IXTltODUCTIOX 



niatiousj so-called from Maryland localities where the strata are typi- 

 cally exposed. 



The Miocene deposits lie nnconformably upon those of the Eocene 

 and overlap them along their western border. In Delaware and south- 

 ern New Jersey they completely transgress the Eocene beds, the latter 

 having disappeared beneath the cover of Miocene strata. 



The Miocene deposits consist of sands, clays, marls, and diatomaceous 

 beds. The last, composed almost exclusively of the tests of diatoms, 

 are chiefly confined to the lower portion of the Calvert formation, where 

 they afford striking, light-colored bluffs along many of the larger stream 

 channels. The nearly-pure diatomaceous earth often reaches a thick- 

 ness of 30 or 40 feet, although the remains of diatoms are found scat- 

 tered in greater or less amounts throughout much of the overlying 

 strata. The greater portion of the Chesapeake group, however, is com- 

 posed of variously colored sands and clays, with which are frequently 

 mingled vast numbers of molluscan shells. Sometimes the shelly mate- 

 rials form so large a proportion of the deposits as to produce nearly 

 pure calcareous strata, which in a partially comminuted state may be- 

 come cemented into hard limestone ledges. The organic remains are 

 very numerous and show clearly the Miocene age of the deposits. Their 

 great number early attracted the attention of geologists, in whose wi-it- 

 ings descriptions of them are frequently found. Several faunas have 

 been distinguished on the basis of which, as well as on stratigraphic and 

 structural grounds, the three well-defined formations, above referred to, 

 have been recognized by the State Geological Survey. 



The Calvert formation consists of clay, sand, marl and diatomaceous 

 earth, the clayey and sandy elements being more or less combined and 

 often filled with great numbers of fossils, affording a fossiliferous sandy- 

 clay. The Choptank formation consists of clay, sand and marl with 

 well-marked beds of fossils scattered through the formation. The St. 

 Mary's formation is characteristically a fossiliferous sandy-clay with 

 here and there beds of clay and marl, the latter often filled with great 

 numbers of fossils. 



Pliocene 



Overlying the Miocene deposits in portions of southern ^Maryland 

 and older deposits along the landward borders of the Coastal Plain is 



