OF NORFOLK AND SUFFOLK. 
121 
with this it may be interesting to notice that the Chalky 
boulder- clay of the Ouse Valley contains much Oxfordian 
debris ; the ice to which it was due, one of the radial lobes 
of the Fenland glacier, having travelled along the strike of 
the Oxford Clay. An interesting exposure of this kind of drift 
may be seen in some brickyards two miles south-south-east 
of Huntingdon. 
It seems that the ice which filled the basin of the Ouse 
and its tributaries at this stage was sufficiently thick to 
enable it to overflow the Chalk escarpment between 
Newmarket and Hitchin, which in places exceeds 500 feet in 
height. Thence it descended the dip slope in a south-easterly 
direction into Essex, at right-angles to its original course to 
the south-west. 
Although the boulder-clay of the Ouse basin has a Jurassic 
matrix, that of the dip slopes of the Chalk range to the south- 
east of it is of the chalky type. Prof. Chamberlin, studying 
ffie Greenland ice-sheet, found that all but the lowest part 
of it was free from detritus. The ice which overflowed the 
Chalk escarpment must similarly have been comparatively 
clean, as it brought over with it but little Jurassic material 
from the lower ground. As soon as it had crossed the water- 
shed, however, it commenced to grind into the underlying 
Chalk, producing boulder-clay of a chalky type. A similar 
explanation may be given of the intensely chalky drift of 
the western slopes of the Lincolnshire Wolds, and its relation 
to the North Sea Drift of the coast region to the eastward. 
A great part of the district covered by the Chalky boulder- 
clay in Norfolk and Suffolk forms plateaux, the surface of 
which has a distinctly levelled appearance, due no doubt 
to the pressure of the superincumbent ice which moved over 
it. No other explanation can be given of this feature which, 
as we have seen before, is also characteristic of the earlier 
glacial drift of north-east Norfolk, than that the deposits in 
question originated as bottom moraines. 
In connection with this it may be mentioned that the Chalky 
boulder-clay often rests upon undisturbed beds of sand. How, 
it may be asked, can this be reconciled with the view that they 
