93 THE GEOLOGY OF ST. MARY's COUNTY 



45 feet. These two localities are sei^arated by a distance of about 35 

 miles, making an average slope toward the southwest of about 1.7 feet 

 I^er mile. This attitude is largely due to the original slope which tlie 

 surface of the formation received when deposited, but in southern 8t. 

 Mary's County an element of tilting seems to have brought the Wicomico 

 lower than elsewhere in Maryland. 



The thickness of the AVicomico formation is variable, but averages 

 about 20 feet. 



Character of Materials. 

 The materials which compose the Wicomico formation consist of 

 clay, loam, sand, gravel, and ice-borne boulders. These, as a rule, do 

 not lie in well-defined beds, but grade into each other both vertically 

 and horizontally. The coarser materials, with the exception of ice- 

 borne boulders, are usually found with a cross-bedded structure, while 

 the clays and finer materials are either developed in lenses or are hori- 

 zontally stratified. The ice-borne blocks are scattered throughout the 

 formation and may occur in the gravel beneath or in the loam above. 

 There is distinguishable throughout a tendency for the coarser materials 

 to occupy the lower portions and the finer materials the upper portions 

 of the formation, but the transition from the one to the other is not 

 marked by an abrupt change. The coarser materials are frequently 

 found above in the loam and finer materials below in the gravel, and are 

 frequently much decayed. 



Stratigrapliic Relations. 

 The Wicomico formation is deposited as a terrace lying unconformably 

 and somewhat irregularly on the older beds of Eocene and Miocene age. 

 This terrace was laid down about the margin of the Sunderland forma- 

 *^^ion and locally is believed to lap up on the thin eroded edges of the lat- 

 ter, which are supposed to run out a short distance beneath it. It is 

 everywhere separated from the Sunderland formation by a well-defined 

 scarp, which is an ancient cliff cut by the waves of the Wicomico sea 

 during the post-Sunderland depression. 



