A slip face is the lee side of a dune where the slope approximates the angle of rest of loose sand that is generally about 33 degrees.
This specific area is a fundamental feature of many types of sand dunes and is created by the continuous movement and deposition of sand driven by wind. It represents the downwind side, protected from the direct force of the prevailing wind, where sand accumulates until it becomes unstable and collapses, or 'slips'.
Key Characteristics of a Slip Face
Based on the provided definition and common understanding in geology and geomorphology, a slip face has several defining features:
- Location: It is located on the lee side of a dune. The lee side is the side sheltered from the wind, opposite the windward side.
- Slope Angle: The slope of the slip face is critically linked to the angle of rest of the material forming the dune, which for loose sand is typically around 33 degrees. This angle is the maximum slope at which a pile of granular material remains stable before slumping occurs.
- Formation Process: Sand is transported up the windward side of the dune and deposited over the crest onto the slip face. When the slope exceeds the angle of rest, gravity causes the sand to slide down in small avalanches or slips, maintaining the characteristic steep angle.
- Surface Appearance: Slip faces often have a smooth appearance interspersed with small tracks left by recent sand slips or miniature avalanches.
Why is the Angle of Rest Important?
The angle of rest is a physical property of granular materials like sand. It's determined by factors such as the shape of the grains, their size, and how tightly they are packed. For dry, loose sand, this angle is remarkably consistent, hovering around 30 to 35 degrees. The slip face is a visible manifestation of this physical limit – the dune simply cannot maintain a steeper slope on its downwind side because the sand particles are no longer stable and slide downwards.
The Dune Lifecycle and the Slip Face
The slip face is integral to the way dunes migrate. As wind continues to move sand from the windward side over the crest, deposition on the slip face causes it to advance downwind. This process of erosion on the windward side and deposition on the lee (slip) face is how dunes effectively "move" across landscapes like deserts or coastlines.
Summary Table:
| Feature | Description |
|---|---|
| Location | Lee side (downwind side) of a sand dune. |
| Slope Angle | Approximates the angle of rest of loose sand. |
| Typical Angle | Generally about 33 degrees for loose sand. |
| Formation | Deposition of sand over the crest followed by gravitational sliding (slips). |
| Role | Crucial for dune migration. |
Understanding the slip face is key to comprehending the dynamics of wind-blown sand environments and the fascinating ways dunes are shaped by nature.
