Read a geologic contact map before drawing a section

1. Define the evidence and convention

A geological map records where geological surfaces meet the land surface, while a cross-section shows an interpretation through depth. Strike is the horizontal direction within a plane; true dip is its steepest downward inclination, perpendicular to strike. A section oblique to that direction usually shows a smaller apparent dip.

2. Connect mechanism and model

The original mapped plane is z=100−x tanδ, with x east and y north in meters. Equal-elevation structure contours at z=100,80,60,40 are north–south lines, spaced 20/tanδ meters eastward. They are contours on the geological plane, not topographic contours. On horizontal ground at elevation 60 m, the contact follows x=40/tanδ.

A vertical section directed at angle α from east traverses eastward distance equal to section distance times cosα. Its slope is therefore tanδ cosα, and apparent dip is arctan(tanδ cosα). A strike-parallel section at α=90° is horizontal within this planar contact even though true dip remains nonzero.

This is an invented coordinate-complete map, not an actual survey or an image of a real locality. Real contact traces depend on both topography and geological structure, and extrapolation between observations adds uncertainty. Preserve map scale, north direction, elevation datum, dip symbol and section orientation before proposing a subsurface history.

3. Reconstruct the worked packet

At true dip 30° and section angle 60°, apparent dip is arctan(0.577350×0.5)≈16.1021°. The 20 m structure-contour spacing is 34.6410 m; the ground contact lies 69.2820 m east of the z=100 line.

4. Predict and investigate

Predict first: Why can the same dipping bed look horizontal in one section but inclined in another?

Read the north arrow and scale, calculate all four structure-contour positions, and draw the ground contact. Rotate only the section direction and compare apparent dip with the fixed true dip. State what new topography would require changing in the map.

5. Test a changed case

Apply the evidence to a changed case

At true dip 45° and a strike-parallel section, find apparent dip, contour spacing and the ground-contact coordinate.

Retain the evidence frame, calculations and assumptions before opening public feedback.

Compare a worked response

Apparent dip is 0°, spacing is 20 m and the contact is x=40 m. The bed is not horizontal; this section follows its strike.

6. Retain a defensible evidence notebook

Preserve first predictions, map orientation, intermediate calculations, units, assumptions and alternatives. These maps, numerical packets and diagrams are original invented instructional cases, not surveyed sites or current hazard forecasts. Independent practical review and genuinely unfamiliar evidence remain separate learner gates.

Mechanism reference, checked 30 October 2026. Original prose, diagrams and numerical exercises. No physical loading, pressurization or field hazard exposure is requested.