Geological mapping relies heavily on understanding the three-dimensional orientation of rock layers and other planar features. Quantifying this orientation, often referred to as the attitude of the plane, is fundamental for interpreting the subsurface structure and history of an area. These precise field measurements provide the necessary data to project geological contacts, reconstruct deformation events like folding and faulting, and ultimately create accurate geological maps. Accurately recording their tilt and direction of slope is a basic skill for anyone studying the Earth’s processes.
Defining Planar Orientation
To describe the attitude of an inclined rock layer, geologists use two mutually perpendicular values: strike and dip. Strike is defined as the compass bearing of the horizontal line created by the intersection of the inclined plane and an imaginary horizontal surface. This strike line represents a line of zero inclination on the rock layer.
Dip describes the angle and direction of the plane’s slope. The dip angle is the acute angle of inclination, measured downward from the horizontal plane, and always ranges from 0° (horizontal) to 90° (vertical). This angle must be measured along the line of steepest descent on the planar surface, which is always perpendicular to the strike line. The dip direction is the compass azimuth that indicates the downhill direction of the plane.
Essential Equipment and Preparation
The primary instrument used to measure strike and dip is a specialized geological compass, often a Brunton or a lensatic compass equipped with a clinometer. The compass component measures the horizontal direction (strike), while the clinometer is a built-in pendulum or leveling device used to measure vertical angles.
Before taking any measurement, a representative, flat, and unweathered segment of the planar feature must be selected. If the rock surface is rough or curved, a flat object like a field notebook or a clipboard can be placed against the surface to provide a smooth, planar reference. Ensuring the surface is true to the overall planar structure is important for obtaining accurate data.
Measuring Strike and Dip in the Field
The initial step in the measurement process is determining the strike of the rock layer. The long edge of the compass is placed flat against the plane, and the instrument is rotated horizontally until the circular bubble level is perfectly centered. Centering this level ensures the compass edge is lying along a truly horizontal line on the inclined rock surface. The compass reading is then recorded as the strike bearing, typically as an azimuth between 0° and 360°.
Once the strike is measured, the next step is to find the dip angle. The compass is turned 90 degrees so that its edge is now aligned perpendicular to the previously measured strike line, pointing down the line of steepest descent. The clinometer arm is then adjusted until its separate bubble level is centered, and the resulting angle is read directly from the clinometer scale. This number, between 0° and 90°, is the true dip angle, representing the maximum inclination of the plane.
The final part of the field measurement is determining the dip direction, which is the compass direction the plane is actually sloping towards. This direction is always perpendicular to the strike reading. A common practical tip is to hold the compass horizontally over the plane and observe the direction an imaginary ball would roll or water would flow downhill. It is also important to avoid taking measurements near metal objects, such as car hoods or belt buckles, as these can cause local magnetic interference and skew the compass reading.
Standardizing Data Recording
After the physical measurements are taken, the data is recorded using standardized conventions to ensure clear communication among geologists. The two most common methods are Quadrant Notation and Azimuth Notation.
Quadrant Notation
Quadrant notation specifies the strike as a bearing relative to North or South, followed by the degrees and the East or West direction (e.g., N30°E). This strike is paired with the dip angle and a general direction (e.g., 45°SE), resulting in a full reading of N30°E / 45°SE.
Azimuth Notation
Azimuth notation is generally preferred for computer-based data analysis and geographic information systems (GIS) because it uses a single, three-digit number for the strike or the dip direction. For the same example, the strike might be recorded as 030°, paired with the dip angle and direction (e.g., 030 / 45 SE). Alternatively, the orientation can be recorded using only the dip azimuth and dip angle (e.g., 120 / 45). On geological maps, the data is represented by a T-shaped symbol, where the longer line shows the strike direction, the shorter line points in the dip direction, and the dip angle is written numerically next to the symbol.