Measuring orientations of veins and faults at Toadstool

Harmon D. Maher Jr., Dept. of Geography and Geology, University of Nebraska at Omaha, 2004

The audience for this document is students who are engaging in the EUR STEP funded Toadstool project this summer. You will need to measure the orientations of a variety of structural features (faults, veins and fractures) at Toadstool. At first you will likely struggle with this, but by the end of the five days you will do it without thinking.

The distillation: Any two lines uniquely define a plane. A horizontal line within the plane is known as the strike line. A line perpendicular to that and within the plane is known as the dip line. If we describe the orientation of these two lines we have by default described the orientation of the plane. The dip line is also the line with the maximum inclination (maximum departure from horizontal). You should practice finding and visualizing the strike and dip lines on an inclined plane.

The orientation of the strike line can be described by the angle between a horizontal line pointing true north and the strike line as measured in a clockwise direction. This is known as the azimuth convention. One can also measure it from true north clockwise or counterclockwise, but in this case you need to indicate which one by labeling the strike angle with NE or NW, indicating which direction from N the strike line is. This is known as the quadrant system. We will use the azimuth convention.

The orientation of the dip line is measured as its inclination from horizontal, i.e. by the angle between the dip line and the directly overhead horizontal line. One still has to attach a direction of dip to this.

An example of the format is as follows: 145 - 45 E, where 145 would be the strike angle, 45 would be the dip angle and E would represent the dip direction. The block diagram below may help visualize. Also check out some of the links below.

A key point to remember when using your compass to measure strike and dip, is that the long axis of the compass is the line you are measuring the orientation of. If you make this line parallel to the strike or dip lines of the beds then you can read its orientation. Your compasses will have inclinometers in addition to magnetic needles so that you can measure the angle from horizontal also.

Links that will help with learning strike and dip, and that I suggest you explore.