CLAYS OF CRETACEOUS FORMATION. 187 



larities are well shown in some of the banks west of Woodbridge. 

 At other localities, on the contrary, the upper surface of the fire 

 clay holds a constant level over somewhat wide areas. Nowhere 

 is this better shown than in the long* exposures in W. H. Cutter's 

 banks (29, 30). (Compare PI. XXI, Fig. 1, with PI. I, Fig. I.) 

 In general, the inequalities of the surface are most marked 

 where the fire clay is immediately overlain by glacial drift or by 

 the yellow gravel (Pensauken) formation (Fig. 35, B). In this 

 case, the irregularities are due to erosion which removed the 

 overlying Cretaceous beds before the much later drift was depos- 

 ited. In those cases not only has a part of the fire clay been worn 

 away, but also a great thickness of overlying Cretaceous beds, 

 and perhaps a part of the Miocene. 



G/acial 

 Drift 

 Lam/nab 



fvg.JS. A. Erosion and partial removal of the black laminated 

 c/oy before the- eflacia/drift was deposited. 



B. Comp/ete rem oral of the black laminated clay. and 

 erosion of the top of the fire clay, before the glacial 

 drift was deposited. 



C-Complete removal of the black laminated clay and of the 

 fire c/ay before the drift was deposited. ~* 



Locally, erosion may have been so extensive as to' remove the 

 fire-clay bed itself. In such cases borings through the drift 

 would strike not fire clay, but the underlying fire sand (Fig. 35 

 C) . Erosion since the deposition of the drift will account for the 

 absence of the fire clay along those streams such as Heard's brook 

 near Woodbridge, where the stream has cut down its valley below 

 the level of the fire clay. In such a case, the fire clay is found on 

 either side the stream at a higher level (Fig. 36, A). In some 

 cases, however, the undulatory upper surface of fire clay is fol- 

 lowed immediately by beds of Cretaceous, sand or sandy clay. In 

 these cases the irregularities are apparently due to erosion by 

 shifting currents, tidal or otherwise, during the deposition of the 



