The Ellicott City Granodiorite is a Silurian or

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. The ...

granitic

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

in Howard and

Baltimore Baltimore ( , locally: or ) is the most populous city in the U.S. state of Maryland, fourth most populous city in the Mid-Atlantic, and the 30th most populous city in the United States with a population of 585,708 in 2020. Baltimore was ...

Counties, Maryland. It is described as a

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumi ...

granodiorite Granodiorite () is a coarse-grained (phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from gr ...

along the margin of the intrusion which grades into a quartz monzonite in its core. It intrudes through the

Wissahickon Formation The Wissahickon Formation is a mapped bedrock unit in Pennsylvania, New Jersey, and Delaware. It is named for the Wissahickon gorge in Fairmount Park, Philadelphia. In Maryland formations, the term "Wissahickon" is no longer used. Rocks in this cl ...

and the Baltimore Gabbro Complex. In 1964, C. A. Hopson grouped the Ellicott City Granodiorite with the

Guilford Quartz Monzonite The Guilford Quartz Monzonite is a Silurian or Ordovician quartz monzonite pluton in Howard County, Maryland. It is described as a biotite-muscovite-quartz monzonite which occurs as discontinuous lenticular bodies which intrude mainly throug ...

and the

Woodstock Quartz Monzonite The Woodstock Quartz Monzonite is a Silurian or Ordovician quartz monzonite pluton in Baltimore County, Maryland. It is described as a massive biotite-quartz monzonite which intrudes through the Baltimore Gneiss at a single locality surrounding t ...

as "Late-kinematic intrusive masses."Hopson, C. A., 1964, The crystalline rocks of Howard and Montgomery Counties: Maryland Geological Survey County Report, 337 p., (Reprinted from Cloos, Ernst, and others, "Geology of Howard and Montgomery Counties," p. 27-215) In 1980, Crowley and Reinhardt of the Maryland Geological Survey remapped the Ellicott City Quadrangle and referred to this unit as the Ellicott City Granite, rather than granodiorite.Crowley, W.P. and Reinhardt, Juergen, 1980, ''Geologic map of the Ellicott City quadrangle, Maryland'': Maryland Geological Survey, scale 1:24,000.
online

Description

Hopson reported the chemical composition (by %) of the Elicott City Granodiorite from two locations. H7-A is on River Road, 0.3 miles east of the Patapsco River Bridge, Ellicott City, and H18-1A is on U.S. Route 29, 200 yards south of

U.S. Route 40 U.S. Route 40 or U.S. Highway 40 (US 40), also known as the Main Street of America, is a major east–west United States Highway traveling across the United States from the Mountain States to the Mid-Atlantic States. As with most routes wh ...

Early quarrying

The 1898 account of Edward B. Mathews of the Maryland Geological SurveyMaryland Geological Survey Volume 2
1898, The Johns Hopkins Press, Baltimore. of the

quarries A quarry is a type of open-pit mine in which dimension stone, rock, construction aggregate, riprap, sand, gravel, or slate is excavated from the ground. The operation of quarries is regulated in some jurisdictions to reduce their envir ...

Ellicott City Ellicott City is an unincorporated community and census-designated place in, and the county seat of, Howard County, Maryland, United States. Part of the Baltimore metropolitan area, its population was 65,834 at the 2010 census, making it the mo ...

begins with a statement that there were two quarries; one on either side of the Patapsco River. The rock on the eastern, or Baltimore County, side is "a fine grained mass, with a decided

foliation In mathematics (differential geometry), a foliation is an equivalence relation on an ''n''-manifold, the equivalence classes being connected, injectively immersed submanifolds, all of the same dimension ''p'', modeled on the decomposition of ...

or gneissic structure," while the rock on the western, or Howard County side, is "more uniform and granitic." The text also refers to the figure of the polished slab on the left: "Here it also has a porphyritic structure in consequence of the development of large flesh-colored crystals of feldspar which are disseminated somewhat irregularly through the rock, as shown in ''(the figure)''." Mathews continued with a description of their historical importance: Mathews described recent (c. 1872-1898) operations at the quarries:

Age

In 1973, M. W. Higgins reported a radiometric ( Rb-Sr age) of 425 Ma, which placed the Ellicott City Granodiorite in the Silurian. In 1998, A. A. Drake revised the age to

based on the granodiorite's relationship with the

.Drake, A.A., Jr., 1998, Geologic map of the Kensington quadrangle, Montgomery County, Maryland: U.S. Geological Survey Geologic Quadrangle Map, GQ-1774, scale 1:24,000

References

{{Reflist Baltimore County, Maryland Howard County, Maryland Granitic rocks Igneous petrology of Maryland Ordovician magmatism

Description

Early quarrying

Age

See also

References