Structural Geology - Attitude of beds, Strike, Dip etc.

1. Attitude of beds, Strike and dip,
Brunton compass and its use; and
Outcrops
Dr. P. T. Hanamgond
GSS College, Belgaum

2. Describing and mapping the orientation of a
geologic structure such as a inclined
sedimentary bed, a fault plane, a joint plane
etc involves determining …
Strike (trend)
Dip (inclination)
Hence, attitude of a bed is defined as the strike
and dip of a bed.
Mapping Geologic Structures
in the field

3. Strike (trend)
The direction of the
line produced by the
intersection of an
inclined rock layer or
fault or a dyke or a
mineral vein with a
horizontal plane.
Generally expressed
as an angle relative to
north.
N37°E – S37°W
N12°W – S12°E
Fracture
Ripple Structure
Dyke
Fault

4. ~??o

5. Dip (inclination)
The angle of inclination of the
surface of a rock unit or fault
measured from a horizontal plane.
Includes both an angle of
inclination and a direction toward
which the rock is inc lined. For Ex.,
82°SE
17°SW
Dip values always are in the range
0-90° . A dip angle of 0° defines a
horizontal attitude. 90° of dip
describes a vertically oriented
plane.
0-20°: Shallow
20-50°: Moderate
50-90°: Steep
Direction of dip

6. In case of horizontal beds, sedimentary beds are
originally deposited as a series of horizontal
layers one on top of another,.. Conformably
Imaginary horizontal plane

7. The principle of superposition states
that, in a series of undisturbed
layers, the oldest layer is on the
bottom and each overlying layer is
progressively younger with the
youngest layer on the top.
When the outcrop lines are parallel
the beds are conformable and have
the same amount of dip.

8. When there are two outcrops with
sudden change in strike and dip, it
indicates that the beds are
unconformable.
Unconformable
Younger
Older

9. True dip – It is the shortest distance of the
dip of a bed or rock formation or it is the
maximum dip or slope of a bed with respect
to the horizon. In the diagram - SS represents
the strike direction and ‘ad’ is the dip
direction.
S
A
B C
E
D
O
S
d
S Sa
Apparent dip – in the given figure SS
represents the strike direction and OD is
the true dip direction which is
perpendicular to the strike. The other dip
directions such as OA, OB, OC, OE are all
called the apparent dips. In other words,
the directions of dip other than the
direction of true dip (OD) are described as
apparent dips. They are less than 90°.
Types of dip

10. Beside true dip and apparent dip the dip is also
differentiated in following ways.
Primary or Initial dip – It is the inclination of strata due
to the original slope of a basin. These dips are generally
less than 35° (generally 0° – 20°).
Secondary dip – It is the inclination induced in the strata
after its deposition due to the tectonic forces to which the
strata is subjected. These dips may be from 0° – 90°.
Local or Regional dip – It is the inclination of rocks
exposed in a limited area of observation.

12. Geologists use a Brunton or Clino compass to measure
strike and dip. When using the compass to determine
the attitude of a plane the edge of the compass is
placed against the inclined surface and then the bulls-
eye bubble is centered. In this configuration the
compass lies in a horizontal plane and its edge is
parallel to the line produced by the intersection of the
imaginary horizontal plane and the inclined surface or
layer. The sighting armature points in the direction
that this line is oriented, and this direction is read
directly from the compass.
GPS

15. Holding a brunton compass

16. Use of Brunton Compass
1. Locate North, Set local declination
2. Measure Bearings
3. Measure Strike and Dip of planes
4. Measure Trend and Plunge of lines
5. Measure Vertical Angles- measuring height /
thickness of a feature

25. OUTCROPS

26. An outcrop is an exposure of a rock formation or a bed
on the surface of the earth.
When the outcrop lines or trend of the beds are parallel
to the contour lines, the beds are horizontal.

27. When the outcrop lines or trend
intersect the contour lines the
beds are dipping or inclined.
When the outcrop lines are
straight and intersect the
contour lines the beds are
vertical.

28. Horizontal
beds
Inclined beds
Vertical beds
Conformable
beds
Unconformable
beds

30. Inlier
D
C
B
A
D
C
B
A
D A
B
C
Younger
Older
Outlier
Inlier – It is an outcrop of older rock
surrounded by the outcrops of younger
rocks.
Inliers are typically formed by the erosion
of overlying younger rocks to reveal a
limited exposure of the older underlying
rocks. These are thus reverse of outliers.
Valley formation is a common process of
erosion, giving rise to inliers.
Outlier – It is an exposure of a younger
bed surrounded by the outcrops of older
formation. It results due to excessive
erosion or due to faulting followed by
erosion.
C
D
B C D E
E
B
A
C
D
E
B
A
Inlier Outlier
TYPES OF OUTCROP
There are two types of outcrop created
because of selective erosion of rocks in
certain areas known as Outlier and Inlier.
These are well exposed in a folded
regions.

31. OUTCROP PATTERN
The outcrop of the top or bottom of horizontal bed follows
topographic contours. The width of the outcrop of a
horizontal bed depends upon the thickness of the bed and
upon the topography.
Strata may be horizontal, inclined or vertical. The thickness
is measured perpendicular to the bedding plane.
Exposed bed Earth Surface The outcrop pattern of
a tilted bed depends on
the angle of tilt and on
the surface topography.

32. The width of the outcrop depends upon the tilt and slope
of the ground. When the bed is vertical the width of the
outcrop is minimum or it is equal to the true thickness of
the bed. If the ground surface is flat, but the bedding
angle is change then also the width of the outcrop varies.
Similarly the change in slope of the ground also matters.
The higher the slope of the ground wider the outcrop.
A
C
B
Thickness of beds A, B and C is the
same but outcrop of B is much wider
than A and outcrop of C is much wider
than B, due to their shallower dip angle

33. When the outcrop runs in the strike direction it tends
to keep an approximately even width. If the
inclination of the ground is towards the dip direction
the width of the outcrop is maximum.
t
d d
t
When the dip is 0° (horizontal beds), the width of the
outcrop becomes infinity, i.e., the outcrop will occupy the
whole of the ground surface.
Horizontal surface-
width is minimum
Inclination of ground towards the
dip – width is maximum

34. A B
Figure 2
C
d
𝜽
ω
G
D
Tt
B A
Vt
Figure 1
WIDTH OF THE OUTCROP (Figure 1): AB or ω
is the width of the outcrop. On the map it is the
distance between the boundary of the upper
surface of the bed and boundary of the lower
surface of the same bed measured in right angles
with the direction of the strike.
THICKNESS OF THE OUTCROP (Figure 1 and
2):
When the rock formation outcrops at surface of the
ground, the distance between the two surfaces of
the outcrop (top and bottom of the bed) is called
the width of the outcrop represented by AB or ω in
the figure 1. The distance GC between the top
surface of the bed measured in vertical direction
from the ground is called its vertical thickness (in
figure 1, it is ‘d’). The distance CD measured along
the direction perpendicular to the surface of the
bed is its true thickness (Tt). Similarly, in figure 2,
the true thickness can be measured at two levels A
and B for upper surface of the bed and lower
surface of the bed respectively.

35. DIP DIRECTION (Figure 3): Suppose
a dyke/vein/a particular bed, is exposed
on the surface and shows only the
width, then to know which side the bed
is dipping, we need to drill minimum two
points at different distances on either
side of the bed( G1, G2, G1’, G2’). Then
if we encounter the surface of the bed at
the two drilled points, measure the two
levels (d1 and d2) and the direction in
which the level is higher the bed will be
dipping in that direction.
G1
C1
C2
d2
G2
d1
Figure 3
G2” G1’

36. RELATION OF TOPOGRAPHY WITH RESPECT TO OUTCROP &
FACTORS CONTROLLING THE WIDTH OF THE OUTCROP
When the topography is horizontal and the formations are
also horizontal the top most formation is continuously
exposed on the surface.
In an area with uneven
topography the boundaries of the
outcrop of a horizontal formation
follows the topographic contours,
whenever such horizontal
formation crosses the valleys their
outcrop pattern becomes U or V
shaped.

37. If the rocks are horizontal or just slightly tilted,
the outcrop follows the contours of the land

38. rocks tilted
and eroded to
a flat
landscape
horizontal
rocks
outcropping in
a terraced
mountain