Strike and Dip - Page: 2

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.

Dip is the measure of how much the bedding planes are inclined from horizontal. Using the clinometer, it is placed along the bedding plane and the angle the beds make with respect to horizontal is measured. In the example at the left, the dip angle is 32 degrees. Next we measure the direction that the dip angle was measured. Dip is always measured perpendicular to strike. It is important to notate the direction of dip since there are two possible outcomes, not notating the direction of dip could lead a geologist to draw inaccurate conclusions about the rocks being measured. In our example at the left, the direction of dip is south 10 degrees west (S10W) or 190 degrees. On a geologic map, the dip is a tick mark perpendicular to the strike line facing in the direction of dip with the angle of the dip written next to the symbol. When measuring strike and dip, always try to place a notebook or other smooth surface along the bedding surface to be measured so any irregularities in the surface are evened out.

There are several methods that can be used for measuring strike and dip. The method outlined above is usable for most small outcrops and surfaces. On very large outcrops, the geologist must move away from the outcrop and select a bedding surface with an attitude representing the entire outcrop. The geologist then moves so the bedding surface appears as a two-dimensional line. This is the projected plane of the bedding surface. The geologist then uses a clinometer as a hand level, creating a horizontal line with respect to the bedding surface and thus allowing the geologist to measure the strike. From the same position, the dip is measured by sighting the clinometer at arms length along the bedding plane.

Sometimes it is necessary to measure strike and dip using a three-point method. This method is useful when the geologist has access to a geologic map, or must be done when the geologist does not have a compass and clinometer. This method is also useful for measuring very gently sloping surfaces or for surfaces that dip over large areas. This method requires the geologist to determine the horizontal position and elevations of three separate points on the bedding plane. The points need to define a triangle with internal angels greater than 20 degrees. When using a map, the sides of the triangle need to be longer than 1.5 cm (.5 in.) when plotted on the map. If no map is available, the geologist can make bearing readings from each of the three points and measure the horizontal distance between them. All of this data is used to create a triangle with points A, B, and C (example at right). Point D is located on one leg at the same elevation as point B. The formula for this is AD = AC x difference in elevation between A and B divided by difference in elevation between A and C. The new line created between Band D is level and represents the strike of the bedding. The bearing is determined from the bearing of BC and the angle DBC, which is read with a protractor. The dip is determined by constructing line AE perpendicular to BD and solving: tangent of angle of dip = difference in elevation between A and B divided by line AE.

Measuring strike and dip is a useful tool for a geologist to for gathering data for a multitude of rocks and bedding layers. Everything from layers of sedimentary rocks, fault plains, joints between rocks, and other structural features can provide valuable information from knowing the strike and. It is always important to determine whether the surface being measured is an actual inclined bed or structure and not a irregular feature like a landslide. Having the right equipment is also important for gathering accurate data in the field. Without the right equipment, a reliable compass and clinometer, the data gathered might be faulty and this would result in inaccurate conclusions being made about the rocks being studied.