MARYLAND GEOLOGICAL SURVEY 53 



THE LOUDON FORMATION 



The oldest sedimentary Paleozoic rocks in Maryland are argillaceous 

 dark slates, sandy shales, blue limestones, white marble, gray sandstone, 

 and quartz conglomerate, immediately overlying the crystalline rocks 

 and known collectively as the Loudon formation, named from Loudon 

 County, Virginia, where all the members are well displayed. Weathering 

 of the unconformably underlying Catoctin schist gave rise to a great 

 variety of sediments, Avhich accounts for the diverse strata composing the 

 succeeding formation. A fine grained, dark slate usually makes up the 

 greater part of the Loudon formation, but almost all of the other varieties 

 of sedimentary rocks, especially coarse and fine conglomerate, shale, and 

 pure limestone are locally developed. 



The formation outcrops in Maryland in depressions and valleys with 

 lines of small hills and ridges. It gives rise to a thin, micaceous, sandy 

 soil of little importance agriculturally. The rocks are exposed in long 

 narrow belts along several lines of outcrop, namely, the east side of Elk 

 Ridge, both sides of South Mountain and both sides of Catoctin Mountain. 

 In the granite and schist area between Catoctin Mountain and South 

 Mountain a few narrow synclines made up of the coarser deposits of the 

 formation are also found. The finer and thinner strata of the formation 

 occur only in the mountain areas mentioned above where the Weverton 

 quartzite overlies the Loudon formation. The limestone occurs as lenses 

 in the slate, and in Maryland has been found only along a line just west 

 of Catoctin Mountain for a distance of a mile or two north of the Potomac 

 River. This limestone is usually metamorphosed into marble, but the 

 marbles are interbedded with slate and schist and are almost always too 

 poorly developed to be worked for commercial purposes. However, almost 

 every outcrop of this limestone has in the past afforded rock for lime. 



The black slate makes up a large part of the Loudon formation in 

 Maryland, especially along the Catoctin Mountain line of outcrop. Here 

 the thickness is not over 200 feet, but along the Blue Ridge at Turner's 

 Gap, 10 miles north of the Potomac River, a thickness of 500 feet has 

 been measured. All trace of the original bedding in these slates has been 



