196 INVERTEBRATE PALAEONTOLOGY 



Following the upheaval that brought Chalk formation 

 to an end, a long interval of denudation supervened. 

 At length the South-Eastern parts of the British area 

 became depressed sufficiently for accumulation of lasting 

 sediment. At the outset of this episode estuarine sands 

 and clays seem to have spread, with practical continuity, 

 from Suffolk to the Isle of Wight ; but this broad area 

 shortly became divided into two troughs separated by 

 the Wealden axis of upheaval. In the London Basin 

 increased, but localized, "sagging" admitted the sea 

 during formation of the London Clay; but a reverse 

 tendency followed, whereby sedimentation (save for 

 fluviatile deposits) was prevented. In the Hampshire 

 Basin a similar series of events occurred, but they were 

 extended over a longer period of time, and produced a 

 greater bulk of sediment. Not until the Upper Eocene 

 (when the London Basin was " full ") did the sea gain 

 undisputed sway in the southern trough. The succeed- 

 ing Oligocene strata mark a renewal of the struggle 

 between land and sea, comprising deposits that are 

 estuarine or lacustrine in the main. With the infilling 

 of the Hampshire Basin, sedimentation (of a durable 

 type) ceased in Britain; the whole area may have 

 undergone considerable elevation during the Miocene 

 period. In Pliocene times the " North Sea " covered 

 parts of East Anglia that are now land, and left thin, 

 but characteristic, layers of " crag " and clay. These 

 are overlain by deposits suggestive of conditions like 

 those prevalent in the Wash at the present day. The 

 fossils of the sub-glacial peats and marls give clear 

 and convincing evidence of the approaching climatic 

 rigours of the Lower Pleistocene ; but apart from 

 their boreal aspect they are essentially modern in facies. 

 Pleistocene deposits are largely Boulder Clay and 

 similar glacial relics; the fluviatile and peaty beds 



