MARYLAND GEOLOGICAL SURVEY 137 



posits are composed of clay, peat, sand, greensand, marl, gravel, iron ores, 

 and ice-borne boulders. The topography of the region shows that the sur- 

 face of the Coastal Plain is made up of five distinct systems of terraces, 

 the oldest lying at the top and the others lying successively beneath in the 

 order of their age. The fifth terrace is now being built by the waves of 

 the Atlantic ocean, the Chesapeake Ba}^, and estuaries. They are uncon- 

 solidated except where locally oxide of iron, lime, silica, etc., has ce- 

 mented the materials into thin ledges. The formations, beginning with 

 the oldest, are known as the Lafayette, Sunderland, Wicomico, Talbot, 

 and Eecent. Each one of these formations corresponds to a distinct ter- 

 race in the topography, the surface of the formation and the physiogra- 

 phic feature of the terrace being one. It has been determined that the 

 Lafayette lies unconformably on older deposits and that a period of ero- 

 sion separates each of the terrace formations. 



An epitome of the oscillations of the Coastal Plain through this time 

 is as follows : 



Subsidence and deposition of the Lafayette formation. 



Elevation and erosion. 



Subsidence and deposition of the Sunderland formation. 



Elevation and erosion. 



Subsidence and deposition of the Wicomico formation. 



Elevation and erosion. 



Subsidence and deposition of the Talbot formation. 



Elevation and erosion. 



Partial subsidence and deposition of the Eecent terrace. 



Comparisons show that the classification adopted by the Maryland 

 Geological Survey is in harmony with that employed by Darton in his 

 latest work on the Coastal Plain, published in the Washington folio of 

 the IT. S. Geological Survey, although somewhat at variance with his in- 

 terpretation of the formations in southern Maryland. Comparisons with 

 the work of Salisbury in New Jersey show lack of harmony throughout. 

 Finally a study of the Coastal Plain deposits from the bottom to the top 

 shows that the Atlantic seaboard has been repeatedly elevated when loaded 

 and depressed when lightened. It would seem that some other theory 

 than that of isostasy must be proposed to account for these movements. 



