HERTFORDSHIRE NATURAL HISTORY SOCIETY. 
xlv 
the form of springs. In this neighbourhood the springs will he 
found where the London Clay, or a clay bed of the Leading Series, 
crops out from under a bed of gravel. Sometimes, as in clayey 
districts, percolation is very slow, and the water soon collects on 
the surface into streamlets or brooks, but over the greater part of 
Hertfordshire percolation is comparatively rapid, the sub-soil being 
gravelly and the gravel reposing on chalk. The water, therefore, 
soon finding its way below ground, there are no small streams on 
the surface, and the rivers are dependent entirely on springs. 
When water flowing over a bed of clay, whether as a streamlet 
on the surface of the land, or by percolation through gravel or sand 
underneath the surface, meets with a pervious bed such as the chalk 
cropping out from beneath the clay, it sinks into this pervious bed, 
usually almost imperceptibly, but occasionally, either from the 
stratum being exceptionally pervious, or from the body of water 
being unusually large, the water may he seen to disappear in 
what are called “ swallow-holes.” This phenomenon is frequently 
observed in chalk and limestone districts. When once the water has 
made for itself a channel in a calcareous rock, by a chemical action 
due to the water holding carbonic-acid gas in solution, the channel 
is gradually enlarged, and, although this action is going on generally 
throughout the rock, as seen by all water derived from limestone 
districts being “ hard,” that is, containing carbonate-of-lime, the 
more rapid action when a channel is formed frequently causes the 
surface of the land to sink, a considerable amount of matter being 
carried away either by chemical or mechanical action. In this 
way the Peak Cavern in Derbyshire has been formed, and the 
Mammoth Cave in Kentucky, and in these cases the course of the 
water can be traced from its disappearance from the surface to its 
appearance again above ground at the mouths of these caves. Here 
we see the water disappear, and we only know for certain that it 
reaches the plane of permanent saturation in the chalk and so finds 
its way to the rivers. 
Some years ago the two Societies visited a picturesque pool near 
Watford, known as Otterspool. Looking into this pool of beautifully- 
clear water several distinct springs could be seen rising out of the 
chalk at the bottom, where the pool is about 16 feet deep. From 
these springs about 300,000 gallons of water a day now rise, and, 
flowing out of the pool in a powerful stream, augment the volume 
of the liiver Colne, and they have been known to yield a million 
gallons a day. The Lev. lames C. Clutterbuck, in a paper on the 
“ Geology and Water Supply of the Neighbourhood of Watford,”^' 
says that it has been supposed that there is some connexion between 
the irruption of water at Letchmore Heath, referring to these 
swallow-holes, and the well-known copious issue of water at Otters¬ 
pool. “ There are,” he adds, “ stories of ducks having found their 
way thither by some subterranean passage; and measurements of 
the level at which the water stands thereabout, and in the direction 
of the pool, show a regularity not easily accounted for but by the 
* 1 Trans. Watford Nat. Hist. Soc.,’ Yol. I, p. 125. 
