Geoscience Reference
In-Depth Information
in an insidious loss of topsoil which causes a slow deterioration of soil potential and
crop yields. Of equal, or in some cases, greater concern are the problems produced by
increased sediments in streams and rivers. Increased turbidity and silting reduce the
quality of trout and salmon rivers, and can also lead to flooding.
Control measures are generally only taken when silting results in blocked drains
and flooding. Apart from returning the arable land to grass or replacing contour
hedges, the only ways of limiting erosion on sloping ground are by reducing run-off
over bare soil. This can be done by early drilling, by cultivating fields on the contour
rather than up and down, and by using tines behind a tractor to break up compacted
tramlines, so that water can penetrate into the soil instead of flowing over the surface.
Erosion of upland grassland and moorland has also become a problem in some
areas, such as the Dark Peak in Derbyshire, around Plynlimon in Wales, Grasmere in
the Lake District and in Teesdale in North Yorkshire. Here, the cause is the increase
in the numbers of sheep since the 1930s, which has been encouraged, especially since
the war, by agricultural policies. The number of sheep in the Peak District has in-
creased three-fold during this period. The deleterious effects of overgrazing were
noted in the Draft Structure Plan for the Peak Park in 1974 and this led to a Moorland
Erosion Study. In an analysis of the problem, R. Evans considered that erosion on
steep grassy slopes was initiated by grazing at a density of about two sheep per hec-
tare, and probably by less than this above 435 metres altitude. On the flank of Black
Tor in the Derwent catchment area of the Dark Peak, a series of scars was monitored
between 1975 and 1976 and showed an increase in bare soil of 42 per cent. Sheep
scars are now obvious on many steep slopes in the U.K.
Water and wind erosion present the obverse and inverse faces of a coin so far
as climate and topography are concerned. Wind erosion is important only in the drier
eastern counties on flat, open expanses of intensive arable agriculture. The soils most
at risk are the light sandy soils and peaty fens of East Anglia, Humberside and the
Vale of York. Here the danger periods are during dry springs, when winds of 15 mph
or more blow for several days at a time over fields with little or no crop cover.
One of the worst fenland blows in living memory occurred in 1968 - in the same
year and area that the disastrous July floods occurred which were mentioned earlier
nessed the scene a few miles away and recorded the events. Rainfall in February of
that year was about 60 per cent of the average for the area, and March had less than
half the average. On the 16th of March, the wind began to blow and continued for the
next thirteen hours at speeds of 22 mph or more, which is considered a strong breeze,
force 6 on the Beaufort scale. Drying winds blew during the following four days at
