THE YORKTOWX FORMATION. 159 



rock of Florida, and doubtless the two have had a similar origin. The shell 

 fragments arc of various sizes and are mixed with greater or less amounts of 

 sand, the materials closely resembling the modern beach shell deposits of the 

 West Indies. It is probable that these fragmental layers accumulated in 

 very shallow waters, the breaking and wearing of the shells being accom- 

 plished by the shore waves. In addition to these fragmental shell beds the 

 Yorktown formation contains layers of sand, blue sandy clay, and also beds 

 in which the shells are excellently preserved. Occasional grains of glauconitic 

 sand are distributed through the beds of light-colored quartz sand. 

 Since the formation of the Yorktown beds there has been considerable 

 removal of the calcareous material by percolating waters, accompanied by a 

 sinking of the overlying beds in many places, with the production of appar- 

 ent unconformities. Occasionally the calcium carbonate removed from the 

 upper beds has been deposited in the form of calcite crystals in the cavities 

 of the shells of lower beds. Thus in many places nearly perfect internal 

 casts of Venus mercenaria and other forms can be found composed of pure 

 calcite. 



Stril-e, dip, and thicJt-ness. — The strike of the Yorktown formation is in 

 a north and south direction, while the normal dip of the beds is toward the 

 east at the rate of a few feet to the mile. Locally, however, as seen especially 

 at certain points in the Yorktown cliffs, the beds are horizontal or have a 

 slight dip to the west. The shallow-water origin of the Yorktown beds 

 accounts for the greater variability in compariscn witli the other Miocene for- 

 mations which were for the most part accumulated in much deeper and more 

 quiet waters. The thickness of the Yorktown formation is approximately 

 125 feet. 



Paleontologic character. — As shown in the following sections the York- 

 town formation contains a very rich and varied fauna. It is one of the most 

 fossiliferous formations represented in the entire series of Atlantic Coastal 

 Plain sediments and for years the beds exposed at Yorktown have been 

 favorite collecting grounds for workers in Tertiary geology. While the fauna 

 is distinctly molluscan in character, 5'et numerous vertebrate bones may be 

 collected along the cliffs at almost any time. The recent discovery of the 

 lower jaw of a walrus" in these beds is of especial interest. 



Area! distribution. — The formation outcrops in Gloucester, James City, 

 York, Warwick, Isle of Wight, and Xansemond counties. It has not been 

 recognized in Maryland but may l>j present beneath the covering of Pleisto- 



aEdward W. Berry and William K. Gregory. Amer. Jour. Sci.. 4tli ser., Vol. xxi, 

 pp. 444-450, figs. 1-4. "l906. 



