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GEOLOGY Rerun:

Aeromagnetic Discovery of a Baltimore Gneiss Dome in the Piedmont
of Northwestern Delaware and Southeastern Pennsylvania

Michael W. Higgins the Setters rocks are absent, and the Cockeysville Marble, or even GSA TODAY | www.geosociety.org/gsatoday
U.S. Geological Survey, Beltsville, Maryland 20705 Wissahickon rocks, rests directly upon Baltimore Gneiss. In
some cases this is due to faulting or to tectonic thinning
George W. Fisher (McKinstry, 1961; Hopson, 1964), but in others it is probably
Johns Hopkins University, Baltimore, Maryland 21218 due to pre-Wissahickon erosion or to nondeposition of the lower
Glenarm rocks (Choquette, 1960; Hopson, 1964). Fisher (1971)
Isidore Zietz has shown that the Setters Formation is not everywhere a
U.S. Geological Survey, Silver Spring, Maryland 20242 quartzose rock, however, and may be locally mistaken for
Wissahickon pelitic schist.
ABSTRACT
The thickness of the Wissahickon Formation is not precisely
In the central Appalachian Piedmont the “basement complex” determined because it is isoclinally folded and because there
is an assemblage of 1,100- to 1,300-m.y.-old gneisses, migmatites, are few marker beds or units. The discovery of previously
and amphibolites that crops out in “domes” mantled by younger unknown domes of the “basement” Baltimore Gneiss in areas
metasedimentary rocks of the Glenarm Series. Aeromagnetic data mapped as Wissahickon has bearing on the thickness of the
and reconnaissance fieldwork indicate that a previously unknown Wissahickon as well as on the structure and regional relations
Baltimore Gneiss dome, here called the Mill Creek dome, is of the Glenarm Series.
present in southeastern Pennsylvania and northwestern Delaware.
The discovery of previously unknown domes of Baltimore Gneiss Figure 1. Generalized map of central Appalachian region, showing domes of
has bearing on the thickness, structure, and regional relations of “basement, complex” Baltimore Gneiss (line pattern) and approximate outline
the Glenarm Series. of newly discovered Mill Creek dome (dot pattern). (Editor’s note: Dot pattern
did not show through on original.)
INTRODUCTION

The oldest rocks in the central Appalachian Piedmont are a
complex of 1,100- to 1,300-m.y.-old (Tilton and others, 1958;
Wetherill and others, 1966; Wetherill and others, 1968; Sinha and
others, 1970) gneisses, migmatites, and amphibolites, collectively
named the Baltimore Gneiss (Williams, 1892; Hopson, 1964). In
Maryland, Baltimore Gneiss crops out in the cores of seven anti-
clinal domes (Fig. 1), true “mantled gneiss domes” (Eskola, 1949;
Hopson, 1964) where the gneiss is unconformably overlain by
metasedimentary rocks of the Glenarm Series (Knopf and Jonas,
1922, 1923; Hopson, 1964; Higgins, 1972). Baltimore Gneiss is
also known to crop out in four anticlinal areas in southeastern
Pennsylvania (Knopf and Jonas, 1923; Bascom and Stose, 1932;
McKinstry, 1961; Gray and others, 1960; Fig. 1, this paper), where
it is unconformably overlain by Glenarm Series rocks, or by
Chilhowee Group rocks correlative with the Glenarm rocks
(Higgins, 1972).

In most of the Baltimore Gneiss domes the gneiss is uncon-
formably overlain by feldspathic mica schist, mica gneiss, feld-
spathic quartzite, and micaceous quartzite of the Setters
Formation (Hopson, 1964). These quartzose rocks are overlain by
marble, metadolomite, and calc-silicate rocks of the Cockeysville
Marble (Choquette, 1960; Hopson, 1964), which, in turn, is over-
lain by a thick sequence of metasedimentary and metavolcanic
rocks of the Wissahickon and James Run Formations (Knopf and
Jonas, 1923; Hopson, 1964; Southwick and Fisher, 1967; Higgins
and Fisher, 1971; Higgins, 1972). In some of the domes, however,

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