60 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



the Antietam sandstone as Lower Cambrian. The same association of 

 species has been found on Observatory Hill, two miles south of Keedys- 

 ville and at a locality about one mile southeast of Smithsburg. 



CAMBRIAN-ORDOVICIAN LIMESTONES 



The second great phase of deposition in the Appalachian region com- 

 prises a group of limestones which, from the fact that these strata form 

 the floor of the great Valley, were first named the Valley limestone. In 

 Virginia, the geographical term Shenandoah limestone was subsequently 

 substituted for Valley limestone of the older geologists. In all the earlier 

 maps of the central Appalachian Valley this limestone was regarded as a 

 single formation and its thickness was supposed to approximate 5000 feet. 

 This calcareous phase of deposition between the Cambrian siliceous rocks 

 and the Ordovician shales is such a conspicuous feature throughout the 

 Appalachian Valley that various local names have been applied to it. In 

 Maryland the name Shenandoah formation was used for this limestone 

 until comparatively recent years when geologic work in adjacent areas 

 of Pennsylvania showed that these strata can be subdivided into seven 

 distinct formations with an aggregate thickness of over 10,000 feet. The 

 names, age, and thickness of these seven formations are as follows : 



Table of Cambrian-Ordovician Limestones in Maryland 



Feet 



Middle Ordovician, Chambersburg limestone 300 



Lower Ordovician, Stones River limestone 1000 



Lower Ordovician ( Canadian )-Beekmantown limestone (Stonehenge mem- 

 ber at base) 2500 



Upper Cambrian (Ozarkian)-Conococheague limestone 1600 



Middle Cambrian. . fElbrook formation 3000 



\ Waynesboro formation 1000 



Lower Cambrian-Tomstown limestone 1000 



All of the above formations may be more or less readily recognized by 

 lithologic peculiarities, by the contained fossils, by the topographic forms 

 and residual debris which their weathering produces and by their known 

 position in the stratigraphic sequence. Limestones of similar aspect may 

 be found common to all the formations, but fortunately the boundary 



