Geoscience Reference
In-Depth Information
scientifi c papers and maps on the regional geology, web-based
satellite image display systems such as Google Earth provide an
easy way of investigating the general lie of the land and making
preliminary observations on a large spatial scale and in some
cases in more detail. A desktop regional study may be used to
gather information on: access to the fi eld locality and within it;
the overall topography; the type and location of potential
exposures; the general structure and strike of the beds.
3
3.2.2 Whole exposure
Arriving at a large exposure for the fi rst time can be both
exciting and daunting. It is often diffi cult to know where to
start so, check whether the exposure is safe (Section 2.11) and
then walk around and view it from different angles before
making a decision.
One way to tackle a large exposure with different rock types
and features is to fi rst of all divide the exposure into 'units'
based upon obvious features such as changes in colour and
weathering characteristics. In most instances you are looking
for somewhere between about 2 and 10 units (Figure 3.1), any
more than this and it is hard to assimilate. If you have more
than about 10 units you should try to group them together.
Decide from a distance what the main characteristics of these
units are and consider making a sketch to show these
characteristics and their relationship to each other (Section 4.3)
before completing more detailed observations close up.
Your preliminary observations of the large-scale features in the
exposure should include the following.
•
Nature of the contacts:
Note whether the contacts are
gradational (e.g. colour change between units 2 and 3 and
within unit 2, Figure 3.1d) or sharp (e.g. contact between
units 1 and 2, Figures 3.1a and 3.1c) and whether they are
planar (e.g. Figures 3.1a and 3.1c) or irregular (e.g. contact
between units 2 and 3, Figure 3.1b and between units 1 and
2, Figure 3.1d). In the case of irregular contacts note the
geometry.
•
Lateral changes in thickness:
Note the position and amount
of any changes. These might relate to processes such as
Figure 3.1
Four photographs and accompanying line drawings of
different exposures showing how they might be divided into major units
for recording and further examination. (a) Jurassic strata faulted against
Triassic strata at Blue Anchor, Somerset, UK (height of cliff
c
. 10 m ).
(b) Carboniferous strata at Bowden Dors, Northumberland, UK. (c) Eocene
strata exposed in the Clarence Valley, South Island, New Zealand.
(d) Cenozoic strata at Choirokitia gorge, Cyprus. (a-d: Angela L. Coe,
The Open University, UK.)
