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 or between two strands of an active strike-slip fault. Sag ponds may be useful identification features in aerial photographs for mapping faults and landslides.
Rotational landslides 
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.
Fault lines 
The relative motion of the two fault strands results in a stretching of the land between them, causing the land between them to sink.
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