MARYLAND GEOLOGICAL SURVEY 87 



XXIV). In southern Maryland the Sunderland formation attains its 

 greatest development and here wraps about the margin of the Lafayette 

 formation and extends up into ancient valleys which penetrate it as 

 reentrants. When the various elevations of these areas are compared, it 

 will be found that the formation has practically no slope along the border 

 of the Piedmont between Perry ville and Washington, but from Washing- 

 ton southward to Charlotte Hall there is a difference in altitude of 40 

 feet throughout a distance of 33 miles, making an average slope of 1.2 feet 

 per mile. Between Charlotte Hall and Eidge there is a difference in alti- 

 tude of 120 feet in a distance of 34 miles, or an average slope of 3.5 feet 

 per mile. It will thus be seen that the Sunderland formation has a 

 gentle decline toward the southeast. This structure is in part due to 

 initial slope and in part to tilting. 



The thickness of the Sunderland formation is as variable as that of 

 the Lafayette. Near Elkton, Cecil county, it attains a thickness of 60 to 

 80 feet, but in other places thins clown and disappears entirely. Taken 

 as a whole, the average thickness of the Sunderland is about 35 feet. 



Character of Materials. — The materials which compose the Sunder- 

 land formation consist of clay, peat, sand, gravel, and ice-borne blocks 

 (Plate VI, and Plate VII, Fig. 2). As explained above, these as a rule 

 do not lie in well-defined beds, but grade into each other both vertically 

 and horizontally. The coarser materials, with the exception of the ice- 

 borne boulders, are usually found with a cross-bedded structure, while 

 the clays and finer materials are either developed in lenses or are hori- 

 zontally stratified. The erratic ice-borne blocks are scattered throughout 

 the formation and may occur in the gravel beneath or in the loam above. 

 There is distinguishable throughout the formation a tendency for the 

 coarser materials to occupy the lower portions and the finer the upper 

 portions of the formation, but the transition from one to the other is not 

 marked by an abrupt change ; and coarser materials are frequently found 

 above in the loam and finer materials below in the gravel. As a whole, 

 the material is coarser in the Potomac and Susquehanna basins than 

 elsewhere. In the vicinity of Congress Heights, the gravels of the 

 Sunderland are frequently cemented by ferruginous material. The 



