ASBOG Field Geology and Mapping 3 — Questions and Answers
Question 1: A geologist observes that bedding planes in a sandstone dip 45° to the east, while cleavage planes dip 60° to the east. The fold that produced this geometry is most likely located:
- To the west, in the upright limb
- To the east, past the fold hinge (Correct answer)
- At the outcrop location
- Beneath the unconformity
Correct answer: To the east, past the fold hinge
When cleavage dips more steeply than bedding in the same direction, the outcrop is on the overturned limb and the fold hinge lies to the east.
Question 2: Which map symbol correctly represents a vertical contact?
- A line with tick marks on the downthrown side
- A line with alternating long and short dashes
- A straight line without any dip tick (Correct answer)
- A line with a T-shaped symbol perpendicular to it
Correct answer: A straight line without any dip tick
A vertical contact or vertical fault is shown as a straight line with no dip tick because the feature is perpendicular to the map surface.
Question 3: In a V-pattern on a geologic map, beds with a V pointing upstream that have dip values less than the stream gradient indicate:
- Vertical beds
- Beds dipping downstream at less than the valley gradient (Correct answer)
- Beds dipping upstream
- Horizontal beds
Correct answer: Beds dipping downstream at less than the valley gradient
When beds dip in the same direction as the stream but at a gentler angle than the valley slope, the V-pattern points upstream but opens more gently than the contours.
Question 4: A geologist encounters a fault with slickenlines plunging 80° on the fault surface. This kinematic indicator suggests:
- Predominantly strike-slip motion
- Predominantly dip-slip motion (Correct answer)
- Oblique slip at 45°
- Pure rotational motion
Correct answer: Predominantly dip-slip motion
Slickenlines plunging steeply (near 90°) on a fault surface indicate nearly pure dip-slip motion.
Question 5: Which property of a rock unit makes it most useful as a regional stratigraphic marker bed?
- Great thickness and variable lithology
- Lateral continuity and distinctive character (Correct answer)
- High resistance to weathering
- Presence of abundant fossils only
Correct answer: Lateral continuity and distinctive character
Marker beds are useful precisely because they can be traced over a wide area and are lithologically or biologically distinctive.
Question 6: When mapping a contact between two units in rugged terrain, the geologist should prioritize which technique?
- Connecting GPS points with straight lines on the map
- Walking the contact and noting its position relative to topography (Correct answer)
- Extrapolating from a single outcrop using assumed constant dip
- Using satellite imagery to trace color differences
Correct answer: Walking the contact and noting its position relative to topography
Walking the contact and recording its position relative to topographic features provides the most accurate three-dimensional placement of the contact.
Question 7: A geologist measures three strike-and-dip readings across a dome structure. The strikes diverge radially and the dips increase outward. This pattern best indicates:
- A plunging anticline
- A structural basin
- A periclinal dome with a flat top (Correct answer)
- A salt diapir
Correct answer: A periclinal dome with a flat top
Radially diverging strikes with outward-increasing dips define a periclinal or doubly plunging dome structure.
ASBOG Field Geology and Mapping 3
A geologist observes that bedding planes in a sandstone dip 45° to the east, while cleavage planes dip 60° to the east.
The fold that produced this geometry is most likely located: