70 The Lowek Cretaceous Deposits op Maryland 



Stratigraphic and Structural Eelations. — The Patapsco forma- 

 tion rests unconformably on the underlying formations and at the 340- 

 foot hill at Eelay transgresses them and rests on the crystalline rocks. 

 Toward the north the Patapsco deposits gradually transgress the 

 underlying formation, and in Delaware and Pennsylvania rest di- 

 rectly on the crystalline rocks at a number of points. Monadnocks of 

 crystalline rock penetrate the Potomac formations at several points in 

 northeastern Maryland and the adjoining portion of Delaware. 



The Patapsco formation was much eroded prior to the deposition of 

 the Earitan, so that marked irregularities are found in the line of con- 

 tact which represent rather pronounced inequalities in the upper surface 

 of the Patapsco. In general the line was well defined, and at some points 

 is marked by a line of broken and redeposited iron crusts. In the ab- 

 sence of the Earitan, which gradually thins out towards the south, the 

 Patapsco formation is overlain unconformably by later Cretaceous or 

 Eocene deposits, while in the absence of both later Cretaceous and Ter- 

 tiary deposits, the Patapsco formation is often overlain unconformably 

 by Pleistocene deposits. 



The internal stratigraphy and structure of the Patapsco formation is 

 somewhat complex, on account of the great difference in the character of 

 the materials, ranging, as they do, from very plastic and highly variegated 

 clays to coarse sands, the latter occurring in lenses and beds which at 

 times considerably complicate the stratigraphy, although they are not 

 sufficiently continuous to make it possible to subdivide the Patapsco into 

 members of more than local importance. Such local lithologic terms 

 have been employed by others, but the very circumscribed limits of 

 these beds render their use very problematical. 



Some warping of the beds evidently occurs along the Fall-line, as 

 shown by the differences in dip, and it is quite possible that actual 

 faults occur, although on account of this continuity of the strata and 

 their frequent cover of later deposits it is impossible to determine this 

 point definitely. As already pointed out, the marked changes in dip 



