J. HOPEINSON-CLIMATE OF HERTFORDSHIRE. 
123 
Whether the Chalk comes to the surface or not, it forms the 
groundwork or foundation of the whole of Hertfordshire except 
a very small portion of the county beyond its escarpment on the 
north-west, where the Gault Clay crops out from underneath it, 
with here and there a thin bed of the Upper Greensand forming 
a porous layer between this clay and the Chalk Marl. This is the 
least permeable of any division of the Chalk, its highest division, 
the Upper Chalk or “ Chalk-with-flints,” which occupies very 
much the largest area in the county, being the most permeable. 
In the south-east the Chalk is overlain by a stratum of the Heading 
Beds, not exceeding 30 feet in thickness and of a mixed nature and 
very variable character, partly pervious and partly impervious, 
which is covered, except along its usually narrow outcrop, by the 
impervious London Clay. Leaving out of consideration the small 
area of Gault Clay and Upper Greensand, the Beading Beds form 
the dividing band between the calcareous rock on the north-west, 
which so freely absorbs moisture that all the rain which falls upon 
it almost immediately sinks into it, and the argillaceous on the 
south-east, which absorbs it so slowly that the rain remains for 
a long time upon its surface, or some portion flows off it. In the 
one case we have a soil which absorbs moisture from the air, and 
in the other, one which may sometimes impart moisture to it. 
The line of division between these two kinds of soil is very 
irregular, but it has a general north-east and south-west trend, 
following, at a distance varying from a quarter of a mile to a mile 
and a half, the Biver Ash downwards from Purneaux Pelham to 
Amwell Magna, the Biver Lea upwards from Hoddesdon to Hatfield, 
and the Biver Colne downwards from North Mimms to Harefield. 
But neither the Chalk on the north-west of this line nor the 
London Clay on the south-east of it everywhere comes to the surface. 
It is only in the north of the county and in the valleys that the 
Chalk does so extensively. In the north it forms open downs, with 
a thin soil and scanty herbage on its steep escarpment; near its 
south-eastern margin there is upon it a line of outliers of the 
Beading Beds, on some of which there is London Clay; and in the 
rest of the county, except in the higher valleys, it is mostly 
covered by superficial deposits, partly pervious, such as Glacial 
gravel and sand, and river-gravels, and partly impervious, such as 
clay-with-flints and boulder-clay. The former is a bed, usually thin 
but of varying thickness, formed by the continuous decomposition 
of the surface of the Upper Chalk and not affecting its permeability 
to any appreciable extent, and the latter is a Glacial deposit only 
here and there of any considerable thickness; but where it does 
occur, which is chiefly in the east of the county, it sometimes makes 
the soil more moist and the atmosphere more humid. 
The surface of the clay area is modified to some extent, on the 
higher hills only, by beds of gravel and sand of Glacial and Post¬ 
glacial age, but these are nowhere of any great thickness. Their 
presence is often shown by the water which percolates through them 
oozing out around their margins, where it is held up by the clay. 
