Geoscience Reference
In-Depth Information
10
Figure 10.11 Contacts like this
one in southern Tibet may be
conspicuous enough to be traced
across diffi cult terrain using
binoculars. (Tom W. Argles, The
Open University, UK.)
10.5.3 Exposure mapping
This technique is typically more detailed than either contact or
traverse mapping, and is used to produce maps at a scale of
between 1:15,000 and 1:1000. Many landscapes are composed
of more-or-less scattered exposures of rock separated by areas
where the rock is obscured, by superfi cial deposits, vegetation,
ice, water and so on. In this environment, the geologist aims to
examine as many exposures as possible, outlining the extent of
exposed rock in pencil and fi lling in each outlined exposure
with the colour assigned to each rock unit that occurs there
(Figure 10.12). The pencil outlines should be traced over in ink
at the end of the day, because pencil will smudge or fade over
a long fi eld season. It is also a good opportunity to review the
day's work. However, exposure mapping is not simply a 'colour
by numbers' exercise! Do not be tempted to leave important
contacts or boundaries to be drawn between different units later.
You should investigate and confi rm the suspected presence of a
boundary in the fi eld where possible, and dash boundaries
where inferred. This constant re-evaluation of the structure will
help you pick up offsets and anomalies more quickly.
Exposures too small to outline can be marked with a coloured
dot and annotated with the rock type if there is space on the
map. Areas of no exposure should be labelled as such, along
with any additional information that may give clues to the
underlying rock type (e.g. 'boggy ground'; 'grass with
sinkholes'; 'thin soil with abundant fl int cobbles'; see Section
10.5.4). Once all the exposures in an area have been mapped,
the intervening blank areas on the map can be lightly coloured
in if the underlying rock unit can be inferred from the adjacent
exposures and/or other clues such as changes in topography,
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