DETAILED GEOLOGIC MAPS, MOST QUANTITATIVE DATA IN GEOLOGICAL SCIENCE: AN ENDANGERED SPECIES?

Geologic maps have been made since William Smith published the first known geologic map of Britain in 1799 and 1815; Maclure in 1809 and 1817 published the first US geologic map. Early geologic maps were mostly small-scale, and were readily modified with detailed mapping. Detailed geologic maps are reproducible data sets that accurately spatially place structural data and locate boundaries between geologic units, within the limits of base map scale. A detailed geologic map should be reproducible a century or more after it was made, except for reinterpretation of individual contacts. The best example may be the Peach, Horne, et al., 1907, geologic map of the Assynt District in Scotland. It was re-mapped in the early 2000s by the British Geological Survey, but contact locations remain largely unchanged. Other sciences, particularly those dependent on instruments, cannot claim data reproducibility for a century or more. Detailed geologic maps are also critical for most engineering applications.

Detailed geologic maps have been made deep into the Appalachian Piedmont, a region marked in the literature as largely devoid of exposure. The key to making detailed geologic maps here is walking all main drainages and tributaries; any exposure on roads is a predictor of extensive exposure in creeks. High quality geologic maps constructed here contain 1200 to 2,000 structural data stations per 7.5-minute quad, and a multiple amount of control on contact locations from mapping float and related soils. We have made detailed geologic maps close to the Fall Line in NC and GA by putting in the required effort—detailed geologic mapping (anywhere) is both mentally and physically challenging for the mapper. Today, <15% of the southern Piedmont has been mapped in detail, with slightly more in the Blue Ridge.

Unfortunately, it takes many months to produce a detailed geologic map, and it is difficult for young geologists to obtain tenure in graduate degree-granting universities by making geologic maps. So, many structural studies today are based on traverses across regions that either have already been mapped in trustworthy detail, or (too many) are based on geologic maps of questionable reliability. Fortunately, geologists in a few universities, state geological surveys, and the USGS are still making detailed geologic maps of parts of the US.