What is Strike and dip of a bed?

 What is Strike line?

"A Strike line is an imaginary line of a plane which is constructed by the intersection of an inclined plane and a horizontal plane". The strike line is measured on the horizontal plane and carried out with respect to the true geographic north. Each strike line can have different elevation values and also may have zero value (e.g. a horizontal bed). On the map, the strike line is drawn through the intersection points of structural traces (an upper trace and a lower trace of a bed) and the contour line. The strike value will be given based on contour values (fig.1 and 2). 

What is Dip-direction of a bed?

"The dip-direction is the inclination direction of a bed on which side it is dipping". The dip-direction is also measured on the horizontal plane and it is always perpendicular to the strike line of the bed (fig.1).

What is the dip of a bed?

"The dip of a bed is the angle of inclination with respect to the horizontal plane. It is always measured along the vertical plane". The angle of inclination can be different because it depends upon the direction of the observer. If an observer is observing along the dip-direction of a bed (perpendicular to the strike), in such case an angle of inclination would be maximum and this maximum angle is called a True dip. A true dip is an actual angle of inclination of a bed. On the other hand, if an observer does observe an inclination along the dip-direction but it is not perpendicular to the strike. In such a case, the angle of inclination is called an Apparent dip. An apparent dip is always less than the true dip (fig.1 and 2).

A Demonstration of the geometrical elements for a plane.

fig.1: Geometrical Elements for a plane: Strike line, dip-direction, and dip of a bed

fig. 2: How to draw a strike line through the intersection points of structural traces and contour lines. Finding out of the true dip along XY traverse direction and an apparent dip along the AB direction.

How to measure the strike, dip, and dip direction of a bed?

Before measuring the geometrical elements for a bed, let us know about some basics of the field. In the field, we to identify an outcrop and its planer and linear features; thereafter apply the Right-hand thumb rule.


An outcrop is a surface exposure of a part of the sub-surface bed-rock (fig.3: a, b, c, de, f, g). These parts are visible over the surface because of the deep weathering & erosion of the surface. An outcrop is regarded as a primary clue for knowing the sub-surface features. Sometimes these outcrops are not visible over the surface, in such a case, we should find out the road cross-section, the naala-section, river channel, etc. If all the mentioned things do not found in the areas, in this case, we should follow the soil sampling, soil characteristics, density of trees, etc. The density of trees can also help to predict the sub-surface conditions like hydrological behaviours. If the density is more, it means the saturation level of water is good in that location.

Bed and Lamination?

A bed or a lamination is a geological planar structure of a rock which was lithified by the sedimentation and lithification processes of the sediment at a location. These sediments were deposited by the geomorphological agents like wind, river, marine, glacier, etc. The bed and the lamination are classified based on the thickness. If the thickness a plane is more than one centimetre, this plane is referred to as a bed, on the other hand, the lamination is considered if the thickness is less than one centimetre (fig.4).

Right-hand thumb Rule?

After the identification of the bed, put your right hand on the bed and observe your fingers. "The thumb points to the strike direction and other four fingers point to the dip direction of the bed (fig.5)". Now, mark the strike and dip-direction lines on the bed with chalk or pencil. Now, align the bar of the compass ( a Clinometerr or a Brunton Compass) toward the thumb direction and measure the strike direction with respect to the true geographic north. We know that the dip-direction of a bed is perpendicular to the strike and measured on the horizontal plane. So, we should keep in mind that the compass must be in a horizontal position (In the case of Brunton compass, the bull's eye should be levelled); and in the case of dip-direction, the crown in the compass (Clinometerr) should be toward the dip-direction (fig.6). 

In the case of measuring the dip of the bed, put the compass perpendicular to the bed and measure the angle on the clinometer scale. Because the dip of the bed is measured along the vertical plane (fig.6).


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