Structural Geology Glossary
Structural geology has a vocabulary of its own, and the terms can trip up students and newcomers to fieldwork. This glossary defines the essentials in plain English — the words you need to describe planes, lines, folds, faults, and the stereonets used to analyze them — with links to deeper guides and free tools where they help.
Apparent dip
The dip of a plane seen in any vertical section that is not perpendicular to strike. It is always shallower than the true dip, and an apparent-dip calculator converts between the two for a given viewing direction.
Attitude
The orientation of a geological plane or line in space. Planes are given as strike and dip; lines are given as trend and plunge.
Azimuth
A horizontal direction measured in degrees clockwise from north, from 000° to 360°. Strike, dip direction, and trend are usually recorded as azimuths.
Bearing
A horizontal angle that expresses a direction. It may be written as a full-circle azimuth from 000° to 360° or in quadrant form such as N30°E.
Bedding
The primary layering of sedimentary rocks formed as sediment is deposited. Each bedding surface is a planar structure whose strike and dip show how the layers are tilted.
Beta (β) axis
A fold axis found on a stereonet as the pole to the girdle traced by folded bedding poles, or as the intersection of great circles of folded planes. It estimates the fold axis orientation.
Cleavage
A closely spaced secondary fabric along which a rock splits, such as slaty cleavage. It forms during deformation, after deposition, and commonly sits at an angle to bedding.
Dip
The steepest angle a tilted plane makes with the horizontal, measured in the vertical plane perpendicular to strike, along with the direction of that downward slope. See the field guide to measuring strike and dip for how to record it.
Dip angle
The numerical steepness of a plane below horizontal, from 0° for a flat plane to 90° for a vertical plane. It is the how-steep part of the dip, separate from the direction.
Dip direction
The compass azimuth of the steepest downhill line on a plane, always 90° from strike. A converter can translate between dip direction, right-hand rule, and quadrant formats.
Equal-angle (Wulff) net
A stereographic net that preserves angles between features but distorts area. It is used for geometric constructions such as rotations and crystallographic work.
Equal-area (Schmidt) net
A stereographic net that preserves the relative density of plotted points. It is the standard net for statistical structural analysis, including contouring and finding clusters.
Fault
A fracture, or zone of fractures, along which the two sides have moved measurably parallel to the surface. Faults are described by their strike, dip, and slip direction.
Fault types (normal, reverse, strike-slip)
Faults grouped by relative motion. Normal faults drop the hanging wall down during extension; reverse and thrust faults push it up during shortening; strike-slip faults slide the two sides horizontally past each other.
Foliation
A pervasive planar fabric in a rock, such as cleavage, schistosity, or gneissic banding. It is produced by the parallel alignment of minerals during deformation or metamorphism.
Fold
A bend or flexure in originally planar layers such as bedding, caused by deformation. Folds are described by their axis, hinge, and limbs and range from gentle warps to tight closures.
Fold axis
An imaginary line that, moved parallel to itself, sweeps out the shape of a fold. In a cylindrical fold it is parallel to the hinge line and is described by a trend and plunge.
Girdle
A great-circle spread of points on a stereonet. When poles to folded bedding scatter along a girdle it signals folding, and the pole to the girdle gives the fold axis.
Great circle
On a stereonet, the curved arc traced where a plane cuts the projection hemisphere. It represents a whole plane, and its curvature reflects the plane's dip.
Hinge
The line or narrow zone of greatest curvature on a folded surface, where the layer bends most sharply. It separates the two limbs of a fold.
Joint
A fracture in rock across which there has been no appreciable movement parallel to the surface. Joints often occur in regularly oriented sets that control weathering and fluid flow.
Limb
The relatively planar flank of a fold on either side of the hinge. The angle between the two limbs describes how tight or open the fold is.
Lineation
Any repeated linear feature in a rock, such as aligned minerals, fold hinges, or slickenline striations. Its orientation is described by trend and plunge.
Outcrop
A place where bedrock is exposed at the surface rather than covered by soil or vegetation. Outcrops are where geologists make direct structural measurements.
Plunge
The angle a line makes with the horizontal, measured in the vertical plane that contains the line, from 0° for horizontal to 90° for vertical. It is the equivalent of dip, but for a line.
Plunging fold
A fold whose axis is tilted from horizontal, so the hinge line plunges into the ground. On a map its outcrop pattern closes in the direction of plunge.
Pole to a plane
The single point that represents the line perpendicular to a plane. Plotting planes as poles on a stereonet plotter lets many measurements be compared as a scatter of points.
Quadrant notation
A way of recording direction relative to north or south, such as N45°E or S20°W. Strike and dip are often written this way, and a converter can switch the format to azimuth.
Right-hand rule
A strike-recording convention in which the plane dips to the right of the quoted strike azimuth. It removes the need to state dip direction separately, and a converter can translate it to other formats.
Rose diagram
A circular histogram whose petals show how many measurements fall within each range of directions. It summarizes strike or lineation trends across an area, and the stereonet plotter can draw one from your data.
Small circle
On a stereonet, a circle that does not pass through the center of the projection. It represents a cone of directions at a fixed angle from a line and is used for rotations and cone-angle problems.
Stereonet
A stereographic projection used to plot 3D orientations on a 2D circle, with north at the top. Planes appear as great circles or poles and lines as points; our guide explains how to read a stereonet.
Strike
The compass direction of a horizontal line on a tilted plane, where the plane meets an imaginary horizontal surface. Strike is always perpendicular to dip direction and is the first value in a field measurement of strike and dip.
Trend
The compass azimuth of the horizontal projection of a line — the direction toward which the line points. Paired with plunge, it fully describes a line's orientation.
True dip
The maximum inclination of a plane below horizontal, measured perpendicular to strike. It is the largest dip value for that plane; any oblique section shows a shallower apparent dip.
