Reports

New interpretations of the eastern Piedmont geology of Maryland


1971, Crowley, W.P., Higgins, M.W., Bastian, T., and Olsen, S.

Guidebook 2


Introduction

The area to be visited by this field trip is the northeastern Piedmont of Maryland (Fig. 1), chiefly Baltimore and Cecil Counties (Figs. 11 and 2) with one stop in Harford County. The stops in Cecil County were chosen by Higgins to illustrate concepts derived from his recently concluded mapping there. The one in Harford County touches on some of the problems considered by Southwick (1969). Those in Baltimore County deal with the recent work of Hopson (1964), Crowley, and Olsen as well as the earlier work of Matthews (1904), Knopf and Jonas (1929), and Broedel (1937). Some comments on the Piedmont archeology of northeastern Maryland are offered by Bastian at stop 4.

The first important work in this region was that of Professor George Huntington Williams and his students at Johns Hopkins University. Williams, an igneous petrologist who had studied under Rosenbusch, centered his attention on the gabbro complex west of Baltimore. His students worked on a variety of Piedmont igneous and metaigneous rocks, ranging from felsic to mafic. The emphasis in all of these studies was an elucidation of the crystallization history of the rocks as deduced from detailed petrographic studies. Conclusions of regional geologic interest were limited for the most part to discussions of cross-cutting relationships. One important result of the work of this period that is relevant to this field trip was the discovery by Bascom (1902), an early student of Williams, that the potpourri of mafic and felsic gneisses in Cecil County included meta-rhyolite and greenstone.

Following Williams' untimely death in 1894 research leadership in Piedmont geology was assumed by Edward Matthews who was elected instructor at Johns Hopkins to offer the courses formerly given by Professor Williams. Although Williams had some concept of stratigraphic relations in the Piedmont, it remained for Matthews (1904) to work out in detail the correct stratigraphic sequence (shown in part in Fig. 12). The complete section is present in Baltimore and westernmost Harford County. In Cecil County only the uppermost part (Wissahickon Formation) crops out; the area there is underlain in large part by felsic and mafic gneisses that did not fit into Matthews' stratigraphic scheme.

By the 30's the emphasis had shifted to structure, and a new crop of geologists returned to the Piedmont armed with Ernst Cloos' concepts of the importance of small-scale planar and linear features in the interpretation of geologic history. A group of papers announcing the results of this new approach were assembled in 1937 into volume 13 of the Maryland Geological Survey. Among those papers that bear on this field trip are Broedel's on the Baltimore gneiss domes, Hershey's on the Port Deposit gneiss complex, and Marshall's on the Cecil County metavolcanics.

Hershey, in his concluding remarks, suggested that stratigraphic-structural evidence indicated a snyclinal geometry for the Port Deposit Granodiorite complex, a conclusion that has also been reached by Crowley farther south in Baltimore County. Recent work by Higgins (1971), however, has shown that what has been called" Port Deposit" inc l,udes rock types of diverse origins, and that the truly plutonic "Port Deposit" occupies a much smaller area than had been previously thought (Stops 3 and 5).

Several papers could be cited in the years following the 30's where the germ of an important idea was expressed. Clifford Hopson weaved many of these ideas along with his own very original thoughts into an elegant synthesis that appeared in 1964. Relevant to this field trip are Hopson's interpretation of the "Sykesville Granite" as a submarine slide deposit, an idea that has been extended by Higgins to parts of the Port Deposit Gneiss (Stop 3) and may be even more broadly applicable (Stop 6). Hopson also interpreted as volcaniclastic rocks certain layered gneisses that had been previously assigned to the Baltimore Gneiss, an interpretation subsequently extended by Southwick to similar gneisses in Harford County (Stop 6).

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