Pediments are gently sloping erosional surfaces that form primarily through the action of running water, particularly during heavy rainfall events. These distinctive landforms are characteristic features often found at the base of mountain ranges or uplands.
The Process of Pediment Formation
The formation of a pediment is a geomorphological process driven by specific erosional mechanisms:
- Erosional Surfaces: Fundamentally, a pediment is an erosional surface—meaning it is shaped by the removal of material, rather than its deposition.
- Sheet Floods: The primary agent in pediment development is the action of "sheet floods." These are broad, thin sheets of running water that spread across the landscape, especially during intense rainfall. Unlike confined streamflow, sheet floods cover a wide area, washing away loose material and gradually lowering the land surface.
- Intense Rainfall Events: The energy required for significant sheet flood erosion is typically generated during periods of intense, heavy rainfall. This high-energy water flow effectively transports sediments across the gentle slope of the developing pediment.
- Cliff Retreat Erosion: Pediments often form at the foot of mountains that are themselves undergoing erosion through processes like cliff retreat. As the cliffs at the mountain front erode and recede, the material broken off (like rock fragments and sediment) can be transported downslope.
- Sediment Cover: While pediments are largely erosional, they may be thinly covered with fluvial gravel. This gravel is transported by the very sheet floods that shape the pediment, having washed down from the foot of the adjacent mountains as a result of the ongoing cliff retreat erosion.
The continuous cycle of intense rainfall, sheet flooding, and the associated transport of eroded material from surrounding uplands gradually carves out these extensive, smooth plains.
Key Factors in Pediment Development
The table below summarizes the critical elements contributing to the formation of pediments:
| Factor | Role in Pediment Formation |
|---|---|
| Nature of Surface | An inherently erosional surface, shaped by material removal. |
| Primary Agent | Sheet floods – widespread, unconfined flows of water. |
| Triggering Event | Intense rainfall events, providing high-energy water for erosion. |
| Associated Process | Cliff retreat erosion from adjacent mountains provides source material. |
| Surface Material | Often thinly covered with fluvial gravel, transported by sheet floods. |
Understanding these processes helps explain the distinctive, gently sloping, and often bare or sparsely vegetated landscapes that pediments represent.
