Geoscience Reference
In-Depth Information
Holocene development of the
British coastline
APPLICATIONS
Imagine for a moment north-west Europe, lying at one of Earth's most sensitive atmosphere-ocean-ice sheet
interactive points, without amelioration by the Gulf Stream. At this latitude our environment would resemble
Labrador, with ice-bound seas for several months each year, a cold and arid atmosphere, more seasonal river regimes
and a subarctic ecosystem. Modern Britons would be true Europeans, able to walk to mainland Europe as our Late
Palaeolithic ancestors could until 8.6 ka
BP
with lower sea levels. Europe itself could probably not have been the
birthplace of great civilizations. This was the scene for most of the past 115 ka, since the last or
Eemian
temperate
(interglacial) stage. Sea level was over 100 m lower than today and the North Sea area was an extension of the north
European plain. Permafrost and tundra conditions prevailed, with intermittent episodes of Early Devensian mountain
glaciation. A late Devensian ice sheet, covering most of Britain and Scandinavia, coincided with worldwide glaciation
to drive sea levels down to - 130 m at the Last Glacial Maximum, 20-18 ka
BP
.
Thereafter, global warming and ice melt ushered in the Flandrian temperate stage. Global sea levels were restored
to 0-3 m above their present level by the Mid-Holocene
hypsithermal
or climatic optimum, c. 5 ka
BP
. Britain's
continental shelf shrank progressively and the coastline became far more indented. Orkney and Shetland became
islands at c. 13 ka
BP
and land bridges with Ireland and the outer Hebrides were drowned by 12 ka
BP
. The Loch
Lomond Stadial ice readvance checked further insularization until after 10 ka
BP
, when the Inner Hebrides, Anglesey
and the Isle of Wight were isolated. The low coastal plain connecting the Thames-Rhine estuary as far north as
Yorkshire and Sussex-Flanders (northern France) was finally breached by the Flandrian transgression c. 8·6 ka
BP
,
which completed the isolation of the British Isles (
Figure 17.16
).
Subsequent minor fluctuations may seem insignificant compared with the overall rise of some 130 m. However, the
last areas to flood - including the Wash, the inner Severn estuary and the Solway Firth and Morecambe Bay fringe
of Lancashire - became the first areas reclaimed naturally during the minor regression (- 1-4 m) which accompanied
cooling after 5 ka
BP
. Extensive peat formation in enclosed muddy estuaries, especially the Somerset and Gwent
levels (Severn estuary) and Fens (East Anglia), records their subsequent environmental and human history. Similar
minor climatic oscillations, such as the
Medieval Warm Epoch
and
Little Ice Age
, between c.
AD
800-1300 and
AD
1350-1850 respectively, caused major socio-economic changes in Europe and altered sea level by ± 1-2 m. The
extent and timing of changes were neither uniform nor synchronous - just as forecast for the twenty-first century.
Sea level first rose by about 1 m and then fell by 1-2 m as European temperatures fluctuated by ± 1·5C. These
changes are small by comparison with the Pleistocene-Holocene transition but were enough to create substantial
problems for coastline and hinterland management for our medieval ancestors (see box on p. 397). This provides a
warning for the twenty-first century (see box on p. 709).
fluxes. They may contain several components at any one
time (
Figure 17.17
).
The turbulent nearshore environ-
ment segregates sand from gravel which sustain gentler
(2-8
bars
develop in the nearshore and offshore zone and
dunes
develop inland of the backshore with good sand
supply and dominant onshore winds. Barriers form at the
coastline or offshore as barrier islands, either by the
creation of tidal passes through barriers or by accretion
in the sheltered back-barrier zone behind an offshore bar.
Offshore systems can migrate onshore, initially trapping
tidal lagoons behind
barrier islands
or
spits
open at one
end, and eventually merging with the coastline. Biogenic
reefs of coral or shells complete the suite of barriers.
Beach morphology is subject to constant short-term
change in response to waves, tides, winds and sediment
) respectively. These
profiles and the different permeability of sand and gravel
also interact with wave style to create local variations in
wave dissipation and morphology. On a larger scale, the
swash from spilling breakers constructs a steep-faced
berm
near the high-water mark which survives all but the most
destructive storm waves. Winter beach erosion alternates
with summer reconstruction on many mid-latitude
beaches in response to seasonal storminess, although
) and steeper slopes (10-20
