Pediments and Pediplains

The Big Picture First

Desert landscapes are not just about sand dunes and wind. A large part of desert geomorphology is actually shaped by water — rare, violent, and short-lived. The section you are studying focuses on one specific landform that forms at the foot of mountains in arid regions: the pediment. Understanding pediments is the key to understanding how entire desert landscapes flatten out over geological time.

What is a Pediment?

A pediment is a gently inclined rocky floor that lies close to the mountains, right at their foot. It may have a thin cover of debris on it, or it may be completely bare rock. The crucial point is that this is a rocky surface — it is not a plain made of transported sediment, but a bedrock surface that has been worn down.

These floors are not perfectly flat. They slope gently away from the mountain front, and this slope is what allows water to flow across them during the rare desert rains.

How Do Pediments Form?

The formation of a pediment is a story of erosion working along the mountain front. The process involves two main agents working together:

• Lateral erosion by streams — streams emerging from the mountains swing sideways, scouring the base of the mountain front.
• Sheet flooding — during heavy rain, water spreads as a thin sheet across the ground, carrying loose material and abrading the rock surface.

Erosion does not begin everywhere at once. It starts along the steep margins of the landmass — that is, the edges of the elevated terrain. It also begins along the steep sides of tectonically controlled steep incision features. These are deep cuts or valleys that have been carved into the landmass along lines of weakness created by earth movements (faults, fractures, joints). In other words, tectonic activity prepares the ground by creating steep, weak zones, and then erosion attacks these zones first.

The Retreat of the Mountain Front

Once a pediment has formed, the landscape develops a characteristic three-part profile:

1. A steep wash slope — the inclined surface of the pediment itself, down which water washes debris.
2. A cliff or free face — the steep, bare rock wall above the wash slope.
3. The mountain mass behind the cliff.

The critical process now begins: the steep wash slope and the free face retreat backwards. This means the entire mountain front does not simply wear down in place — instead, the cliff face eats its way backward into the mountain, like a wall being pushed back. As the free face retreats, the pediment at its foot extends further and further into the mountain mass.

This backward retreat is the mechanism by which the pediment grows. The mountain front is progressively consumed, and the rocky floor expands.

What is a Pediplain?

Although the textbook text you have ends mid-sentence, the concept of the pediplain follows directly from the process described. When the retreating free faces and wash slopes from opposite sides of the mountains eventually meet, the mountains are completely worn away. What remains is a vast, gently undulating rocky plain — this is the pediplain.

The pediplain is the end product of pediment formation and extension. It represents the final stage of desert landscape evolution, where the once-rugged mountain terrain has been reduced to a low, rolling, rock-cut surface.

Key Points to Remember …