Field identification criteria of fault and fold

Fault

Direct evidences

• Displacement or offset of beds, dykes, veins, streams, roads or ridges.
• Repetition or omission (missing) of beds in a sequence.
• Slickensides (polished, grooved surfaces) and striations on the fault plane showing the direction of movement.
• Fault breccia, fault gouge or mylonite along the fault zone.
• Drag folds (bending of beds near the fault plane).

Physiographic (topographic) evidences

• Fault scarps (straight steep cliffs) and triangular facets.
• Linear valleys, straight stream courses and sudden changes in stream direction.
• Offset or beheaded streams, waterfalls and rapids along a straight line.
• Line of springs and seepages, and a sharp line of vegetation change.
• Alignment of sag ponds, lakes and hot springs.
• Abrupt change of rock type across a straight line, and earthquake epicentres in a line.

Fold

• Repetition of beds in reverse order on both sides of an axis (A B C D C B A).
• Opposite dips on two sides of a line (axis), either converging (syncline) or diverging (anticline).
• Outcrop pattern on a map: a series of curved or zig-zag outcrops (V-shaped patterns) with plunge.
• Age relation: in an anticline older beds are in the core; in a syncline younger beds are in the core.
• Drag folds and minor folds in the small scale, and variation of strike or dip along a traverse.
• Topography: ridges and valleys following the folded beds, and plunging noses on hill slopes.
• Cleavage or joints (fan or radiating pattern) related to the axial plane.