1. GEOTECHNICAL PROPERTIES (CE1203)
Basic Geological Mapping
Ms Ikmalzatul

2. What are geological maps?
A geological map shows the distribution of various types of bedrock in an area. It usually consists of a topographic map (a map giving information about the form of the earth’s surface) which is coloured to show where different rock units occur at or just below the ground surface.
What is a geological map used for?
The most obvious use of a geological map is to indicate the
nature of the near-surface bedrock. This is clearly of great
importance to civil engineers who, for example, have to
advise on the excavation of road cuttings or on the sitting of
bridges; to geographers studying the use of land and to
companies exploiting minerals.

3. STRIKE AND DIP
Strike is the orientation of the line of intersection between the horizontal plane and the rock bed. These are expressed as a
compass bearing (eg. NW)

4. STRIKE AND DIP
Dip is the angle that the bed makes with horizontal, measured in the vertical plane, in the direction perpendicular to the strike

5. WORKING OUT SUB-SURFACE GEOLOGY

6. STRIKE LINES
A strike line is a horizontal line drawn on a planar surface. This can be a bedding plane, fault, etc.
Strike lines are sometimes called “structural contours” because just like conventional contour lines they join points of equal elevation on a surface.
In order to draw one you must first find at least two points of equal elevation on the same plane.
You can choose any planar geological feature but for your exercises the plane is the boundary between two different rock types.

7. In this case the boundary between rock type G and H have been chosen
STRIKE LINES
Step 1 – Choose a plane
In this case the boundary between rock type G and H have been chosen

8. Use topographic contours to find elevation
STRIKE LINES
Step 2 – Find at least two points on the plane with the same elevation.
Use topographic contours to find elevation

9. Make sure to label the strike line correctly
STRIKE LINES
GH400
Step 3 – Connect the points of equal elevation to draw a strike line (structural contour) on the G-H boundary plane
Make sure to label the strike line correctly

10. STRIKE LINES
GH400
GH300
Step 4 – Now draw another strike line at a different elevation on the same plane (G-H boundary) using the same technique

11. STRIKE LINES
GH400
GH300
The strike of the bedding is the compass bearing of the strike line.
In this case, the strike is about ENE

12. DIP
Dip is the line of steepest slope on a plane and it is always at right angles to the strike.
Beds dip from the higher elevation strike line to the lower one

13. CALCULATING DIP ANGLE
GH400
GH300
500m
In the field, the dip is measured using a geological compass but on a map it can be calculated using:
rise/run

14. THE END