128 PHYSIOGRAPHY AND GEOLOGY OF THE COASTAL PLAIN PROVINCE. 



County, the Calvert is found some miles to the east of the "fall line" but to 

 the south it extends much farther west and is found lying upon the Pied- 

 mont crystallines. Throughout much of the Virginia Coastal Plain out- 

 liers of the Lafayette alone extend farther westward. 



The Calvert formation occupies the higher-lying lands along the west- 

 ern portions of its area of outcrop with the Eocene and Potomac formations 

 in the lower parts of the valleys and gradually dips to lower and lower levels 

 as it passes to the east, until it finally sinks beneath tide level. The line 

 along which it disappears is in the vicinity of Nomini Bay on the Potomac 

 Eiver and at about the same longitude along the Eappahannock River. It 

 disappears at West Point where the Mattaponi and Pamunkey rivers 

 unite to form York Eiver. From that point the eastern limit takes 

 a southwesterly course to a point a short distance south of Peters- 

 burg where it disappears entirely. The formation has not been recog- 

 nized in Sussex, Greenesville, and Southampton counties. In the first two 

 the St. Mary's is in contact with the crystalline rocks of the Piedmont 

 Plateau. 



Paleontologic character. — Possils are especially abundant in the Calvert 

 strata of Virginia, and represent a wide range of species. As shown in the 

 faunal tables on a later pa^e, fossils belonging to most of the larger groups 

 of animals occur in tlie Calvert, and certain forms are to be found in almost 

 every place where the formation outcrops. Besides the faunal remains, 

 the Calvert has yielded an abundance of plant fossils. Most of these are 

 microscopic, and belong to the group of the diatoms, but Berry, in 1909, 

 described 14 species of higher plants from the diatomaceous earth of Rich- 

 mond. About 200 species of diatoms have been described from the diato- 

 maceous earth of Eiclimond and Petersburg, and careful examination would 

 no doubt reveal the presence of many more. 



DETAILED SECTIONS. 



The Calvert formation is exposed in hundreds of places throughout its 

 area of outcrop and in almost every case is easily recognized. Most of the 

 sections are along the tributary streams where only a few feet of materials 

 are exposed, while the he&i are those along the larger streams. The sections 

 given below are t^'pical of the formation and may be duplicated in scores of 

 places throughout the State. 



