MIOCENE. 1:^5 



MIOCENE. 



THE CHESAPEAKE GROUP. 



The Miocene deposits of Virginia form part of a broad belt of forma- 

 tions of tliat age which extend from New Jersey southward 1u the South 

 Atlantic States. The strata attain considerable thickness and constitute the 

 most important element in the Coastal Plain series with the exception of 

 the Cretaceous beds which are usually thicker than the Miocene although 

 the latter, because of a gentler dip, outcrop over a much wider belt in Vir- 

 ginia and Maryland. In Xorth Carolina, on the other hand, through the 

 removal of much of the Miocene by erosion the underlying Cretaceous beds 

 have a more extensive outcrop. 



The materials composing the Miocene formations are largely fine- 

 grained, light-colored sands which are mostly unconsolidated except whei-e 

 locally cemented by carbonate of lime derived from the fossil shells which 

 form such an important feature of the deposits. Diatomaceous earth is also 

 characteristic of the Miocene deposits and occurs in thick beds in its lower 

 portions. A dark blue, often greenish, clay is likewise common in some 

 of the deposits. 



It has been found possible, on the basis of the fossils and the lithological 

 characteristics, to divide the Miocene of the Chesapeake Bay region into 

 four formations, known as the Calvert, the Choptank, the St. Mary's, and 

 the Yorktown. These were first grouped together under one formation 

 which was named the Chesapeake formation because of its great development 

 in the Chesapeake Bay drainage basin. The name is now retained as a 

 group name, and very properly, because the four Miocene formations exposed 

 in the region of the Chesapeake Bay form a group of deposits formed under 

 very similar conditions and possessing many striking similarities which dis- 

 tinguish them from the underlying Eocene and the overlying Pliocene ( ?) 

 and Pleistocene strata. The surface of the Chesapeake group is, for the 

 most part, covered on the divides by deposits of Pleistocene age, but along 

 the estuaries and trilnitary streams there are many excellent exposures. 



Except for the comparatively superficial deposits of the Pleistocene, the 

 Miocene strata are much more widely distributed in the Coastal Plain of 

 Virginia than the strata of any other age. They make their first appear- 

 ance in the high divides of Stafford County a few miles northeast of Fred- 

 ericksburg where they overlie the Nanjemoy deposits. From this point 

 they extend in an almost continuous sheet to the south and southeast and 



