Strike and Dip© Geoff Habiger
May 29, 2001
Understanding geologic structures is an important part of understanding the geology around us. My previous article discussed folded rocks and the different types of folds. In that article I mentioned a principle known as dip. Dip is one half of the method geologists use to help understand and visualize the direction and angle that rocks tilt into the earth’s surface. Since there are only a few areas around the earth that have not been subjected to tectonic forces and where the rocks lie in a completely flat, horizontal surface; understanding how to determine the dip and the strike of rock layers is an important aspect of geology. In this article I will present a simple and easy to use method for determining the strike and dip of tilted or folded bedding.
To begin to understand strike and dip, you need to have the right equipment. Geologists either use a Brunton compass, which is excellent for measuring bearings like a regular compass, but can also be used as a clinometer (for measuring angles) and a hand level. The Silva Ranger Compass can measure bearings as well as strike and dip, but cannot be used as a hand level. Strike and dip can also be measured using a regular compass and creating a clinometer out of a protractor. The protractor is attached to a stiff piece of cardboard and orientated so the bottom of the cardboard is perpendicular to the straight edge of the protractor. Mark the bottom edge of the cardboard so you will know that this is the surface that is placed on the rock surface to be measured. A string with a weight attached is placed at the center of the protractor allowing angles to be measured. When you approach an outcrop of rock that is tilted, image a horizontal line intersecting the surface of the bedding plane (such as from a body of water). This horizontal line is used to define the strike of the bedding. The compass is placed on the bedding surface so it is level and the orientation of the beds is measured. Generally, geologists notate the direction as relative to north, but azimuth readings can also be taken. In the example to the right, the strike is measured as north 80 degrees west (N80W) or as 280 degrees. When mapping the rocks, the strike line is drawn as a straight line in the direction of strike.
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Great article Geoff and a great set of diagrams to support it as well!
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