11-i THE GEOLOGY OF PRINCE GEORGE S COUNTY 



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Sand forms a rather unimportant part of the Lafayette deposits. 

 Such as is present seems to have been derived mainly from the 

 Potomac beds. Lenses of sand occur at many places in the gravel 

 deposits, but do not commonly form beds of great thickness or extent. 

 The sand usually serves as the matrix for the gravels or else is inti- 

 mately mixed with the loam. 



Throughout the county the Lafayette is capped by a deposit of 

 loam varying in thickness from a few inches to 10 feet or more, with 

 an average of about 5 feet. ISTear Washington this loam contains 

 considerable iron and has here and there a decided orange color. 

 To the east and south this color becomes much less pronounced. In 

 many places the loam resembles the loess of the upper Mississippi 

 Valley in color and also in texture. On the broad Lafayette plain 

 in the southern part of the county the loam shows a very pronounced 

 mottling of drab and brick-red. This is particularly noticeable when 

 the material is wet. It is seen in numerous road cuts, especially 

 west of Brandywine. The Lafayette loam is in some places highly 

 argillaceous, in others decidedly arenaceous. As a general rule, it 

 is of very fine texture. Although the loam capping is relatively free 

 from bands of gravel, they are not entirely absent. Single pebbles 

 are not uncommon in the loam and locally there are well-defined 

 beds of gravel and sand. The following section, taken l^^^ miles 

 southeast of Piscataway, makes these relations clearer : 



Section 1% miles southeast of Piscataway. 

 Lafayette: Feet. 



Fine grayish-yellow loam -5 



Medium-coarse gravel in a matrix of gray sand 4 



Yellow cross-bedded sand 3 



Unassorted gravel mixed with gray sand 5 



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Physiographic Expression. — As described under "Topographic 

 features," the deposits of this formation form a plain of depo- 

 sition which is well-developed in many places in the Coastal Plain 

 and slopes gradually toward the sea. In the vicinity of Ana- 

 costia the base of the Lafayette plain is at a height of 280 feet and 

 its surface at 300 feet. At Charlotte Hall in St. :^Iary's County, 

 30 miles to the south, the base is not visible, but the surface lies at 



