Softer rock underlying hard rock
Explanation
Waterfalls are significant erosional landforms formed by the action of running water (rivers), typically in the upper course. Their formation is often driven by the process of differential erosion, where rock layers of varying resistance erode at different rates.
Analysis of Waterfall Formation
- Geological Structure: Waterfalls frequently develop where a band of resistant rock (hard rock, such as granite or sandstone) lies horizontally or gently dips over a band of less resistant rock (soft rock, such as shale or limestone).
- Mechanism of Undercutting:
- As the river flows over the hard cap rock and plunges down, the force of the falling water creates a plunge pool at the base of the falls.
- The swirling water, aided by rock fragments (abrasion) and hydraulic action, attacks the base of the cliff.
- Since the base consists of softer rock, it erodes much faster than the overlying hard rock.
- This basal erosion creates a hollow or cave-like depression behind the falling water. This process is known as undercutting.
- Collapse and Retreat: The undercutting leaves the hard rock overhanging without support. Eventually, the overhang collapses under its own weight and gravity. This process repeats, causing the waterfall to retreat upstream and leave a gorge in its wake.
- Conclusion: The water power primarily acts to erode and undercut the softer rock underlying the hard rock, rather than the hard rock itself or the river bed uniformly.
Key Takeaway
Undercutting in waterfall formation is the result of differential erosion, where the river erodes the softer rock layer at the base more rapidly than the resistant hard rock on top, leading to the collapse of the overhang.