The Ellicott City Granodiorite is a

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

granitic pluton in

Howard Howard is a masculine given name derived from the English surname Howard. ''The Oxford Dictionary of English Christian Names'' notes that "the use of this surname as a christian name is quite recent and there seems to be no particular reason for ...

and

Baltimore Baltimore is the most populous city in the U.S. state of Maryland. With a population of 585,708 at the 2020 census and estimated at 568,271 in 2024, it is the 30th-most populous U.S. city. The Baltimore metropolitan area is the 20th-large ...

Counties, Maryland. It is described as a biotite

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 gra ...

along the margin of the intrusion which grades into a

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

monzonite in its core. It intrudes through the Wissahickon Formation and the Baltimore Gabbro Complex. In 1964, C. A. Hopson grouped the Ellicott City Granodiorite with the Guilford Quartz Monzonite and the Woodstock Quartz Monzonite 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 Ellicott 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.

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 at Ellicott City 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 topological manifold, ''n''-manifold, the equivalence classes being connected, injective function, injectively immersed submanifolds, all of the same dimension ...

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 right: "Here it also has a

porphyritic Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning ...

structure in consequence of the development of large flesh-colored crystals of

feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagiocl ...

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

. In 1998, A. A. Drake revised the age to

based on the granodiorite's relationship with the Woodstock Quartz Monzonite.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