Geoscience Reference
In-Depth Information
your Jacob staff above the point where you started (Figure
2.15).
5. An entire slope can be measured this way by cumulatively
moving up the slope to the point where the last
measurement was taken. Beds and boundaries between the
sighting points will need to be interpolated.
For particular situations it
may be necessary to
improvise a scale: e.g. mark
a thin, straight piece of wood
in black and white stripes
(Figure 2.13b) or, for
measuring inaccessible cliffs,
paint a length of rope and
then take photographs while
the rope is hanging over the
cliff.
If you have a fi eld partner you can use a pole or folding rule of
known length as a crude Jacob staff when measuring up a
slope or a large thickness of dipping strata. Hold the pole
roughly perpendicular to the strike of the strata to be measured.
With your fi eld partner standing about 5 m away, get them to
fi rst, by eye, tell you how to alter the angle of the pole so that
it is perpendicular to the strike. Then, the person not holding
the rule can obtain a good estimate of the stratal thickness that
is equivalent to the length of the pole by extending an
imaginary line by eye perpendicular from the top of the staff to
the strata.
2
2.6 Classifi cation and colour charts
Various well-established comparison charts can be used to
provide a semi-quantitative description of the rock and any
changes in it. These include grain-size charts and rock
classifi cation diagrams. The fi gures commonly used are included
in the appendices at the end of this topic but charts such as
those for grain size and texture can also be purchased for use in
the fi eld. The grain size chart should be used by placing the edge
of the card on top of a clean fresh surface of rock and comparing
the grain size on the chart to that of the rock until a match is
found for the average grain size and if appropriate the maximum
and minimum size (Figure 2.16). If the grain size is small it
might also be necessary to use a hand lens on the card and rock.
If the rock is poorly consolidated, scatter a few representative
grains across the grain-size images to determine the average
size.
Weak hydrochloric acid can be used to test for carbonate. A
fresh surface of the rock free from any coating minerals and
weathered coating must be obtained before dropping acid on
the rock. The acid will fi zz strongly with fresh calcium
carbonate but much less strongly for dolomite (calcium-
magnesium carbonate). Freshly powdered dolomite will fi zz
more readily. Health and safety precautions for the handling of
acid should be followed. In sedimentary rocks an easier and
less destructive method of determining whether there is
carbonate is to test the hardness of the rock. The most common
colourless minerals in sedimentary rocks are quartz and calcite
(with feldspar less frequently). Quartz will scratch steel
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