The correct option is 1 and 3 only.
Explanation
In arid and semi-arid regions, landform evolution is significantly influenced by the mechanics of slope retreat and water action. Pediments and pediplains represent specific stages in the erosional cycle of desert landscapes.
Statement 1 is Correct: Pediments are gently inclined rocky floors located at the foot of mountains. They typically exhibit a low angle of slope and may be bare or covered with a thin veneer of debris. They serve as a transition zone between the steep mountain front and the lower basin.
Statement 2 is Incorrect: Pediments are not formed by vertical down-cutting, which is characteristic of youthful streams creating deep valleys or gorges. Instead, pediments form through the lateral erosion of the mountain front and sheet flooding. As the steep mountain front retreats backward (parallel retreat of slopes), the gently sloping bedrock surface (pediment) extends at the expense of the mountain.
Statement 3 is Correct: A pediplain is a low-relief, extensive erosional plain formed by the coalescence of several pediments. As the mountain fronts continue to retreat, the intervening pediments merge, eventually reducing the high-relief landscape into a featureless plain, often leaving behind isolated remnant hills known as inselbergs.
Key Takeaway: Pediments are erosional bedrock surfaces formed by the lateral retreat of mountain fronts in arid regions, while pediplains result from the merging of these pediments over geological time.