Brief notes on Dip and Strike, Clinometer, Compass and its uses

As we know, most sedimentary rocks were originally deposited; on flat or very gently inclined surfaces. Sometimes, of course, as a result of special circumstances, certain beds may have started with an initial inclination, as in current bedding; but where we find great thickness of strata tilted into conspicuously inclined positions, it is to be borne in mind in general that the beds have been tilted by move­ments that occurred after their deposition. The attitude of an. inclined bed has got two important components as

1. Dip

2. Strike.

In specifying the attitude of any inclined bed, i.e., to define its position in space, it is required to determine the dip and strike of the concerned bed.

1. Dip. What it is?

It is essentially an angle of inclination of the bed. It is defined as the amount of inclination of a bed with respect to a horizontal plane; measured on a vertical plane lying at right angles to the strike of the bedding.

The dip of a bed has got two components, like direction as well as magnitude. So the dip of a bed is a vector quantity. The amount of dip is the angle which varies from ‘0°’ to ’90°’ according to the disposition of the bed. The direction of dip is the geographical direc­tion, along which a bed has maximum slope.

In case of horizontal beds, the dip is ‘zero’ degree and for a vertical bed the dip is ’90°’. Accordingly, the symbolic representa­tion of a horizontal and vertical bed in a map is also different, which may be seen from the following figures:

Types of dip:

There are two types of dip as

(i) True dip (ii) Apparent dip.

(i) True dip:

It is the maximum amount of slope along a line perpendicular to the strike, in other words, it is the maximum slope with respect to the horizon.

It may also be stated as the geographical direction along which the line of quickest descent slopes down.

(ii) Apparent dip:

Along any direction other than that of the true dip, the gradient is scheduled to be much less and therefore it is defined as the apparent dip. The apparent dip of any bed towards any direction must always be less than its true dip.

2. Strike:

The direction of the line along which an inclined bed intersects a horizontal plane is known as the strike of the bed. It is a scalar quantity, as it has got only one component, i.e., direction but not the magnitude. The strike of the bed is independent of its amount of dip.

Relation between dip and strike:

The direction of dip and strike of any inclined bed must lie at right angles to each other. True dip is in the direction along a perpendicular to the strike.

Importance of strike and dip:

In structural geology, strike and dip are quite important for the following purposes:

(a) To determine the younger bed or formation:

It is well known that younger beds will always be found in the direction of dip. If we go in the direction of dip relatively beds of younger age will be found to out crop and older beds in the opposite direction.

(b) In the classification, and nomenclature of folds, faults, joints and unconformities the nature of dip and strike is of paramount significance.

Thus the attitude, which refers to the three-dimensional orientation of some geological structures, is defined by their dip and strike.

Clinometer-compass:

It is essentially equipment for geolo­gical field work, which contains;

(i) a circular plane graduated from 0° to 360°

(ii) a magnetic needle which gives the direction of north and south and it is also helpful in the determination of the strike of any inclined bed;

(iii) a pointer attached with a freely oscillating pendulum for the determination of dip of any inclined bed, and their direction;

(iv) it contains a beam, which can be only rotated through an angle of 90^r, is having a small hole at one end and a vertical window at the other, with a vertical separator. It is used to take bearing (which is ‘back bearing’) at the time of determining one’s location in the field.

Thus clinometer compass is used for determining the dip and strike of beds in the field, for surveying etc.