Geology of Norfolk. 
4G7 
of a valley, if they are surrounded by still higher hills, the warp 
is deeper than in the bottom of a valley near its head. 
In winding valleys a gradual slope and a deep warp are founl 
on one of its sides, Section VII., and steep escarpments with 
little or no warp upon the other, and these change reciprocally 
from side to side of the valley. 
This has evidently been caused by the action of currents of 
water, of which the valleys were the channels during the deposi- 
tion of the warp, and which have flowed at higher levels than the 
existing streams, and even through valleys now perfectly dry. 
The phenomena are analogous to those of river erosion, in which 
the current is deflected from side to side, and thus produces 
alternately bluff banks and low alluvial tracts ; wearing away the 
banks at the points where it strikes them, and depositing the ma- 
terials on the opposite side. The extent to which the denudation 
has been carried further modifies the quality of the soil. When, 
as in Section IV., the valley through its whole course is excavated 
entirely on the sands and gravels of the upper drift, we have a 
sandy loam on an absorbent subsoil, the depth of the soil in- 
creasing as we descend the valley. 
When the denuding action has been carried further so as to 
reach the clay of the lower drift, as shown in Sections VI. and 
VII., we have stronger loams upon a retentive base ; soils which 
in their natural state are often cold and wet, but become highly 
productive when drained. Where the upper drift has been 
w holly swept off large areas, we have a warp of clay-loam, vary- 
ing in depth with the form of the surface, on a subsoil of clay. 
Outlying patches of the upper drift, remaining on a partially 
denuded surface, are covered Avith loamy warps, varying in depth 
with the form of the ground, and in strength with the extent of 
surface of the lower drift exposed in their vicinity, while the 
subsoil varies from absorbent to retentive, with the depth of the 
outlying patches of upper drift. 
These variations will be better understood from the section 
which I have given across Norfolk, from Sidestrand on the north 
to the Waveney on the south. Section II., than from anv map, 
unless constructed on a scale much too large to accompany this 
paper. 
Poringland Heath shown in that section, and Strumpshaw 
Hill, which is off the line of it, but is shown in Section I., are, in 
Smith's county map and sections, erroneously laid down as 
patches of London clay. The gravel of the upper drift, about 
30 feet deep, which caps them, was evidently mistaken by him 
for the upper marine of the Paris basin, of which the Bagshot 
sands are considered the English equivalent : the blue till for 
the London clay, the brown and yellow varieties, with much 
