Geologic Map Legends
Eastern Piedmont Plutonic 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.

GRANITIC SERIES Pegmatite Dikes Quatrz-albite-microcline-perthite-muscovite granite pegmatites associated with gneiss domes; age 425 +/- 20 m.y.* by radiogenic dating. Woodstock Quartz Monzonite Massive biotite-quartz monzanite; well-developed granular textures; age 420 +/- 50 m.y.* by radiogenic dating. Guilford Quartz Monzonite Biotite-muscovite- quartz monzonite; occurs as discontinuous lenticular bodies; age 420 +/- 50 m.y.* by radiogenic dating. Ellicott City Granodiorite Ranges from biotite granodiorite along margin of body to quartz monzonite in core: age of quartz monzonite core 420 +/- 50 m.y.* by radiogenic dating. Norbeck Quartz Diorite Ranges from weakly foliated quartz diorite to strongly gneissic and schistose rock with recrystallized textures; more massive rocks have igneous textures; age 510 +/- 10 m.y.* by radiogenic dating. Port Deposit Gneiss Moderately to strongly deformed intrusive complex composed of gneissic biotite quartz diorite, hornblende-biotite quartz diorite, and biotite granodiorite; all rocks foliated and some strongly sheared; age 550 +/- 50 m.y.* by radiogenic dating. Kensington Quartz Diorite Moderately to strongly deformed; igneous textures generally destroyed; composition ranges from quartz diorite to granodiorite; comprises thin concordant sheets r wedges localized along plunging crest of Baltimore anticlinorium. Gunpowder Granite Remobilized Baltimore Gneiss. Fine- to medium-grained quartz monzonite with biotitic schlieren; grades into Baltimore Gneiss in core of Towson Dome; age 550 +/- 50 m.y.* by radiogenic dating. HARFORD COUNTY Muscovite Quartz Monzonite Gneiss Well foliated to nearly massive quartz monzonite gneiss, generally medium-grained and even textured but locally porphorytic and pegmatitic. Quartz Gabbro and Quartz Diorite Gneiss Mixed rock zone of greenish-black, uralitized, quartz-bearing gabbro to dark gray, weakly gneissic, pyroxene-hornblende-boitite diorite.

GABBROIC SERIES Georgetown Mafic Complex Poorly exposed complex of tonalite, dark quartz diorite, gabbro, amphibolite, and undifferentiated basic rocks. Relay Quartz Diorite Intensely foliated, fine-grained, light-colored; ranges from quartz diorite to albite granite; age 550 +/- 50 m.y.* by radiogenic dating. Ultramafic Rocks Chiefly serpentinite with partly to completely altered dunite, peridotite, pyroxenite, and massive to schistose soapstone; talc-carbonate rock and altered gabbro are common in some bodies. Baltimore Gabbro Complex Hypersthene gabbro with subordinate amounts of olivine gabbro, norite, anorthositic gabbro, and pyroxenite; igneous minerals and textures well preserved in some rocks, other rocks exhibit varying degrees of alteration and recrystallization, and still others are completely recrystallized with a new metamorphic mineral assemblage. HARFORD COUNTY Metagabbro and Amphibolite Weakly to strongly lineated metagabbro and epidote amphibolite. Ultramafic and Gabbroic Rocks Mixed metagabbro, serpentinite, metapyroxenite, and actinolite-, chlorite-, and epidote-bearing schists.

*Radiogenic date from Wetherill et al., 1966