Did You Know #5 – “Guilford Granite” is an Official Geologic Unit?

The granite industry in Howard County was so geologically significant that the three main commercially quarried granites were named after Guilford, Ellicott City and Woodstock.  The US Geological Survey describes our local granite as “Guilford Quartz Monzonite,” dating back approximately 420 million years.  Guilford Granite is only found here in the Guilford and Atholton areas of Howard County, including an area around the Middle Patuxent River and represents only 1.5% of the geological units in the county. 

This granite differs only slightly from the Ellicott City granite which is described as “Ellicott City Granodiorite” and also dates to about 420 million years ago. Ellicott City granite is found in both Baltimore and Howard Counties.  Woodstock Granite is another famous Howard County granite and a little darker in color than the Guilford granite. The Guilford, Ellicott City and Woodstock granites made a large economic and social impact on Howard County both with the B&O railroad, building and monument stones, and crushed stone for road bed materials.

The stones and quarries along the Wincopin Trails and the Patuxent Branch Trail south of the I-95 overpass are gabbro, which has a much darker color. Gabbro also has a higher iron content than granite and can appear with a rusty color on it.  This stone was usually crushed and mostly used for road bed construction.  Another fabled stone has been called “Blue Stone”, which seems to actually be a granite-type of stone called Gneiss which appears in the Sykesville Formation along the Middle Patuxent River (National Park Service 2020). 

Please forgive me for oversimplifying the geology lesson and if anyone has suggestions for correcting or clarifying this material, please let me know!  Thanks. 

Sources:

Cleaves, E.T., Edwards, J., Jr., and Glaser, J.D., 1968. Geologic Map of Maryland: Maryland Geological Survey, Baltimore, Maryland, scale 1:250,000.

Howard County GIS. 2020. Interactive Map. https://data.howardcountymd.gov/InteractiveMap.html

Merrill, George P. and Edward B. Mathews. 1898. Volume 2. Part II. The Building and Decorative Stones of Maryland. Maryland Geological Survey. The Johns Hopkins Press, Baltimore.  P. 157 (Downloaded from Google Books)

National Park Service. 2020. National Building Stone Database. Accessed 4-7-2020. https://www.ncptt.nps.gov/buildingstone/stone/potomac-bluestone

Watson, Thomas Leonard. 1910. Granites of the Southeastern Atlantic States, US Geological Survey Bulletin 426. Chapter 2, pp 54-56. U.S. Government Printing Office, Washington, DC - 287 pages. (Downloaded from Google Books)

Wetherill, G.W., Tilton, G.R., Davis, G.L., Hart, S.R., and Hopson, C.A., 1966. Age measurements in the Maryland Piedmont: Journal of Geophysical Research, v. 71, p. 2139-2155

US Geological Survey. 2020. Geologic units in Howard County, Maryland. https://mrdata.usgs.gov/geology/state/fips-unit.php?code=f24027 retrieved 4-7-2020