![]() Specifies a plane that dips at 45° with strike roughly NNW. It’s a good idea to add a rough compass direction to the dip measurement, just as a check that right-hand rule measurement has been done correctly. Right-hand rule: When you are facing in the strike direction, the plane dips downward to your right.Īn equivalent statement is that strike is always 90° counter-clockwise from the dip direction. In most Canadian geological field work, the right-hand rule (‘ RHR‘) is used to avoid this ambiguity. There are two directions in which we could measure the strike, 180° apart! The dip direction is clockwise from one, and counterclockwise from the other. Strike, dip, and dip-direction of a plane. The strike of a plane is the azimuth of a horizontal line that lies in the plane. For this reason, many geologists prefer to measure the strike, which refers to the direction of a horizontal line drawn on the surface. However, dip direction is not easy to measure accurately with many compasses, because the slope of the plane varies rather gradually on either side of the dip direction. The dip direction of the plane is the azimuth of the steepest line in the plane. If we measure the direction in which the plane slopes downhill, then we are measuring dip direction. There are several different directions that we might measure. The azimuth of a plane is a bit more complicated. The dip of a plane is the inclination of the steepest line in the plane. If you are in doubt, imagine water running down the surface it will take the steepest path, in the direction of dip. It’s important when measuring dip to measure the steepest possible slope in the plane. To specify the orientation of a plane, we also need two measurements, an azimuth and an inclination. ![]() Of course, the compass must be adjusted for the particular area in which you are working. Most geological compasses have a mechanism for compensating for declination. ![]() Currently (2020) the declination in Edmonton is about 014°. Magnetic declination varies from place to place and varies slowly over time. The magnetic declination is the azimuth of the Earth’s magnetic field. In most parts of the Earth, the magnetic field is not aligned exactly north-south. However, you need to be prepared to understand measurements recorded as quadrants, especially when reading books and geologic reports published in the U.S.Īzimuths are typically measured with a compass, which uses the Earth’s magnetic field as a reference direction. Here are the four azimuths above, converted to the quadrants representation:īecause it is more confusing, especially when doing calculations, we will not use the quadrants method much in this manual. In the quadrants method of measuring bearings, angles are measured starting at either due north or due south (whichever is closest), and measured by counting degrees toward the east or west. In the United States, bearings are often specified using quadrants. The degree symbol is often omitted when recording large numbers of azimuths.Ĭonfusingly, there are other methods of specifying an azimuth. This helps to avoid confusion with inclinations (below). Notice that it is best to use a three digit number for azimuths. Figure 1: Compass used to measure an azimuth – in this case the strike of a bedding plane. In most geologic work, bearings are specified as azimuths.Īn azimuth is a bearing measured clockwise from north.Īn azimuth of 000° represents north, 087° is just a shade north of east, 225° represents southwest, and 315° represents NW. To describe almost any structure, we need to say something about its orientation (also known as its attitude): Does it run north-south, or perhaps east-west, or somewhere in between? A direction relative to north is called a bearing. Notice that although several of the above descriptive observations lead to kinematic inferences, only the last one allows us to make dynamic conclusions! joints and faults produced by the failure of rocks in response to stress (and which therefore reveal the orientation of stress at some time in the past).the fabric or foliation produced by alignment of sheet silicate minerals such as mica in metamorphic rocks, which reveals the direction of flattening during deformation.tabular igneous intrusive bodies such as dykes and sills.and the alignment of elongate clasts or fossil shells in sedimentary rocks (which reveals current direction).the fabric or lineation produced by alignment of amphiboles seen in metamorphic rocks (which reveal the direction of stretching acquired during deformation).glacial striae (which reveal the direction of ice movement).The following are examples of linear features that one might observe in rocks, together with some kinematic deductions from them: \)Īlmost all work on geologic structures is concerned in one way or another with lines and planes.
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