HOW DO YOU MAP A CONTINENT?

Construction of a geologic map of any area requires many decisions. What units can I map? What aspects of the geology should be emphasized? What shall I omit? What base map should be used? What existing data are available? When the map to be prepared aims to depict the grand geologic architecture of a continent the questions and problems multiply exponentially.

In 1980 the steering committee for the Decade of North American Geology project agreed that: “...the scope [of the DNAG project] should extend from the Arctic Ocean on the north to the southern edge of the Caribbean plate; from the mid-Atlantic Ridge on the east to the Pacific plate in the approximate vicinity of Hawaii. The emphasis should be placed on the geology of the continent; the adjacent sea floor should be carried as it is related to the continental story”. I rashly agreed to undertake to deliver on this ambitious order in collaboration with John Wheeler of the Geological Survey of Canada, and Brian Tucholke of the Woods Hole Oceanographic Institution. Originally the task was to be completed in 5 years; ultimately it took 24 years of close collaboration and friendly but vigorous debate among the compilers.

The questions we considered included the selections of the units to be mapped, determination of how they would be identified with letter symbols, and how they would be identified by color and pattern, the degree of generalization required in complex areas, how to distinguish on shore from sea floor units of the same age, and how the outlines of the continent could emphasized. Brian Tucholke was faced with the Herculean task of developing techniques for depicting sea floor geology on a map of this scale, a feat that had never before been attempted. With the publication of the map, demand has risen for development of a digital version, a task that is now under way in the USGS National Geologic Mapping Database Project.

In the end, we hope we have produced a map that will contribute to the vision of Bert Bally, who wrote in the summary volume of the DNAG project: “…Today mankind finds itself as a significant geologic agent, modifying the evolution not only of North America but of the Earth as a whole. An understanding of the evolution of our continent will put our own activities in a proper and responsible perspective”.