• Lab 1. Orientation of Lines and Planes

Do the questions in any order to avoid traffic congestion around the rock samples. Rock samples may not be available outside the lab hours.

*An asterisk indicates a question for self-marking. These must be passed in for the lab to be verified as complete, but will not be individually graded. Answers will be verified by a teaching assistant and/or posted for checking in next week’s lab.

  1. * To make sure you are conversant with both the quadrant convention (widely used in the USA) and the azimuth convention (used in Canada and most of the rest of the world) for recording bearings, translate the azimuth convention into the quadrant convention, and vice versa, for the following bearings.

a) N12E

b) 298

c) N62W

d) S55W

  1. * Rock samples containing planar structures are set up in the laboratory. (a) Using a compass-clinometer, measure the strike of a planar structure. To do this, hold the compass in a horizontal plane so that the needle swings freely in the Earth’s magnetic field. Then place the compass so that its horizontal edge is against the surface, keeping the compass level. Note the reading of the compass needle (your instructors will show you how to read the particular model of compass). (b) Now measure the dip. To do this, turn the compass so that it is in a vertical plane (the pendulum or spirit bubble – depending on the type of compass – should swing freely in the Earth’s gravity field). Place the compass so that its edge is in contact with the surface along the steepest slope. Read the dip (your instructors will show you how to read the number for the particular model of compass). (c) Record the strike (right-hand-rule) and the dip. (d) Also record the dip direction (N, S, E, W) as a check. (e) Repeat for the other samples as directed. (Note that the answers you get will probably not be true orientations because the Earth’s magnetic field will be distorted by metal objects in the building: in other words, the declination of the Earth’s magnetic field is highly variable indoors.)

* When you are done, have a teaching assistant check and initial your answers.

  1. * Translate the following orientation measurements from the dip-direction and dip (e.g., 060°, 45°) convention into the North American right hand rule convention, adding an alphabetic dip direction as a check (e.g. 330°/45°NE).

a) 177°, 13°

b) 032°, 45°

c) 287°, 80°

  1. * Translate the following orientation measurements from the strike, dip, and alphabetic dip-direction (e.g., 087°/ 21°N) into the North American right hand rule convention.

a) 087°/21°N

b) 005°/73°W

c) 042°/30°SE

  1. * Rock samples containing linear structures are set up in the laboratory. (a) Using a compass-clinometer, measure the trend of a linear structure. To do this, hold the compass in a horizontal plane so that the needle swings freely in the Earth’s magnetic field. Then place the compass so that its horizontal edge is over the plunging line, keeping the compass level. Note the reading of the compass needle (using the same method as before). (b) Now measure the plunge. To do this, turn the compass so that it is in a vertical plane (the pendulum or spirit bubble – depending on the type of compass – should swing freely in the Earth’s gravity field). Place the compass so that its edge is in contact with the line. Read the plunge (using the same method as for dip). (c) Record the plunge and the trend. (d) Add the trend direction (N, NE, E…) as a check. (e) Repeat for the other sample(s) as directed. (Note that the answers you get will probably not be true orientations because the Earth’s magnetic field will be distorted by metal objects in the building: in other words, the declination of the Earth’s magnetic field is highly variable indoors.)

* Have a teaching assistant check and initial your answers.

  1. * Topographic and geological surfaces are not always planar. When a surface is curved, the strike and dip vary from place to place. When this happens, contouring is a good way to reveal the shape of the surface. Map 1 shows an area of map in which the elevation has been measured at a large number of points. Place tracing paper over your map. Using a contour interval of 100 m, thread contours through the measured points. In making your contour map, remember the following points:

    • Maps and map scales: You should be able to convert a map scale expressed as a representative fraction (e.g. 1:50,000) to a map scale expressed in metric or imperial length units (2 cm = 1 km) and draw a scale bar based on either. You should understand and should be able to look at a map with topographic contours and identify hills, valleys, and predict which way streams are flowing.
    • Your contour map is a hypothesis: it should be the simplest map that is consistent with the data, so your contours should be as smooth as possible; avoid sharp bends and changes in the spacing of contours, unless required by the data;
    • Each contour has a high side and a low side. The 200 m contour (for example) separates ground that is above 200 m (on one side) from ground below 200 m (on the other);
    • Contours can never branch;
    • Rivers flow at the lowest point of a valley and must always flow downhill in the same direction.

Add arrows to show the direction of flow.

  1. Map 2 shows the configuration of the topographic surface and the trace of the top surface of a unit of banded iron formation. Determine the orientation of the surface, and shade the area of the map where the iron formation crops out.

  1. * Map 3 shows the same area as Map 1. Place the contour map you made in the earlier question over Map 3 to compare the contours. If there are differences, try to evaluate whether the two interpretations are equally valid. Show with dashed lines any places where you think changes to your map are required by the data

  1. A thin coal seam was observed at point X on Map 3. Unlike the topographic surface, it is perfectly planar; the orientation is everywhere 010°/14°. Determine the spacing and orientation of the structure contours, and draw this second set of contours on Map 3.

Complete the trace of the coal seam. You may assume that the soil cover is negligible.

Lab 1 Map 1 DrawTopoContours 20200116 copy

Lab 1 Map 2: Determine the orientation of a dipping surface

Lab 1 Map 3: Draw an outcrop trace

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Geological Structures: a Practical Introduction by John Waldron and Morgan Snyder is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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