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The directional aspects of tilted bedding planes, fold axes, faults, and other geologic structures can be completely described using two types of measurements: strike and dip.


Strike (illustrated by line s-t below) is the compass direction of a line marking the intersection of an inclined plane with a horizontal plane such as the Earth’s surface.

James D. Dana, New Text-book of Geology (New York: Ivison, Blakeman & Company, 1883).


In this class, we will measure strike using an ordinary compass with a bubble leveling device attached.  Many compass direction systems are possible, but we will be using the "azimuthal system" indicated below.



The directions for finding strike are:


1) Place the side edge of the compass against the bedding plane.

2) Use the bull's eye leveling bubble to make the compass horizontal (while maintaining step 1 above).

3) The strike of the bedding plane is indicated by the needle direction.  For consistency, record the lower value of the two supplementary angle (for example if the needle points to 120 degrees and 240 degrees, you should record 120 degrees).




Dip (line d-p above) is the maximum angle between the inclined plane and the horizontal plane.  Dip is always perpendicular to strike, and has both a compass direction and an angle.


James D. Dana, New Text-book of Geology (New York: Ivison, Blakeman & Company, 1883).


The directions for finding dip are:


1) Determine the dip direction.  This is always perpendicular to the strike direction!

2) Use the inclinometer to measure the amount of dip in degrees (a plane lying flat along the horizontal as zero dip).





Reporting Strike and Dip on a Map


Geologist frequently need to record strike and dip on a map (bird's eye view).  The procedure is quite simple:


1) Draw the strike line with the correct orientation (0-360 degrees).

2) Draw the dip line perpendicular to the strike line, and in the correct direction of the dip.  Ask yourself...which direction would your head point if you were standing on the inclined plane?

3) Record the amount of dip (0-90 degrees) at the tip of the dip line.



4) The following notations are used to record the direction of anticline and syncline fold axes.






The Brunton Compass-  In real life, geologists use an instrument called a Brunton compass to measure strike and dip.  This device contains a compass, leveling bubble, and inclinometer all built into one unit.  This versatile device can also be used as a transit for the purpose of triangulation.