Physical Geology Interactive: The Lab - Geologic Maps (Exercises-Activity 1)

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.

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: