THE ZEN OF GEOLOGIC MAPPING

A geologic map is the expression of a hypothesis. If you make a blob map, you have a blobby hypothesis. (E-an Zen, circa 1985)

On this bicentennial of William Smith’s geologic map it is worth reflecting on E-an’s statement from 30 years ago. A geologic map is not mere description: there is never enough outcrop nor time to examine what outcrop there is. A map comprises complementary hypotheses about the distribution of earth materials and the earth history (subsidence, deposition, deformation, erosion) that produced this distribution. The distribution of earth materials that we observe informs a hypothesis of earth history and this hypothesized history informs our prediction (another hypothesis) about the distribution of materials where we have not observed them. Each subsequent observation has the potential to falsify our hypotheses and lead to their revision. Geologic mapping is science sensu Popper.

Our map-hypotheses are underlain by a great deal of theory: foremost Steno’s principles of original horizontality, superposition, and primary lateral continuity, but also theories about depositional processes, plate tectonics, fault mechanics, ore-forming processes, and others. Where the theory of primary lateral continuity may be inappropriate (e.g., surficial-deposits maps, accretionary complexes, suspect-terrane maps, some igneous terrains), mapping is much more difficult. As our theories improve we make better geologic maps, thus it is commonly worthwhile to remap every few decades. Theory enables automated checking of maps in a GIS for some kinds of blunders and interpretive errors.

Perhaps the most important theory underlying our maps is that the Earth has a history. A blob map—or any other map that is unrestorable—violates this theory and thus is less likely to be correct than a more interpretive map.