A trellis drainage pattern occurs where subparallel streams erode a valley along the strike of less resistant formations. These beds are usually steeply dipping and may be part of a fold system. The tributaries often intersect at right angles where a notch called a water gap cuts through a harder formation. If the notch and the surrounding formation are later uplifted, the water gap becomes a wind gap
A trellis pattern may also contain right-angled bends controlled by joints or faults. Such a pattern is called fault trellis drainage.
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