Maryland Department of Natural Resources


The geology of the crystalline rocks near Baltimore and its bearing on the evolution of the eastern Maryland Piedmont

1976, Crowley, W.P.

Report of Investigations 27


Extensive subdivision of rock units in the east-central Maryland Piedmont requires a radical restructuring of its classic stratigraphic framework. The Baltimore Gneiss is divided into five units and its type locality redefined. The Glenarm Series is changed to the Glenarm Supergroup. The Setters Formation and the Cockeysville Marble are each divided into six units of member and lens rank. The Wissahickon Formation is elevated to group status and divided into six formations and numerous units of lesser rank.

The large mass of marble at Hydes is shown to be a synclinal lens within the Wissahickon, not an unroofed gneiss dome in Cockeysville Marble.

The Baltimore Mafic Complex (formerly Baltimore Gabbro Complex) is elevated to group status and divided into four formations. Felsic gneisses closely associated with the Mafic Complex and formerly grouped together as Port Deposit Gneiss are separated into three formations. Rocks of supracrustal origin, flanking the Mafic Complex on the southeast, are assigned to three new members of the James Run Formation and to the newly erected Jones Falls Schist.

The Baltimore Gneiss represents a pile of predominantly felsic, volcaniclastic rocks that were deposited, metamorphosed, folded, and intruded by granite in the Late Precambrian.

Resting unconformably on the Baltimore Gneiss, the Setters Formation records an initial period of clastic sedimentation in coalescent basins on a shallow marine shelf, followed by development of a carbonate bank complex preserved as the overlying Cockeysville Marble.

Emergence of a land mass to the southeast led to an influx of clastic sediments (Wissahickon Group) accompanied in places by mafic volcanics, all of southeasterly derivation. The basal Wissahickon rocks include quartzite and marble in places, indicating localized persistence or re-establishment of shelf conditions. In general, the Wissahickon coarsens upward from meta-shale at the base through meta-graywacke and meta-diamictite. Exotic slide masses of mafic and ultramafic rock occur throughout the lower Wissahickon clastic sequence, and were derived from a composite allocthon (Baltimore Mafic Complex and associated rocks) which arrived during early Wissahickon time.