Geoscience Reference
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bank and wrecking the nearby buildings, and these are the sorts of local events that do
result in change. Despite the excitement, individual changes of this sort are small and
can usually be regarded as local modifications. However, over time, the accumulated
effects of many such modifications can cause whole landscapes to change.
Size and time clearly both play key parts here. The collapsed cliff or eroded river
bank will probably be tens to hundreds of metres long at most, while the larger land-
scape features picked out in this topic are tens or even hundreds of kilometres across.
Noting the length scales involved in this way is an important way of keeping such dif-
ferences clearly in mind.
Moreover, while local events such as the destruction of landforms or buildings
may be immediately newsworthy, more long-term patterns of change in the natural
scenery are rarely apparent during the life spans of people, and even during the hun-
dreds of years of written records. So it becomes necessary to use indirect and circum-
stantial evidence - to play the detective - to find out what long-term changes have been
going on.
An important step in thinking about the natural landscape is to look at it in terms
of modifications to complex surfaces defined by the ground. On land, we tend to be
most aware of erosional processes removing material, but it is important to realise that
the material removed has to end up somewhere - and this will involve its deposition
later, on land or in the sea. How much material was removed from the cliff during
the storm or from the banks of the river during the flood? Where did the lost materi-
al go, and how did it change the landscape when it was deposited at its new destina-
tion? Knowledge of these surface modifications can provide a yardstick that allows us
to compare different sorts of changes happening over different periods of time and at
different scales, and can help us to work out their relative importance, quoting amounts
and rates. For example, a flooding river may remove a hundred metres of river bank,
modifying the local landscape a little in the process. However, this modification is
unlikely to have much impact on the scenery, unless followed many, many times by
similar modifications, over centuries to hundreds of thousands of years. In this way a
series of such floods can erode and move material that, in the long run, may be of suf-
ficient volume to significantly change the landscape, for example lowering a hill slope
or filling a valley bottom.
The majority of- but not all - surface modification processes act to reduce or
flatten topography, mainly by eroding the higher features but also by filling in lower
ground with sediment. So logically landscapes might always be regarded as tending
towards a flat surface. Our understanding of the processes involved suggests that any
land area with mountains or hills will be eroded downwards to an increasingly flat sur-
face as time passes, although the rate of erosion will reduce as the topography becomes
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