What geometric condition would most likely expose an older geologic unit on a hillside that lies above a younger unit?

• A. Steep slope with dip opposite to slope
• B. Erosional unconformity
• C. A gentle slope where the dip and slope are in the same direction and the dip is steeper than the slope
• D. Fault offset

Answer: (C)

The key idea is how bed orientation relative to the hillside controls where an older unit becomes exposed. For an older rock layer that sits atop a younger one to crop out on a hillside, that older layer must cut through the surface as it slopes downward toward the landscape. When the beds dip in the same direction as the slope and the dip is steeper than the slope, the edge of the older bed reaches the surface along the hillside, so you see the older material exposed there. If the dip were opposite the slope, it would tilt away from the hillside and not break the surface there, so the older unit wouldn’t be exposed on that slope. Erosional surfaces or faulting can create exposures too, but they involve different processes or geometries beyond this simple dip-slope interaction.