Physical Resources / 121Water flowing across the surface carries particles in suspension, which never touch the ground,
particles that slide or roll over the surface, and particles that are repeatedly lifted and dropped. Some
of the water finds its way into natural depressions and flows along them. This widens and deepens
them, forming ‘rills’. Rills are small enough to be removed by ordinary cultivation, so they are
temporary, but unless they are removed they may grow much larger, into ‘gullies’. These are more
difficult to remove, because ordinary farm implements cannot cross them.
Routine cultivation destroys rills and ensures that gully erosion is rare on cultivated land, but on land
that is never ploughed it can be serious. Tracks used by vehicles can turn into streams in wet weather,
and from streams into badly eroded gullies. Paths much used by walkers suffer in the same way. A
vicious spiral develops, in which the worn-down path becomes wet, walkers avoid the mud by walking
to the side, and the gully widens as well as deepens. Some years ago in England the southern end of
the Pennine Way national trail had to be rerouted, so severe was the erosion on Kinder Scout.
Two steps are involved in the removal of soil: the detachment of particles, by rain splash or wind;
and their transport by water or wind. The remedy lies in minimizing both.
When rain falls on to vegetation, or wind blows across ground covered by vegetation, particle
detachment is greatly reduced. The raindrops and wind are dispersed and their energy dissipated
as leaves absorb the shock and rebound, like springs. For arable farmers this may be more difficult
than it sounds; land is usually bare between the time of sowing and the emergence of the crop.
Nevertheless, some sort of cover is valuable on vulnerable soil. In some places this is achieved by
planting crops in alternate strips, such as grain and grass, or by leaving stubble lying on the
surface after harvesting. In areas with a temperate climate, such as Britain, cereals are sown in
autumn, as soon as possible after the completion of the previous harvest. This allows the seed to
germinate and provide a vegetation cover through the winter, and a crop that starts growing rapidly
in spring. It minimizes erosion, but is possible only where winter temperatures do not fall so low
as to kill the young plants. In regions with a more extreme climate, cereals must be sown in spring
and the soil must remain bare through the winter, although in this case erosion is reduced by the
freezing of the surface or by a covering of snow.
Contour ploughing, in which the plough follows the contours of the land, produces parallel furrows
oriented at right angles to the slope. Soil eroded from the plough ridges is trapped in lower furrows.
In a field ploughed up and down the hill, with furrows parallel to the slope, soil can be swept
downslope in furrows that quickly become rills.
Where slopes are steeper and the land is farmed intensively, terraces may be desirable. There are
many types and Figure 3.15 shows cross-sections of two of the most common. Broad-base terraces
are made by cutting trenches not quite at right angles to the slope and at intervals down it, using the
excavated soil to build ridges on the downslope sides. Soil being washed down the hillside is held
before it has travelled very far and the gently sloping trenches carry the water into drains or ditches.
This avoids the situation in which soil erodes from the top of a field and accumulates at the bottom,
making the soil quality uneven.
Bench terraces require more drastic engineering, because the hillside must be converted into a series
of level strips, like a staircase, excavated soil being used to construct a bank along the downslope
edge of each terrace. The technique is very effective, and is widely used in the tropics, but because
the terraces must follow the contours they can have irregular shapes that make machine cultivation
awkward.
Surplus water can be removed by ‘grass waterways’. These are wide strips running down the
slope and sown to grass. In effect, they are controlled gullies and can follow the routes of actual