A trellis drainage pattern is indicative of which geological condition?

• A. Unfolded beds
• B. Folded beds
• C. Uniform rock resistance
• D. Dune-like deposition

Answer: (B)

A trellis drainage pattern shows the land is structurally controlled by folded, layered rocks with alternating hardness. In folded terrains, long, parallel ridges form from the more resistant beds, while the valleys carve through the softer beds. The main stream tends to stay in the resistant layer, and tributaries cut across bedding planes to join the main channel at roughly right angles, producing the ladder-like, trellis appearance. This pattern is typical where anticlines and synclines set up different resistances in a regular, arranged fabric of rock. It wouldn’t arise from unfolded beds, uniform rock strength, or dune-like deposition, which produce different drainage styles.