220 NEOZOIC TIME. [CHAP. XI. 



formed in anything but a large lake or series of lakes. 

 No one, perhaps, has studied them more carefully than 

 Professor Prestwich, and in 1854 he referred their forma- 

 tion to river-action on a large scale, and looked to the 

 granitic districts of the south-west for the supply of the 

 argillaceous material. 1 Mr. Gardner also speaks of them 

 as fluviatile ; but considering the area over which they are 

 known to extend in unbroken continuity, they can hardly 

 be called fluviatile deposits in the ordinary sense of the 

 term. That the lakes were shallow and prolonged into 

 extensive arms and inlets is very probable, and they were 

 doubtless fed by streams which had little velocity, but 

 meandered quietly over the plains of chalk that surrounded 

 the lacustrine areas, so that parts of the water-covered tract 

 might be regarded as expansions of the river valleys, and 

 in that sense fluviatile. 



If this is a correct idea of the conditions under which 

 the Reading Beds and Plastic Clays were formed, it is 

 clear that some barrier must have been raised which pre- 

 vented the access of the sea that had at first occupied a 

 portion of the lacustrine area, for there cannot have been 

 much difference of level between the marine and the lacus- 

 trine waters. It is not difficult, however, to understand 

 the construction of such a barrier if the existence of land 

 over the Wealden area is granted, for this would itself 

 form a portion of the barrier, and the erosion of its shores 

 would afford a supply of flint pebbles and sand which the 

 set of the currents might carry northward and pile up in 

 the form of shingle banks and sand dunes across the 

 shallow waters of the London area. Seabord lakes and 

 alluvial levels, shut in and protected from the inroads of 

 the sea by such banks, are of common occurrence ; and it 

 is a curious coincidence that pebble beds should exist at 

 various horizons in the Woolwich and Reading Beds, and 

 1 " Quart, Journ. Geol. Soc.," vol. x. p. 136. 



