Geoscience Reference
In-Depth Information
N
S
Apparent dips
A
B
61
49
51
60
48
25
400
200
0
400
200
0
48
61
50
51
26
60
BRL
BO
c
HL
c
BRL
PoB
CdL
BO
CO
LSh
HL
LSh
CDL
CDL
(e)
line of section must be corrected to lower values,
because in effect the cross-section cuts these strata
obliquely, and their dips will hence be apparent dips on
the cross-section. The more oblique the strike to the
section line, the lower the apparent dip will be.
(Appendix 10, Figure A10.1 shows a graph you can
use to determine apparent dips.) Draw short lines for
the dip data at the correct angle just above the profile
initially, as a guide.
10
Transfer the position of these features directly onto the
topographic profile on the graph paper. Dip measure-
ments can also be transferred onto the section. If there
are none on the line of section, data lying short
distances away from the section line can be projected
at right angles onto the line. However, only where
strata strike at exactly 90° to the section line can their
dip angles be drawn directly onto the cross-section.
Dips of strata striking at more oblique angles to the
Burrington
Coombe
Black
Down
Cheddar
Gorge
Axial
plane
N
S
A
B
Height
above
OD
400
200
0
400
200
0
PoB
LSh
BO
BRL
BO
BRL
LSh
c
HL
(no vertical exaggeration)
KEY
Hotwells Limestone Fm
HL
Cheddar Limestone Mbr
CdL
'chinastones' (calcite-mudstone)
Burrington Oolite Subgp (Lst)
c
BO
Clifton Down Limestone Fm
Cheddar Oolite Mbr (Lst)
CDL
CO
Black Rock Limestone Subgp
BRL
LSh
PoB
Lower Limestone Shale (Lst, mst)
Portishead Fm (Sst)
(f)
draw these parallel to the first horizon. In some cases,
you may know or be able to determine the throw on
faults, and their attitude; you may also be able to
define fold structures. Generally, you will be able to
extend your section further below the surface in folded
areas; indicate uncertainty with question marks.
Finally, add a key to unit colours and other symbols.
Choose an important marker horizon or stratigraphic
boundary, and draw this across the section, matching
its dip to the dip measurements where possible, and
interpolating between the different points on intersec-
tion with the topography. With this guide to the overall
structure, add other unit boundaries in the same way;
unless you have evidence for varying strata thickness,
Figure 10.19
Stages in drawing a geological cross-section. (Map for b and d: Ordnance Survey 1:25,000
Scale Raster © Crown Copyright 2009. An Ordnance Survey/EDINA supplied service. Geological Map Data ©
NERC 2009.)
Colour the outcrops of geological units as continuous blocks,
even where inferred. Use colours thoughtfully: for example,
choose pale colours for widespread units and stronger colours
for those with limited outcrop (e.g. dykes, thin strata). Colour
on a computer has the advantage of being standardized (e.g.
using CMYK values). For hardcopy maps, various media
can be employed (e.g. watercolours, coloured pencils).
Every unit should be identifi ed by a code, in many cases an
abbreviation of the unit name (e.g. MMG for Mercia
