What feature forms where a river meets softer rock after flowing over more resistant material, with waterfalls retreating to leave a gorge?

What feature forms where a river meets softer rock after flowing over more resistant material, with waterfalls retreating to leave a gorge?

• A. Watershed
• B. Traction
• C. Waterfalls
• D. Upper course

Answer: (C)

When a river moves from more resistant rock to much softer rock, the softer rock erodes faster, so the base of the harder rock is undercut. The overhang of the hard cap rock eventually collapses, creating a vertical drop. Over time, the waterfall retreats upstream as erosion continues, leaving behind a deep, narrow valley downstream—the gorge. This pattern is the classic formation process of a waterfall, which is why that feature fits the description best. The other terms don’t describe a landform created by this differential erosion and headward retreat: a watershed is a drainage boundary, traction is a mode of transport for bedload, and the upper course is simply the start of a river, not a feature formed by this process.

When a river moves from more resistant rock to much softer rock, the softer rock erodes faster, so the base of the harder rock is undercut. The overhang of the hard cap rock eventually collapses, creating a vertical drop. Over time, the waterfall retreats upstream as erosion continues, leaving behind a deep, narrow valley downstream—the gorge. This pattern is the classic formation process of a waterfall, which is why that feature fits the description best. The other terms don’t describe a landform created by this differential erosion and headward retreat: a watershed is a drainage boundary, traction is a mode of transport for bedload, and the upper course is simply the start of a river, not a feature formed by this process.