stream that flows through resistant formations because its course was established at a higher level on uniform rocks before downcutting began.

a stream that incises (erodes) its channel down into older rocks and rock structures (such as folds or faults) that were buried under some younger rock or sediment. Usually, the superposed stream is established on the younger material first, then regional uplift forces the downcutting of the stream channel.

A stream that cuts across resistant bedrock units. This can occur when the stream's course was determined at a previous time and on a previous landscape.