122 MR. F. W. HARMER ON THE GLACIAL DEPOSITS 
were over-ridden by a great thickness of ice ? Prof. 
Chamberlin’s researches in Greenland throw light also on 
this point. He states that the talus slope of detritus which 
precedes the advancing ice sheet in that country forms an 
embankment, over which the latter advances without 
disturbing any beds of incoherent sand which may underlie it. 
THE EROSION OF THE NORTH SEA DRIFT BY THE 
INLAND ICE STREAM. 
The fact that the North Sea Drift ceases to the south of 
Norwich, the Chalky boulder-clay coming on abruptly and 
lying side by side with it at the same level, was for many 
years a puzzle to me ; it was not until I began to study on the 
spot the glacial deposits of the region to the north-west, and 
especially those of Lincolnshire, that the following explanation 
suggested itself. 
Originally, as we have seen, the whole of Norfolk, a part of 
Suffolk, and probably much of the Fenland, was covered by 
the morainic deposits of the earlier glaciation, i.e., by the 
North Sea Drift. The subsequent invasion of all this 
region, except that part of Norfolk to the north of the line 
A B C of the contour map, lying under the lee of the Lincoln- 
shire Wolds, by the inland glacier from the north-west, 
destroyed and incorporated in the moraine of the latter 
that of the North Sea ice. On this view the igneous erratics 
occasionally found in the Chalky boulder- clay, similar to 
those found in the North Sea Drift, are derivative. Cases 
are occasionally met with near the line A B, where the older 
deposits have been disturbed by the impact of the later ice. 
Mr. H. B. Woodward noticed, for example, in one of the 
railway cuttings north-east of Fakenham, near the junction 
of the Chalky North Sea marl with that of the later Chalky 
boulder-clay, that “ all kinds of clay, sand and gravel were 
jumbled up together ” ; this is what we might expect to find 
where an ice-stream had invaded the moraine of an earlier 
glaciation.* 
* Mem. Geol. Survey, Fakenham (1884), p. 16. 
