In a trellis drainage system, what geologic feature primarily causes its formation?

A trellis drainage system typically forms in areas where the geology consists of deformed strata with alternating layers of varying resistance to erosion. The structural complexity of these layers leads to distinctive river patterns. In such a system, rivers tend to flow parallel to the more resistant rock layers, creating a main channel that may be interrupted by smaller tributaries that cut across the weaker strata. This gives the appearance of a trellis, where streams resemble the framework of a garden trellis.

The alternating layers of rock that are more resistant to erosion create a pattern of valleys and ridges. As streams navigate these changes in rock type, they are influenced by the geological structure, resulting in a characteristic drainage network. Thus, the features that dominate a trellis drainage pattern are primarily the varying strengths of the geological layers, rather than uniform layers of rock, flat areas, or volcanic formations.