Contour maps showing areas of equal joint spacing in outcrop can provide important evidence for locating faults at the surface in areas of extensive alluvial and vegetative cover. The technique has been applied to an area of detailed joint mapping in the Allegheny Plateau of south-central New York and contiguous northern Pennsylvania. Contour maps of joint spacing indicate that the spacing between joints, within a joint set, increases as some faults or fault zones are approached. The maximum spacing of joints is adjacent to faults.
A contour map of the northwest-striking joints appears to identify and delineate thrust faults (that is, east-striking faults), whereas a contour map of the northeast-striking joints seems to show tear faults (that is, north-striking faults).
Although not all known faults are indicated by this technique, several mapped faults as well as several suspected faults are apparent on the contour maps.