82 THE GEOLOGY OF ST. MART's COUNTY 



graphically higher than those which constitute the various formations 

 of the Columbia Group. Since its deposition it has suflEered considerably 

 from erosion. 



Area! Distribution. 

 The Lafayette is one of the most widely developed formations of 

 the Coastal Plain, extending from Xew Jersey southward to Florida, 

 and thence around the Gulf coast through Texas into Mexico. In 

 Maryland it has suffered considerably from erosion since it was deposited, 

 and is found best developed between Anacostia Eiver and Charlotte Hall. 

 The representatives in St. ]\Iary"3 County, then, constitutes the southern 

 portion of this area. In this county the Lafayette formation occurs as 

 isolated outliers which are grouped about K"ewmarket and Charlotte 

 Hall. One of these, the largest, extends from the headwaters of Indian 

 Creek to Charlotte Hall, and smaller areas continue the formation south- 

 ward along the divide to the vicinity of Mechanicsville. Another area 

 lies southwest of Xewmarket and continues over into Charles County. 

 Many streams have forced their headwaters back into the body of the 

 Lafayette formation, so that it no longer is as continuous as when first 

 deposited, but has developed a sinuous outline. The streams which did 

 this cutting and destroyed the continuity of the Lafayette formation in 

 St. Mary's County no longer exist. Their valleys are filled in with 

 deposits belonging to the Columbia Group. A younger generation of 

 streams, however, is rapidly pushing their headwaters backward to 

 continue the work of destruction left unfinished by their predecessors. 



Structure and Thickness. 

 The base of the Lafayette formation is nowhere visible within St. 

 Mary's County, but the surface lies at an elevation of about SOO feet. 

 It is developed as a flat-topped terrace, and it is probable that its average 

 thickness is about 30 feet. Within this county no dip or slope are dis- 

 cernible, but taking the formation as a whole from Washington City 

 southward to Charlotte Hall it has been foimd to gently decline toward 

 the southeast at the rate of 5.5 feet per mile. It is probable that this 



