218 
ST. ALBANS AND ITS NEIGHBOURHOOD. 
Chalk, the spring being converted into a “ swallow-hole.” It is 
only when the river has made its bed impervious by depositing 
alluvium in it that this does not happen; then there can be 
neither spring nor swallow-hole ; and this frequently occurs. 
The saturation of the Chalk is a work of time, and as far more 
rain percolates in winter than in summer, owing to the greater 
amount then evaporated and taken up by growing vegetation, 
our rivers are usually highest in spring and lowest in autumn. 
Experiments with percolation-gauges at Nash Mills near 
Hemel Hempstead and at Lea Bridge have shown that in the 
six summer months (April to September) about 6 per cent, of 
the rain which falls passes through three feet of soil or chalk 
with grass growing on the surface, and in the six winter months 
(October to March) about 46 per cent., giving an average annual 
percolation of 26 per cent, of the rainfall. When grass is not 
growing on the surface the percolation is much greater. 
The rain which percolates into the Chalk will at first tend to 
make the surface of completely saturated chalk—the plane of 
permanent saturation—follow the contour of the ground, but 
as the water is held up below by saturated chalk which by the 
force of gravitation must always be tending to have its upper 
limit of saturation horizontal, there is a greater distance between 
the plane of saturation and the surface of the ground on the 
hills than there is in the valleys. This plane, however, always 
rises from the valleys towards the hills, or from any point where 
water is abstracted, whether it be naturally by a spring or 
artificially by pumping, the depression then being known as the 
cone of exhaustion. It is of course an inverted cone. 
The head of the valley of the Yer is between Dunstable and 
Kensworth, at 550 ft. O.D. The river now seldom rises higher 
than Markyate Street, at 400 ft.; it frequently rises no higher 
than Bedbourn, at about 300 ft. ; and except at its perennial 
sources there it has not a single tributary. Although beyond 
our area it is necessary to state that for more than two miles 
above Markyate Street it has formed a continuous deposit of 
gravel in its main valley, and for nearly two miles in a subsidiary 
valley ; and that two former tributaries, one on each side of the 
river, have cut valleys for four miles above Eriars Wash, which 
is two miles below Markyate Street, one down to the Chalk 
through clay-witli-flints, and the other through that and brick- 
earth. Within our area, from Bedbourn, three miles below 
Briar’s Wash, there radiate four dry valleys varying from about 
two to five miles in length (one with two subsidiary valleys) cut 
through clay-with-flints into the Chalk, the presence of river- 
gravel in some of them testifying that they have been formed 
by streams running with considerable velocity. 
The higher part of the valley of the Yer is in the Middle 
Chalk, the lower part is in the Upper Chalk, the dip of the 
Chalk to the south-east being greater than the inclination of 
the valley in the same direction. The river has cut its way 
