Laboratory Manual for Introductory Geology 4e

Let’s begin our discussion of geologic maps by considering the various features
that can be portrayed on these maps. (You will practice mapping these features in
Exercise 10.11.)
■ Rock units: Geologic maps show the different rock units in an area. These units
may be bodies of intrusive igneous rock, layers of volcanic rock, sequences of sedimentary
rock, or complexes of metamorphic rock. The most common unit of sedimentary
and/or volcanic rock is a stratigraphic formation, as noted earlier. A formation is commonly
named for a place where it is well exposed. A formation may consist entirely
of beds of a single rock type (e.g., the Bright Angel Shale consists only of shale), or it
may contain beds of several different rock types (e.g., the Bowers Mountain Formation
contains shale, sandstone, and rhyolite).
Typically, geologic maps use patterns, shadings of gray, or colors to indicate the area
in which a given unit occurs. On geologic maps produced in North America, an abbreviation
for the map unit may also appear within the area occupied by the formation.
This abbreviation generally has two parts: the first part represents the formation’s
age, in capital letters; the second part, in lowercase letters, represents the formation’s
name. For example, O indicates rocks of Ordovician age (Oce 5 Cape Elizabeth Formation,
Osp 5 Spring Point Formation), and SO indicates Silurian or Ordovician age
(SOb 5 Berwick Formation, SOe 5 Eliot Formation). For further explanation of geologic
time periods, see Chapter 12 and specifically Figure 12.8, the geologic time scale.
■ Contacts: Different kinds of contacts are generally shown with different types of
lines. For example, a conformable or intrusive contact is a thin line, a fault contact is
a thicker line, and an unconformity may be a slightly jagged or wavy line. In general,
a visible contact is a solid line, whereas a covered contact (buried by sediment or
vegetation) is a dashed line (see dashed line in Fig. 10.9).
■ Strike and dip: On maps produced in North America, geologists use a symbol to
represent the strike and dip of a layer. The symbol consists of a line segment drawn
exactly parallel to the direction of strike and a short tick mark drawn perpendicular
to the strike and pointing in the direction of dip (FIG. 10.10a, b). A number written
next to the tick mark indicates the angle of dip. (It is not necessary to write a
number indicating the strike angle because that is automatically represented by the
map trend of the strike line.) Different symbols are used to represent bedding and
foliation; in this manual, we use only bedding symbols.
■ Other structural symbols: The explanation also includes symbols representing the
traces of folds and faults. FIGURE 10.10c illustrates some of these symbols.
FIGURE 10.10 Indicating
features on geologic maps.
N
N
35˚
35
(a) A block diagram showing
the dip angle.
(b) A geologic map showing a
strike and dip symbol.
U D
Anticline
Syncline
Plunging
anticline
Plunging
syncline
High-angle fault;
U moved up,
relative to D.
(c) Basic structural symbols used on geologic maps.
Thrust fault;
teeth are on the
hanging-wall side.
258 CHAPTER 10 INTERPRETING GEOLOGIC STRUCTURES