ASBOG Field Geology and Mapping 2 — Questions and Answers
Question 1: When measuring strike using the right-hand rule, the geologist holds their right hand along the strike line with fingers pointing in which direction?
- Down-dip
- Up-dip
- In the direction of dip
- So the thumb points in the dip direction (Correct answer)
Correct answer: So the thumb points in the dip direction
The right-hand rule states that when you align your right hand along the strike with the thumb pointing in the dip direction, fingers curl in the direction of dip.
Question 2: A geologist records a fold whose axial plane dips 30° and whose hinge line plunges 20°. This fold is best described as:
- Reclined
- Inclined plunging (Correct answer)
- Recumbent
- Upright
Correct answer: Inclined plunging
An inclined plunging fold has an axial plane that dips at an intermediate angle and a hinge line that plunges in the same vertical plane.
Question 3: On a geologic map, a contact that is concentric with topographic contours and closes on itself typically represents:
- A thrust fault
- A dike
- An intrusive body or dome (Correct answer)
- A normal fault
Correct answer: An intrusive body or dome
Circular to elliptical contacts that parallel topographic contours indicate an intrusive body such as a pluton or a structural dome.
Question 4: Which approach is MOST appropriate when mapping in areas with poor outcrop exposure?
- Assume continuous flat-lying beds between outcrops
- Use float, soil color, and vegetation patterns to infer lithology (Correct answer)
- Only map where rock is exposed
- Extrapolate strike from aerial photos alone
Correct answer: Use float, soil color, and vegetation patterns to infer lithology
Indirect evidence such as float, soil characteristics, and vegetation changes helps infer subsurface geology where outcrops are absent.
Question 5: The apparent dip of a bed is always _____ the true dip, except when the cross-section is cut perpendicular to strike.
- Greater than
- Equal to
- Less than (Correct answer)
- Unrelated to
Correct answer: Less than
Apparent dip is always less than true dip because you are measuring across an oblique section rather than perpendicular to strike.
Question 6: A reverse fault in map view typically displays which relationship between hanging wall and footwall contacts?
- Hanging wall rocks are younger than footwall rocks
- The fault contact dips away from the observer
- Older rocks are thrust over younger rocks (Correct answer)
- The fault scarp faces the direction of net slip
Correct answer: Older rocks are thrust over younger rocks
Reverse and thrust faults place older (deeper) rocks over younger rocks because the hanging wall moves up relative to the footwall.
Question 7: When sketching a stratigraphic column in the field, the geologist should record which of the following for each bed?
- Only thickness and color
- Lithology, thickness, grain size, sedimentary structures, and contacts (Correct answer)
- Formation name and GPS coordinates only
- Fossil content and chemical composition only
Correct answer: Lithology, thickness, grain size, sedimentary structures, and contacts
A complete stratigraphic column requires lithology, thickness, grain size, sedimentary structures, and contact types to allow later interpretation.
ASBOG Field Geology and Mapping 2
When measuring strike using the right-hand rule, the geologist holds their right hand along the strike line with fingers pointing in which direction?