THE UPPER CRETACEOUS OF NEW JERSEY 79 



of the New Jersey Cretaceous at the base of the Navesink marl in 

 Monmouth County. But there are other facts to be considered. 



One of the most cha*racteristic faunal features of the Navesink 

 marl is a conspicuous shell bed about 12 feet above the base of the 

 marl in Monmouth County. It is usually about 1 foot in thickness, 

 and is composed almost exclusively of the shells of Gryphaea vesicu- 

 laris and Ostrea larva, with occasional specimens of other pelecypods 

 and gasteropods. At the base of the Navesink in Monmouth County 

 there is sometimes an arenaceous, more or less abundantly fossilifer- 

 ous bed, which Cook designated as the "sand-marl." At Atlantic 

 Highlands this bed is 3 or 4 feet thick, and is evidently the bed which 

 Clark mentioned in his description of the Mount Laurel sand at that 

 locality, and which Knapp regarded as forming the top of his Weno- 

 nah sand. In passing to the southward this arenaceous basal mem- 

 ber of the Navesink seems to become more and more conspicuous, 

 replacing higher and higher beds of the green- sand marl, until at 

 Mullica Hill it extends up to and even includes the conspicuous 

 shell layer of the formation. This arenaceous facies of the Navesink 

 frequently abounds in fossils, although they are usually imperfectly 

 preserved casts, and the fauna is always characterized by the typical 

 Navesink species Belemnitella americana. 



It is believed that Clark's conception of the Mount Laurel sand 

 formation has grown out from this changing facies of the Navesink 

 to the southward, and, in the absence of sufficient data concerning 

 the fauna of the beds immediately beneath those with the Belemni- 

 tella fauna, he has extended the Mount Laurel formation downward 

 to include the entire sand bed to the top of the Marshalltown clay. 

 On the other hand, Knapp and Kiimmel have extended the Wenonah 

 formation upward to include all the sand to the south, so that their 

 upper boundary line of that formation marks a higher and higher 

 geologic horizon in that direction. From the standpoint of the 

 faunas the major division line in this portion of the Cretaceous beds 

 mus*t be drawn where the Belemnitella fauna is introduced, and 

 although the Wenonah fauna of Monmouth County has not yet 

 been detected in the more southern portion of the area, neither has 

 the Belemnitella fauna been observed in the lower portion of this 

 Mount Laurel- Wenonah sand, 18 feet beneath the top of the sand 



