MARYLAND GEOLOGICAL SURVEY 57 



the low land of the typical Coastal Plain and on the west of it are the 

 rolling uplands." 



St. Mary's County contains only one type of Coastal Plain topography, 

 which is the Western Shore type, but the broad flats along the north 

 bank of the Potomac Eiver suggest an Eastern Shore topography. Its 

 former level surface has been so extensively dissected by streams which 

 run east and northeast into Chesapeake Bay and the Patuxent Eiver on 

 the one hand and south into the Potomac Eiver on the other, that the 

 country now possesses the character of a rolling upland, such as is custom- 

 ary to associate with the eastern margin of the Piedmont Plateau. The 

 surface, although resembling a dissected plain, is in reality made up of 

 three distinct systems of terraces, which lie above one another like steps 

 in a flight of stairs. The oldest, which is topographically highest, occu- 

 pies the center and the other terraces are grouped about it in concentric 

 arrangement in order of their age. 



The oldest terrace, having been subjected to erosion longer than the 

 others, is more dissected and its surface, which was originally level, has 

 now been modified so as to present a gently rolling aspect. The next 

 younger terrace, although it also has suffered from erosion has not yet 

 reached the advanced stage of the oldest, while the terrace which is 

 topographically lowest and therefore the youngest of the three has suf- 

 fered least of all by erosion and, in fact, has been subjected to the work 

 of streams for so short a time that its surface for the most part retains 

 its originally level and unbroken character. 



Each of these terraces is separated from the one just below by a well- 

 defined scarp-line similar in appearance to the sea-cliff which separates 

 the lowest terrace from the modern beach. In approaching the main 

 divide of St. Mary's County, one travels for some distance over an 

 unbroken flat, which constitutes the lowest and youngest terrace. The 

 surface of this plain gradually rises toward the interior. At its inner 

 margin, which is about 45 feet in height, it is terminated by an abrupt 

 scarp of 10 to 20 feet, which leads up to the surface of the middle terrace. 

 This also is a flat, lying higher than the former and extensively eroded 

 by the headwaters of streams which rise within it. This middle flat in 

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