Laboratory Manual for Introductory Geology 4e

EXERCISE 10.2
The Basic Types of Contacts
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In the figure below, each arrow points to one of the four basic kinds of contacts. Add the labels.
Paleozoic
sedimentary strata
Cretaceous granite
Precambrian gneiss
10.2.2 Describing the Orientation of Layers:
Strike and Dip
You can efficiently convey information about the orientation, or attitude, of any planar
geologic feature, such as a bed or a contact, by providing two numbers. The first
number, called the strike, is the compass direction of a horizontal line drawn on the
surface of the feature (FIG. 10.3a). You can think of the strike as the intersection
between a horizontal surface (e.g., the flat surface of a lake) and the surface of the
feature. We can give an approximate indication of strike by saying “the bed strikes
northeast”, or we can be very exact by saying “the bed has a strike of N 45° E,” meaning
that there is a 45° angle between the strike line and due north, as measured in a
horizontal plane. The second number, called the dip, is the angle of tilt or the angle
of slope of the bed, measured relative to a horizontal surface. A horizontal bed has
a dip of 00°, and a vertical bed has a dip of 90°. A bed dipping 15° has a gentle dip,
and a bed dipping 60° has a steep dip. The direction of dip is perpendicular to the
direction of strike (FIG. 10.3b, c).
Because strike represents a line with two ends, a strike line actually trends in two
directions: a strike line that trends north must also trend south; one that trends
northwest must also trend southeast. How do we pick which direction a strike line
trends? By convention, strike is read relative to north, so you will generally only
see strikes described as angles east or west of north, or due north, east, or west
(FIG. 10.3d). Thus, the beds on the left side of Figure 10.3a strike N 38° E or 38°
east of north. Also by convention, beds that dip directly to the north are considered
to strike west, while those that dip directly to the south strike east.
10.2 BEGINNING WITH THE BASICS: CONTACTS AND ATTITUDE
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