200 CLARK AND BIBBINS — GEOLOGY OF THE POTOMAC GROUP 



clays, etcetera, mostly barren of iron, which lie between this line and 

 the base of the next succeeding formation. In general the demarka- 

 tion is well defined, and in many local sections, notably in the Timber- 

 neck area, the contact is sharp, leaving little room for doubt that we 

 have to do with something more than local changes in lithology or the 

 effects of other varying local conditions. To the seaward, however, the 

 line is by no means as clearly defined, and there is a suggestion of gra- 

 dation, as at Federal hill. Again, the circumstance that the two forma- 

 tions are on the whole argillaceous, and the character of their clays 

 often very similar, increases the difficulties of tracing the details of this 

 boundary, and to this may be added the fact that the contrast in the 

 floras and faunas is by no means as well marked as in the case of the 

 Arundel and Patuxent. On the whole, therefore, it may be said that a 

 more intimate relation exists between the Arundel and Patapsco than 

 between the Arundel and Patuxent. 



Areal distribution. — The Patapsco formation outcrops in an irregular, 

 crescentic belt, deeply dissected along the drainage lines and often hav- 

 ing its subaerial portion interrupted at the principal waterways. It ex- 

 tends from below Indian head on the Potomac to and beyond the head 

 of the Chesapeake bay. To the northward the outcrop is narrow. It 

 broadens toward the center of the belt and narrows again to the south- 

 ward by virtue of the transgression of the superjacent terranes. In Cecil 

 county, where the Arundel formation is wanting, the Patapsco beds re- 

 pose directly on those of the Patuxent terrane, and the same is true 

 over a number of smaller areas to the southward. The most notable body 

 of Patapsco materials occupies the highlands between Branchville and 

 Bowie. 



Characteristic local section. — The following section occurs at Red hill, 

 Cecil county : 



Feet 

 Raritan 1. Coarse reddish sand and coarse, evenly bedded, dark-brown iron 



sandstone 10 



" 2. Yellow and buff sands and corrugated iron stone containing fer- 



ruginated coniferous wood ; sand beds near the base, which is 



marked by springs 10 



" 3. Tough white clay 7 



Patapsco 4. Massive variegated red and drab clays, the latter slightly lignitic 

 and with obscure leaf impressions ; lenses of white sand toward 



base 130 



Patuxent 5. Sands not exposed at surface 60 



Economic products. — The Patapsco formation, although on the whole 

 an argillaceous terrane, is' often arenaceous, particularly to the land- 

 ward, where it yields building sands and iron sandstone for road metal. 



