beach and Porlock Beach
  • Hurlstone Point
  • Porlock Bay
  • Bossington Hill
  • Sparkhayes Lane
  • The route of the 5 metre contour line
  • Shade in the area enclosed by the 5 metre contour line between Porlockford and Bossington and label as Porlock Marsh
  • The photograph in Resource 9 is of the cliffs at Gore Point to the west of Porlock Bay.  Explain using labelled diagrams to assist, the erosional processes that would have led to cliff collapse and the exposure of pebbles and shingle here, when sea levels rose at the end of the last ice age.
  • Using a copy of the outline map of Porlock Bay in Resource 10 draw annotated diagrams to explain how depositional processes would have created first a pebble and shingle spit and then finally a barrier which eventually enclosed Porlock Bay.

Go to Question 2  - How did the Porlock pebble ridge alter the morphology of the coast?