A circular feature on a geologic map in an area of moderate relief is most likely produced by which geological structure?

• A. A structural dome
• B. An intrusion
• C. A horizontal bed
• D. A fault

Answer: (A)

A circular feature on a geologic map in an area with moderate relief is most consistent with a structural dome. A dome is an upwarping of rocks that causes beds to dip away from the center in all directions, producing a circular or oval pattern on the map. The surrounding strata tilt outward from the center, so you see a concentric arrangement of rock units around a central uplift. In moderate relief, this uplift can be gentle yet still map as a distinct circular feature.

This differs from a horizontal bed, which would appear flat with no circular pattern; a fault would show a linear displacement along a fault line rather than a circular outcrop; and an intrusion, while it can be circular, would typically present a distinct igneous body with a recognizable contact and lithology contrast rather than the broad, outward-dipping pattern of a dome.