482 A. W. GRABAU PALEOZOIC DELTA DEPOSITS OF NORTH AMERICA 



The Longwood shales and sandstones. — This formation holds its char- 

 acter and thickness for some distance along the Front Eidge of the north- 

 ern Appalachians. At Culvers Gap, Xew Jersey, it is about 2/200 feet 

 thick, and consists of red sandstones alternating with red shales. The 

 sandstones are often cross-bedded and very commonly carry an abundance 

 of red clay galls. Xow and then a bed of greenish or grayish shale of 

 slight thickness may occur, but the formation is prevailingly and strik- 

 ingly of a red color. At the Delaware Water Gap, where the thickness is 

 much the same, the character also has not changed much. The Shawan- 

 gunk, which is here 1,900 feet thick, has some ferruginous la} r ers, and it 

 alone was referred to the Medina and the Oneida, while the red and green 

 shales overlying were referred to number V of the Pennsylvania series 

 and made to include both Clinton and Salina. As a matter of fact, no 

 Clinton occurs here, as that belongs wholly below the Shawangunk, with 

 which this section begins. The subdivisions given by the Pennsylvania 

 Survey for these shales are as follows : 



Feet 

 Strata concealed by Cherry River Valley 740 



Feet 



Upper red shale 155 



Variegated shales and sandstones 450 



Lower red shales 740 



Lower olive shales 340 



■ 1,685 



2,425 



The lower series is more sandy and is to be included with the Shawan- 

 gunk. The lower red shales contain many sandstone bands, which by 

 their cross-bedding, their clay galls, etcetera, indicate the mode of their 

 formation. The clays also commonly contain mud cracks, though these 

 are not always easy to observe on account of the ready shattering of the 

 rock. Some of these mud cracks from the Schuylkill section of the Long- 

 wood, in rather greenish beds, penetrate to a depth of 3 or 4 inches or 

 more, indicating a prolonged period of desiccation, during which the 

 parched ground was baked hard and cracked deeply. On the Delaware 

 the central part of the Longwood contains more greenish sands and shale 

 layers than is found in the sections either north or south. At the "Wind 

 Gap nearly 2,400 feet of red shales and sandstones occur, while the Le- 

 high shows nearly 2,500 feet of these strata, according to the recent 

 measurements of Mr. Billingsley. Chance's measurements give only 

 1.275 feet for the Lehigh Gap, but there is a concealed portion which 

 Lesley thought is also occupied by the shale and of which he says "an 



