86 THE GEOLOGY OF ST. MAEY's COUNTY 



Structure and Thickness. 



In the northern portion of St. Mary's County the base of the Sunder- 

 land formation lies at about 0'> or 100 feet, while in the southern 

 portion near Eidge it is obscured by deposits belonging to the Wicomico 

 formation and is not visible. In the vicinity of Charlotte Hall the 

 surface of the Sunderland lies at about 180 feet and at Ridge at about 

 60 feet. This difference in elevation of 120 feet between the two locali- 

 ties, which are 34 miles apart, amounts to a slope toward the southwest 

 of 3.5 feet per mile. This gentle decline is probably in large part 

 due to the original slope of the surface which was imparted to it while 

 it was undergoing deposition beneath the surface of the Sunderland 

 sea. There is, however, a slight element of tilting downward toward 

 the southeast which is more prominent in the southern portion of the 

 county than elsewhere. The surface of the Sunderland at Eidge is the 

 lowest which has been observed anywhere in Maryland; it is. probably 

 due to both tilting and initial slope. The deformation in this region 

 exhibited in the Sunderland formation is also found in both the Wicomico 

 and Talbot formations, which here also lie lower than elsewhere in 

 Maryland. 



The thickness of the Sunderland formation is variable, but averages 

 about 35 feet. 



Character of Materials. 

 The materials which compose the Sunderland formation consist of 

 clay, loam, peat, sand, gravel, and ice-borne boulders. 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 ice-borne 

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

 clays and finer materials are either developed in lenses or are horizontally 

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

 and may occur in the gravel beneath or in the loam above. There is 

 distinguishable throughoirt 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 



