Two geologists disagree about the origin of a roughly circular lake basin.

Geologist A (Glacial Scour): The basin was carved by an advancing glacier during the last ice age. Glacial scour produces a U-shaped bedrock cross-section and leaves unsorted rock debris (till) resting directly on the bedrock floor.

Geologist B (Volcanic Collapse): The basin is a collapsed volcanic caldera. Caldera collapse produces a roughly circular basin floored by layered volcanic ash and pumice, with no till present.

A new core sample through the basin floor reveals a U-shaped bedrock cross-section and a layer of unsorted till resting directly on bedrock; no volcanic ash or pumice is found anywhere in the core.