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Wissahickon Formation with intercalated hornblende gneisses
(amphibolites). Bascom and Miller (1920), on the other hand,
mapped all the coarse-grained schists and gneisses in this region
as Baltimore Gneiss, including large areas of rock now known to
be Wissahickon and making no distinction between the older
rocks and the overlying Glenarm Series.
The distinctive magnetic expression, coupled with the
known outcrops of marble and the recently emphasized fact
that the Setters Formation is not everywhere a quartzite or
quartz schist (Hopson, 1964; Fisher, 1971), led us to suspect
that a dome of Baltimore Gneiss might be present in the area of
the magnetic low. Reconnaissance field work has indicated that
this is indeed the case.
The marble that crops out in the valley of Pike Creek near
Pleasant Hill, Delaware, in the Mill Creek Valley near Hockessin,
Delaware, and in the valley of Broad Run southeast of
Landenberg, Pennsylvania (all these outcrop areas coincide gener-
ally with the gradient at the edges of the deep magnetic low; see
Fig. 2), is lithologically identical to parts of the Cockeysville
Marble (see Hopson, 1964, p. 69–70). Although outcrops of the
marble are few, even in the areas listed above, topography and
soils suggest that its distribution is more continuous than shown
on existing geologic maps. Feldspathic and micaceous quartz
schists that probably belong to the Setters Formation are intermit-
tently exposed adjacent to the marble valleys. The rocks that coin-
cide with the main part of the deep magnetic low (Fig. 2), and
apparently lie stratigraphically beneath the quartz schists and
marble, are a complex of biotite gneisses and migmatites, with
numerous intercalations of coarse-grained amphibolites (Table 1);
except for the greater abundance of amphibolites, these rocks are
strikingly similar to the Baltimore Gneiss in the Maryland domes
(see Hopson, 1964). They are lithologically distinct from the
Wissahickon schists and gneisses that are exposed in the area—
for example, in the valley of Pike Creek, on the other side of the
marble outcrop belt.
CONCLUSIONS
Aeromagnetic data and reconnaissance field work indicate that
a previously unknown dome of Baltimore Gneiss is present in
southeastern Pennsylvania and northwestern Delaware. The
gneiss in the dome appears to be surrounded by a discontinuous
outcrop belt of feldspathic and micaceous Setters Formation
quartz schists, and these schists, in turn, by a discontinuous
outcrop belt of Cockeysville Marble. Wissahickon pelitic schists
and gneisses flank the marble. We propose that this dome of
Baltimore Gneiss be called the Mill Creek dome, for Mill Creek
County, Delaware, which encompasses most of it.
Detailed geologic mapping is still needed to determine the exact
distribution of the geologic units associated with the Mill Creek
dome and to determine the detailed structure of the dome. The
aeromagnetic maps (Henderson and others, 1963; I. Zietz and J.R.
Kirby, unpub. data) should prove valuable aids to this mapping.
REFERENCES CITED
Bascom, Florence, and Miller, B.L., 1920, Description of the Elkton-Wilmington Tilton, G.R., Wetherill, G.W., Davis, G.L., and Hopson, C.A., 1958, Ages of
quadrangles [Maryland-Delaware-New Jersey-Pennsylvania]: U.S. Geol.
Survey Geol. Atlas, Folio 211, 22 p.
Bascom, Florence, and Stose, G.W., 1932, Description of the Coatesville-West GSA TODAY | www.geosociety.org/gsatoday
Chester quadrangles [Pennsylvania]: U.S. Geol. Survey Geol. Atlas,
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City, Maryland: U.S. Geol. Survey Geophys. Inv. Map GP-613.
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Air and vicinity, Harford, Baltimore, and Cecil Counties, Maryland:
U.S. Geol. Survey Geophys. Inv. Map GP-482.
Choquette, P.W., 1960, Petrology and structure of the Cockeysville Formation
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Eskola, Pentti, 1949, The problem of mantled gneiss domes: Geol. Soc.
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Fisher, G.W., 1971, Kyanite-, staurolite-, and garnet-bearing schists in the
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Gray, Carlyle, and others, compilers, 1960, Geologic map of Pennsylvania:
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ACKNOWLEDGMENTS
L.C. Pakiser reviewed the manuscript. J.R. Kirby assisted in
valuable discussions related to the aeromagnetic map.
Publication authorized by the Director, U.S. Geological Survey.
Original Geology manuscript received 26 Feb. 1973; manuscript
accepted 3 July 1973. 1
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