[Rockhounds] Potomac Marble (2)

[Frederick Olmstead]

Building Stones of Our Nation's Capital: Washington's Building Stones [3 
of 4]

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"Calico Rock," a limestone conglomerate in the Leesburg Member of the 
Balls Bluff Siltstone, is the most striking building stone quarried near 
Washington, D.C. This stone was deposited at the mouths of ancient 
rivers in a series of alluvial fans in the western edge of the Triassic 
Lowland province in Maryland and Virginia. Called Potomac marble by 
builders, it is not marble but rather consists mainly of limestone and 
quartz pebbles and multicolored fragments, as much as 12 inches across, 
naturally cemented together. Samples of Potomac marble from various 
localities show marked differences in color. The Potomac marble in the 
Capitol is predominantly gray but shades to a rich reddish brown. The 
inclusions are gray, beige, yellow, black, white, brown, orange, and 
reddish brown in an indescribable variety of combinations. White veins 
of calcite accentuate the effects of the different colors.

 

	Potomac marble was first used in 1815 by Benjamin L


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http://pubs.usgs.gov/gip/stones/stones3.html


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