Legacy data versus future data

The body of North American geologic-map information has two parts:

• “legacy data” archived in paper maps and digital files as the result of historic geologic mapping;
• new data developed through future geologic-mapping.

Incorporating these two kinds of data sets into geologic-map databases involves different kinds of strategies, each posing its own challenge to science language.

North American legacy geologic maps are rich in geologic terminology. Typically, these terms are contained either in map-marginal descriptions of map units, or in pamphlets and reports that accompany the geologic map. Unfortunately, legacy maps rarely cite the classification systems the map maker used to name and describe earth materials. Consequently, it is left to the map user to interpret the meaning and usage of terminology.

For high-level terms (e.g., sedimentary rock, terrigenous-clastic sediment, plutonic rock, metamorphic rock, volcanic rock) the meaning may be universally understood. However, for deeper-level terms (e.g., shale, mud, basalt, quartz latite, quartz monzonite, granodiorite, volcaniclastic, slate, lahar, greenstone, gneiss, layered gneiss) the meaning may not be clear because many terms have inconsistent usage depending on when and where the map maker learned his or her craft. As a result, the map user commonly must interpret the meaning of earth-material terms according to his or her own experience.

This problem is compounded by two factors:

• some geological terms have acquired usages that border on the generic or commonplace, and lack strict definitions or meanings (e.g., sandstone, granite, shale, gneiss);
• some terms have been used as though they were lithologic names (e.g., alluvium, greenschist, till, turbidite, metasediment, loess, debris flow, lahar). This practice has blurred the distinction between lithologic description and genetic interpretation.

As a group, the SLTT committee had to wrestle with these issues, and decide whether our science-language approach (1) should try to accommodate historical usage that is diverse, inconsistent, and in some cases generic, or (2) should reflect the needs and requirements of future geologic-map makers for science language that is stable, logical, and consistent. Ideally, any such decision will reflect the policy of the data developer, which usually means the management policy of the geologic-mapping agency or entity.

For legacy information, two contrasting data-management choices apply:

1. modern databases can archive and organize legacy terminology verbatim, without attempting to translate such terms into modern science language;
2. modern databases can interpret and translate legacy terms in the context of modern science-language structures, preserving archival terminology where it is clearly understood in terms of a modern standard but using more generalized terminology where the specific original meaning can not be reconstructed. (NOTE: It is essential that legacy geologic-map unit descriptions and supportive descriptions and interpretations be archived exactly as indicated by the original map author. This should be accomplished by embedding legacy information in a text field or other field dedicated to such a purpose.)

Ultimately, earth-science agencies will have to choose between these two data-management policies: SLTT is not mandated to make such decisions on behalf of its constituent agencies. The fact remains, however, that legacy geologic maps include a wide variety of earth-material terms, many of which have similar, if not identical, meanings. Our purpose was to review how such terms have been used historically, and to judge how useful they are for storage, manipulation, retrieval, and analysis in geologic-map databases.

In many instances, we found that traditional earth-material nomenclature lends itself well to database applications. However, we also found that some traditional names and classification schemes do not adapt themselves easily to database requirements. In such instances, we had to modify existing names slightly, abandon some terms, or propose new names. The result is a hierarchical classification of earth materials that accommodates two objectives:

1. the language allows legacy map terminology to be brought into modern geologic-map databases, using archival terminology where appropriate or by using generalized terminology where the specific original meaning is not clearly determinable;
2. the language allows future geologic mappers to archive information about earth materials in a manner that is consistent, uniform, flexible, and forward-looking.