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

To correct for magnetic declination, the azimuth ring on the Brunton compass must be adjusted. For an East declination, the index pin is turned the corresponding number of degrees counter-clockwise (to the East on the dial) from 0°. This adjustment ensures that the compass needle's reading relative to the adjusted dial will indicate true north, thereby providing an accurate geologic strike measurement.

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

The foundational step for a geologically accurate cross-section is establishing the ground surface. This is done by creating a topographic profile along the chosen line of section. It is crucial that this profile has no vertical exaggeration (a 1:1 scale) initially, because exaggerating the vertical scale will distort the apparent dip of geologic contacts and structures, leading to an incorrect interpretation of the subsurface geology.

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

The standard symbol for strike and dip on a geologic map uses a long line to represent the strike direction of a planar feature (like a sedimentary bed or fault). The shorter, perpendicular line points in the direction of the dip. The accompanying number indicates the angle of the dip in degrees from the horizontal.

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

The true stratigraphic thickness (T) can be calculated using the formula: T = W * sin(d), where W is the outcrop width on a horizontal surface and d is the dip angle. In this case, T = 200 meters * sin(30°). Since sin(30°) = 0.5, the true stratigraphic thickness is 200 * 0.5 = 100 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

Studies analyzing the transfer of data from field notes to published maps have shown that the most frequent blunders are human errors in transcription. These include using the wrong symbol for a structural feature (e.g., mistaking a bedding symbol for a cleavage or joint symbol) and errors in transcribing the numerical value of the dip angle.

ASBOG - Association of State Boards of Geology Practice Test

ASBOG - Association of State Boards of Geology Practice Test

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