The Geology of Pacific Northwest Volcanoes, Mountains and Earthquakes
Lab 5: Geologic Maps
Name: _______________________________________________ Date: ___________
We will use some basic geologic mapping principles to complete this geologic mapping lab.
Generally, geologists can see how bodies of rock or sediment are positioned three dimensionally where
they crop out (stick out of the ground as an outcrop) at Earth’s surface. They record this outcrop data on
flat (two-dimensional) geologic maps using different colors and symbols to represent the locations of
different layers or formations of rock and their directions and angles of tilting or other deformation. This
map data are used to infer the geometry of geologic structures.
Structural geology symbols and abbreviations used on geologic maps.
The Geology of Pacific Northwest Volcanoes, Mountains and Earthquakes
Lab 5: Geologic Maps
RULES FOR INTERPRETING GEOLOGIC MAPS
1. Anticlines have their oldest beds in the center, and their limbs (sides) dip away from the fold
axis.
2. Synclines have their youngest beds in the center, and their limbs (sides) dip toward the fold axis.
3. Plunging anticlines plunge toward the nose (closed end) of the V-shaped outcrop belt.
4. Plunging synclines plunge toward the open end of the V-shaped outcrop belt.
5. Streams cut “V” shapes into tilted beds and formation contacts that point in the direction of dip
(except in rare cases when the slope of the stream is greater than the dip of the beds and
formation contacts).
6. Streams cut “V” shapes into horizontal beds and formation contacts that point upstream. The
formation contacts are parallel to topographic contour lines, and the stream drainage system
developed on horizontal and/or unstratified formations has a dendritic pattern that resembles
the branching of a tree.
7. Vertical beds do not “V” where streams cut across them.
8. The upthrown blocks of faults tend to be eroded more (down to older beds) than downthrown
blocks.
9. Contacts migrate downdip upon erosion.
10. True dip angles can only be seen in cross section if the cross section is perpendicular to the fault
or to the strike of the beds.
The strike and dip symbol is used on maps to indicate the attitude (position relative to horizontal) of
rock layers. The long line segment in the symbol indicates strike (a compass direction), and the short line
segment points in the compass direction of dip (always perpendicular to strike). A number beside the
symbol (if given) indicates the angle of dip
(tilting). Compass directions can be expressed
as bearings in quadrant or azimuth form.
Azimuth bearings should be expressed as
three digits in order to distinguish them from
two-digit dip angles. Strike can be expressed
as a bearing in either direction.
The Geology of Pacific Northwest Volcanoes, Mountains and Earthquakes
Lab 5: Geologic Maps
Part I (2 pts). This is a copy of the geologic map from the ppt presentation.
(1) Add symbols to show the attitude (strike and dip) of the
formations.
(2) Notice the abrupt change in dip at the contact between
the Blue Fm and Yellow Fm Figure 10.1. This contact could be
at least two different kinds of geologic structures. What are
they?
____________________________
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Part II (3 pts). For each map below, record the strike of the
formations and their contacts. Then use the Rule of Vs to
determine the dip direction and sketch formation contacts on
the geologic cross section. Cut a V-shaped notch in a 3 X 5
card. Hold the card vertically and look on the edge. See Vs at
different dips of the card.
The Geology of Pacific Northwest Volcanoes, Mountains and Earthquakes
Lab 5: Geologic Maps
Part III (15 pts). Three Paper Models; Cut and fold them as noted on each model.
Model 1: This model shows Ordovician (O), Silurian (S), Devonian (D), Mississippian (M), Pennsylvanian
(IP), and Permian (P) formations striking due north and dipping 24° to the west. Provided are a complete
geologic map (the top of the diagram) and three of the four vertical cross sections (south, east, and west
sides of the block diagram).
Finalize the vertical cross section on the north side of the block so it shows the formations and their
dips. On the map, draw a strike and dip symbol on the Mississippian sandstone that dips 24 degrees to
the west.
Model 2: The geologic map is complete, but only two of the cross sections are available. Letters A–G are
ages from oldest (A) to youngest (G).
Complete the north and east sides of the block. Notice that the rock units define a fold. This fold is an
anticline, because the strata are convex upward and the oldest formation (A) is in the center of the fold.
It is symmetric (non-plunging), because its axis is horizontal. On the geologic map, draw strike and dip
symbols to indicate the attitudes of formation E at points I, II, III, and IV. Also draw the proper symbol on
the map (top of model) along the axis of the fold.
How do the strikes at all four locations compare with each other?
How does the dip direction at points I and II compare with the dip direction at points III and IV? In your
answer, include the dip direction at all four points.
Model 3: Complete the remaining two-and-a-half sides of this model, using as guides the geologic map
on top of the block and the one-and-a-half completed sides. On the map, draw strike and dip symbols
showing the orientation of formation C at points I, II, III, and IV. Also draw the proper symbol along the
axis of the fold.
How do the strikes of all four locations compare with each other?
How does the dip direction (of formation C) at points I and II compare with the dip direction at points III
and IV? Include the dip direction at all four points in your answer.
Is this fold plunging or non-plunging? ________________