Geoscience Reference
In-Depth Information
(a)
(b)
Hind-dunes
+
+
+
+
-
Dune-inland flux
Blow-out
Dune meadow
Sand mobility
Carbonate (shells) and salinity
Wind speed
Nutrients and
plant diversity
Fore-dunes
+
Dune slack
m
+
+
+
+
20
-
-
+
Blow-out
0
Dune-beach
flux
Beach
Fore-dune
Dune slack
Hind-dune
Meadow
Shrub
woodland
Ephemeral pond
Freshwater lens
500 - 1000m
Beach-nearshore
flux
Saltwater
incursion
Figure 16.7
The form and some environmental gradients of a coastal sand dune system, showing the direction in which
parameters increase (+) or decrease (-).
Source: Partly after Carter (1993)
Medieval storms and coastal dunes
HUMAN IMPACT
The Medieval Warm Epoch and subsequent Little Ice Age represent the principal climatic oscillation of the last
millennium (see
Chapter 9)
and the switch from one to the other was marked by 'trade mark' climatic instability and
more extreme weather events, as we experience today. Then, as now, the long British coastline was vulnerable to
rapid change through the coincidence of meteorological, eustatic and geomorphic shifts. Marine transgression,
however minor, is likely to have pushed sediment shoreward and abandoned it during the subsequent regression.
Some material was recovered in storm surges reaching beyond the range of transgression. There are many cases
of ports and farmland lost to storms, particularly on the European North Sea coast. Elsewhere onshore gales mobilized
large volumes of dry sand in the backshore (see
Chapter 17)
and formed coastal dunes.
Many events of rapid dune development and the ensanding of settlements occurred on the coast of Wales during
later medieval times. Approximate ages and rates of formation are known from radiocarbon dating, the dates of
buildings and historical records. Dune slack peats yield carbon-14 dates showing rapid northward growth of a small
dune system at Ynys Las in the mid-fifteenth century. The dunes surmount an older shingle ridge which bars the Dyfi
estuary at Borth. Llangennith Burrows, a 3 km
2
dune 'wedge' enclosing the northern end of Rhossili Bay in G
^
r
(Gower), finally ensanded a Norman church there after
AD
1200. Smaller dunes also bar Oxwich and, at Three Cliff
Bay, they ensanded Pennard castle, church and vicarage 50 m above sea level by
AD
1525. The dunes cap a shingle
storm bar, virtually blocking Pennard Pill's estuary to the sea and causing it to silt up with a tidal saltmarsh (
Plate 16.6
).
Dune formation initiated in a single storm on 6 December 1330 blocked the harbour and estuary at Aberffraw, on
Anglesey, and ensanded valuable arable land. Aberffraw was the ancestral court of the kings of Gwynedd and Princes
of Wales. The mark of their eventual conqueror, Edward I, helps us to date the impressive dune systems of northern
Cardigan Bay. Two large morfa or coastal marshes, each some 25 km
2
, flank the coast for 20 km between the
Mawddach estuary and Vale of Ffestiniog (see
Figure 17.18
).
Their seaward edge is barred by shingle ridges capped
with large dunes. Edward I's military strategy depended on a ring of seventeen castles isolating the granaries of
Anglesey from the natural Welsh fortress of Snowdonia and - crucially - therefore stocked and garrisoned by sea.
Harlech castle, built from
AD
1283 with a water gate and small harbour, like many others, was cut off from the sea
by
AD
1385 through dune formation 1 km seaward of the original rock shore (
Plate 16.7
).
Dune formation and other
coastal impacts of this stormy advent of the Little Ice Age changed the configuration and dynamics of substantial
coastal stretches of Britain and Europe, and built structures of known age help to date the geomorphic events (see
Chapter 23,
p.571). Modern tourism already threatens dune systems and their protected hinterland through trampling
and blow-out along beach access routes, which rising sea level may now exploit.
