What is the mostly likely setting for this drainage pattern? Multiple Choice The rocks have a relatively equal resistance to erosion. Drainages have followed a series of fractures that branch off of one another. The area has folded or faulted rocks with different resistances to erosion.
The Correct Answer and Explanation is:
The most likely setting for the described drainage pattern — where drainages have followed a series of fractures that branch off one another — is:
“Drainages have followed a series of fractures that branch off of one another.”
Explanation:
Drainage patterns describe the arrangement of rivers, streams, or channels on the landscape and are closely related to the underlying geology, including rock types, structures, and erosional resistance. The pattern where streams follow a series of fractures or joints branching off from one another is called a dendritic drainage pattern.
Here’s a detailed breakdown:
- Equal resistance to erosion (First option):
If rocks have a relatively uniform resistance to erosion and there are no significant structural controls like fractures or folds, the drainage tends to develop a dendritic pattern. This pattern resembles the branches of a tree, with many small tributaries joining to form larger streams. However, dendritic patterns generally do not reflect fracturing but rather homogeneous rock and random erosion paths. - Drainages following fractures branching off (Second option):
When streams align themselves along fractures, joints, or faults, the drainage pattern reflects the structural control of these weaknesses in the rock. Fractures provide paths of least resistance for water to erode channels. If these fractures branch off systematically, the drainage pattern mimics this fracturing, forming what is called a trellis or rectangular drainage pattern, depending on fracture orientation. - Folded or faulted rocks with different resistances (Third option):
If an area has folded or faulted rocks with alternating layers of strong and weak rocks, streams preferentially erode the weaker layers. This results in a trellis drainage pattern, where tributaries enter the main stream at right angles, reflecting the folded topography and varied erosion resistance.
Why the correct answer is the second option:
- The description “Drainages have followed a series of fractures that branch off of one another” directly indicates structural control by fractures. These fractures serve as pathways for water flow, eroding along these lines and creating a drainage pattern that mirrors the fracture network.
- This pattern often forms in areas with fractured bedrock, where streams exploit these weaknesses.
- The other two options imply either uniform rock resistance with random drainage or folded/faulted geology with varying erosion resistance—neither of which matches the direct branching fracture-controlled drainage.
Summary:
- Fracture-controlled drainage: Streams follow joints/fractures, forming branching patterns aligned with fractures.
- Uniform resistance drainage: Streams form dendritic patterns without structural alignment.
- Folded/faulted rocks: Streams form trellis patterns aligned with weaker rock layers.
Hence, the best match for the described pattern is the second option:
Drainages have followed a series of fractures that branch off of one another.