Geoscience Reference
In-Depth Information
10.5.2 Contact mapping
One important objective of geological mapping is to trace out
the contacts between different geological units. In many cases,
careful examination of aerial photographs or satellite images
(including online sources) prior to (or during) mapping may
pay rich dividends, since these may pick up subtler
differences in soil type and vegetation than can be seen
while on the ground. Contacts may be rapidly traceable
across rugged terrain, and even under superfi cial deposits, in
this way. Because contacts typically juxtapose rock units that
have different properties (hardness, permeability, composition),
there are often landscape clues to their presence (Section
10.5.4).
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All that is required thereafter is to 'ground-truth' (Worked
Example 10.1) your interpretation by visiting selected
exposures at intervals along (or as close as possible to) the
contact identifi ed from the aerial photograph or satellite image.
For a well-exposed contact you may simply be able to follow it,
marking it on the map as you go. Exposures either side of the
contact may contain clues to its proximity (e.g. minor faulting,
sedimentological variations, contact metamorphic effects;
abundant xenoliths). The fi eld map in Figure 10.7a (p. 219)
shows a contact mapped in this way between granite (red) and
volcanic rocks (green), its position constrained by exposures
marked on either side.
This technique is generally suited to mapping at scales of
between 1:50,000 and 1:15,000, but it can also be used for very
detailed mapping of small areas. In areas of good exposure, this
method can build up a geological map quickly, especially if the
structure is reasonably simple. The technique can be
compromised by diffi cult terrain, poor exposure or complex
geology. However, in poorly exposed terrain, contacts may still
be traced by combining information from the few exposures
with other information (Section 10.5.4): landscape features,
drainage and mapping of the drift deposits that obscure the
contact. Obvious contacts may be traced further using
binoculars, especially in rugged landscapes with extensive
exposure (Figure 10.11, p. 226).
To map a contact, mark it as a solid line on the fi eld map
where it is exposed, and as a dashed line where inferred
(e.g. under vegetation, superfi cial deposits, ice, water, etc.;
see Figure 10.7a). Confi rm which rock units lie adjacent to
the contact (these may change along a major fault), and use
colour to show their outcrop. Shade actual
exposures
of rock
more heavily than areas where rock units obscured by
superfi cial deposits or vegetation are inferred (i.e. their
outcrop).
