Geoscience Reference
In-Depth Information
How accurate the measurement needs to be, or whether an
estimate is suffi cient, depends on the objective of the exercise
and the quality of the exposure. For example, if all you need is
a general description of a sandstone body it may be suffi cient
to describe it as a sandstone with beds of variable thickness
between about 10 cm and 2 m. However, if you need to sample
the sandstone or determine how the thickness of the individual
units varies laterally then it will be necessary to measure the
thickness of each of the units. Equally in most cases there is a
need to record the azimuth (direction relative to north) and the
magnitude of the vertical angle or dip to the nearest couple of
degrees rather than just the general direction. This is because of
the need to convey important information on the direction of
different processes (e.g. folding or palaeocurrents) and,
importantly, enable an accurate record of the geometry of rock
units to be calculated and recorded.
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2
3
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2.2 The hand lens and binoculars
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The hand lens is an essential piece of equipment for the detailed
observation of all rock types and fossil material. Most have a
lens with 10× magnifi cation and some contain both a 10× and a
15× or 20× lens (Figure 2.1). If your eyesight is poor, a better
quality lens will often help, especially a larger lens. It is also
possible to obtain lenses with built-in lights, which can enhance
the image considerably, e.g. Figure 2.1; lenses 2 and 3.
Figure 2.1
A variety of different
hand lenses. (1) Standard 10
×
single lens; (2) 10
×
lens with
built-in light - the lens casing
matches the focal length; (3) 8
×
lens with built-in light; (4) 10
×
and
15
×
dual lens.
To use the hand lens, ensure that you are standing fi rmly or
sitting down. Examine the specimen carefully fi rst with the
naked eye to fi nd an area where it is fresh rather than
weathered or covered in moss or lichens or algae, and also so
that you can see where there are areas of interest such as well
defi ned grains or crystals. If necessary, to ensure that when you
look through the lens you have the correct area, place your
fi nger tip or thumb tip as a marker adjacent to the area of
interest identifi ed with your naked eye. Place the lens about
0.5 cm away from your eye. Then, gradually move either the
rock if it is a hand specimen, or yourself and the lens if it is an
exposure, until the majority of the fi eld of view comes into
focus (usually about 1-4 cm away; Figure 2.2). Not all of the
rock's surface will be in focus at the same time because of its
unevenness. You will need to rotate the hand specimen or move
your position to look at different areas. In the case of some
metamorphic rocks and carbonate sedimentary deposits it is
also useful to examine a weathered surface because the minerals
or grains sometimes weather out and are often easier to see.
Figure 2.2
Photograph to show
correct use of the hand lens. Note
that the person is holding the lens
close to his eye. The lens is
fastened on a lanyard around his
neck for ease of access and use.
Binoculars can be very useful during fi eldwork. They can be
used to assess access, for instance in mountain regions.
However, their most common use is to obtain a better view of
