TOWARD A COMMON MAP LEGEND FOR ONTARIO

Ontario encompasses over 1 million km², 3 major Precambrian geological provinces (Superior, Southern, Grenville) and a variety of younger Paleozoic rocks and Quaternary sediments. The Ontario Geological Survey (OGS) typically produces maps at several scales: detailed (1:20 000), regional (1:50 000 and 1:250 000), or provincial (1:1 000 000). Historically, creation of map legends has been individualistic, dependent on both the mapping geologist and the character and scale of the map area.

With the advent of increasing interjurisdictional collaborative mapping projects, digital collection of field data, and a focus on compilation maps, there is an increasing need for the OGS to develop a common map legend for all OGS Precambrian geology maps.

Creating a common map legend is more challenging than simply creating a standard scheme for rock nomenclature, although the latter is a necessary prerequisite. A map legend must also be able to reflect relative age relationships, formal stratigraphic nomenclature where established, be applicable to areas of well known and poorly known geology, yet be independent of map scale. Furthermore, the methodology used must be relatively easy for the end user to understand and, for hard-copy maps, must contain as few characters as possible in order to avoid clutter and be comprehensible.

This presentation presents a first attempt by the OGS to create a common map legend. At its essence is a basic 3 or 4 letter alphanumeric code: legend/colour code_rock class_root rock name[±key modifier]. For example, 5Ig[bi] is a biotite granite, assigned colour code of unit 5. Comments on the merits and drawbacks of this scheme are welcomed by the authors.