When most visitors think of Holkham they think of its
unspoilt beach and stunning sand dunes. The sand dunes are a key feature of the
area and this has led to many people asking me, how they were formed.
There are approximately 11,897 ha of sand dunes in England
with 1,200 ha here in Norfolk.
Dune grassland and dune slacks support a wide variety of
plants and are rich in invertebrates. The dune system at Holkham is protected
by a number of conservation designations, such as SSSI and SAC. This is the
reason why horses and bikes are not allowed access through the dunes.
Along with the beach, the dunes are one of Holkham's
most famous features.
The sand dunes at Holkham were formed because there is a
large beach plain, so plenty of sand. The surface dries out between high tides,
and then dry sand is blown landwards by strong onshore winds and deposited along
a shingle ridge above the high water mark. At Holkham the dunes are unusual as
they are rather alkaline due to the amount of shells in the area.
Embryo Dunes are
the first stage. There is very little vegetation and sand is only just starting
to accumulate.
Lots of bare sand and
little vegetation.
Foredunes are part
of the sand dune system on the side nearest to the sea. They are comprised of
wind blown sand and scattered plants, such as sand couch-grass and sea rocket.
As embryo dunes build up, the surface is raised so as to be out of reach of all
but the highest tides.
Large hillocks of Marram grass.
Yellow dunes are
yellow in colour because of a lack of soil. They are dominated by marram grass.
Marram grass is tall, robust, flexible in the wind and very effective at
trapping sand. Their binding rhizomes help stabilise the sand dunes and enable
other species to grow, such as sea sandwort. The dunes grow to around 5m.
Vegetation has become varied and dense.
Grey dunes are
grey in colour because of the presence of soil. As dead leaves from the marram
grass breaks down so plant nutrients are released into the soil improving the
conditions for plant growth. Due to an increasing distance from the sea
vegetation diversity increases, though the plants are still highly specialised.
Marram grass is still abundant but other species are able to colonise the area,
such as red fescue and bird’s foot trefoil. Grey dunes grow to between 8 – 10 m
high.
Core samples have shown that the Holkham dunes are
200 years old!
Dune Slacks occur
when the surface of the sand dune is eroded down to the water table and wet
sand is exposed. A secondary dune slack can have the appearance of a bomb
crater! Dune slacks can be extremely species rich. As well as rare orchid
species there are a number of uncommon species of butterfly, dragonfly and the
very rare vulnerable natterjack toad.
Dunes slacks are rare and unique habitats.
Through careful sympathetic
management the warden team aim to ensure that there is a succession of dune
slacks at different stages of re-colonisation, thereby providing a habitat for
rare flora and fauna, to control the invasion of non-native species and to
protect the sand dunes from erosion by recreational use.
When you next walk along the dunes at Holkham National Nature
Reserve why not try and identify these stages. The coast is always changing so
there is always something to see!
Happy Hunting
Jonathan Holt
Warden
Warden