Dip and Strike

Basically the beds of the sedimentary rocks are found in horizontal disposition. But due to the earth movement and ground disturbance, these beds of the sedimentary rocks get inclined and loose their horizontal position. The tilted strata and the inclination of rock beds is known as ”dip”. The object of measuring dip and strike of rocks is to obtain information on their three diemension position.

(1). Dip:

The angle of inclination of a rock bed with the horizontal plane is called the ”dip”. It is measured in plane perpendicular to the strike. In addition to the amount of dip, its direction must also be stated. The dip angle is measured with a clinometer and its direction is measured with a compass. An example of recording dip is 60° N, 45° East. This means that the beds dip at an angle of 60° in a northeasterly direction.

Appearent dip and True dip: The true ”dip” is defined as the maximum angle of dip on a rock bed. It is measured in the direction at right angles to the strike. The measurement of a ”dip” from the ”dip direction” perpendicular to the ”strike line”. Note that the ”dip direction” is always perpendicular to the ”line of strike”.

While a ”dip” is measured in a vertical plane not perpendicular to the ”strike line” is known as an apparent dip. Or a dip measured in any other direction than the true dip, is called the ”appearent dip”. An apparent dip will always have a value less than the true dip. The amount of dip decreases as the dreiction of dip moves round towards the strike direction. Along the strike direction, however, the dip wil be zero.

How to read and interpret a geologic maps

(2). Strike:

Strike is a line formed by the intersection of the dipping rock-bed and the horizontal plane. The trend of a rock bed on the ground surface is called the strike. In other words the ”strike” may be defined as the direction of a line formed by the intersection of a bedding plane and a horizontal plane. The strike is always at right angles to the true dip direction Thus the strike is a horizontal llne on a surface of rock beds. The driection of the strike is measured by compass with reference to the true north and south. For example, if the strike direction of a particular bed is 30° east of north, the strike will be recorded as N30°E – S30°W.

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