476 
Geology of Norfolk. 
smaller scale, but more frequently from outlying portions of the 
upper drift, covered by a deep or thin warp, remaining on the 
surface of the till. 
Sub-Districts. 
If we divide Smith's largest district of strong soil by a line 
drawn from Wymondham to Attleborough, wheat and bean land, 
consisting of the denuded surface of the till covered bv a warp 
of clay loam, will be found to prevail over the southern portion — 
the outlying patches of upper drift constituting exceptional cases 
— whde in its northern part, and in the small district lying north 
of the Wensum, the proportions are reversed, the loams of the 
upper drift covering the greater portion, and the till being only 
exposed, except in artificial excavations, in the bottoms of the 
valleys which intersect it. In the portion of the district south of 
Wymondham the Suffolk plough and the Suffolk course of hus- 
bandry prevail. 
IV. District of the thin upper Drift. 
This comprehends the whole of Smith's light land district 
lying on the west of his district of strong soil. It occupies the 
summit level of the county, on which a thinner deposit of the 
upper drift appears to have been thrown down, originally, than 
in the eastern side. It covers also with a thin coating the greater 
part of those areas which, following the usual course adopted in 
geological maps, he has represented as occupied by the rock 
nearest the surface. On the summit of the watershed the drift 
varies in depth, from less than three feet to more than thirty. 
Between the chalk and the warp there are generally interposed 
two or three feet of rubbly chalk, mixed with patches of sand and 
yellow clay. This is covered, in some cases, by merely a thin 
sandy warp ; in others by thirty or forty feet of sand, gravel, and 
coarse shingle, with little regularity of stratification, and having 
masses of transported chalk and Kimmeridge clay enveloped in 
them. The thin upper drift of the watershed is covered in 
general by a more sandy warp than is found on the lower 
summits; there is, however, in diflerent parts considerable vari- 
ation in the quantity of argillaceous matter contained in it. This 
increase takes place here, as in other districts, in hollows and 
situations where during the submergence water would have stag- 
nated and deposited the finely divided matter held in suspension, 
while sharp sand and gravel are exposed in situations which, 
from the contour of the surface, would have been swept by brisker 
currents. These prevail in the southern portion of the district. 
