HOW TO MAP

GEOLOGICAL MAPPING

“ A geologist must remember that accurate geological maps are the basis
of all geological work, even laboratory work, for it is pointless to make a
detailed investigation of a specimen whose provenance is uncertain” Barnes Basic geological mapping.


Your indispensable guides – Geological
Society handbooks


Where will you be mapping?-1
• Semester 2 - Easter: one week
residential course (Leader
Prof Eelco Rohling, March 31
April 7
• A series of exercises
involving mapping, logging
and construction geological
cross-sections from field data.
The locality (Tenby, South
Wales) will also provide an
introduction to a wide range
of rock types and a large
variety of field relationships,
sedimentary and tectonic
structures.


Where will you be mapping?-2
•Summer term weekend May 11 – 14 (Brittany

residential field course)
•Basic field skills but in an area of more complex
igneous and metamorphic geology


Where will you be mapping?-3
• SOES 2012 (M Geol, Geology Single
Honours, Geology with...., MGeoph,GS).
• First summer vacation: residential field
course at Ingleton (Leader Dr D Teagle, First week Summer vacation)
• This course provides basic training in field
mapping and involves the production of a
geological map of the Ingleton area.


Where will you be mapping?-4
• SOES 2022 (M Geol, Geology Single Honours,
Geology with....and Geoscience pathway for
OESS).
• Easter vacation: residential field course, Almeria:
leader Heiko Pälike
• This course provides an opportunity to map, log and
interpret a variety of different rock associations
helping to build a comprehensive experience of field
geology. The focus is provided by detailed study of
some well exposed examples of geological processes.
It also affords an introduction to fieldwork in an
overseas environment.

Where will you be mapping?-5
SOES 3025/3027
(M Geol, Geology Single Honours, Geology
with....).
• Year 2 Summer term (after semester 2 examinations): one
week course (Anglesey: leader Dr Dorrik Stow) This
completes the core component of fieldwork training. It
focuses upon the challenges of mapping in an area of more
complex geology where emphasis is placed upon the
collection and presentation of data and putting into practice
methods introduced in lectures and the laboratory.
•

NB OC/GY and OESS oceanography pathway students
take SOES 3018 (Plymouth). Geophysical Sciences
students take SOES 3021, Geophysical field methods


“ There is no substitute for the geological map and section - absolutely
none. There never was and there never will be. The basic geology must
come first - and if it is wrong, everything that follows will probably be
wrong” - Wallace (1975) Mining Engineering, 27, 34-36.


Geological mapping: an ability to:
•observe and record field data
selectively and systematically for the
construction of a geological map at a
given scale;
•compile a field note book containing an

accurate record of all field data,
including field sketches and
photographs;
•compile field sheets representing a
visual summary, using accepted
conventions, of all the mapping data
recorded in the field notebook;
•construct an accurate and well drafted
interpretative geological map using
conventional map symbols;
•construct a representative geological
section across the mapped area
(Geological Society of London)


LOCATE

A fundamental skill is to be able to correctly position yourself on your
base map


LOCATE – HOW?
•Check grid orientation on map
•Check compass, set declination
•Triangulate (use compass to
take bearings on three prominent
features). Aim for small (< 1mm)
triangle of error.
•Pace and bearing. Pace out
distance from known feature

marked on map (use compass to
take bearing on feature and mark
faintly on map so line can easily
be removed)
•GPS -widely used in industry:
new course SOES 2023 next year


LOCATE

On a 1:10 000 map, 1 cm on the map represents 100 m in the field. You
should be able to pinpoint yourself to within 10 m in the field, so when
you translate this to the map, the margin for error is 1 mm
Remember, when mapping at 1:10 000 scale, millimetre accuracy is
expected


OBSERVE

Spend some time looking at the exposure. What is the rock type? –
Mineral composition? Grain size? Texture? What structure can I see? –
Bedding? Tectonic fabric? Sedimentary structures?


OBSERVE
Observations are recorded in two
ways;
•On the field map (field sheet or slip)
•In the field notebook (information
which cannot be accommodated on

the field map – see ppt on Blackboard
- geological field work).
• Map data is also normally recorded
in the field notebook using a locality
number reference system on the
map and a grid reference in the
field notebook
•Thus you can navigate between the
two.


ON THE MAP

•All exposures ringed by a solid line, colour coded by formation
with an abreviated lithological descriptor e.g. f.gr.rd. sstn (= fine
grained red sandstone
•Information about what can seen (can be pictorial)
•All structural data, unless there is too much to fit it all on the
map, in which case, representative structural data


You can put all sorts of things on the map, e.g. where you took a
photograph or collected a sample


Ringed areas of exposure should be moderately coloured; surrounding
unexposed areas should be shaded very lightly with the same colour.


RECORDING EXPOSURES

Map A: good features
•Represents exposure shape on
the map as accurately as possible,
bearing in mind the scale
•Accurate placement of boundary
due to accurate recording of
observations
•‘V’ effect in valley where
boundary is inferred
•Map B: poor features
•Exposures too blobby and not to
scale
•Boundary poorly placed due to
above features
•‘V’ effect in valley not
interpreted


MAPPING TECHNIQUES
Traversing
•Walking along a predetermined route
plotting in the geology on the way
•The geology in between traverses is
interpolated, using whatever clues can be
gleaned from the topographic map
•This method is used predominantly for
reconnaissance work
•Large areas of ground can be covered

MAPPING TECHNIQUES
‘Green line’ mapping
•Exposures ringed by green lines using
a very fine (0.1 mm) nib size
•ALL exposures are portrayed. There
should be evidence on the map that all
the ground has been covered
•When all exposures have been
mapped, boundaries between the units
can be drawn on the map
•The classic technique for 1: 10 000
mapping

Barnes: Plate 1


MAPPING TECHNIQUES
Boundary mapping
•One of the main aims in compiling a
geological map
•Locate a boundary and follow it
across the map
•One of the easiest methods of
mapping where exposure is good
•Very effective in conjunction with
aerial photographs
• Does not work well if exposure is
poor

STRUCTURAL
DATA

Use compass to transfer reading directly onto the map, e.g. strike
direction
Draw appropriate symbol on map in black ink with fine pen at point of
measurement
Indicate dip (plunge) amounts in figures on map. Record compete data
in notebook
Use red ink for faults if possible


MAPPING IN POORLY EXPOSED AREAS

You need to look for and record on the map indirect evidence, e.g.
•Soil colour, spoil from man-made holes, fence posts, animal burrows
•Float (loose rocks or stones which may reflect the underlying bedrock) but be wary
of fluvially transported material
• Vegetation can reflect underlying bedrock e.g. limestones – beech, juniper


MAPPING IN POORLY EXPOSED AREAS
Feature mapping
•Map topographic features that may
indicate underlying differences in
the geology
•Use them to help locate
boundaries in unexposed areas
•Map convex and concave breaks
of slope

Convex break of slope,
tick on downhill side

Concave break of
slope, tick on uphill
side


FEATURE MAPPING

Break of slope used to help locate position of fault (top
left)