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Baltimore Co., Maryland, USA

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Most of Baltimore County is underlain by the complex rocks of the Piedmont Province. These are diverse igneous and metamorphic rocks that have been folded, faulted and intruded by magma multiple times during Paleozoic mountain building episodes. The oldest rock is the Baltimore Gneiss, a Precambrian sequence of highly metamorphosed volcanic and sedimentary rocks. Overlying these are Cambrian sediments that were metamorphosed into schist, quartzite and marble. A large gabbroic pluton, he Baltimore Gabbro Complex was intruded and later metamorphosed. Plate movements also incorporated chunks of ultramafic oceanic crust into the sequence. These are metamorphosed into a string of serpentinites. The most notable of these underlie the Bare Hills, where these rocks produce distinct soil and vegetation patterns. A number of granitic rocks also intruded during the various mountain building episodes. Along the east side of the county lies a cover of Cretaceous sediments from the Coastal Plain Province. Oxidized siderite nodules from the Cretaceous rocks were worked in many localities for iron ore dating back to pre-Revolutionary War time.

Most of the listed minerals are from mines and quarries in the Piedmont Province. The gabbro, gneiss and granite have long provided crushed and building stone. The ultramafic rocks were worked not only for crushed stone but also for chromite and asbestos. The marble, worked for crushed stone and agricultural lime. provides excellent collecting where exposed in quarries. A number of small iron mines, some dating back to pre-Revolutionary War times. Urbanization has covered or obscured many of the deposits. However considering the diversity of the geology, any new excavation or quarry is worth examining for mineral specimens.

Mineral List

Mineral list contains entries from the region specified including sub-localities

112 entries listed. 74 valid minerals.

Localities in this Region


The above list contains all mineral locality references listed on This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to visit any sites listed in without first ensuring that you have the permission of the land and/or mineral rights holders for access and that you are aware of all safety precautions necessary.


Singewald, J. T. (1911) Report on the iron ores of Maryland: Maryland Geological Survey Vol. 9 Part 3: 183.

Mathews, E.B. & E. H. Watson (1929) The mineral resources of Baltimore County: Maryland Geological Survey: Baltimore County Report, p 219 -304.

Ostrander, C. W. & W. E. Price Jr. (1940) Minerals of Maryland: Natural History Society of Maryland, 92 p.

Bennet, R.R. & R. Meyer (1952) Geology and groundwater resources of the Baltimore area: Maryland Geological survey Bulletin 4, 573 p.

Kuff, K. R. (1975) Mineral land inventory of Baltimore County and Baltimore City: Maryland Geological Survey: 1:62,500 map.

Reinhardt, J & W. Crowley (1979) Baltimore east: Maryland Geological Survey: Geological Quadrangle map 1:24,000.

Crowley, W. & J. Reinhardt (1979) Baltimore west: Maryland Geological Survey: Geological Quadrangle map 1:24,000.

Bernstein, Lawrence (1980) Minerals of the Washington D. C. area: Maryland Geological Survey Educational Series 5, 148 p.

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