Geologic Maps Lab

Plunging Folds: folds in which the fold axis is dipping; the direction the fold axis is dipping is called the plunge direction

Basin: a fold in which the limbs dip toward the center in all directions (picture an upright bowl)

Dome: a fold in which the limbs dip away from the center in all directions (picture an overturned bowl)

Geologic Map: contains geologic units and structures displayed upon a topographic base

• the term "units" refers to geologic formations (see below)
• the term "structures" refers to faults and folds

Formation: a distinctive body of rock that is large enough to symbolize on a geologic map

The map symbol for a formation contains an abbreviation for its age of origin followed by an abbreviation of its name in lower-case letters (ex., Kgr for the Cretaceous age Glen Rose formation).

Other Map Symbols (Table 18-1, p. 362):

Common map symbols are a light solid line for a depositional or igneous intrusive contact, dashed or dotted where there is uncertainty. A fault is symbolized by a heavy solid line, dashed or dotted where there is uncertainty. A thrust fault is symbolized by a heavy solid line with triangular "teeth". Note that with fault map symbols the "tick" marks point in the direction of dip of the fault plane.

Anticlines and synclines are indicated by a line that describes the fold axis, and by opposing short arrows oriented in the direction that water would flow down the limb of the fold.

The Map Explanation:

The map explanation is a list of formations shown on the map; the formations on the bottom of the column are the oldest; getting younger as you move towards the top

INTERPRETATION OF MAP OUTCROP PATTERNS

• If contour lines AND contact lines parallel one another, the beds are nearly horizontal (approximately a 0° dip)
• For uniformly dipping strata, a thick bed has a broad outcrop and a thin bed has a narrow outcrop (p. 388).
• For stratum of uniform thickness, if it dips gently the outcrop is broad, and if it dips steeply the outcrop is narrow (p. 389).
• A "zig-zag" or serpentine pattern indicates the presence of plunging folds
• Where contour lines or a horizontal bed cross a stream, the outcrop pattern forms a "V" that points upstream.
• The outcrop pattern of a vertical bed does not deviate where it crosses a stream (p. 390).
• Outcrop patterns of dipping strata may V upstream or downstream depending upon the dip of the strata and shape of the valley (p. 390).

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