The attitude of a rock unit is given in terms of the strike and the dip of the rock unit's surface.
The attitude of a rock unit, any planar geological feature or any other surface is a description of that surface in terms of its orientation in three-dimensional space given as strike and dip. The strike of a surface is the compass direction that a horizontal plane intersecting that surface runs. The dip of a surface is the angle measured between the imaginary horizontal strike plane and the slope of the rock unit's surface. Use a few simple tools to measure the strike and dip of any surface and to define the attitude of that surface.
Instructions
Measure the Attitude of Rock Units
1. Use a carpenter's bubble level to define an imaginary horizontal plane, which intersects or bisects any surface. Place the bubble level on the surface whose attitude you wish to measure. Adjust the level against that surface until the bubbles are centered in their tubes. Draw a line using the straight edge of the level as a guide. The line drawn is the strike of the surface. Measure the direction of strike with a compass in terms of north as a bearing in degrees or as a compass direction.
2. Place a golf ball on the strike line you drew previously. Release the ball. The direction that the ball rolls is perpendicular to the strike line by definition and represents the dip of the surface whose attitude you are describing. Gravity will carry the ball along the steepest angle, which is the true dip of the unit. Anything less is known as an apparent dip.
3. Place the end of the bubble level perpendicularly against the strike line you drew previously, and raise the level until the bubbles are centered in their tubes. Use a protractor to measure the number of degrees in the angle between the horizontal level and the surface of the rock unit perpendicular to the strike in the same direction that the golf ball rolled. This is the dip of the unit, and its compass direction is the strike direction plus 90 degrees by definition because it is perpendicular to the strike. Give dip directions in terms of compass azimuth -- southwest or northeast, for example -- rather than bearings in degrees.
4. Use a Brunton pocket transit with a built-in clinometer to measure the strike and dip of the rock unit and to check the results you obtained previously. Place the side of the Brunton pocket transit against the rock unit and adjust the compass in space until its internal bubble level is centered in its circle. Take the compass bearing in terms of degrees from north. This is the strike of the unit and should be the same as the strike you obtained using a bubble level and ordinary compass.
5. Turn the Brunton pocket transit on its side perpendicular to the strike line determined previously. Use the clinometer view finder portion of the Brunton to view the angle of the dip of the unit's surface. This dip angle should be the same as that which you determined previously using the bubble level, protractor, compass and golf ball.
6. Record the attitude that you have determined in terms of strike and dip on a published map or field drawing of the rock unit. Draw an oriented "T"-shaped symbol to represent the attitude. The longer top of the "T" is the strike oriented to the map, and the shorter stem of the "T" is the dip given as an angle in degrees and an azimuth direction. Take several readings at various locations on the rock unit surface, and record them to confirm your measurements and to observe any variations.
Tags: rock unit, bubble level, perpendicular strike, strike line, that surface, unit surface, Brunton pocket
