THE ST. MAKY'S FOEMATION. 141 



to the Calvert formation. It is possible that the Choptank may be rep- 

 resented, as it gradually thins out, in the low country lying between the 

 known outcrops of the Calvert and St. Mary's formations but buried beneath 

 the cover of Pleistocene formations. No outcrops or well borings occur in 

 this intermediate region and as no evidence is afforded for the presence of 

 the formation it has been considered in the mapping as having finally 

 thinned out north of the Virginia shore of the Potomac. 



The St. Mary's Fonnation. 



Name.- — This formation receives its name from St. Mary's County, 

 Maryland, where highly fossiliferous beds of this age are found and where 

 the lithologic characters of the formation are also well shown. It was so 

 named by G. B. Shattuck in 1902.* 



Stratigraphic relations. — The St. Mary's formation overlies the Calvert 

 formation unconformably except in the western portions of Sussex aud 

 Soutluuiipion counties and the eastern part of Greenesville County, where 

 the St. Mary's rests directly on the crystalline rocks due to its transgression 

 by the Calvert formation. This relation exists in North Carolina where 

 the St. Mary's is the oldest Miocene formation present. Along the Eoan- 

 oke River it unconformably overlies either the crystalline rocks of the 

 Piedmont Plateau or the strata of the Lower Cretaceous. In the southeast- 

 ern portion of the State the St. Mary's formation is unconformably over- 

 lain by the Yorktown deposits which constitute the latest . Miocene forma- 

 tion of the State. In the region of outcrop the St. Mary's, like the Chop- 

 tank, is frequently concealed from view over the divides and in the valleys 

 of the larger streams by thin deposits of Pleistocene materials which rest 

 unconformably upon the eroded surface of the St. Mary's strata. 



Lithologic character. — In lithologic character the St. Mary's formation 

 is very similar to the Calvert except that deposits of diatomaceous earth 

 are lacking. Dark blue to bluish-black compact clay is a very common con- 

 stituent, while layers of sand and shell marl such as occur in the Choptank 

 formation are of secondary importance. The sands and clays are frequently 

 rich in calcareous matter from the disintegrated molluscan shells. The 

 shell marl has been employed for agricultural purposes in a great many 

 places. In the vicinity of Williamsburg the St. Mary's formation contains 

 considerable glauconitic sand irregularly distributed through the beds of 

 yellow and buff quartz sand. Professor W. B. Rogers described this occur- 



aSeience, Vol. xv, 1902, p. 906. 



