You will need the following equipment:
Magnetic compass or an accurate compass on your GPS unit.
Print-out of an inclinometer (angle estimator) or a good knowledge of angle measurements.
Optional: carpenter’s level.
As you walk along the stream bank underneath the bridge, notice that the rock layers are all tilted off horizontal. In what direction and how much tilt is the subject of the measurements that geologists call “strike” and “dip.”
Knowing the strike and dip of a rock layer is very useful in being able to predict where that layer will be located underneath the surface. If a particular rock layer contains desirable minerals, geologists can calculate how deep and where to drill or mine in order to access that layer.
“Strike and dip” refers to the orientation of a rock layer in relation to a horizontal plane. The orientation of the tilted layer of rock can be described by two angular measurements. The strike of a rock bed is the compass direction of a horizontal line drawn along the plane of the bed. Compare it to a horizontal line along the top of a sloping roof. (Strike and Dip.)
Dip is the amount of slope of a rock bed in number of degrees from horizontal and is measured at 90º to the strike. In the house illustration, the dip is comparable to the slope of the roof. (Leet and Judson 436, 648; Schott)
Strike
Strike is sometimes difficult to visualize. Imagine a book placed inside a half-full aquarium. If the book is tilted, the line that the surface of the water makes on the cover of the book represents the strike of the book. It is a horizontal line along the plane of the cover. The compass direction that the line is running (northwest-southeast, for instance) is the strike direction. Even if you set the book on its corner, you will still have a horizontal line where the water surface intersects the book. ("Orientation Description")
To estimate the strike of this rock outcrop, you will need a compass. The compass on your GPS unit may be accurate enough, but a magnetic compass would be better. To measure the outcrop underneath the bridge, move to the middle of the width underneath the bridge to help reduce the effect on the compass caused by the iron above you. Make sure the rock surface you are going to measure is smooth, free of debris, and representative of the whole outcrop. Strike is traditionally reported in terms of North, so start out facing north.
Hold the long edge of your compass (or the west side if your compass is round) against the smooth surface of a rock layer, getting the compass as close to horizontal as you can. Read the bearing (the degree number or compass direction which is in line with the strike direction) as follows: There are several types of compasses. If your compass's East and West are in the normal positions, count the number of degrees between the north arrow and the North marking, but reverse the E/W directions. If yours has East and West in the reverse positions from normal, just read the bearing where the north arrow is pointing. If you have an orienteering compass, "put red Fred in the shed" and read the number of degrees and direction along the sighting arrow. On your GPS unit, simply read the direction and degrees at the top of the compass. Record the strike direction. For instance, NW (or 300º).
Strike direction = ______________________________________
Here’s how to measure the strike of a rock layer more accurately, using an ordinary carpenter’s level and a compass. Hold the level against the flat surface of a rock layer, as close to horizontal as you can. Rotate it until the bubble indicates that it is level. Then hold the long side of the compass against the middle of the horizontal surface of the level (once again, to reduce any effect due to any metal in the level). Read and record the bearing. (Compton 30)
Dip
Dip is the amount of slope of a rock bed in number of degrees from horizontal and is measured at 90º to the strike. Steeply inclined beds have a large dip angle while nearly horizontal beds have a small dip angle. (Leet and Judson 436; "Structural Geology")
Without expensive specialized equipment, you will have to estimate dip angle. No slope at all is 0º; vertical is 90º. About halfway in between is 45º. If the slope is steeper, it might be 60º; smaller would be about 30º.
You can get a better estimate by printing this inclinometer (angle estimator) onto a piece of paper or cardstock. Attach a short string at the black dot, and attach a metal nut or other small weight at the other end of the string so that the weight swings freely somewhere between the numbers and the bottom of the paper. Hold the indicated edge of the angle estimator against the rock surface. Read the number where the string crosses the circle edge; this is the dip angle in degrees.
(Dana)
In addition to an angle, dip needs to have a direction. Dip direction is exactly 90 º to the strike. For instance, a bed with a N-S strike would have either a W or E dip. At this outcrop, just use your compass to estimate the dip direction.
Record the dip direction and angle. For instance, SW (or 225º) at 65º.
Dip = ______________________
To log this earthcache, send me an email (rkclimber4Jesus@bellsouth.net or through my profile page) with your estimates of the strike and dip of this outcrop.
1. Your strike estimate (either in degrees or compass direction) needs to be within 45º to be acceptable.
2. Your dip angle estimate needs to be within 20º.
3. Make sure you remember to indicate dip direction.
Otherwise I’ll send you back under the bridge to try again!
Don't miss the additional earthcache and the traditional cache nearby!
Works Cited:
Compton, Robert R. Manual of Field Geology. New York, London, Sydney: John Wiley & Sons, 1962. 30. Print.
Dana, James D. Dip. 1883. Drawing. http://www.geologyclass.orgWeb. 10 Apr 2014. .
Leet, L. Don, and Sheldon Judson. Physical Geology. 4th Edition. Englewood Cliffs, New Jersey: Prentice-Hall, 1971. 436, 648. Print.
"Orientation Description." University of Wisconsin -Geoscience. University of Wisconsin-Madison. Web. 10 Apr 2014. . Page 4.
Schott, Ron. "Measuring and Recording Strike and Dip." GEOL B10L - Intro Geology Lab. N.p., 17 Nov 2013. Web. 10 Apr 2014. .
Strike and Dip. 2013. Graphic. n.p. Web. 10 Apr 2014. .
"Structural Geology." http://www.geosci.ipfw.edu/. Indiana University - Purdue University Fort Wayne, 04 Apr 2014. Web. 10 Apr 2014.