Geology Reference
In-Depth Information
sandy sediments. Later, during their campaign
against the German tribes, the Romans used the river
to transport supplies. Although there is no archae-
ological evidence for this, they may have dammed
some channels, so concentrating discharge into a sin-
gle channel that would then broaden and deepen and
start behaving as a meandering river.
2
Natural meandering river with modification starting
in medieval times.
Under natural conditions, the
Lippe actively meandered across the floodplain, erod-
ing into meanders and eroding avulsion channels
during floods, and the Aue consisted of several
small channels that carried discharge during floods.
Starting about 1,000 years ago, several meanders were
artificially cut to shorten the navigation route and
new towpaths built in sections of the avulsion chan-
nels. Shipbuilding started in Dorsten in the twelfth
century, and it is known that a towpath was built
next to the river at variable heights to move the
ships. The artificial cutting steepened the channel
gradient and encouraged meander incision. In the
nineteenth century, a higher water level was needed
for navigation on the river, and sediments from sec-
tions with steep embankments and natural levees
were used to narrow the channel. The result was
another bout of channel incision and the building of
a new towpath. Recently, the towpath has widened
owing to flood erosion, and the river is building a new
terrace between the higher level of the Inselterrasse
and the Weichselian Lower Terrace.
A drier and windier world
The Earth is calm at present. During periods of
the Pleistocene, and notably around the last glacial
maximum, some 18,000 years ago, it was much windier
and, in places, drier. Many aeolian features are inherited
from those windy times in the Pleistocene when episodes
of aeolian accumulation occurred in the world's drylands.
Some sand seas expanded considerably and accumulated
vast quantities of sand. Areas of expansion included the
Sahel in northern Africa, the Kalahari in southern Africa,
the Great Plains in the central USA, and large parts of
Hungary and central Poland. Grass and trees now fix
many of these inherited sand accumulations. Inherited
Pleistocene landforms include the largest desert dunes,
mega-yardangs as seen in the Tibesti region of the Sahara,
and loess deposits, some 400 m thick, that cover about
10 per cent of the land area. High winds of the Pleistocene
were also the main contributors to the large thickness of
dust in ocean floors.
How do geomorphologists distinguish ancient dune
systems from their modern counterparts? Several lines of
biological, geomorphic, and sedimentological evidence
are used to interpret the palaeoenvironments of aeo-
lian deposits (e.g. Tchakerian 1999) (Table 14.3).
Dune surface vegetation is a piece of biological evi-
dence. Geomorphic evidence includes dune form, dune
mobility, dune size, and dune dating. Sedimentological
evidence includes granulometric analysis, sedimentary
structures, grain roundness, palaeosols and carbon-
ate horizons, silt and clay particles, dune reddening,
scanning electron microscopy of quartz grain micro-
features, and aeolian dust.
By using methods of palaeoenvironmental recon-
struction and dating, reliable pictures of Pleistocene
changes in the world's drylands are emerging.
The Kalahari sand sea was once much larger, cov-
ering 2.5 million km
2
. This Mega-Kalahari sand sea
now consists mainly of linear dunes bearing vegeta-
tion interspersed with dry lakes (Thomas and Shaw
1991). Luminescence dating shows that the three chief
linear dunefields present in the Mega-Kalahari - the
northern, southern, and eastern - were active at dif-
ferent times during the late Quaternary (Stokes
et al
.
1997). In the south-western portion of the sand sea,
This example shows how difficult it can be to reconstruct
the history of river valleys, and how humans have affected
rivers for at least 2,000 years.
AEOLIAN CHANGES
'The Earth's most imposing aeolian landforms are
inherited rather than products of contemporary
processes' (Livingstone and Warren 1996, 125). Why
should this be? The answer seems to lie in the changing
windiness of the planet and in the changing distribution
of arid desert environments.
