“Lineation” includes all linear structures without regard to origin. It may be due to: primary or secondary flowage; rotation around axes; intersection of planar structures; slippage; and mineral growth.
Lineations are mostly normal to, or in the direction of, rock movement and only rarely oblique. This rather simple and rigid relationship renders observation on lineation an indispensable tool in the determination of movement directions.
Lineations should be as carefully and accurately observed and described as refractive indices of minerals, or shapes and sizes of fossils. Their orientations should appear on all geological maps together with other structure symbols.
In this paper an attempt is made to show the kinds of lineation that occur and how they have originated. Study and analysis of lineation frequently permits deduction of movement directions.
Fabric analyses are mostly linked to field direction through a lineation but very frequently without sufficient definition and description of the type of lineation. Mere reference to “lineation” or “b axes” is wholly inadequate in view of the fact that 15 different kinds of lineation may occur.
Fold axes are frequently normal to the direction of movement or pressure, but can also parallel it.
Girdles in fabric diagrams are commonly indicative of movement in the girdle plane, but can also form by movement normal to it.
The literature shows that lineations have been described for over 100 years and the author tabulated the evolution of terms, their usage by different authors, and the historical sequence.
The regional arrangement of lineations in Maryland and Pennsylvania is shown in Plate 10. Five distinct zones can be seen: (1) a belt of steeply-dipping lineations responding to local uplift; (2) a broad belt in which fold axes and lineations dip gently; (3) areas in which lineation is steeply dipping or almost horizontal, (4) a broad and dominating region including South Mountain where all lineations are dipping south-eastward within the cleavage plane; and (5) a zone where local conditions seem to determine orientations of linear structures but without the influence of igneous intrusions. It is thought that these zones represent fundamentally differing structural areas.
In the second part of the paper the literature on the subject has been compiled as far as possible, and short notes or citations are added to indicate each author’s trend of thought on the subject.