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MAPS
Thus it represents a part of the earth surface as seen when looking down from a great height.
There are different types of maps - viz: topographic maps, geologic maps, economic geological
TOPOGRAPHIC MAP
This shows the diversity of landform by means of contours. It shows the ground or earth features
like the coast lines, rivers, hills, vegetation, cities, highways, dams, bridges, etc. It also shows the
relief of the mapped area by indicating the range of elevation, i.e. height above sea level for
several points in the area. Geologist usually use topographic maps as base maps, i.e. field-guide
1. Orientation: Maps are conventionally oriented in a way that the top is the North, the bottom is
the South, the right is the East and the left is the West.
North
West East
South
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2. Latitudes or Parallels: The equator is the line mid-way between the North and the South pole.
It is perpendicular to the polar axis. The equator bisects the earth into 2 equal halves. Latitude
denotes the angular distance North or South of this midline. The equator is labelled the zero
degree latitude, and it is the line of origin from which other latitudes are measured.
N N
45oN 45oN
45o 45o
o
Equator (0 ) 50o 60o
50oS
60oS
S S
3. Longitudes or Meridians: The longitude crosses latitude at right angles, and the longitudes
merge at the poles. The prime meridian, or zero meridian, runs through Greenwich in England,
and it is the reference for longitude measurements. It can be measured in any direction, i.e.
Eastward or Westward.
N
000o
180o
S
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4. Scale: It gives the relationship between the size of the map and the actual size of the area
represented on the map, i.e. the relationship between the drawn object on paper, which is the
map, and the actual physical object on the ground. The scale can be stated in 3 different ways:
a) Verbal or Statement Scale: a statement scale in the form of an equation, e.g. 1cm = 1km; or
This means that one unit of anything on the map is equal to one million units on the ground. This
c) Graphic or Bar Scale: a bar marked off to show scaled distances in metres, kilometres or any
(i) the primary part which is to the right of the zero. This part normally displays the whole
numbers;
(ii) the secondary part which is to the left of zero. This usually shows the fractions.
A graphic scale is always included on the map. If a map is to be reduced or enlarged, only the
5. Direction:- This is a straight line on the map or ground measured from a standardized common
base line called the zero direction. The north is regarded as line of zero direction. direction can be
a) Azimuths: are directions expressed as angles measured clockwise from the north throughout
the full range of the directional circle. The South is 180o, East is 090o, Southwest is 225o. Three
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W 270o E 090o
SW 225o
S 180o
b) Bearings: are measured within each of the 4 quadrants of the directional circle. They are
always measured as angles from the South or North, e.g. azimuth 090 is N90oE, Southeast is
Base Line or Base Directions: The North is the generally accepted base direction for both
(i) True North: This is the direction to the North pole, represented on the map by the meridians. It
(ii) Magnetic North: This is the direction established by the needle of the compass. It doesn't
coincide with the true North. The angular difference between the two is called the Magnetic
Declination (M.D.). For practical field mapping, the magnetic compass is used. Therefore, the
measurements made are for the magnetic North as base direction. Corrections have to be made
The earth's magnetic field usually causes the magnetic declination. The magnetic north does not
T.N.
M.N.
It shifts gradually every year. Every large-scale or regional map will always give information on
the magnetic declination of the area at the time of making the map. It will also give the predicted
annual change in that area. For example, M.D. annual change of -0.1 implies a movement of
magnetic North towards the true North at a rate of 0.1 degree per year. M.D. annual change of
+0.1 implies a movement of the magnetic North away from the true North (i.e. movement of
(iii) Grid North: This is the direction of the North-South grid lines on a map. It is a North line
6. Topographic Contour Lines: A contour line connects points of equal altitude above or below
the sea level or ordinance datum (O.D.). The vertical distance between two contour lines is called
the contour interval, and it is the difference in height between any one contour and the next one.
The contour lines are drawn with reference to the datum plane or zero contour. There are two
(a) Form Lines: These are the thinner lines and do not have any value printed on them.
(b) Strengthened Contours: These are thicker lines and they bear printed values. They are more
Form Line
Generally, the contour lines on a map show the distribution of the high and low grounds. Contour
2) All contour lines close somewhere, although it may be outside the map at hand.
3) Contour lines never cross each other. On a vertical cliff, several contour lines may become
7) Contours bend upstream in valleys and cross the streams at right angles.
9) An isolated close contour has the same elevation as the next adjacent contour.
10) All points inside a depression contour are lower than that line of the depression.
EXERCISE: The map below contains point elevations measured in an area. Contour line 400m
O.D. has already been sketched out for you. Using contour interval of 50m, draw the remaining
contours. Later on, you will draw the topographic profile from A to B.
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Topographical map forms the base for the compilation of geological maps. In topographical
- The distribution of the contour lines is an expression of the foundation geology of the area, i.e.
- If the regional pattern of the contour lines is wavy and sharp, then the strata are horizontal.
- Resistant beds are expressed by parallel ridges while valleys are dissected by streams.
- If the dip of a bedding plane is less than 40o, its direction may be determined from the spacing
of the contours. Closely spaced contours represent an eroded scarp face while the widely spaced
contours represent the dip slope, i.e. the plane along the direction of the dip of the bed.
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TOPOGRAPHIC PROFILE
The topographic profile is a single line representation of a surface, i.e. it is a trace showing the up
It should be ensured that this line passes through the important features needed to be shown on
the profile.
- Make a short mark on the paper for each of the contour lines and all the points of interest that
fall on the profile line. - Indicate the elevation of the contour lines as they are marked.
- Label other features that are marked, e.g. major roads, rivers, mountains, plateau, etc.
4) Layout a suitable vertical and horizontal scale. The horizontal scale used for the profile is
usually the same as the scale of the map. At times, the scale along the vertical direction could be
changed, i.e. the scale may be enlarged or exaggerated along the vertical.
However, from experience, it is strongly recommended here that the scale of the map should
always be used along the vertical axis. Any exaggeration is not necessary for geological studies.
5) Project each contour line up to its proper elevation along the vertical scale and mark that point
- Give a title, show the scales that have been used along the horizontal and vertical axes.
- Indicate directions of the end points of the profile, i.e. whether it is North or South, etc
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EXERCISE:
The topographical map below has two lines of section. Cross-section from A to B is drawn at the
GEOLOGICAL MAPS
A geological map is a graphic picture of the sum total of data accumulated on the field. It is a
representation on a plane surface, at an established scale, of the physical features of a part or the
whole of the earth's surface from any desired surface or subsurface data, by means of signs and
symbols.
A solid geologic map shows the distribution of the various rock units as they occur at the bedrock
surface. The map shows the bedrock surface irrespective of whether the surface is covered with
Geologic maps usually illustrate geological features such as rock units (folded, horizontal, etc)
and bedding, faults, folds, intrusions, unconformities, river traces, etc. Geologic map can be
Formation:- The fundamental unit of the geologic map is the formation. A formation is a
unit. A formation may consist of one rock type e.g. sandstone, or a mixture of two e.g. sandy
On a geologic map, a formation is designated by a distinctive colour and symbol. The symbol
may be a number or letter. The boundary between any two geologic formations is called
Outcrop:- The term outcrop refers to a point on the ground surface where bedrock is exposed
Legend:- The legend of a geologic map is a listing of the formations which occur within the
mapped area. For each formation, the legend indicates the distinctive colour and symbol, the
nature and the type of rock. For sedimentary rocks, the legend is arranged in orderly sequence
with the oldest formation at the bottom, the younger ones above and the youngest at the top.
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If igneous and metamorphic rocks occur within the mapped area, they are often arranged in a
separate legend. Some conventional signs and symbols used on geological maps are:
Conglomerate Clay
Marl Coal
Volcanics Porphyries
(Basalt, Andesite, etc) (Pegmatite, etc)
Granite Breccia
Dyke rocks
(Dolerite dyke, etc)
Quartzite Slate
Alluvium Fossil locality Borehole
050
Attitude:- dip & strike
18 ` ` dip trend
32
Major anticlines:-
22
or 5
25
horizontal plunging
Major synclines:-
or 8
horizontal plunging
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Minor folds:-
15
horizontal plunging
Lineations or trends:-
10
horizontal plunging
Fault:-
Normal Reverse
Geological boundary:-
GEOMETRY OF OUTCROPS
The determination of geometric forms of outcrops enables the definition of the overall shape and
orientation of a rock body or formation. The outcrop surface is considered in terms of a plane and
the parameters measurable on the plane are used to characterise the whole formation.
The Plane:- A plane is a general term for a 2-dimensional geologic form that is without
curvature. Ideally, it is a flat or level surface. Examples of geologic planes are bedding planes,
Natural rock surfaces are supposed to be planar. In the determination of the orientation of rocks,
only the natural planar surfaces are searched for, and it is only on them that measurements are
made. The planar structure most frequently encountered in sedimentary outcrops is the bedding
plane. Other planar structures usually encountered in crystalline rocks are cleavage, foliation,
Some of the terms employed in defining the orientation of planes of outcrops in space are
ATTITUDE:- The attitude of a plane in space is determined by the values of its dip and strike.
DIP:- is the angle at which any plane is inclined away from the horizontal. It is at right angle to
the strike and is measured in a vertical plane. The true dip is the direction of maximum
C
A.D. T.D.
angle of true dip
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The diagram above shows a rock bed inclined at an angle BÂC to the horizontal AC. The angle
BÂC, which is the maximum inclination to the horizontal, is the angle of full or true dip. With
the exception of the direction at right angle to the direction of true dip, which is called the strike
of the bed, every other direction across the bed has some dip.
The dip in any direction other than the true dip is called an apparent dip. Apparent dips are
always smaller than the true dip. There is no dip along the strike of beds. The closer the direction
of apparent dip approaches the direction of true dip, the nearer will the angle of apparent dip
approach the angle of true dip. Also, the nearer the direction of apparent dip approaches the
Whether apparent or true, dip ranges between 00o and 90o. It is measured on the field with a
clinometer.
STRIKE:- is the direction of a horizontal line on an inclined plane. Dip and strike directions are
perpendicular to each other. Strike ranges from 000o to 180o. It is measured on the field with a
compass.
INCLINATION:- is a general term for the vertical angle between the horizontal and an inclined
plane or line.
- If the boundary of a rock bed or outcrop crosses a contour line at two different points, such
points can be joined by a straight line and the straight line is a strike line or stratum contour on
- Where the same rock boundary cuts another contour line at two different points, another strike
- As many strike lines as possible can be drawn in this way using the same rock boundary. Such
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- The direction of strike can then be measured using the base direction of the map.
- With at least two strike lines drawn, the dip or gradient of the beds can be calculated. The dip or
gradient is the change in value of the vertical distance with respect to the horizontal distance, i.e.
- The dip is calculated by measuring the perpendicular distance and the elevation difference
between two chosen strike lines on the same bedding surface using the following equation:
- The difference in elevation is the contour interval between the two strike lines being used, while
the horizontal distance is the distance measured on the map between the two strike lines
- The dip direction is the direction towards which the surface elevation falls, and should be
In the geological map shown above, six beds numbered P, Q, R, S, T, & U are shown. Strike
lines 200 & 300 are drawn for boundary between formations Q & R. The dip direction for this
plane is also shown in between the strike lines. Read the strike direction and calculate the dip
amount for this plane. Draw the strike lines for other boundaries, read their strike directions and
- In the figure above, the sandstone bed boundary cuts the contours each at a point on the map.
- The boundary of the rock bed is assumed to be a plane surface, hence there is an even gradient
between A and B. Between these two points, the surface falls 300 metres.
- As the contour interval is 100 metres, we divide line A-B into three equal parts. Points C' and
D' will then have elevations 400m and 500m O.D. respectively.
- If D is joined to D' and extended while C is joined to C' and also extended, two strike lines will
be obtained.
- Other strike lines parallel to these two may be drawn across the map to complete the stratum
contours.
- The strike direction, dip value and dip direction can then be got as in the first method.
Problems involving the use of information from borehole are called 'three-point problems'.
- In the figure above, let the boreholes be at A, B, C, and at topographic heights of 500m, 675m
- Suppose a sandstone bed lies at the surface at A and at depths of 675m and 320m in boreholes
B and C, respectively.
- The elevation of the sandstone relative to the sea level will then occur as follows:-
- This means that the surface of the rock bed falls 300m from A to C, 500m from A to B and
200m from C to B.
- If we divide line A-B into 5 equal parts; - points A', A'', A''', and B' will have elevation 400m,
- If line A-C is divided into 3 equal parts, points C' and C'' will have elevation 300m and 400m
O.D. respectively.
- From similar reason, it can be seen that B'' lies at elevation 100m on line B-C.
- Considering the above, points on same elevation can be joined to obtain strike lines on the
A' will join to C'', A'' to C', A''' to C and B' to B''.
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EXERCISES:
Map A:-
Draw in the strike lines. Determine the dip and strike directions. Determine the amount of dip.
Map B:-
Draw in the strike lines. Determine the dip and strike directions. Determine the amount of dip.
GEOLOGICAL SECTIONS
A geological cross section is an aid of visualizing the structures portrayed on the 2-dimensional
(ii) filling in the structural data. These are added to the topography and extrapolated into the
underground.
- The line of section is chosen to show the required structural features to best advantage.
- In general, the line should cut across the strike direction, as close to the perpendicular as
possible. This is the section that provides the greatest structural details. It is also the direction
that one should try to follow on the field during geological field mapping exercises.
This is as follows:
- Mark accurately, the various contacts and attitudes of planes, layers, etc, on the profile paper.
- These data are then projected onto the section using the values of dip angles and dip directions.
- It should be noted that where the geological boundaries on maps are almost parallel to the
contour lines, the beds are horizontal or almost so, i.e. with little or no dip.
- Accurate extrapolation of these data downward into the underground where observed data are
lacking is a complex process. The understanding of this can only come with thorough
blank.
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(iii) The section should always be accompanied by a legend explaining any symbol or colour
carried on it.
- the section should be labelled, indicating the geographical co-ordinates and a title of the area.
- the section line should be drawn on the geologic map to assist the reader.
A simple geologic map is drawn below, with its section shown from X to Y. Study the section
Below is a simple geologic map. Draw the section from C to D, and also from X to Y.
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Instead of using the mathematical instruments to project the rock boundaries using the angles of
dip, the strike lines can be plotted directly. However, this does not give accurate boundaries as in
the projection method but it is nonetheless a quick method of viewing the general orientation of
the beds.
- For every boundary, plot out the elevation of all the strike lines that cut it along the line of
section.
- Connect the marked points, producing the line down below the drawn profile.
1) The mode of succession of the rock units can be determined from the dips of the beds as
shown in the section. Generally, older beds dip towards younger ones.
Normally, sedimentary deposits are approximately horizontal at the time of deposition in quiet
water, with the youngest bed at topmost and oldest at bottommost parts. However, in most
localities, earth movements during and after deposition of rocks cause inclination of strata so
that they outcrop at the surface in succession, one after another, with the older beds dipping
1) Mathematical Method:-
This involves the application of simple rules of trigonometry to determine the true thickness of
the rock bed. There are three possible approaches based on the relationship between the
orientation of the bed and the slope of the land surface. Three different maps, each having bed
with unique relationship to the topography, will be used for this exercise.
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ON MAP 1:
The map here shows a bed of rock which dips in the opposite direction to the slope of the land
DA = direction along the dip between the bottom and top of the rock bed.
AE = difference in elevation between D and A; i.e. the contour interval between D and A.
CB = ABcosθ
ON MAP 2:
The beds here dip in the same direction as the slope of the ground surface.
AD = direction along the dip between the bottom and top of the rock bed.
AB = difference in elevation between D and A; i.e. the contour interval between D and A.
AE = ACcosθ
ON MAP 3:
The rock bed here dips in the same direction as the slope of the ground surface. Slope of the
ground surface is gentle here. The orientation of the bed is as presented below:
AB = direction along the dip between the bottom and top of bed.
DB = difference in elevation between A and B; i.e. the contour interval between A and B.
θ = angle of true dip of the bed, i.e. maximum inclination of bed from the horizontal.
EC = CBcosθ
With respect to the techniques discussed above, there are eight possible orientations of beds
In all the three cases treated above, it can be seen that the thickness could be obtained graphically
- No exaggeration of any axis should be made, i.e. only the scale of map should be used on both
There is a big danger in this method when the scale of the map is such that 1mm on the map may
mean several metres on the ground. For example, if a map has a scale 1/50,000 i.e. 1mm on the
map to 50 metres on the ground and the thickness of a coal seam (which may not be more than 1
to 8 metres on the ground) is to be calculated. A pencil trace may be up to 2mm thick, and an
error of this magnitude on the paper when taking ruler measurements may not be uncommon.
This level of error will give you an error margin of up to 100 metres on the ground !. Therefore,
your coal seam which is far less than 10 metres thick in reality may be measured as being 100 or
more metres thick. This will amount to a serious blunder which any serious-minded investor will
never forgive. It will be the case of a cockroach for a horse and can cost you your job.
In mapping an area, it is usual to observe or encounter only a few outcrops of rock boundaries.
Positions of the complete boundaries are calculated from these when plotted on map.
This method for completion of outcrop is only possible where the series of beds are conformable
so that the strike and dip are uniform and the same for all the beds.
Procedure:- Look for any of the outcrop boundaries that cuts a contour line at 2 points. If there is
any, join those 2 points with a strike line. Look for any other contour line where you can draw
In the map provided above, the sandstone/sandy shale boundary can be used for this.
- The strike lines establish the strike as east to west and the dip due South.
- Since the beds are conformable, i.e. with uniform strike and dip, the strike lines will be evenly
- Other strike lines of equal intervals can be drawn parallel to the 2 strike lines already drawn.
- The beds are assumed to be whole number multiples of one hundred metres in thickness, so the
strike lines for the sandstone/sandy shale contact will also be strike line (though of different
- For example, the strike line (at the topmost) for the conglomerate/sandstone contact is 500m but
- The outcrops of the various contacts between the beds can be drawn where the strike lines, with
their values appropriate to the particular surface, intersect contours of like value.
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- Problems involving the use of information from boreholes, or from the subsurface generally, are
- If the height of a bed is known at 3 or more points, it is possible to find the direction of strike
- The principle has many applications to mining, opencast, and borehole problems encountered
- The height of a bed may be known at points where it outcrops or its height may be calculated
- If the height is known at three or more points, only one possible solution exists as to the
EXERCISE 1:-
The map below shows part of the outcrop of a coal seam. Calculate the dip and strike. Complete
the outcrop. At what depth would the seam be encountered in a borehole sunk at point M ?.
Procedure:
- The height of the seam is known at 3 points, A, B, C, where it crosses the 700m, 800m and
- Join A to C and bisect at D. Point D will lie on the seam at a height of 800m.
- Since B is a point in the seam at the same height as D, D is joined to B to form the 800m strike
line.
- Draw the 700m and 900m strike lines parallel to D-B through A and C, respectively. If these
lines are carefully drawn, the spacing between them should be equal.
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- Draw further strike lines of equal intervals parallel to the three strike lines already drawn.
- Draw enough strike lines to cover the whole map. All lines should extend through the whole
- Now, trace out the position of outcrop of the coal seam;: the guiding principle is: where ever the
strike line of the seam crosses or intersects a contour line of equal value, the seam will outcrop at
that point. We can therefore find a number of outcrop positions or intersections on the map, and
CAUTION: An outcrop trace must not touch any strike line or contour line except at appropriate
intersection only.
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Note:- The map portrays a coal seam and an average seam is of the order of 2 metres or less in
thickness. On the scale of 1cm = 1,000m, its thickness is such that it can be represented
=> To determine the depth at which the seam will be encountered in a borehole sunk at point M,
a line that passes through point M and parallel to the seam’s strike lines should be drawn.
=> Using the average strike line interval and spacing, the value of this strike line M should be
calculated relative to the other strike lines. The value of this strike line M is the elevation (O.D)
=> Looking at the map, the ground surface elevation at point M was about 575m O.D. The
difference between this surface elevation and the strike line M value obtained is equal to the
EXERCISE 2:-
The map below shows 3 boreholes A, B & C. The geological log of each borehole is as follows:
A B C
MARL 50 - -
LIMESTONE 300 - -
CONGLOMERATE ? ? ?
Determine dip and strike of the beds. Draw the geological map and construct the section. At what
Part I:-
- Looking at the 3 borehole logs supplied, five rock formations were encountered at borehole A.
Out of these five, the thicknesses of the first 4 formations were given. Thickness of the last
formation, i.e. conglomerate, was unknown because the borehole, probably, did not fully
- The two uppermost units in A were absent in borehole B. Only the last 3 formations were
present in this borehole. Thickness of the last formation was unknown here, meaning that this
- Comparing the logs of A & B, the shale unit in A was 200m thick while it was 100m thick in B.
But this shale was encountered at deep depth in A while it was found on the surface in B. It can
then be reasoned that the shale must have been affected by surface erosion and transportation at
site B thereby reducing its natural thickness, while it must have been protected by marl and
limestone beds on top of it at site A. Therefore, the maximum thickness of the shale was as found
in A, i.e. 200m.
- Although the limestone in borehole A was absent in B and C, but it can be seen to be protected
by marl layer on top of it. So, the maximum thickness of the limestone must be 300m.
- The thickness of sandstone layer was 400m in A & B, but was recorded as 300m in C. As can
be seen, this sandstone was protected on top at both A & B, but it has no top protection cover at
C. So, erosion of some of the sandstone must have taken place at site C. Therefore, the maximum
- Borehole C, like A & B, ended inside the conglomerate. Maximum thickness of the
- In borehole A, marl occurred at the topmost. The 50m indicated cannot be the natural maximum
thickness since the bed must have been affected by surface processes of erosion.
- In summary, the maximum thicknesses of the topmost and the bottommost beds were unknown
while the thicknesses of the layers in between are known. Total thicknesses of beds given in A =
950m, B = 500m and C = 300m. These are equal to the depths from the ground surface to the
Part II:-
- Our major assignment in the problem defined above is to trace out the boundaries between
Conglomerate & Sandstone; Sandstone & Shale; Shale & Limestone; and Limestone & Marl.
In 3-point problems like this, drawing of rock boundaries begins from the bottom, i.e. the
bottommost boundary is drawn first, then the next one above it, etc.
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- Starting with the Sandstone/Conglomerate boundary now, calculate the elevation of this
- On the map supplied, point A lies on surface elevation 750m O.D. Depth to the
conglomerate/sandstone boundary was 950m below the ground surface here. Therefore, elevation
of this boundary relative to sea level will be (750-950)m = -200m O.D. i.e. 200m below sea level
- On the map, point B lies on surface elevation 400m O.D. Depth to conglomerate/sandstone
boundary here was 500m below ground surface. Elevation of the boundary relative to sea level at
B will be (400-500)m = -100m O.D, i.e. 100m below the sea level.
- Point C lies on surface elevation 700m O.D. Depth to conglomerate/sandstone boundary here
was 300m. Elevation of the boundary relative to sea level at C will then be (700-300)m = 400m
- Having obtained the sea level elevation of the bottommost rock boundary at the 3 sites
- Going from A to B on the triangular plot, there is an increase of 100m in elevation. Since 100m
is the contour interval unit on the map, then line AB is made up of only one unit of contour
interval.
- Now, going from B to C on the plot, there is an increase of 500m in elevation. So, line BC is
made of 5 units of contour spacing as provided on the map. We then cut line BC into 5 equal
parts. The elevation points on the line from B to C will be -100, 0, +100, +200, +300, and +400.
- Going from A to C on the plot, there will be an increase of 600m in elevation, i.e. line AC is
made of 6 units of contour spacing of the map. Therefore, line AC should be cut into 6 equal
parts. The elevation points on the line from A to C will be -200, -100, 0, +100, +200, +300, and
- Using a long ruler, join points of equal values occurring on line BC and AC. Extend each of the
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lines across the full page of the map. You should have 5 lines, to be marked -100, 0, +100, +200,
and +300.
Each of these lines is a strike line occurring on the planar surface of the conglomerate/sandstone
boundary at depth.
If the drawings and measurements were carefully made, the strike line intervals should be the
same.
- Use the strike line interval obtained to construct other strike lines above and below the triangle
On the whole, there should be about 12 strike lines on this map, namely from top to bottom of
map: -400, -300, -200, -100, 0, +100, +200, +300, +400, +500, +600, and +700. The lines should
be so marked.
- Look at the strike lines one by one and mark out those that intersect contour lines of equal
values. These points of intersection define the orientation of the rock boundary.
Trace out the rock boundary with a smooth curve as appropriate using these points. Both ends of
CAUTION:- A rock boundary trace must not touch any contour or strike line, except at the
- After getting the conglomerate unit delineated, use appropriate symbol to cover the area.
All the steps described in the last 4 pages (Parts I & II) are shown on the map below.
Follow the remaining procedures in Part III and complete the map.
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Part III:
Boundary between Sandstone & Shale: The is the next boundary in succession to be traced out.
- The maximum thickness of sandstone was 400m. The sandstone/shale boundary therefore
- Therefore, add 400 to the value of each strike line earlier used. The new values will now be
(from top to bottom): 0, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000 & 1100. The lines
should be so marked.
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PLEASE! Don’t erase old values. Just cross them out neatly. You may re-use them for other
calculations again.
- As done earlier, mark out points of intersections between strike lines and contour lines of equal
values. Trace out the rock boundary as appropriate. In tracing the rock boundary from this stage,
the orientation of the boundary must be guided by that of the previous one already traced.
Boundary between Shale & Limestone: The maximum thickness of shale was 200m. The
shale/limestone boundary, therefore, existed at 200m above the last sandstone/shale boundary, or
- Add 200 to the value of each strike line used last. The new values will now be (from top to
bottom): 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200 & 1300. The lines should be
- As usual, mark out points of intersections between strike lines and contour lines of equal
REMEMBER! Boundary trace must not touch a contour or strike line except at the appropriate
point of intersection.
Boundary between Limestone & Marl: The maximum thickness of limestone was 300m. The
limestone/marl boundary therefore existed at 300m above the last shale/limestone boundary or
- Add 300 to the value of each of the strike lines last used. The new values will now be (from top
to bottom of map): 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500 & 1600. The
lines should be so marked and earlier values neatly crossed out as usual.
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- As before, look for and mark out points of intersections between strike lines and contour lines
of equal values. Connect these points with a smooth line. Use appropriate symbols to cover the
= To obtain the depth at which a borehole sunk at point H will encounter the sandstone bed, we
will go back to the top of the sandstone, i.e. the shale/sandstone boundary. Re-list the strike line
= Draw a line passing through point H and parallel to the strike lines.
= Using the average strike line interval and spacing, the value of this strike line H should be
calculated relative to the other strike lines on the shale/sandstone boundary. The value of this
strike line H is the elevation (O.D) of the shale/sandstone boundary or the top of sandstone bed at
point H.
= Looking at the map, the ground surface elevation at point H was about 400m O.D. The
difference between this surface elevation and the strike line H value obtained is equal to the
FOLDS
Sedimentary rocks cover over 3/4 of the surface of the earth. Most of these were originally laid
down in almost horizontal layers. The sequence of deposition is normally conformable on the
depositional surface. The topmost bed will be the youngest and will lie on the next youngest bed.
Folded Strata:
Compressive forces developed within the earth's crust (i.e. tectonic forces) cause many of the
By description, anticlines are upfolds and synclines are downfolds. The limbs are the sloping
sides of a fold.
The axis of a fold is a median line along the apex of a fold. The axis may coincide with the crest
(i.e. the highest part of an anticline) or the trough (i.e. lowest part of a syncline), though such is
The axial plane is the vertical plane about which the dip of the beds changes in direction and
often in amount.
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Symmetrical Fold: is a fold in which the axial plane divides the fold into exact reverse mirror
images of each other. In symmetrical folds, the axial plane coincides with the crest of the fold.
Asymmetrical Fold: is a fold where the two sides are not images of each other.
The simplest type of fold is shown in the diagram above where the bed is folded into an anticline
(or upfold) and a syncline (or downfold). The examples above are symmetrical folds, and they
hinge about a vertical plane with the limbs of the folds equally disposed on either side of it.
Overturned Fold (or Overfold): is a fold which has one or both limbs folded beyond the vertical
so that the apex or axis overhangs (in an anticline) one of the limbs. In the case of an overturned
syncline, the axis is said to underhang. The direction of the axis of the fold is the strike of the
fold. This always coincide with the strike of the beds involved in the fold.
Plunging (or Pitching) Fold: is one which has its axis inclined in reference to a horizontal line.
The inclination is measured in degrees. The fold has its axis tilted, hence the inclination
Isoclinal Fold: is one whose limbs are dipping in the same direction with the same angle of dip.
Symmetrical Fold:-
The map shows two symmetrical anticlines and synclines. Strike lines drawn through the rock
boundaries show that there are both easterly and westerly dips.
Caution: Don't extrapolate any strike line. They can all be visibly drawn by connecting points of
outcrops (i.e. similar pair of points of contact between a rock bed and a contour line). All the
strike lines are found to be equally spaced over the whole map. This shows that the folds are
symmetrical.
Asymmetrical Fold:-
Draw the strike lines and determine the values of the easterly and westerly dips. Don't extrapolate
Overfold:-
Along section X-Y on the map, the beds are repeated about the arcuate outcrop of the sandstone.
The dips for all the beds are generally westward, but vary in values. Determine these dip values.
This constancy in direction but variation in amount of dips is characteristic of overfold beds.
Recumbent Fold:-
In the map below, the conglomerate/limestone contact occurs at two elevations, 850m and
1050m, each follows the position of the contour of that value. This shows that both contacts are
Strike lines along the contact in the western part of the map indicate a westerly dip on the 1050m
The continuation of this dip would bring the 1050m contact to intersect the 850m junction. The
interpretation of this disposition is shown as a typical recumbent fold in the geological cross
The map below covers a series of isoclinal folds. The strike lines indicate that the beds have a
uniform westerly dip. Draw the geo-section from X to Y. In the section drawn, the axial planes
will be found to lie parallel to each other. Note that the crest of each fold is not coincident with
The map shows a group of asymmetrical plunging folds. To indicate clearly the nature of these
folds the strike lines or stratum contours have been drawn for you.
Characteristically, the outcrops of the plunging series lie en echelon as shown in the map. The
outcrops open out in the direction of plunge in the synclines and close with the plunge in the
anticlines. In this and comparable situations, strike lines will no longer remain parallel across
large areas, but will converge in relation to the closure of folds. This is a major consideration in
On the map, study both contacts of the marl (mudstone) with shale. Strike lines on the left
hand contact show that the beds dip in a south-easterly direction while the strike lines on the right
hand contact show a south-westerly dip. Those strike lines cross each other to the north and so
form chevron-shaped lines, running parallel to each other to produce an en echelon pattern in the
syncline. These two boundaries on either side of the mudstone therefore dip towards each other
while the axis between them dips southward. {ascertain this}. This structure is an asymmetrical
plunging syncline.
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Strike lines on both contacts of the conglomerate with the sandstone also form an en
echelon pattern, but with the dips away from the northerly trending axis of the fold. This is an
Notice that the section drawn does not give any indication of their plunging character.
EFFECTS OF FOLDING:
The general effects of folding are changes in dip value and dip direction. These lead to variation
in the true thickness of the beds affected. Recumbent folding leads to repetition of beds in a
vertical column.
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FAULTS
A fault is a break or fracture of rock masses in the earth's crust along which an observable
displacement on either side of the surface of fracture has occurred. Faults occur in all types of
rocks, but they are best observed in sedimentary rocks where the displacement of strata may be
identified and measured. The strata on one side of a fault may be vertically or horizontally
displaced hundreds, or even thousands of metres, relative to the strata on the other side of the
fault. Horizontal displacement of several kilometres is not uncommon. The movement affects a
zone rather than a single surface, i.e. effect of faulting is more extensive than the traceable
fracture that is seen on the surface. Tensional, compressional and torsional forces operate in the
(iii) offset and displacement of normally adjacent parts of folds, dykes or other rock layers.
The surface along which the movement has taken place is referred to as the fault plane. Such
(i) Fault Plane:- is the fracture surface along which relative movement of a series of rocks has
taken place. In the diagram below, surface MNOP is the fault plane.
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(ii) Hanging Wall & Foot Wall:- The fault plane is usually inclined. The part of the rock mass
lying above the fault plane is the hanging wall, while that part below is the foot wall. In the
diagram, the MN block is the foot wall while the OP block is the hanging wall.
(iii) Upthrow & Downthrow Sides of the Fault:- These terms refer to the relative position of the
rock masses on either side of the fault plane. The side where the movement has been downwards
is the downthrow side and that in which the movement has been upwards is the upthrow side.
(iv) Dip of the Fault:- This is the angle between the fault plane and the horizontal. It is the
maximum angle of inclination of the fault plane away from the horizontal. It is shown in the
(v) Hade of a Fault:- is the angle between the fault plane and the vertical. Geometrically, it is the
(vi) Throw of a Fault:- The vertical throw of a fault is the vertical displacement of the severed
(vii) Heave of a Fault:- This is the horizontal displacement of the severed ends of a bed of rock.
The heave is very important in mining where the separation of this kind indicates a barren
1) The Relative Movement of the Hanging Wall & the Foot Wall:-
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But where the foot wall has an apparent downward movement, the fault is called a REVERSE
fault.
2) The Relation of the Dip & Strike of Fault Plane to That of the Beds Affected:- If the strike of
the fault plane is parallel to the direction of maximum dip of the bed, the fault is called a DIP
fault. If the strike of the fault plane is parallel to the strike of the beds, the fault is called a
STRIKE fault. If the strike of the fault plane is oblique to the strike and dip of the beds, the fault
3) Direction of Slip (or Movement) on the Fault Plane:- In faulting, movement may be wholly
horizontal and parallel to the strike of the fault plane. In this case, the fault is called a STRIKE
There are two types of strike slip faults. These are the Dextral strike slip faults and Sinistral
strike slip faults. When a strike slip fault is viewed from the ground while standing on one of the
displaced blocks, if the adjacent block on the other side of the fault has been displaced to the
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right, the fault is called a dextral strike slip fault. But if the displacement is to the left, then the
Alternatively, movement may be wholly along the direction of maximum dip of the fault. In that
In some cases however, the movement may be a combination of the two, i.e. an oblique
4) Types of Movement:-
(a) Horst:- When the area between two parallel groups of normal faults is upthrown and
upstanding, the fault system is called a horst. The area may be plateau-like or ridge-like.
occurs when the area between two groups of parallel normal faults is downthrown. The system is
Example A:
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Here, a seam of coal has undergone normal faulting and the severed edges of the seam have been
drawn away from each other as a result of the faulting. This results in creation of a zone in which
coal is absent, i.e. the barren ground. This barren ground is the heave of the fault. It is often a
region of much fragmented rocks. It causes difficulty in places where roadways and underground
Example B:
In this case, the coal seam is broken by a reverse fault. Here the severed ends of the seam have
ridden over each other so that a borehole sunk at A would pass through the same seam twice.
In the map above, the sandstone bed is broken by a dip fault R-S which trends from North to
South. In order to find the throw of this fault, the first thing is to determine the dip and strike of
the sandstone by drawing strike lines on the same surface of the same bed.
Procedure:- There are two approaches to the determination of vertical throw of a dip fault. These
This is a graphical construction method. After determining the strike, dip and gradient, the next
(i)- select two points, which are at the same elevation, on the same surface of the bed but on
- Join AC.
(ii)- Through A, draw a line in the direction of the dip. Produce this line close to edge of the map,
(iv)- Use the scale of the map and measure line AB. This is the horizontal distance between the
- Since the dip has already been determined as 1 in 2 to the south, elevation of the bed is
- A is at 700m O.D. so that a fall of 200m to B means that B will lie at 500m O.D.
- But B and C lie on the same strike line. Thus, before C could have been at 700m O.D., there
must have been either an upthrow of 200m to the East, or a downthrow of 200m to the West.
b) Mathematical Method:-
After the determination of the strike and dip, a mathematical treatment can be used if it is found
out that
(ii) the throw of the fault is not likely to be an even number of metres; or
Throw = D x T
The map below shows a sandstone bed broken by a reverse strike fault. In order to calculate the
effect of the fault, first determine the dip and strike of the two outcrops east and west of the fault.
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The eastern outcrop strikes north to south and dips west, with a gradient of 1 in 2. The western
outcrop has same strike and dip directions as that in the East.
- If the sandstone continued to dip from A' westward without the intervention of the fault, it
would have fallen to an elevation of 300m O.D. at A. This can be seen from a consideration of
- However, the outcrop at A is at 800m O.D. There is therefore, a difference of 500m between the
actual elevation on map and the expected elevation if there had been no faulting.
- It is then deduceable that the fault has upthrown the sandstone 500m on the western side of the
fault.
(i) - select two points of like elevation on the same surface of the bed that occurs east and west of
the fault.
(ii)- Through d, draw a line in the direction of dip to intersect at d' the strike line that comes
through g.
That is, the throw of the fault is 500m upthrow to the west of the fault.
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UNCONFORMITIES
There are three major view points to the concept of unconformity, i.e. the occurrence of an
This concept equates deposition and time. Thus, an unconformity represents unrecorded time.
unconformity. Major breaks in sedimentation can usually be demonstrated easily, but minor
3) STRUCTURE: Structurally, an unconformity may be regarded as the plane (or planar surface)
separating or truncating older series of rocks below from younger rock series above, representing
deposition, or possibly some combination of these factors. It may be parallel to the upper strata or
at an angle with the upper strata or be irregular. Subsequent earth movements may have folded or
faulted it.
Unconformity indicates a change, either temporal or permanent, in conditions. This may be due
TYPES OF UNCONFORMITIES
There are several types, but the most common ones are:
1. Angular Unconformity:- Here, the lower and older series of beds dip at a different angle to the
younger upper beds. This also includes the case where unfolded younger strata rest upon folded
older strata. The unconformity is easily recognized by change in dip of beds or truncation of
The intrusions into the lower series occurred before deposition of the upper series. This is so
2. Parallel Unconformity:- In this case, the lower and upper series of beds have the same amount
top of intrusive igneous rocks or metamorphosed rocks which have been exposed at the surface
determine the dip and strike of the beds. Do this for every contact of the outcrops and compare.
A geological section needs to be drawn to establish or confirm the type of unconformity. In the
IGNEOUS INTRUSIONS
Igneous rocks are formed by solidification of hot mobile rock material called magma. In
Nigeria, both extrusive and intrusive igneous rocks occur. Igneous rocks do not occur with the
regular arrangement which is characteristic of sedimentary rocks. They occur in irregular masses
or in sheets intruded from the earth's interior into the pre-existing rocks above.
They are classified, on structural basis, into two groups namely - discordant and concordant
intrusions.
The discordant ones are dykes, cone sheets, batholiths, volcanic plugs, stocks, bosses, veins, etc.
A batholith is an irregularly-shaped large mass of plutonic igneous rock of deep seated origin. It
grows broader with depth and only appears at the surface as a result of erosion and denudation. It
is the deep-seated reservoir of magma from the earth's mantle and it gives rise to all other
intrusive bodies.
A sill is a sheet of igneous rock injected along the bedding planes of the rocks. The thickness is
approximately uniform, and usually relatively thin compared with its lateral extent.
A dyke is a vertical or near-vertical intrusion, cutting through the overlying sediments. Dykes are
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commonly dolerite or basaltic. However, ring dykes, which are granitic, occur in Nigeria within
A volcanic plug is the solidified material filling a vent or pipe of a dead volcano.
If or when a volcanic plug has resisted weathering better than the surrounding rock-mass, it will
stand alone as a column of solidified igneous rock, e.g. the Wase rock in southern Plateau state.
It is possible, in certain cases, to date a dyke or an intrusion relative to the bed it cuts. The
Take a close look at the map drawn below and study its section provided below.
MINOR FEATURES/STRUCTURES
These are features on rocks, so small that they cannot be represented on maps. However, they are
very good diagnostic features of the rocks exhibiting them. They can be used as clues to the past
1. Joints:- Joints may be localized. They are fractures along which there has been no
displacement. They occur in sets and have different origins. There are tensional, cross,
When mapping, the orientation of each joint and its estimated length have to be determined on
2. Minor Folds & Faults:- These may result from local movement of material. The features are
common with gneisses and gneissic rocks. The most common localized faults are the strike-slip
faults - both the dextral and sinistral varieties. Drag folds are also common in rocks of the
When mapping, one needs to sketch their diagrams as they exist on the field, with the correct
orientation in the field note book. Photographs of these features may be better taken at times.
When mapping on the field, their widths and orientations must be noted where found. The nature
of the boundary between the vein and surrounding rock should also be noted, i.e. whether
boundary is sharp, or gradational or wavy, or otherwise. This tells about the event or conditions
4. Tension Gashes:- These are local fractures or openings in rocks due to tensile stresses. The
openings usually taper off at the edges. They may be a few centimetres long, but usually not more
than few metres in length. They may or may not be filled with minerals like quartz.
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When encountered during mapping, their orientations, lengths and thicknesses (at the middle
point) should be accurately determined on the field and recorded in field note book.
5. Boudinage Structure:- This is found in places where hard competent rocks are set in less
competent or incompetent rocks. The incompetent rocks would yield easily to forces that tend to
fold them while the hard rocks would be jointed and broken up into pillow-like segments called
boudins.
When encountered on the field during mapping, they should be properly sketched in the field
note book.
It tells about the palaeocurrent direction (i.e. the direction of movement of the fluid that
deposited the rock materials at the in the ancient times) as well as the mode of fluctuation of the
transporting energy. These should be sketched accurately in the field note book. Where possible,
photographs should be taken, especially when there are colour or textural changes.
7. Mylonitic Structures:- These are crushed zones of rocks. Rock materials along shear zones,
e.g. fault zones, are usually crushed into finer particles by effect of movement of the shear zone.
When encountered during mapping, it should be well sketched in the field note book, noting the
8. Augen Structures:- These are eye-like features found commonly in feldspathic granites.
Feldspars occupy the centre of the feature. They can be found in the Older Granite rocks.
During mapping, they should be well-sketched where found, frequency noted, and the general
9. Rock Boundaries/Contacts:- These features are not commonly found exposed on the field
because of the processes of weathering which result in the covering of these contacts with
residual soils. As such, rock boundaries are most commonly inferred or extrapolated when
Where this contact is then found exposed on the field, it is a lucky find and should be carefully
studied and sketched, with correct orientations. Such find help in reducing the level of
uncertainty when inferring the rock boundaries. Photographs of exposed rock contacts are
An outcrop of a bed that is completely surrounded by outcrops of younger beds is called an inlier.
Whereas, an outlier is an outcrop of a bed that is entirely surrounded by outcrops of older beds
,and so separated from the main outcrop. They can result from down-faulting.
By faulting:
Structurally, an outlier depicts a structural syncline while an inlier depicts a structural anticline.
One can get synformal anticline or antiformal syncline. This may occur when beds have been
The map below contains an inlier. Study its section drawn underneath. Draw your own section
from L to M.
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GENERAL EXERCISES:
Draw the sections and identify the structures present in Maps 1-3. Calculate thicknesses of beds.
MAP 1:
MAP 2:
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MAP 3: