Paper No. 15
Presentation Time: 4:30 PM


THOMPSON, Margaret D., Geosciences Department, Wellesley College, Wellesley, MA 02481,

Massachusetts geologically spans the Appalachian orogen, and the geologists who translated this complexity into maps of the state were also leaders of the Geological Society of America from its beginnings. The 1916, 1:250,000 scale map by Amherst College Professor Benjamin K. Emerson who presided over the Society in 1899 extended and elaborated on the first (of any) state maps in 1833 and 1841 by his Amherst predecessor, Edward Hitchcock. The 1916 document recorded a huge stratigraphic step forward with granites, for example, subdivided into Archean and Paleozoic phases rather than collectively treated as the oldest rocks. Likewise noteworthy was its more detailed depiction of belts between Pittsfield in western Massachusetts and Worcester in the east based largely on field studies by Emerson himself. It is no criticism to say that his tectonic grasp of these belts was limited by general adherence at that time to contracting earth hypotheses reviewed in his presidential address (GSA Bulletin, v. 11).

Replacing Emerson's map with the 1983 Bedrock Geologic Map of Massachusetts required a team effort involving USGS geologist (and 1992 GSA president) E-An Zen as editor working with compilers Norman Hatch, Nicholas Ratcliffe, Rolfe Stanley, Peter Robinson (all Northeastern Section chairs), Richard Goldsmith (vice-chair), David Wones (1979 MSA head) and many others. The improved stratigraphic resolution of metamorphic units in this map owes much to the thinking of Harvard's Marland Billings who led GSA in 1959. The new map also broke ground by incorporating emerging lithotectonic concepts of suspect terranes and results of pioneering U-Pb geochronology. Voluminous Dedham Granite intrusions that dominate the currently recognized Avalon terrane in SE Massachusetts and appear in 1916 as Devonian (?) were thus properly re-assigned to latest Precambrian time. My collaborations at MIT and Boise State University continue to clarify the chronology of these and overlying units in the Boston Basin (last synthesized by Billings in 1976 when no isotopic age constraints were available).

This brief, necessarily simplified history pays tribute to those who advanced geologic map-making in Massachusetts over long years and, with it, the stature and reach of our Society. Parallel histories of many other field areas can surely be written.