The side of a sand dune that has a steep slope is called a slip face.
Understanding the Slip Face
The slip face is a defining characteristic of sand dunes and is located on the downwind side of the dune. This steep slope forms through a process of sand accumulation and collapse.
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As on Earth, the downwind side of a sand dune has a steep slope called a slip face. Sand grains blowing across the windward side of a dune become sheltered from the wind by the dune itself.
How a Slip Face Forms
The formation of the slip face involves several key steps driven by wind:
- Windward Side: Sand grains are carried up the gentler, windward slope by the wind (saltation and creep).
- Crest: As the sand reaches the crest of the dune, the wind speed decreases significantly because the dune itself provides shelter on the downwind side.
- Deposition: The sand grains, losing momentum, fall out of the air or roll down onto the leeward side.
- Avalanche: As sand accumulates on the leeward slope, it reaches an angle of repose – the steepest angle at which unconsolidated material remains stable. When this angle is exceeded, the sand avalanches or slides down, creating the steep slip face.
Contrasting Sides
Sand dunes typically have two distinct sides:
| Dune Side | Characteristic Slope | Wind Interaction |
|---|---|---|
| Windward | Gentle | Exposed to wind; sand is pushed or bounced along. |
| Downwind | Steep (Slip Face) | Sheltered from wind; sand accumulates and avalanches. |
The slip face is crucial to the migration of sand dunes. As avalanches occur on the slip face, the dune effectively moves in the direction of the wind.
