MARYLAND GEOLOGICAL SURVEY 111) 



portions of the county to 180 feet in the southern portion. Through- 

 out this tract the original surface of the formation was nearly level, 

 but the streams which now flow across it have locally produced a 

 gently rolling surface. In the vicinity of Laurel there are many 

 gravels present in the surface soils that may belong to this formation 

 though not so represented on the map. The Potomac strata in that 

 locality contain gravel bands, and it seems more probable that the 

 surficial gravels are the residual materials of these Cretaceous beds 

 or of the Lafayette formation that occurs in the immediate vicinity. 



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

 derland formation consist of clay, sand, gravel, and ice-borne 

 boulders. As explained above, these materials 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, have in the main a cross-bedded structure, but the 

 clays and finer material are either developed in lenses or horizontally 

 stratified. The erratic ice-borne blocks are scattered through the 

 formation and may occur in the gravel, sand, or loam. The coarser 

 material throughout the formation tends to occupy the lower por- 

 tions and the finer the upper portions, but the transition from one to 

 the other is not marked by an abrupt change, and at many places the 

 coarse materials are present above and the finer materials below. As 

 a whole the material is coarser on the western side of the County, in 

 the Potomac basin, than elsewhere. In the vicinity of Congress 

 Heights, the gravels of the Sunderland are rather commonly 

 cemented by ferruginous material. The ferruginous conglomerate 

 used in the wall about the grounds of St. Elizabeth's Asylum was 

 obtained from beds of consolidated Sunderland deposits. Many of 

 the pebbles of the Sunderland are much decayed, but in general they 

 show less decomposition than the Lafayette gravels. 



Physiograpliic Expression. — The Sunderland deposits occupy and 

 form the Sunderland plain mentioned in the discussion of topog- 

 raphy (p. 80). This plain is separated from the Lafayette terrace 

 In- a well-defined scarp. This scarp has suffered considerable modi- 

 fication since its formation, and where it was not prominent it has 



