316 HARMER : THE PLEISTOCENE DEPOSITS OF EAST ANGLIA. 
the Pleistocene epoch, I will attempt to deal more fully with 
some of the points raised in the paper referred to, especially as 
to the probable origin of the Chalky Boulder-clay, the most 
important of our more recent deposits. It may be desirable, 
however, in the first instance, to carry our thoughts back for 
a moment to the geographical conditions of the district at the 
period immediately preceding the Glacial epoch. 
During the deposition of the oldest of our Pliocene beds, 
the Coralline Crag, the North Sea seems to have existed as a 
basin closed to the north, but under the influence of the tidal 
currents of the English Channel, communicating with the latter 
by a strait over some part of the south-east of England ; the 
moUuscan fauna of these beds was almost exclusively southern, 
resembling generally that of the Mediterranean at the present 
day ; early in the Red Crag period, however, connection with 
northern seas was opened up, perhaps more or less suddenly, 
while that with the south was closed by a combined tectonic 
movement of upheaval and subsidence (apparently continuing 
into the Pleistocene period), to w^hich I have elsewhere referred.* 
At a somewhat later stage, northern and Arctic shells invaded the 
Crag basin in considerable numbers and variety, the southern 
forms from that time gradually disappearing, while, coincidently, 
the sea retired towards the north ; by the time the Chillesford 
stage of the Crag was reached, the molluscan fauna of the German 
Ocean had acquired a boreal and very recent facies. The 
southern shore of the Crag basin then lay as far to the north 
as the north coast . of Norfolk, and East Anglia was united to 
Holland. The Chillesford Clay, an estuarine deposit which can 
be traced in a sinuous course from Chillesford in Suffolk to 
Burgh in Norfolk, where, in my opinion, it suddenly disappears, 
as shown in the sketch-map (Fig. 3), probably represents one of 
the channels through which, at this time, the Rhine reached the 
sea. 
A slight submergence afterwards permitted the German 
Ocean to re-invade North-east Norfolk, possibly nearly as far as 
Norwich, introducing marine beds containing in great abundance 
* " The Pliocene Deposits of Holland." Q. J. G. S., Vol. 52, p. 748, 
1896. 
