Sands of Time Title
This site provides detailed information on the sand dunes of the Sefton coast in North West England
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Coastal Change
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Dune Slacks
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Dune Slacks
 

Most of the dune system is characterised by very dry conditions. In large dune systems such as the Sefton Coast, however, there are a number of dune slacks, wet and sometimes very large depressions in the dunes. They are formed in two ways.

Primary dune slacks are formed on rapidly accreting coasts where the top of the beach is cut off from the sea by a dune ridge forming further out on the beach. This is occurring on the Sefton Coast at Birkdale, a feature known locally as the green beach. Primary dune slacks are long and narrow and parallel to the coast.

 
The Green Beach at Birkdale
The green beach at Birkdale. A new dune ridge (left) is cutting off
a low section of the beach (right) that might eventually
form a primary dune slack.
 

Secondary dune slacks result from a blowout, where erosion down to the water table has occurred. If the erosion is extensive, a large flat area of wet sand is exposed. Most of the dune slacks on the Sefton Coast are secondary dune slacks and they are orientated at right angles to the coast, parallel to the prevailing wind direction.

 
Erosion to the water table to form dune slack
Erosion to the water table to form a
secondary dune slack
 

The dune water table fluctuates annually because of the differences in rainfall and evapo-transpiration between the summer and winter months. Because of this, many dune slacks are flooded during the winter period.

 
Dune slack at Ainsdale in winter
Dune slack at Ainsdale during the winter
 

The wet sand created by secondary dune slack formation is colonised by plants and a succession occurs. This succession on wet sand, however, is very different to that on the dry dunes. A range of wetland plants are important and early vegetation can be extremely species-rich with plants such as

  • Sedges Carex spp Bog pimpernel Anagallis tenella Variegated horsetail Equisetum variegatum Early Marsh Orchid Dactylorhiza incarnata Marsh Pennywort Hydrocotyle vulgaris
  • Water mint Mentha aquatica

together with a number of mosses and liverworts

 
Dune slack vegetation in the early stages of succession
Dune slack vegetation at the early stages of succession
 

Creeping Willow Salix repens, is an important species of the dune slack habitat and begins to outcompete these plants when it gets established in the later stages of dune slack succession. In this situation the vegetation can consist of only two major species, Creeping Willow and the moss, Calliergon cuspidatum.

 
Dune
Old, wet, dune slack vegetation dominated by Creeping Willow
(here with no leaves in winter) and the moss Calliergon cuspidatum
 

Older dune slacks may become drier and the later stages of succession may lead scrub invasion and eventually woodland. Grazing will maintain the diversity of more specialised dune slack plants which are found at the earlier stages of succession.

 

 

For more information about this project email dunes@hope.ac.uk at Liverpool Hope University.
  Go to the site of Liverpool Hope University    

Liverpool Hope University worked with English Nature and the
Sefton Coast Partnership to implement the Sands of Time project.