Geoscience Reference
In-Depth Information
However, where more diffi cult three-dimensional features
need to be recorded such as folds or sedimentary structures
there are two simple solutions depending on the feature being
recorded.
•
Draw two or three sketches of faces that are approximately
at right angles to each other to illustrate the three-
dimensional characteristics. It is important in this case to
choose the most representative faces.
•
Record the features as a small-scale map and cross-section
(for instance the anticlines and synclines* in Figure 4.8).
4
Another useful technique when sketching structures is to
ensure that the geometries are drawn at the correct angle. This
means checking the apparent angle of any dipping surfaces and
drawing them to within about 5° (see Section 4.3.1 for how to
do this with a compass-clinometer) as well as showing the
geometric relationship between the different parts correctly. For
instance in cross-stratifi ed deposits the beds generally dip at
between about 10° and 25° and never higher than 40°; the
individual layers in cross-sets do not cross-cut each other; and
the base of the beds run in tangentially to a bottom surface that
represents true bedding (e.g. Appendix A6; Figure A6.14a and
b). Fossils are often best also recorded with photographs.
Sketching has the advantage of making the geologist observe
the features, but fossils are often so complex that a photograph
is quicker and easier. If the fossil is partly obscured or has a
complex three-dimensional form (e.g. some burrows), or you
need to show the overall context, then sketching may also be
necessary to show the overall form and how the different parts
are related (see also Section 6.2.2). This ensures that you have
a record of the interpretation of how the different parts fi t
together to enable the photographs to be put into context.
Tracing the structure: A
simple technique that can
help with the recording of
more complex features is to
trace a lamination or
bedding plane through the
structure with your fi nger or
hand. This can help to give a
better sense of the geometry
and to split the structure into
its component parts.
4.3.4 Sketch maps
Sketch maps can be constructed to show areas of interest at a
high spatial resolution so that the area can be relocated at a
later date, or to remind you how the different data sets that you
collected are related laterally. They can also be used to depict
geological relationships schematically that cannot be shown on
a single sketch. Sketch maps need only to be roughly to scale;
they should, however, include an approximate scale and a
north arrow. You should include suffi cient topographical
features, such as the edge of a cliff face or buildings or the sea,
as well as the access route in order to put the specifi c
geological material into context.
*The terms anticline and syncline
imply that the folded strata are the
correct way up (i.e. younging
upwards). The terms antiform and
synform refer simply to whether the
fold closes downwards or upwards
no matter which way the younging
direction lies.
Figure 4.9 (p. 70) shows a number of examples of different
styles of location maps from fi eld notebooks. Remember that
