GEOTECHNICAL PROPERTIES (CE1203) Basic Geological Mapping Ms Ikmalzatul

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.

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)

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

WORKING OUT SUB-SURFACE GEOLOGY

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.

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

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

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

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

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

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

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

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