WHAT IS A GEOLOGIC
MAP AND HOW IS IT USED?
G. Richard Whittecar
Department of Geological Sciences
Old Dominion University
Norfolk, Virginia 23529
Level: High School and College
Anticipated Learning Outcomes
- Students will understand how a geologist uses a geologic
map to predict locations of unseen rocks after they record observations
on base maps, classify data, and analyze map patterns.
- Students will understand relationships between outcrops,
contacts, formations, and geologic structures.
- Large box or tray half-filled with clean, dry sand.
- Numerous circles (1-2" wide) of stiff, colored paper.
- String, colored and lead pencils, and adhesive tape.
Procedure (before class)
- Mold surface of sand into realistic "landscape"
(flat or hilly).
- Fasten string across box to form coarse grid of squares;
prepare copies of base map showing box outline and grid.
- Each paper circle represents an outcrop of a sedimentary
rock (colors represent different rock formations) lying at a given attitude
(angle of the circle stuck into the sand). Place "outcrops" across
the "landscape" according to a predetermined pattern to represent
flat, tilted, folded, and/or faulted strata.
- Each student should do the following for each "outcrop":
a. Locate the "outcrop" on her/his base map and mark its location
with a small colored dot.
b. Draw a short line on top of that dot to indicate the strike direction
of that "outcrop."
c. (optional) Draw a short line on top of that dot to indicate the dip
direction of that "outcrop."
- After numerous "outcrops" are marked, students
should draw black lines ("contacts") to separate areas of the
map with different colored dots. Lines should roughly parallel the strike
lines of nearby outcrops. Lightly color all formations on the map.
- Ask students to predict what rock types are hidden underground
between selected "outcrops" and to explain their logic.
- Students will benefit greatly from repeated trips to
the "field" (terrain model) as they draw contacts and other map
- If paper squares are used for outcrops, the edges often
confuse students' recognition of strike directions.
- Chips of gravel make good "outcrops" of igneous
- A very large number of geologic structures and situations
can be represented by this method. Be creative!
Whittecar, G.R. 1984. Terrain models in field geology courses.
Journal of Geological Education 32:153-155.