Geologic Mapping & Field Techniques Flashcards

A bed is recorded as striking N30E and dipping 40° to the SE. What is the dip direction azimuth?

Answer: 120°

Dip direction is perpendicular to strike on the down-dip side, so N30E + 90° to the SE gives 120°.

Beds dip downstream at an angle steeper than the stream gradient. Which way does the outcrop V point where the beds cross the valley?

Answer: Downstream

Under the rule of Vs, beds dipping downstream more steeply than the stream gradient form Vs that point downstream.

A cross-section is drawn at an oblique angle to strike. How does the dip shown on the section compare with the true dip?

Answer: It is less than the true dip

Apparent dip on any section not perpendicular to strike is less than the true dip.

Local magnetic declination is 12° E. A Brunton without a declination correction reads a magnetic bearing of 050°. What is the true bearing?

Answer: 062°

With east declination, you add it to the magnetic bearing to get the true bearing: 050° + 12° = 062°.

On a standard USGS-style geologic map, what does a dashed contact line usually mean?

Answer: An approximately located contact

Solid lines mark accurately located contacts, dashed lines mark approximately located ones, and dotted lines mark concealed ones.

A basalt dike cuts across folded sandstone and shale but is cut off by a flat erosion surface overlain by conglomerate. Which sequence is correct from oldest to youngest?

Answer: Sandstone/shale, folding, dike, erosion, conglomerate

Cross-cutting relationships and superposition put the folded host rocks first, then the dike, then erosion and deposition of the conglomerate.

On a 1:24,000-scale USGS 7.5-minute quadrangle, 1 inch on the map equals how many feet on the ground?

Answer: 2,000 ft

24,000 inches equals 2,000 feet, so 1 inch on the map represents 2,000 feet.