Hello everyone I hope you are doing well.
Welcome back again to this online structural
geology NPTEL course. This is our last week.
That is week number 12 and we are in our lecture
Number 33. As we have concluded in the last
lecture of the previous week that we more
or less have learnt the basics of structural
geology.
So it is a very beginning we understood the
different processes. Then we understood stress,
strain. We learnt how to measure dip and strike
different structural elements out of that
we moved to the rheology the deformation mechanism
and after that we learnt a series of important
structures their characteristics, geometries,
how to interpret them and these included fold,
fault, joints, buddinajs and so on.
In this lecture will particularly focus on
the basics of Litho-structural mapping. As
a geologist it is very very important that
he or she knows the basics or the methods
of geological mapping. Nowadays there are
many techniques which are digitized or there
are many equipments which are essentially
helpful or reduces time and also you do not
have to go to the field always.
Some you can use some satellite images and
you can use several techniques or modeling
techniques to interpret some very interesting
geological feature that sometimes we cannot
interpret going to the field. But as an undergraduate
student it is important that you understand
the basics of structural geology and its mapping
techniques.
This lecture will particularly focus on that
having said this I must say that structural
mapping or geological mapping is something
that one has to learn in the field and because
this is an online course it is not possible
to demonstrate all these techniques. What
I have included in this lecture is mostly
some very basics in the sense that so far
we have learnt about the structures their
geometries and so on. These we mostly considered
about their geometries there dispositions
and so on and we restricted them inside a
block diagram either in three dimensions or
two dimensions.
Now we have not considered that how they would
look like in the field. Well we have seen
some outcrop patterns of superposed folds
and so on also it we learnt about them in
the fault lectures. But again and the surfaces
that we constructed that this would be the
map view was essentially a flat surfaces is
not it.
But you know that earth surface is not flat
even within 10 or 15 meters it can be extremely
unrelated and therefore the interaction of
the structures with the earth surface is something
very important to consider and understand
to particularly figure out the local and regional
structures.
In this lecture will mostly focus on that
and we will figure out the processes that
one can have some ideas about litho-structural
mapping. We will learn some very basics will
not go into the complex structures we even
do not go even deformed structures for example
will not going to include fold, fault and
so on but we mostly restrict our self on a
flat bed which is dipping differently and
we will see how and why we can interpret different
structural features out of it.
Continuing this we will learn in this lecture
some very basic formats and composition of
a geological map. Then will try to understand
what is topography and its interaction with
the lithology and mostly bedding planes or
any planer fabrics that how they interact
with each other.
After that we will see when you are discussing
this topography and lithology will discover
that there is something very interesting which
is rule of V and finally will figure out that
even if a bed or a feature is not exposed
on the surface that where you cannot measure
dip and strike but you can have the line which
is connecting the two different lithologys
on the surface of the Earth which is contoured
which is which has a topography then how to
calculate the strike and dip with the help
of structural contour.
So, let us start with the very first Idea
and the books I have suggested you at the
very beginning of this lectures may not be
very very helpful in understanding this lecture
or feature analysis related to structural
maps and so on.
So here are 3 books that I personally like
and I also would like to recommend this books
for this particular class and also a for your
future reference. The first one is geological
structures and maps written by Professor Richard
lisle this is a published from Elsevier 3rd
edition 2004.
The second book is structural analysis and
synthesis it is written by Rowland Etel published
in 2007 from Blackwell and the third one is
one of the latest once it talks about geological
field techniques Angela Coe edited this book
along with 4 or 3 coauthors I exactly do not
remember but this is published in 2010 and
it is from Wiley and Blackwell.
Now, I personally would recommend the first
book a must to have if you would like to continue
with geological mapping and to be very specific
with the structural geology. The second and
third book interestingly deal with many many
techniques that you should learn in the field
it gives you the very basic ideas what is
dip? What is strike? How to measure it? How
to hold the compass? How to take the reading?
How to take the reading On the Notebook? How
to interpret them? How to construct things
in the field directly and so on.
These are very handful but again these are
not the scope of this lecture I hope in future
I can come up with another course I have to
design it differently on geological mapping
and so on particular with Emphasis of structural
elements. But these 3 books are very very
important and in this lecture whatever I am
going to show you is mostly derived from the
book of Professor Lisle the illustrations
and diagrams I have redrawn it but it is essentially
from his book.
So what is the geological map? You know all
this what is but this is how more or less
you can define it a geological map a describe
it in way it you do not have to define everything.
A geological map shows the distribution of
various types of Bedrock in an area. The map
is usually prepared over a topographic map
which takes into account the various forms
and elevation depression of the Earth surface.
In the map the different lithologies are generally
shaded or coloured or symbolled with or without
structural data and may other data to show
where different Rock units occur at or just
below the ground surface.
So many things are written here let us take
the highlights of these statements. So for
this first of all a geological map essentially
gives you the surficial data maybe few meters
or few tens of meters and so on. So what you
see on the surface is essentially included
in the geological map looking at a geological
map you may not conclude what is happening
at 10 or 15 kilometers down or even 1 kilometer
down it may be completely different.
And then therefore a geological map essentially
represents the various features various Rock
types that you see in the field and you compose
them together with the different Rock type
that we have seen their contacts and so on.
And generally the map if it is very large
scale map it is done over a topographic map
which you can consider that this is a reference
of your area and the topographic map is very
very useful we have a couple of slides and
topography and it is a topographic map and
its usefulness and so on.
So generally large scale maps are drawn over
topography topographic maps but you can also
considered a plane paper mapping and so on.
If we have to do a very detailed mapping in
say a 10 meters by 10 meters area then topographic
map is not that much useful because topographic
maps are generally very large scale. Again
these are different techniques but there are
large scale mapping and plain paper mapping
in large-scale mapping you need the help of
topographic map but in plain paper mapping
no.
The primary thing in a geological map is mostly
the lithological contacts and their disposition
on the surface but if say a lithological contact
is faulted then you can use some symbols and
therefore you start introducing the structural
elements on your map or maybe you have series
of lithologies planar lithologies and their
deep into a certain directions so you can
include the strike and dip in the map and
therefore you again introduced the different
structural features.
For example you see that in the field this
is synform so I can use the symbol of synform
you can use a symbol of anti form and so on
and by this slowly add many many many information
as much as you can based on you need to the
geological map. People also add many other
features say geophysical measurements say
you can add density you can add many other
features conductivity in the corresponding
to particular lithology of particular feature
of your map.
So geological map is essentially very very
important not only for geologists not only
for interpreting the structure original tectonics
or know what is there but it is essentially
important also for engineers for constructing
dams or buildings or roads and so on. So yes
this is very very important we have to learn
how to read and how to interpret and how to
prepare it is a geologist that is a primary
job of us.
So here is an example of a geological map
I took it from geological survey of India
and this is what you see the map of Sikkim
is a state in the North Eastern part of India.
So the state is like this this part is the
Whitish part that you see this snow-covered
and people could not figure out the lithology
and so on but you see that this map shows
very nicely the different lithologies with
different colours.
So this pink has a different lithology, this
purple has a different lithology, this red
has a different lithology, this grey has a
different lithology and they are disposed
in a different way in a map and you can see
many other maps just type geological map and
you will see many different features in this
map.
Also you can see that here these the structural
data are included there the dip and strike
are included here and so on, so this also
tells you a very very important information
about this area and a geological map essentially
comes with something called Legend. So you
have to explain what are the different colours
or what the different symbols are you have
to explain what are the different features
that you are using the line drawings and so
on.
These are also very important and finally
what is most important in any geological map
is that you provide the scale this is very
important will learn about it that what scale
I am looking at that if I measure 2 centimeters
in this map that means how much I am measuring
in the real scale is it hundred meters is
it 1 kilometer or is it 4 kilometer or even
more and this is also very important that
you indicate what is the direction of North
sometimes people indicate the direction of
North at the same time the indicate also the
latitudes and longitudes in the map that also
tells you where exactly in the earth you have
mapped or the map is located.
So these are the very important aspects of
this map apart from the different colours
indicating different lithologies the different
symbols of structural data and so on. You
have to have a legend, you have to have a
scale in your map. You have to assign what
is a north direction and if possible in the
map you also if it is large scale map is suggest
what is the latitude and longitude your mapping
area is.
Now how this map is constructed again I am
not going into the detail but I tell you it
is not a very easy task. So there are series
of processes geologist in the field first
has to go and then he has to record the nature
of rock where it is visible at the surface
and it is not visible everywhere that is quite
common.
Then Rock outcrops and characteristics such
as Rock composition, distribution and relationship
of structural elements fossil contents etc,
the geologists to do record in his notebook
and so on an nowadays maybe in the mobile
phone, a laptop or with some digital mapping
softwares.
Now using all these details the geologist
then distinguishes different units in the
field at he has she has seen and then he tries
to plot them separately in the map or on the
base of the topographic map. Now the geologist
can include some additional information for
example of geophysical data is taken into
account when the geologist decide to add other
parameters like density, strength, seismic
velocity and so on the map. Never the less
there are always parts of the map where more
uncertainty exist about the nature of the
Bedrock and it is important for the reader
of the map to realize that a good deal of
interpretation is used in the map making processes.
What I mean by this that it comes actually
from this place that where it is visible at
the surface. I do not see rocks everywhere
but you map it and then you use your intelligence
to interpret that if I see this rock here
and if I see the same rock here how they are
connected maybe your interpretation is wrong
but based on your experience based on your
intelligence based on your theoretical background
you conclude this is how it should be someone
can challenge it but this is how we always
work.
But once you look at a geological map you
be sure that a lot of uncertainties are involved
in this map so do not take any map as granted
unless I mean particularly if it is really
required for some very special jobs so better
you go to the field match it that is this
is what is happening otherwise you take it
if it is from a good source.
And final part is of course the interpretation
that once you have a geological map this is
a job done but after the map is ready then
it is very important that you interpret the
structure you interpret the feature you interpret
the lithology and so on but this is something
else that you would learn in a different class
a different lectures.
So, as I was talking about their many modern
techniques which are very very useful but
it is also important as a student you go to
the field with your field instructor structure
with your teacher and learn how to hold a
compass learn how to take back bearing learn
how to take front bearing. You take your steps
with a measuring tape and then do Different
techniques because not always not everywhere
you will you can use all this modern techniques.
So let us talk about the topographic map for
a while you know that what is a topo map we
generally called it topo map or topo sheet,
so a topographic maps represent numerically
the complex curves and elevations of earth
surface with the contour lines. Now a control
line is a line joining the points of equal
elevation on a surface and the contour lines
I represent the intersection of those curves
with imaginary horizontal surface at regular
intervals.
We will see topographic map and other things
with some good illustrations later but a contour
line or a series of contour lines should have
some very special characters because the contour
lines indicate equal elevation on a surface.
Every point along a control line is the exact
same elevation.
Contour lines therefore can never cross each
other if you see a Contour map going like
this going like that and then another country
map like this then this is something very
very strange and you discard the map immediately.
So this is not something you should look at.
A contour line must close on itself. So that
is also something that you need to see maybe
it is not closing in your map area but it
has to close somewhere and the lowest closing
point is of course sea level will see that.
The map distance between two adjacent contour
lines may vary but there elevation difference
should remain constant. What I mean by that
that say you have a contour line going like
this and you write this is say 80 meters then
you have the next control line something like
that this is 70 meters and this is your saying
on a map say this is you have a scale for
that.
See the map scale this distance and this distance
is essentially different but the change of
elevation from here to here is 10 meters and
here to here is as well 10 meters so does
not matter how far the contour lines are.
But two adjacent contour lines always should
represent a very similar elevation and that
gives you an another idea when the 2 adjacent
contour lines are closely spaced that means
this is steeper compared to when they are
spaced far from each other that means they
have a gentle slope will see the soon.
So the use of topographic map is very significant
and topographic map also do contains significant
information. It tells you about the roads,
buildings, urban development, railways, airports,
names of the places and geological features,
geographical features, then administrative
boundaries, state and international boundaries,
reserves and so on. It also tells you about
the water bodies of this area like lakes,
rivers, streams, swamps, a coastal flats excreta.
It also tells you because it is a topographic
map about the relief of this region say mountains,
valleys, contour and cliffs, depressions,
basins and so on.
It also tells you about the vegetation of
this area so whether this area is a jungle,
wooded area, reserved forest, vineyards lands
or orchards and so on. All these things all
this information you get from topographic
maps. So a topographic map it is not necessarily
we geologists use it is used in every community
particularly those who deal with the nature.
In addition to that geological structures
such as bidding contacts faults and folds
also do interact and intersect the topography
along some lines and this is exactly what
you are going to learn in this lecture after
a while.
So topographic map generally come with different
scales and the scales are given generally
in this for that what topographic map you
have and I said 1 to 50,000 what does it mean
it means that 500 meters is equal to 1 cm
so this is how is it is given. So if I tell
that I have a map of 1 to 50,000 that means
in the map if I measure 1 centimeter by my
scale or ruler that distance represents 500
meters in the actual area. So similarly you
can have 1 to 20,000 that means 200 meters,
1 to 24,000 that means 240 meters and so on.
And then finally 1 to 20 Lakh ratio that means
1 centimeter in the map is equivalent to 20
kilometers in the region.
So this column represents this things in centimeters
or meter scale and their it is inch and miles
scale. So very similarly 20 kilometers in
1 to 20 Lakh map scaled map it should be 1
inch in the map if you can measure that represents
about 32 miles in the actual area.
There are some terminologies that involved
in the topographic map so we learnt few of
them but let us have a look in in this list.
So contour lines contour lines are isolines
that show equal elevation on a map at defined
intervals this you have learnt. Then magnetic
North not according to the Earths magnetic
poles rather than its Geographic poles therefore
if you have magnetic pole then you must have
declination, so declination from the true
North is given in mils 1 mils is equal to
1 divided by 6400 of 360 degrees.
The true North is the Geographic north and
this is what we all understand when you talk
about North declination have learnt so a measurement
of the degree to which a grid or magnetic
North varies from the true North. Then grid
is a network of uniformly spaced lines on
the face of a map intersecting at right angles
and usually running north south and east west.
Grids are often numbered and can be used to
define position by rectangular coordinates
and grids generally does not care about the
contour lines so does not matter how steep
the slope is or how gentle the slope is or
whether it is flat or not grids ignore everything
and generally run either north-south or east-west
or if you have define them in a different
way. And then you have Meridian. Meridian
is a starting point or line usually line of
longitude for a numbering system if the numbers
continued to the section of the earth into
grid.
Now this is how the topographic maps are indexed.
So this is our India. So India is generally
gridded with this numbers for example we can
have this 45 here or 63 and so on. Then this
is a single grid now within this grid you
actually divide this grid in 16 sub grids
and then you define them as it is written
here A, B, C, D, E, F, G, H up to P and then
each of these grids A for example here it
is given here then again you sub divided by
16 grids and then you again numbered them
1, 2, 3, 4, 5, 6, 7, 8 and so and so this
process continuous.
So for example if I want map of D here then
I have to say that I need 55 D and if you
need a map of 55D3 that means you have this
grid 55 then you have this D and then D is
divided again in 16 segments and then you
are looking for something here. So more numbers
and alphabets you add to your topo map more
high resolution you go with the mapping process
and the topo map as well has legends so these
are the different things that you see in the
topographic maps.
So, it is important when you look at it for
the first time you actually read this and
see what is what that is that is also important
so that once you read it will remember so
you do not have to look back every time unless
you forget. But it is important ones you read
a map you see what it is and not necessarily
in each and every map you see this Legend
is attached maybe you scan something for you
crop something and then the scanner thought
that ok legends are not important so he cut
it out or cropped it out and you do not have
it so it is better you remember what is what.
Now this is how a topographic map looks like.
So this is something digitally constructed
so you see the colours are varying. So the
highest elevation is marked by red and slowly
it is going to the cooler colours to the blue
and you see that these black lines these are
actually making your equal elevations in this
region. But this image here is representing
in a much better way.
So what you see here we have a valley, a river
is flowing through this valley and we have
2 little elevations here. We also see that
in this elevation the slope is going down
here the slope is going down here it is going
down this side. This is one of the maximum
elevations we see. We also see that slope
is here very gentle here the slope is extremely
high. We also see some little rivers or whatever
channels are flowing along these valleys and
so on.
And this is where you have the sea and on
the sea you have also some cliffs where the
slope is almost perpendicular. Now if I have
to represent in the topographic map then it
looks like this. So here you see that it is
contoured and by this you can actually figure
out though it is map view but you can figure
out towards which direction the slope is changing.
For example this is 260 this is 200 this is
100 so you can clearly figure out that this
is how the slope is decreasing and once you
know that then you can also figure out that
this is how it is decreasing here but here
you see the spacing is much much higher compared
to the spacing here. So this is a steep slope
and this is exactly what we have seen here.
Now this is the river is flowing and we also
see that here we have the highest elevation
and at highest elevation if you see a concentric
circle that is the highest elevation of this
region and that must be closed by a circle
and then the other contours must should follow
it in different ways, but this is how it is.
Now it is also important that you train your
eyes so when you look a topographic map you
try to visualize that what is the elevation
of that region, so I have given 4 example
here 2 are in this slide so the first column
is outcrop map or plan view and you see here
from 50 that is the lowest elevation in meters
to 300 it is going. So 300 is the highest
peak here and here is the highest point and
it is marked by a concentric circle.
If we have to make a cross section from A
to B of this region then I should get something
like this. So if I see something like that
and values are given then it should be like
this. Now if the values are opposite way for
example this one is 300 and this one is 50
then A to B in this case should be something
like that. So these are actually depressions
but here the values are increasing towards
core of this contour so this is you have 2
hillocks here 1 hillock this one another hillock
this one this one and this one.
Very similarly you see here as I talked about
so here we start again at 50 and end up 350
meters. In this side the contours are closely
spaced in this side contours are not that
closely spaced. So here the slope is very
gentle here slope is extremely steep. So just
you look at this contour pattern and you can
figure out what is the section or how it should
look like when you actually see them in the
field.
This is another one so you see we have 2 concentric
circles but this concentric circle closes
at 200 meters and this closes at 350 meters.
So therefore this must be of higher elevation
compared to this and if you make a section
you actually can see that this is your 350
and this is somewhere is your 200. So this
is how you actually visualize and this is
another one we have a single peak and we get
a single peak here will see in one of the
next slide that how to construct this elevation
map profile map from this contour maps and
this is exactly where it is.
So we will have a lab demonstration on this
but I explain you briefly. So whenever you
see a topographic map and if you have to draw
the profile the first thing you have to decide
that from which section you would like to
draw the profile. For example here the profile
should be drawn along this line, so this is
X and this is Y.
Now we have series of contour lines so for
example this contour line is 75 meters and
then this contour line is closing here and
then we have 150 meters here which is this
one and if this is 150 this is 75 then this
one should be even lower. But if we do this
then we can figure out that it is cutting
across this XY line the number of contours.
So what you do you take a strip of paper which
is this one and align the age of this paper
along XY line and once you aline it then you
mark this places or points where it is intersecting
the contour lines. So here for example number
the 1, 2, 3, 4, 5 and so on. Once you are
done then it is important that you draw a
Cartesian co-ordinate system where your horizontal
axis should be the distance XY and this should
be your elevations.
And this distance you can fix by yourself
depending on the scale you would like to see
but make sure that these are all equal. Say
for example you can figure out that this is
75 meter this is 150 meters and so on.
Now you arrange your strip here this paper
strip that you have where you have marked
this 1 then 2 then 3 then 4 and then 5 intersections
of the strip of the papers so this comes here.
And you see that 1 your value is almost at
the sea level so because this is sea so this
close to 0 so at 1 you have somewhere here.
Then at point 2 here the value is 75 meter,
so you have to plot a point here at 75 meter.
At 3 your value is 150 meters the value of
the point 3 so you come here and the value
is 150 meters. At 4 again this contour is
coming back and we are at 150 meters so we
come here and plot it like this and then at
5 again we are touching this 75 contour so
it is like this, of course you can grid your
map first so that you do not have to do it
but once you are expert you can do it just
by putting your scale accordingly the way
you need. But these are the points that we
got and simply you can connect these points
like this. Now here you can make it flat or
you can make it little curved there is no
harm. So this is the elevation or profile
along XY line of this.
So does not matter how is your topography
and so on you can cross section say A to B
say this is 20 this is 30 this is 40 this
is 50 and this is 60. So if you draw a profile
along this again we have to put a strip paper,
paper strip here. So you mark this point,
this point, this point, this point, this point,
this point and so on we go on and then you
again may make your grid and then you know
the values you plot it you will get series
of points and then you are done. But we will
have little lab demonstration on this that
how to construct the profile from a topographic
map.
Now let us come to a very important part that
so far we have figured out what is topographic
map? How to get the profile and so on. But
you may have different lithologies which are
intersecting or interacting with the topography.
For example if you have a flat horizontal
bed and if you have a flat topography then
you do not see the other beds which is below
the flat bed because it is only single bed
you will see because bed is flat topography
is flat.
Now if your bed is again flat but your topography
is in a particular slope in a very gentle
slope then topography would go down and at
one point of time it would cut the contact
between one bed to another bed. So other bed
will be exposed on the topography. And this
can be a valley topography, a mountain in
topography and so on.
So there are 3 possibility that you have horizontal
beds with one is flat topography, slope topography
and Valley topography. Then you can have uniformly
dipping beds and again you can have flat topography,
slope topography and valley topography. And
this is exactly what rule of V is we learn
about it soon. But let us try to understand
this process in a better way.
Say for example you have a bed here like this
which is a green one which is sloping and
this is some other rocks. So we have a bed
which is sloping in this way and then you
have some other rocks around this. Now with
time what can happen it can erode uniformly.
So you see that this got eroded and because
this bed is sloping then you can clearly see
that this is a horizontal surface. So you
see that you have say lithology A then lithology
B and again lithology A. So you have A then
B then A and they are absolutely fine no problem.
Now if the erosion happens and you get a sloppy
topography so the topography is uniformly
sloping here as you can see here so this is
the slope. Then you of course generate some
contour lines, these is white dotted lines
are your contour lines and again there is
no variation here except bed thickness changed
on the exposed surface area. But now if there
is a river flowing here as we see here so
it would cut a gorge or make a Canyon or whatever.
Then the question is how on this river valley
this bed would look like? Because it would
have also contours like this. So would it
look like this or would it be a straight line
straight bed or it would be like this and
so on. You may guess how it should look like
but there is a rule and this rule is known
as rule of Vs. We will see this soon but before
that let us see some other interesting features.
What do you see here in the first image or
first illustration that is the surface of
the Earth for example which has a flat sloping
surface and then you have to 2 different lithologies
say this is A the green one and this light
cream one is B and their contact is somewhere
here. So when it intersects like this and
this is a topography, so this is the lowest
elevation that you can see here 20 meters
and it goes up to 90 meters towards this direction
and this is the line it is cutting or intersecting
the topography the boundary between A and
B. So in this side you have A and in the side
you have B.
You can also see that if this topography is
not as flat as we have seen or maybe river
is flowing along this then you have very similar
thing A and B the beds are dipping exactly
similar way. So to strike and dip of these
two contact lithological contacts are very
very similar but if I change the topography
in this case if I have an undulatory topography,
Then the interaction of this topography with
this uniformly dipping bed would be along
this lines, so here you have A and here you
have B. So if I try to see them this is a
block diagram if I see them in a topographic
map their appearance would be something like
this if I have to plot it.
So here because the topography is flat we
clearly see that the flatbed interacting uniformly
dipping bed which has no deformation it is
not folded or something like that this is
very much straight line. But same flatbed
if the topography is undulating then which
is given by this little red dashed lines then
it is not anymore a straight line.
So there must be happening something and I
would like to also a emphasize the fact that
we have already talked about that you see
here this appears like V or in this case this
is like V with a mirror image. So we will
learn about it later.
So we see here that same bed but in one case
we have flat topography another place we have
undulatory topography and we may have 2 different
intersections on the topographic map. But
if we have same topography but the bed is
dipping differently then what should be the
condition?
Here at the examples the first one again we
have A and B the bed is dipping very gently.
Okay you can see that this is the deep of
the bed this is the lithological boundary
and this is the topography where you have
some river network and the interaction would
be something like this if the bed is moderately
dipping in this case the interaction is something
like this with the topography on the surface.
And if the bed is highly dipping then the
interaction is something like this.
So in the topographic map we see them differently.
So same topography but bed is dipping in different
ways the appearance or exposers of these topographic
maps would be completely different. So this
is the take home message from this slide and
the previous slide and based on that a geologist
have constructed the rule of V. Again you
see that this is a V this is again going to
be a V and this is a V of different shape
and so on. Let us see how does it look like.
Now will look up the V rules and will look
after one and then the next one and then we
have 6 illustrations to demonstrate what is
the V rule. But before we go to the actual
V rule let us talk about what does it say
to us.
So the first illustration in all figures will
see that on the left hand side that means
this one is like a V block that used in lathe
machine something like that. But this has
a slope say for example this contour me have
100 meters this is say 10 meters spacing so
110 then 120 then 130 and then 140.
So from here to here we have changed of elevation
of about 40 meters and then the countries
are running like this inside the valley and
you can imagine that a river is flowing in
this direction. So this is the downstream
and essentially then this would be upstream.
So river itself as a slope and this is also
maintain a slope the walls of the valley and
this is horizontal.
So this concept will first look what happens
if we have a horizontal bed in this condition.
So horizontal bed means that this dip is 0
here and we see if the bed is horizontal then
the V rule suggest and this is the a top view
on the topographical map, so this is a topo
map.
So the way horizontal Bed would intersect
the valley or it is something like this. So
it would the V that would be created it would
point upstream. So this is exactly what to
see here and the angle of V is very similar
to the angle of the contours or contour lines
you can think of. If this is alpha then this
has to be also alpha. What happen if the bed
is a vertical? Is given in the next slide.
If we have a vertical bed that means this
is 90 degree and again this is the downstream
that is the upstream and the vertical bed
interestingly would not produce any V in this
section in the top view on the topo map view.
So it would not be influenced by any of the
contour lines and it would run straight as
it was.
Now if we have the beds which are dipping
along the slope of the stream. That means
the stream has a slope along which it is flowing
and the bed also has a very similar slope
of the stream. So this is the deep of the
bed and this is also the stream the way it
is flowing.
So, if this is a horizontal line if I can
reproduce it here so this angle and this angle
are very similar. In that case the V rule
suggests that you would not produce any V
shape. However the 2 projections of the beds
on this V they would slowly try to intersect
towards the downstream.
If the bed is gently dipping towards the downstream,
so this dip is very very low, low angle dipping
bed. This is again the downstream then the
V would be something like that and in the
plan view it would be very interesting that
when it was horizontal we saw that this angle
of the V and this angle of the V were equal.
But in this case if the bed is gently dipping
towards the downstream then the angle made
by the bed inside the valley the V angle if
this is alpha and if this is alpha 1 then
Alpha is less than Alpha 1. So V points also
upstream in this case and it is also sharper
than the contour lines.
Now if we have steeply dipping bed towards
the downstream. So bed the dip of the bed
is very high in that case we will see that
V is pointing downstream in this manner.
Now we look at something very interesting
that how we can figure out the strike and
then the dip of the bedding plane which is
uniformly dipping inside the surface and it
is intersecting with different topographies.
And if we remember the definition of the strike
then you can figure out the fact that strike
was nothing but the intersection of an inclined
plane with the horizontal plane and horizontal
plane is imaginary so they would producer
a line.
The azimuth of the line is the strike, so
from that idea of the definition of strike
we actually can construct on the topographic
map even if we do not go to the field. If
we have this intersection line on the topographic
map of a bedding plane then we can construct
something what is known as structural contour.
So if a dipping surface crosses valleys and
ridges we can construct strike lines which
are known as structural contour to precisely
determine the strike.
Now a map showing outcrops of a surface together
with topographic contours can be used to construct
structure of contours for that surface. And
when we construct these structural contours
it has 2 underlying principles. The first
principle is where a surface crops out the
height of the surface equals the height of
the topography.
So that means if I have an intersection point
between the contour line of a particular value
and also the intersection line of the two
surfaces on the earth surface then if I find
a similar point somewhere and if I connect
these 2 points so these 2 points would have
also very similar elevation value and is the
height of a planer surface is known at a minimum
of 3 places the structure contours for that
surface can be constructed.
Let us see how does it work I think instead
of reading the text let us do this directly.
We have seen this illustration before this
was an uniformly dipping bed. So what is important
to draw the contour lines structural contour
lines that you have to figure out the intersection
between a fixed value contour line for example
this 20 meters which the intersection of the
2 lithologies which is coming up on the surface.
So in this case this is one point and this
is one point. So these two points are relevant
for 20 meter contour lines. Now interestingly
if this point is A and this point is B as
both point A and B are falling on the contour
line so the elevation of A and B should be
same. At the same time this point A and B
also showing you the fact that the elevation
of underlying bed or the intersection between
these two layers is green and creamy layer
also has same elevation values at these two
points. So I can actually construct a line
like this and this line indicates that this
bed has or this litho boundary has an elevation
of 20 meters.
Let us go to the next contour which is 30
meters. Now again in a similar principle we
can figure out that where this contour is
intersecting the boundaries between the 2
lithologies in this case it is here and this
is coming around and in this case this is
also here. So, similarly if I considered this
A1 and B1 then point A1 and point B1 has similar
elevation and because these 2 points are sitting
at the intersection between the 2 lithologies.
So the lithology itself has also similar elevation.
So I can draw another line here suggesting
that this is 20 meters this is 30 meters.
These 2 lines indicate that the boundary between
the 2 litho units have similar elevation along
these lines.
Similarly, we can construct for the 40, so
40 meters contour so it goes like this comes
back here again to draw a line like this.
We can figure out the 50 meters here and then
it goes and it comes here again interesting
these 50 meters also crosses another point
here and that is actually the perfect one
because now we have 3 points. So we can construct
a line of same elevation of this plane and
this is of 50 meter this was of 40 meter.
We can similarly do for 60 meters here and
it is coming here and again it is possible
that we can. What is interesting you see that
these lines are parallel to each other. Dotted
lines that we have drawn these 20 meters,
30 meters, 40 meters, 50 meters and 60 meter
suggesting the elevation of the bed or of
the lithogical contact. These are parallel
so first hand they are telling you that the
bed is uniformly dipping and the next one
is 80 meters and it is also running parallel.
Now these lines what this lines actually do
signify this signify as I talked about that
the same elevation of the bedding plane. The
contour lines that you have seen here these,
these do indicate same elevation on undulatory
surface and these lines indicate the same
elevation of contact or lithological boundaries
or a bedding plane or an uniformly dipping
plane and these are known as structural contour.
And we see that these lines actually are horizontal
lines at different elevations. So the bed
is inclined and I have their intersection
line on the horizontal plane, which are all
these lines. So these lines the orientation
of this line if this is the strike line as
well. So in this case the strike line is oriented
east west.
So this is how you construct the strike lines
and if we have to see it in a different form
it is something like that. So you can construct
series of structural contours and structural
contours are essentially different to that
of the topographic contour unless I let you
decide what it is. There should be a specific
conditions, say for example I tell you if
the bed is horizontal then the structural
contour should be exactly similar to that
of your topographic contour.
Now this is how we have constructed the strike
and now will learn how to figure out the dip
angle of the bedding plane or the litho contact
from the structural contour. So the first
thing you have to do you have to draw the
structural contours. So in this case this
figure this is figure A, show the set of structural
contours for the surface defined by the base
of a sandstone bed. This example I took from
the book of lisle.
This is the north direction, so if this is
a structural contour then the strike is 120
degrees. So this is the strike line. So from
North if you count it would be 120 degrees.
Now to find the angle of dip we must calculate
the inclination of a line on the surface at
right angles to the strike. So the dip we
have to calculate at a right angle of the
strike line that is the definition the true
dip.
Now one can be confused here that which way
the dip direction is it is on this side or
it is on this side. Now that is not very tough
job because you see that structural contour
is increasing the side here it is 160 here
it is 210 so that means the bed must be dipping
in this side not in this side. So Dip direction
is towards this side.
Now, to figure out the dip angle what you
have to do, so this is the section that will
be working on you simply have to draw a line
like this say AB and also scale is given you
can have this distance and in a very similar
way the way we walked on the paper strip and
so on. You can actually figure out this thing
so it first cuts 180 then 190 then 200 and
then 210. At this point it cuts here say it
cuts hear say this 1, 2, 3, 4. So this is
1 this is 2 this is 3 and this is 4.
So it is possible that you can get a line
like this and the slope of this line is actually
the dip. Now how to calculate the slope of
course you know the distance and you know
this distance as well from the scale. So dip
angle as it is written here is related to
the spacing of the countries that is tangent
or angle of dip is contour interval.
So in this case this is 10 meters divided
by spacing on map between the contours and
this is exactly what you can figure out. So
once you know the dip and strike from the
topographical map this kind of basics actually
help you to understand the problems like 3
point problems and so on.
Particularly when people do bore hole in an
area that do not see the actual Rock. So what
I mean by this lets to a very simple block
diagram. See you have a dipping plane at the
subsurface and this is of your interest and
what are you doing you doing some boreholes
say you have done one borehole here you get
something here you get something here and
maybe one here you get something here.
So every time you touch your desired bed at
different points. So here on the plan you
actually have 3 points where are you touch
the desired bed at 3 different values of X
Y and Z and if you sure that this is an uniformly
dipping bed then using this basic of finding
dip and strike you can figure out that what
is the dip and strike of this bed which you
actually do not see.
So this is something very interesting of this
kind of problems there should be some problems
associated with this and the demonstrations
of this lab works would be given along with
this this weeks lecture series.
So with this I conclude this weeks lecture
because we will have a hand full of demonstrations
which are very useful and would be uploaded
in this week by the teaching assistants. So
I request you to check them and practice.
I not only conclude the lecture of this week
as your instructor but I also sign off from
this course because this is last lecture.
I thank you very much for joining this course
and I particularly enjoyed a lot in teaching
this course. I learnt a lot as well, I hope
this course was useful for you and I still
remain at your (())(60:54) distance if you
have any ideas that you would like to discuss
with me your more than welcome to write me
through my email. So, thank you very much
stay well. I hope I will see you again with
another series of lectures. Thank you.
