VEGETATION SURVEY OF NORFOLK. 
745 
On heavy lands they usually do not exceed twenty to twenty- 
five acres. 
Geology. 
The vegetation of a district is so dependent on its geology 
that no understanding of it is possible without a knowledge of 
the main geological features. Chalk is the basement rock, 
exposed only where the thick drift deposits have been denuded. 
Pebbly gravel and sand of the Norwich Crag, exposed on the 
slopes of the primary and tributary valleys of the Wensum, 
Bure, and Tas, are the only Tertiary beds known to occur. At 
Attlebridge, Crostwick, Horstead, Newton Flotman, Saxlingham 
Nethergate, Spixworth, and Stratton Strawless, these beds have 
been fossiliferous. In various parishes flints have been found 
with a patination apparently peculiar to the “ stone bed ” at 
the base of the Norwich Crag, and are probably derived there- 
from by glacial erosion. The lowest of the beds resting on 
the chalk is the pebbly series, considered by some geologists 
to be Pliocene, belonging to the uppermost division of the 
Norwich Crag, and by others to mark the commencement of 
the drift period. This bed is well seen in pits at Alderford 
Common, Attlebridge, Great Witchingham, Spixworth, Drayton, 
St. Faith’s, and Flordon. Contorted drift (lower boulder clay), 
with boulders derived from palaeozoic or igneous rocks, occurs 
at Flordon, Morton, Lyng, Sparham, Weston, and Great 
Witchingham. Chalky boulder clay occupies most of the 
surface in the Flordon district and the western part of the area, 
and occurs in most parishes throughout the district. It was 
once more widely spread, but has been partially denuded in 
some places and entirely so in others. Sections show this at 
Weston, Hockering, Attlebridge, Taverham, and Ringland. A 
pit near Ringland Hills shows chalk with chalky boulder clay, 
glacial gravel and sand and Norwich Crag, including the stone 
bed, tilted into a vertical position. At Taverham a pit less 
than half a mile east of Ringland ford showed tongues of 
chalky boulder clay beneath disturbed chalk, lenticular masses 
of clay and pockets of red and white sand with flint pebbles. 
