How can the size of cave-wall scallops indicate past water speed?

In the study of caves, scallops are small, scooplike depressions found on passage walls that serve as paleo-flow meters, allowing geologists to estimate the velocity of water that once flowed through the system.1

The size of a scallop is inversely proportional to the flow velocity of the water. This means that smaller scallops indicate higher water speeds, while larger scallops indicate slower water speeds. Specifically:1

• High velocity: Scallops measuring only a few centimetres wide correspond to flow velocities on the order of a few metres per second.1
• Low velocity: The largest scallops, which can exceed one metre in width, indicate much slower flow velocities of only a few centimetres per second.1

Because these features are asymmetrical—featuring a steep wall on the upstream side and a gentler slope on the downstream side—they also function as reliable indicators of the direction of ancient water flow, even in passages that have been dry for hundreds of thousands of years.1