MARYLAND GEOLOGICAL SURVEY 61 



THE STRUCTURE OF THE COASTAL PLAIN. 



The materials of which this region is built consist of clay, loam, sands, 

 gravel, and boulders. These deposits are loose and unconsolidated, ex- 

 cept where local ledges of ironstone have been developed. Although the 

 materials which have built up St. Mary's County have been deposited at 

 various times and belong to a large number of geological horizons, still 

 they all lie either horizontal or nearly so. Tliose which have been tilted 

 most, seldom exceed a dip of 12 feet to the mile. The structure of the 

 region, therefore, has not materially influenced the drainage, and the 

 streams flow from its surface as if they were flowing from a country 

 composed of unconsolidated deposits of clays, sands, and gravel hori- 

 zontally bedded throughout. 



Topographic HIST0RY^ 



A detailed study of the topographic features which have been described 

 above and of the materials out of which the land is composed has re- 

 vealed many of the incidents which have produced the present relief. 

 An outline of the topographic history will now be given under the follow- 

 ing four stages, beginning with the oldest : 



1. The Lafayette Stage. 



3. The Sunderland Stage. 



3. The Wicomico Stage. 



4. The Talbot Stage. 



5. The Eecent Stage. 



THE LAFAY-ETTE STAGE. 



At the close of the Miocene period the area now occupied by St. 

 Mary's County, together with contiguous regions, was raised and sub- 

 jected to a long period of erosion. This was followed by a deformation 

 of the Middle Atlantic slope of such a character that the Coastal Plain 

 was depressed more than the Piedmont region to the west. At this time 

 St. Mary's County was entirely submerged and the highest terrace was 

 deposited as a thin veneer throughout the region. It is not known how 

 far inland the Lafayette sea advanced, but it is probable that its shore- 



