220 
ST. ALBANS AND ITS NEIGHBOURHOOD. 
A long and narrow dry valley emanates near Chorl End 
between Dunstable and Luton, has in it most of Harpenden, 
turns south at No-Man’s-Land Common, passes Sandridge, 
then widens, and is met between Oaklands and Smallford by 
a little brook flowing into the Colne near Colney Heath. Its 
length above the source of this brook is fourteen miles. For 
seven miles at least, from Badger Dell near Caddington to 
Harpenden Common, perhaps even by No-Man’s Land to 
Sandridge, a former stream, once for more than two miles 
forming the county boundary of Herts and Beds, has deposited 
in it a narrow band of river-gravel. Here and there its former 
course is now marked by a pond. Below Harpenden the map 
of the Geological Survey indicates it as glacial, but it has even 
here more the appearance of a fluviatile gravel. 
The Lea tells a similar tale, although the evidences of its 
shrinkage are not so striking. In a paper in our ‘ Transactions ’ 
(Yol. XI, p. 236) Mr. James Saunders showed that while the 
river formerly rose at Houghton Kegis, at the time he wrote, in 
1902, it had receded more than two miles, rising in Leagrave 
Marsh, and that while the springs there once formed a rapid 
stream at least three yards wide and nine or ten inches deep, it 
was then only about three feet wide and two or three inches 
deep. The river has not now a tributary until the Mimram 
flows into it near Hertford, although it has several subsidiary 
dry valleys. Moreover, the whole of the valley of the Lea 
within our area, and down to Water End just outside it, was 
once a lake known as Lea Mere, fordable at Batford, Pickford, 
and Mereford (now Marford), and with two islands above 
Wheathampstead. 
It is thus evident that our rivers have been much reduced in 
volume by the lowering of the plane of saturation of the Chalk, 
which has been greatly accelerated in recent years by the large 
quantity of water abstracted by pumping from great depths, 
and by increased drainage causing their flooding with heavy rain 
and thus preventing the water from being stored up for their 
constant supply; and also that this lowering accounts for the 
many dry valleys which the presence of river-deposits, whether 
gravel or alluvium, shows to have been excavated by the action 
of water flowing over the surface. But we must not conclude 
that no lowering of the ground can take place without surface- 
erosion. The frequent phenomenon of streams in a Chalk 
district disappearing and again reappearing shows that there 
is often a flow of water underground in the direction of the 
stream, and by this the chalk will be dissolved and the surface 
of the ground may be lowered. The work of rain on calcareous 
soils is incessant, its erosive action not being confined to the 
surface but being constantly active wherever it penetrates, 
either mechanically by friction or chemically by the dissolution 
of the chalk or limestone by carbonic acid gas (carbon dioxide) 
taken up from the atmosphere or from decaying vegetable matter. 
