Geoscience Reference
In-Depth Information
volcanic ash layer, a porphyritic sill, a calc-silicate layer or a
sedimentary bed with characteristic ironstone concretions.
Recognizing and recording these marker layers is invaluable for
understanding the stratigraphy and structure of any region. It is
also important to take into account localized variation and to
group or split units appropriately.
10
10.4.3 Sketch cross-sections
For a geologist, tracing strata such as marker layers across
the land surface leads naturally on to questioning the
subsurface structure. It is often useful to sketch out rough
cross-sections based on the data you collect while mapping,
especially in areas where there are distinct topographic
features (Figure 4.3d, p. 60) or folding or faulting has affected
the rocks (Figure 4.8, p. 69). Fieldwork is frequently conducted
along more-or-less linear traverses (Section 10.5.1), and
if these cut across the strike of the strata at a high angle,
cross-sections can readily be sketched from the data
collected.
Section 10.6.2 details how to construct an accurate cross-
section, but a sketch section can be as rough as you like - it
serves simply to help you develop possible interpretations of
the geological structure or test different hypotheses. A sketch
section could be simply a quick drawing of how one stratum
might be folded across an area, or it may involve rather more,
as in the following description. First, pick a notional line on
the map that cuts at right angles across the dominant strike of
the strata as the line of section. In your notebook, or on a
separate piece of paper, draw a rough representation of the
topography along this notional line. Then quickly mark on the
locations of the main boundaries between geological units as
short lines dipping below the topographic surface, taking the
dip angles from the nearest measurements to the line of
section.
Start extending the strata boundaries below the topographic
surface. This will give you a fi rst impression of the relative
thicknesses of the different units. To continue sketching the
section, you may need to consider:
matching the surface dip measurements to the structures at
depth;
erecting hypotheses to explain repetitions of strata (i.e.
folding, faulting);
potential changes in the thicknesses of strata;
any alternative structural confi gurations that might fi t the
recorded data.
