ADVERTISEMENTS:
The procedure of drawing a section consists of three steps: 1. Drawing a Topographic Profile 2. Determination of Dip and Strike 3. Construction of Geological Structures.
Step # 1. Drawing a Topographic Profile:
Suppose it is required to draw a topographic profile along a XY line on the geological map. The map is first folded along the XY line on a drawing paper. A horizontal line equal to the XY line is drawn, which serves as a base line to the Geological sections to be made over this line.
The folded map is placed and the positions of the various contour lines along XY lines are marked. From these points vertical lines are drawn their length being proportional to the values of corresponding contours as per the scale of the map. Then a Free Hand Curve joins their tops.
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Topographic Profile:
The vertical lines drawn above the base line are not desired and therefore should be removed.
Step # 2. Determination of Dip and Strike:
Generally on Geological maps both the contour lines as well as boundaries of rock beds are shown. To determine the dip of a rock bed by graphical method such a contour is selected which cuts the particular rock boundary at two points.
A line called Strike line joins these two points. Two Strike lines from contours of different values are drawn for the same boundary. Then the distance between them along XY line is measured.
And a line having the same length is drawn on the drawing paper somewhere below the profile already made. From one end a perpendicular of length equal to the strike of interval is constructed as per the scale of the map.
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The angle formed by joining its top with the other end of the line will be the dip angle. And its directions will be from the strike line of higher value to the one that has the lower value. In this way dip of various rock beds shown on the map are determined.
Step # 3. Construction of Geological Structures:
Now the map is folded along the XY line and is placed over the base line of the profile.
The points of intersection of the boundary lines between different beds and the XY line are transferred. These points are then projected to the surface profile. From these points lines are drawn according to the Dip of the respective boundary surfaces. This is usually done by drawing angles of Dip first at the base line and then drawing parallel lines from the corresponding points at the surface profile.
ADVERTISEMENTS:
In the section thus constructed, the various structures present are completed as follows:
i. If unconformity is present, an undulating line shows its plane.
ii. If a fold is present, anticlinal or synclinal structures are made accordingly.
iii. If a fault is present, strata must be shown dislocated along it.
iv. Dykes are shown running vertically. Sills are shown running parallel to the bedding planes.