Activity 1: Strike and Dip

Geology exists in three dimensions. Yet when we display it on a map, we see only two dimensions. To convey accurately the nature of geologic objects, we must use a shorthand notation to specify orientation in three dimensions. For a planar feature, its three-dimensional orientation is described by its strike and dip. Strike is the line formed by the intersection of a horizontal plane with the plane of the feature (Fig. 1). The strike of a rock layer fixes its orientation with respect to compass direction. The angle the layer dips into the ground is its dip. It is always perpendicular to strike. To visualize strike and dip, consider a tilted board placed in a tub of water. The board’s strike corresponds to the line traced on the board by the surface of the water. The dip is the direction water slowly poured on the board would flow down.

Fig. 1: Strike-and-dip relations.

Fig. 2: The strike-and dip-symbol and the determination of strike.
The strike-and-dip symbol is T-shaped (insert of Fig. 2). The long line is parallel to the strike, and the cross member indicates the direction of dip. These two lines are always at right angles to each other. A number next to the strike-and-dip symbol indicates the number of degrees the bed dips from the horizontal (Fig. 2). The quantitative value of the strike is not shown explicitly with the strike-and-dip symbol. It is obtained by measuring the angle between the strike line of the symbol and a line pointing north (Fig. 2). Strike is expressed in terms of quantrant, S45ºE, or as an azimuth, 135º.
In addition to the general strike-and-dip symbol, three unique orientations of rock layers require special strike-and-dip symbols (Fig. 3).

 Horizontal strata Vertical strata Dipping strata Overturned strata Fig. 3: The four types of strike-and-dip symbols.

Before doing this activity, you can practice with strike and/or strike-and-dip using two interactive practice exercises.

 To complete Activity 1, select one of these files: