CROMER FOREST-BED. 
189 
there is a gravel of a very similar character, and suggests that 
the stones may have been brought to Norfolk by some old river 
traversing the Ardennes. The plateau gravels above the Meuse 
are, however, full of veined quartzites of a character unknown in 
the Forest-bed. The Forest-bed river appears certainly to have 
flowed from that direction; but it seems unnecessary to bring 
the materials such a long distance, though a large stream with 
floating ice could probably do so. The whole of the evidence 
seems to show that in the Forest-bed we are dealing with the 
old alluvial and estuarine deposits of the Khine—a view held by 
various writers ever since the Forest-bed was first known. 
The changes that took place during the Newer Pliocene period 
may be thus summarized. During the formation of the Coralline 
Crag there was a considerable submergence of the land, and free 
connexion between the German Ocean and more southern seas. 
Afterwards the land rose, and the connexion with all seas, 
except the Arctic, was cut off.* A further rise caused the 
estuarine and freshwater Forest-bed to spread over the southern 
half of the North Sea. 
If the marine fauna of the different Crags is examined, a 
gradual dying out of southern forms and increase of northern 
may be perceived, till at last in the Chillesford and Weybourn 
Crags the mollusca have a thoroughly arctic facies. This change 
has generally been referred to the greater intensity of the cold; 
but in working out the physical geography of the Pliocene 
Period, I have been led to agree with Prof Prestwich, that it 
is not altogether due to general climatic changes. When the 
connexion with southern seas was cut off, the direct cooling 
action of northern currents, without any from the south, must 
have had a considerable effect on the temperature of the water.f 
But probably the change in the fauna was principally due to 
the sea being fully open to the north, so that there was a constant 
supply of arctic species brought by every tide or storm, while 
at the same time the southern forms had to hold their own with¬ 
out any aid from without; and if one was exterminated, it could- 
not be replaced. In this way, of two species, a southern and a 
northern, equally fitted for any station, the northern would have 
the best chance of surviving, and would probably exterminate 
the southern. The fact that not a single southern species 
appears for the first time in the Fluvio-marine, Chillesford, or 
Weybourn Crags seems clearly to show that they could not 
migrate into the district, owing to some barrier. 
Taking now the land fauna and flora, it is seen that the same 
elevation which would raise a barrier to the migration of marine 
species, would form a highway for the land and freshwater forms. 
See Goclwin-Austen On the Kainozoic Formations of Belgium, Quart. Journ. 
Geol. Soc.y vol. xxii., p. 238, 1866 ; and Prestwich On the Structure of the Crag 
Beds, ibid.y vol. xxvii., p. 475 (1871). 
f See Prestwich, op. cit., p. 478. 
