Sag pond

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A sag pond is a body of water collected in the lowest parts of a depression formed either near the head scarp of rotational landslides[1] or between two strands of an active strike-slip fault.[2] Sag ponds may be useful identification features in aerial photographs for mapping faults and landslides.

Rotational landslides[edit]

Deep-seated landslides may exhibit rotational failure along a curved slip surface resembling a circular arc in a vertical cross-section along the axis of soil movement. Rotational failure occurs when a bowl-shaped block of soil rotates to reduce the slope of the ground surface. The uphill edge of undisturbed soil surrounding the bowl may be left exposed as a steep scarp face indicating the uphill boundary of the landslide. The lower edge of the rotating bowl tends to break apart and collapse into translational downhill movement. A sag pond may form below the head scarp if rotational momentum carries the ground surface of the rotating soil block past horizontal.[3]

Fault lines[edit]

The relative motion of the two fault strands results in a stretching of the land between them, causing the land between them to sink.

External links[edit]


  1. ^ "Earthquake Images". Idaho Geological Survey. Retrieved 2013-02-10. 
  2. ^ Nevada-Government Dictionary on Water Words, retrieved April, 3rd 2012
  3. ^ Samford, Jerrold "Narrows Landslide, Giles County, Virginia" in Virginia Minerals volume 29, number 4 (November 1983) p.37