Interpretation of geological data is based on both personal judgement and previous experience of related scenarios. In combining such information geologists employ heuristics (rules of thumb), and are therefore subject to biases that are well known in cognitive psychology and are common to all expert judgements. Here we analyse dynamic uncertainty in an evolving geological interpretation. Through a well-designed elicitation process we show how the inclusion of multiple experts influences interpretational bias. In particular, group convergence of opinion is observed, and we show how this can be differentiated from ‘herding' behaviour similar to that observed in economic bubbles by forcing a consensus to be reached. Thus we can identify when and why the judgement of a single geological expert should be treated with caution. This process can be applied to any geological interpretational scenario.