All existing fractures can be imitated in four scale models which show tension or shear fractures with or without rotational deformation. These four experiments can easily be produced by deforming soft clay which rests on a moving square of wire cloth.
En echelon zones are easily produced by placing clay above the edge of a moving tin plate. Tension and shear fractures open in the flexure zone and are unfailing indicators of relative displacement.
Normal or gravity faults occur preferably above domes and anticlines or other areas of distension. Salt-dome patterns may readily be imitated by pushing a plug into a clay model from below. A systematic study of fracture anomalies may prove useful in the detection of areas of uplift above salt domes.
Longitudinal and transverse normal faults in North dome of the Kettleman Hills are readily imitated in a clay model and may be due to vertical uplift with or without lateral compression. The en echelon arrangement of the Kettleman Hills is probably not due to north-south movement toward a weak zone trending northwest. True en echelon folds would trend west-northwest but not nearly north-northwest. The echelon arrangement may be only the alternation of folds or may be due to northeast-southwest movements, possibly in combination with subcrustal blocks and faults.
The San Andreas-Garlock fault system can be imitated only if considerable rotation has opened the angle between shear planes from near 60° to 90° or to 120°. Asymmetry of the fault pattern suggests clockwise rotation. A more complicated experiment in which three blocks are placed as suggested by Ralph Reed seems to come nearer to a fitting imitation but may still include too large an area.
The value of experimentation lies in experience gained by the experimenter rather than in the exact duplication of known fracture patterns. The intense occupation with mechanical principles and the close visible relationship between cause and effect is sobering as well as stimulating.