132 . THE GEOLOGY OF PKINCE GEOKGe's COUNTY 



SEDIMENTARY RECORD OF THE MIOCENE. 



Eocene sedimentation was brought to a close by an uplift by which 

 the shore line was carried far to the east and probably all of the 

 present State of Maryland became land. This was followed by a 

 resubmergence and another cycle w^as conuiienced. The deposits of 

 the Miocene were laid down upon the land surface which had just 

 been depressed beneath the water. Sluggish streams brought in fine 

 sand and mud, which the waves and ocean currents spread over the 

 sea bottom. 



JSTear the beginning of Miocene submergence, certain portions of 

 the sea bottom received little or no materials from the land, and the 

 water in those places was well suited as a habitat for diatoms. These 

 must have lived in the waters in countless millions, and as they died 

 tlieir silicious shells fell to the bottom and produced the beds of 

 diatomaceous or infusorial earth which are so common in the lower 

 part of the Calvert formation. Many Protozoa as well as Mollusca 

 lived in the same waters and their remains are plentifully distrib- 

 uted throughout the deposits. During the Miocene epoch the condi- 

 tions seem to have been favorable for animal life, as may be inferred 

 from the great deposits of shell marl which were then formed. 



After the deposition of the Calvert formation the region was again 

 raised and subjected to erosion for a short period, and then sank 

 once more beneath the sea. The Choptank formation was laid down 

 contemporaneously with the advancing ocean. This formation lies 

 unconformably upon the Calvert, and farther north transgresses it. 

 In neighboring regions to the south of this county a third Miocene 

 formation, the St. Mary's, was deposited conformably upon the Chop- 

 tank at a later period. 



SEDIMENTARY RECORD OF THE LAFAYETTE FORMATION. 



At the close of the Miocene the entire region was uplifted to form 

 land. Streams at once began to carve valleys on the featureless sur- 

 face. These conditions continued until the country was reduced 

 approximately to a base-level, so that the weathered products of the 

 Piedmont were not carried oif by the sluggish streams. Then a sub- 



