much further north the mouth of the river lay we do not know, for in Norfolk the 

 delta*deposits consist largely of coarse gravel, and beyond we might expect a wide 

 expanse of muddy alluvium. The necessary results of this advance of the delta must 

 be taken into account. The addition of 600 kilometres to the -course of the Rhine, and 

 the consequent rise of the riverbed, must affect the flow for long distances above the 

 head of the delta, and far above the seaslevel. 



Before the delta began to form, the Rhine and Maas poured their waters, perhaps 

 independently, into the Pliocene sea, and the erosion of the Rhine and Maas valleys 

 near the sea reached its maximum depth, leaving successively lower and lower terraces. 

 But then, as the gravelly delta advanced over the marine Pliocene deposits and obstructed 

 the flow, the water was ponded back, the level of the delta rose, and new high4evel 

 terraces may have been formed. The newest of these high*level terraces, corresponding 

 with the maximum extension of the gravekdelta into Norfolk in Cromerian times, may 

 in the upper Rhine Valley coincide in level with some far more ancient terrace, perhaps 

 of Lower Pliocene or even of Miocene date. 



The fluctuations of the position of the mouths of the Maas, Rhine, and Elbe, 

 which flowed into the intermittently subsiding area of the North Sea, were very compli* 

 cated, and we are not yet in a position to write the complete history of these fluctua* 

 tions. At certain periods the rivers entered the sea directly they left the hills, at others, 

 as already observed, their combined deltas extended hundreds of kilometres further. 

 We know of at least three periods of great extension. The earliest was in Cromerian, or 

 latest Pliocene times, when coarse Rhine*gravels were carried as far as the coast of 

 England, where they yield peculiar mammals, of which Elephas mevidionalis, Rhinoceros 

 ettuscus, Equus stenonis, and Cervus verticornis are perhaps the most abundant. A second 

 extension seems to be represented by the Pleistocene mammalian fauna of the Dogger 

 Bank, where in former years were dredged many bones, of which the most abundant 

 were the mammoth (Elephas primigenius), Rhinoceros antiquitatis, Bison priscus, Equus 

 caballus, Cervus tarandus, and walrus (Trichechus rosmarus). 



Recently a third extension of the delta has been recognised, in the peat trawled 

 from the submerged plateau of the Dogger Bank. This extension, from the character 

 of its temperate flora, seems to be of Early Neolithic date, and equivalent to the lowest 

 "submergedsforests" or submerged peat*beds of England and the Netherlands. 



These two later extensions of the delta, however, do not concern us here. They 

 probably did not much affect the surface of the high plateau, for the deposits of the 

 Dogger Bank lie at a considerable depth below the sea (20 metres and more). The 

 equivalents of these strata may be looked for in the Maas and Rhine valleys in the low 

 terraces, for the outlet of the rivers, though far away, was not blocked, as it must have 

 been in Cromerian times, when coarse Rhine*gravels were deposited in Norfolk to a 

 height of 5 or 10 metres above the present sea4evel. 



The problem of the tectonic movements in the delta and in the North Sea basin 

 is a most difficult one, and brings in additional complications. Intermittent movements 



