Understanding geology through maps. Strata and plane-dipping features / Понимание геологии с помощью карт. Слои и наклонные поверхности

We are very familiar with topographic contours of landforms. Since landforms are complex surfaces, topographic contours (and bathymetric contours below water level) follow contorted paths tracing out a horizontal line on the map. It is often convenient to imagine oneself “walking out contours” and so visualize the land surface and its slopes. The closer contours are spaced, the steeper the surface. Occasionally contours merge, one above the other, where a cliff is located.
Most geological surfaces are much simpler than topographic surfaces and many are essentially planar. Bedding planes for strata of constant dip direction and constant dip therefore have straight “structure contours”. Since the dip angle is constant, these structure contours will be equally spaced and parallel. The closer the structure contours, the steeper is the dip of the bedding plane. Note that “structure contour” is preferred to “stratum contour” since the construction may be applied to any geological surface, not just a bedding contact. It is important to remember that the structure contour defines a surface, e.g., the top or the bottom of a bed. Therefore, care must be taken to work consistently only with one geological surface <...>