46 TRANSACTIONS OF THE [1889 



intervals all the way down to the Fort Washington peninsula, 

 under which they pass to reappear near the base of the cliffs on 

 the north shore of Piscataway Creek. The Albirupean clay 

 stratum has also suffered very unequal denudation and erosion 

 in the old basin south of the Eastern Branch. It appears to be 

 only two to five feet thick north of Rosier Bluff, while at the 

 bluff itself it forms a high, mixed, irregular bed, apparently 

 about ten feet in thickne^. Below Fort Foote it is again 

 reduced to a thin belt, and is scarcely observable at points 

 leading in the direction of Broad Creek. Back in the country, 

 however, it seems to rest in heavier beds, and its exposures 

 vary in thickness at almost every change of locality. A similar 

 irregularity of surface characterizes this formation near the 

 Patuxent river, while at its best exposed position along the 

 Severn river, north of Round Bay, it stands almost complete in 

 high bluffs, with the white sand towering above it in monu- 

 mental piles. 



The Albirupean, wherever observable in place, shows a dip 

 of more than ten degrees, but along the Potomac river its dips 

 cannot be satisfactorily studied, notwithstanding the fact that it 

 can be discovered at or below tide as far down as to the vicinity 

 of Pomonkey Creek, in Charles county. South of the Piscata- 

 way, however, the white Albirupean clay is not always strictly 

 in place, and its representative is rather a mixed clay which is 

 partly derived from the upper part of the Baltimorean forma- 

 tion. The white clay keeps on steadily beneath the surface, so 

 that on Pomonkey Creek, at a distance of two miles from the 

 mouth of Piscataway Creek, it is reached at a depth of nearly 

 forty feet below the surface of the Quaternary beds, which there 

 rise about twenty-five feet above the level of high water. 



It is, accordingly, over the surface of this uneven bed of clay 

 that the Cretaceous system of Maryland is laid down. Such 

 conformability as it appears to have with the underlying Albi- 

 rupean is only of that kind which might have resulted from the 

 deposition of soft and freely movable material upon a yielding 

 surface. Viewed comprehensively, however, so as to take in its 

 full width along the Severn, Patapsco, Northeast and Elk 

 rivers, the Cretaceous is observed to dip at a much lower angle 



