Final answer:
A trellis drainage pattern occurs where hard and soft rocks exist parallel to each other, leading to streams joined by tributaries at right angles, and is common in areas with alternating bands of resistant and less resistant sedimentary rocks.
Step-by-step explanation:
The type of drainage pattern that occurs where rock that is resistant to weathering alternates with rock that erodes easily is a trellis pattern. A trellis drainage pattern develops where hard and soft rocks exist parallel to each other, leading to parallel streams that are periodically joined by tributaries at nearly right angles. This pattern resembles a garden trellis, hence the name. Over time, the softer rock erodes away, creating valleys or troughs, while the harder rock remains as ridges.
The trellis pattern is common in regions where sedimentary rock layers have been folded or tilted and then eroded, creating alternating bands of resistant and less resistant rocks. It's indicative of former geological activity, such as the movement of tectonic plates that created fold mountains or where differential erosion has laid bare the varying rock types.