Vertical or inclined diagrammatic slices
through the upper part of the crust, showing how structures,
rock units, and other features vary with depth. Cross sections
are extremely useful because geologic maps provide only a
two-dimensional view of three-dimensional structures. Cross
sections are generally highly interpretative, based on knowledge
only of the surface geology. Data from drill holes, or
knowledge of the local stratigraphy, however, may help in
constructing cross sections. Most cross sections are drawn
perpendicular to the local or regional strike so that the dip of
structures is accurately portrayed. Cross sections oriented
obliquely to strike will show a dip less than the true dip of
the structure.
In areas of complex structure, several parallel cross sections
may be shown as fence diagrams, or block diagrams
may be drawn to give a three-dimensional view of the structure.
Many cross sections, particularly in tectonics, are shown
with the vertical scale exaggerated relative to the horizontal
scale. This has the unpleasant effect of distorting the way
structures appear, gives erroneous impressions of the nature
of forces involved, and can change the relative thickness of
units with different dips.
See also STRUCTURAL GEOLOGY.
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