Which of the following statements is incorrect?

Potholes form because of stream erosion aided by the abrasion of rock fragments.

Waterfalls are also transitory like any other landform and they too recede gradually.

Valley types depend upon the type and structure of rocks in which they form.

Pebbles and boulders get collected in potholes and get rotated by flowing water and consequently the gorges are formed.

Valleys start as small and narrow rills that gradually develop into long and wide gullies, which consequently form valleys.