3IARYLAXD GEOLOGICAL SDKVEY 119 



At Sotterly Wharf, on the Patiuseut Eiver, there is a firm ledge of 

 Miocene rock whicli has been used for such purposes. Elsewhere ferru- 

 ginous sandstones and conglomerates from the Pleistocene deposits 

 supply the small local demand for rough building stones. 



THE MARLS. 



Extensive deposits of shell marl have a wide distribution throughout 

 the Atlantic Coastal Plain and have been worked, at intervals, since 

 the earty part of the last century, when their value as fertilizers was first 

 determined. However, their importance in the enrichment of soils de- 

 ficient in lime has never been generally recognized. At present their 

 use in Maryland has been almost entirely discontinued, although the 

 deposits are practically inexhaustible. At the Xomini Cliffs, just across 

 the Potomac Eiver, in Virginia, shell marl has been dug for shipment 

 and used in the manufacture of artificial fertilizers. 



The Calvert, Choptank, and St. Marj^'s formations all contain beds 

 of shell marl which are exposed in the river cliffs and valley slopes in 

 many places throughout the county. The Talbot also contains deposits 

 of shell marl at Langleys Bluff, on Chesapeake Bay, about 5 miles 

 south of Cedar Point, and at Wailes Bluff, on the Potomac, about 1 

 mile above Cornfield Point. The shell beds of the Choptank are the 

 most important and are particularly well exposed at Drum Cliff, on 

 the Patuxent Eiver. In places the shells are mixed with so much sand 

 that the lime forms only a small percentage, but in other places the 

 amount of lime exceeds 90 per cent. 



The value of the shell marls and methods for using them are thor- 

 oughly discussed by Professor H. J. Patterson in a Bulletin of the 

 Marjdand Agricultural Experiment Station (No. 66, May, 1900). He 

 states that the lime has an especially beneficial effect upon sandy soils, 

 such as prevail throughout St. Mary's County, in improving their physical 

 characteristics. This it does through its cementing action which ren- 

 ders such soils less porous and thus enables them to retain moisture 

 better. Chemically, lime corrects the acidity of the soils through its 

 neutralizing effect upon acids, acting upon other soil constituents, ren- 



