ASBOG - Association of State Boards of Geology Field Geology and Mapping Questions and Answers

Question 1

A geologist is mapping a sedimentary sequence in an area with a known magnetic declination of 12° East.
When using a Brunton compass to measure the strike of a sandstone bed, which of the following adjustments is necessary to obtain the true geologic strike?

Accepted Answer

The azimuth ring must be rotated so the index pin is set to 12°.

Answer

The compass must be held perfectly level using the bull's-eye level.

Answer

The clinometer must be set to zero before taking the measurement.

Answer

No adjustment is needed; the raw reading provides the true strike.

Question 2

While creating a geologic cross-section from a map, which of the following is the most critical first step to ensure the accurate depiction of subsurface structures?

Accepted Answer

Drawing a topographic profile along the line of section without vertical exaggeration.

Answer

Coloring the different geologic units on the map before starting the section.

Answer

Assuming all contacts are vertical to simplify the initial drawing.

Answer

Extrapolating the surface geology to a depth of at least 5,000 feet.

Question 3

On a standard geologic map, what does a strike and dip symbol that looks like a 'T' with the number '35' next to the shorter line represent?

Accepted Answer

A planar feature striking parallel to the long line and dipping 35 degrees in the direction of the short line.

Answer

A horizontal bed with an elevation of 35 feet.

Answer

A vertical fault with 35 feet of offset.

Answer

A fold axis trending in the direction of the long line with a plunge of 35 degrees.

Question 4

A geologist is mapping a contact between a sandstone and a shale unit on a hillside. The outcrop width of the sandstone unit on the map is 200 meters. The beds dip at 30 degrees into the hill, and the ground surface is perfectly flat. What is the true stratigraphic thickness of the sandstone unit?

Accepted Answer

100 meters

Answer

173 meters

Answer

400 meters

Answer

200 meters

Question 5

Which of the following represents the most common type of error or 'blunder' made when transferring structural data from field maps to a final published geologic map?

Accepted Answer

Using the wrong symbol for a measurement (e.g., bedding vs. cleavage) or recording an incorrect dip value.

Answer

Incorrectly calculating the true stratigraphic thickness.

Answer

Misidentifying the rock type of a mapped unit.

Answer

Failing to correct for magnetic declination during fieldwork.

Question 6

During a field mapping project in a structurally complex area, a geologist encounters an outcrop where the bedding planes are contorted, and a consistent strike and dip measurement is impossible. What is the BEST practice for documenting this location in the field notes and on the map?

Accepted Answer

Use a specific symbol for contorted or convoluted bedding and provide a detailed sketch and description in the field notebook.

Answer

Leave the area blank on the map as no reliable data can be collected.

Answer

Average the various orientations to create a single representative strike and dip.

Answer

Record the measurement of the most dominant fold limb and ignore the others.