Arrange the following sequential stages in the retreat of a waterfall and the subsequent formation of a river gorge, from the initial bedrock arrangement to the final landscape feature.
- 1A river flows over a horizontal band of hard, resistant rock overlying a layer of soft, easily eroded rock.
- 2Hydraulic action and abrasion erode the softer underlying rock faster, creating a plunge pool and leaving an overhang of hard rock.
- 3The unsupported overhang of resistant rock eventually collapses into the plunge pool under the force of gravity.
- 4Continuous repetition of undercutting and collapse causes the waterfall to retreat upstream, leaving behind a steep-sided gorge downstream.
Answer
The correct order of stages in waterfall retreat and gorge formation begins with a river flowing over hard rock overlying soft rock, followed by undercutting of the softer rock to form an overhang, then collapse of the unsupported overhang into the plunge pool, and finally continuous headward retreat leaving behind a steep-sided gorge.
The evolution of a river gorge begins when running water encounters alternating rock strata, where a resistant rock layer overlies softer rock. Hydraulic action and abrasion erode the softer stratum beneath, creating an overhang above a plunge pool. When the overhang loses support, it collapses under gravity. As this sequence of undercutting and collapse repeats over time, the waterfall retreats upstream, leaving behind a narrow, steep-sided valley termed a gorge.
Step-by-Step Solution
Key Concept
Waterfall Retreat and Gorge Formation