WATERFALLS

• stage 1

  • formed where there is a junction between a hard rock capping upstream and soft rock downstream
  • differential erosion - the river erodes the softer rock and the water falls vertically from the hard rock to the soft rock below
• stage 2

  • splash back causes hydraulic action to weaken the rocks behind the fall of water
  • pieces of soft rock break off and an indentation is formed at the base
  • continued undercutting means that the hard rock capping loses support and overhangs the drop
• stage 3

  • the unsupported cap rock breaks off
  • the fallen rock breaks up and is caught in the turbulent flow of water
  • some rocks become trapped and drill into the bed which creates a plunge pool
  • undercutting continues creating a new overhang
  • as this repeats the waterfall retreats upstream leaving a gorge downstream