|Geologic Map Legends|
|Eastern Piedmont Metasedimentary Rocks|
The information contained on this page was adapted from Maryland Geological Survey's Geologic Map of Maryland (1968). This information reflects geologic interpretations from over 20 years ago and do not necessarily represent an accurate interpretation of currently accepted geologic theory. We present this information for historic purposes only. Do not use this information for anything other than illustrative purposes. When a corrected and updated geologic map of Maryland is available you will see a notification on our web site.
The designation "Wissahickon formation" is no longer used in Maryland. The legend presented here was reproduced from the 1968 Geologic Map of Maryland, but does not reflect current theory. The "Wissahickon' group has been broken up into various divisions by several authors. The nomenclature is somewhat confusing, and is summarized at the bottom of this page. Also, the rocks that were formerly included in the "Wissahickon" are probably Upper Cambrian or Lower Ordovician in age, and not Precambrian.
|*Radiogenic date from Wetherill et al., 1966|
These rocks are now considered upper Cambrian to Lower Ordovician, not Precambrian
County Geologic Map
(Southwick and Owens, 1968)
(Also see Southwick and Fisher, 1967)
of Investigations 27
Plate 2, generalized geologic map of Baltimore and Harford Counties (Crowley, 1976)
of Investigations 67
Plate 1a , inset map of the geology of the Delta Quad
|Op||Peach Bottom Slate||pb||Peach Bottom Slate||pbsl||Peach Bottom Slate|
|Oc||Cardiff Metaconglomerate||cc||Cardiff Metaconglomerate||pbcc||Peach Bottom Cardiff Metaconglomerate|
|pCwu||upper pelitic schist||ps||Prettyboy Schist||pb||Prettyboy Schist|
|py||unnamed phyllite (2)|
|pg||Pleasant Grove Schist||pg||Pleasant Grove phyllite and phyllitic schist|
|pCwg||metagraywacke and fine-grained pelitic schist||o||Oella Formation
Piney Run Formation
|pc||Peters Creek (3)|
|pCwc||metaconglomerate||sy||Sykesville Formation||rp||Rocks Park metaconglomerate (4)|
|pCwb||boulder gneiss, metagray-wacke||dc||Deer Creek Complex: schist, metagraywackes, and intrusives|
|pr||Piney Run Schist (5)|
|pCwl||lower pelitic schist||Loch Raven schist||(not recognized in Delta Quad)|
|pzm||Muscovite quartz monzonite gneiss||glg||granitic leucogneiss|
|pzb||Baltimore gabbro||bg||Baltimore Mafic Complex Bel Air metagabbro||bac||Bel Air Complex metagabbro|
|pzug||ultramafic and gabbroic rocks||u||undifferentiated allocthonous ultramafic rocks||u/s||ultramafic intrusive bodies (u) locally serpentinized (s)|
(1) The term "Wissahickon" was first used by Bascom (1902) in Pennsylvania, then Mathews (1904) in Maryland. Southwick and Fisher (1967) recognized five formations within the Wissahickon: Lower Pelitic Schist, Boulder Gneiss, Metaconglomerate, Metagraywacke, and Upper Pelitic Schist. Crowley (1976) recognized six formations within the Wissahickon: Loch Raven Schist, Oella Formation, Piney Run Formation, Sykesville Formation, Pleasant Grove Schist, and Prettyboy Schist. The Pleasant Grove Formation, in particular, had previously been included in the Peters Creek Schist (Knopf and Jonas, 1923) or Peters Creek Formation (Knopf and Jonas, 1929b). Hopson (1964) considered the Peters Creek Formation to be a discontinuous turbidite facies within his Western Sequence of the Wissahickon (comparable to the upper pelitic schist and metagraywacke of Southwick and Fisher (1967).
(2) This unnamed phyllite was included as a "gray phyllite" on the first draft of the geologic map at 1:24,000 scale, but was omitted from a final draft at 1:24,000.
(3) On the earlier draft map, Muller showed the area of Peters Creek as Piney Run Formation
(4) Comparable to the Deer Creek quartzite of Leslie (1892, p. 130-132).
(5) On the inset map showing bedrock geology on Plate 1 of Report of Investigations No. 67, Muller shows the small area of Piney Run schist (pr) in the southwest part of the Delta 7.5-min. Quadrangle as Deer Creek Complex (dc) on an unpublished 1:100,000-scale map that was prepared at roughly the same time (1990). Muller's stratigraphic placement of this Piney Run is lower than that of Southwick and Fisher (1967) and Crowley (1976), presumably because Muller interprets it as part of an upthrust sheet.