152 ON THE WATER IN THE CHALK 



open, must have observed that in journeying along any of the 

 railways which radiate from the metropolis, except only those 

 which go direct to the coast of Essex and Suffolk, he passes 

 through a district of considerable width, in which the rock 

 underlying the surface soil, or even the surface itself, is chalk. 

 A more careful observer will see, by observing the chalk strata, 

 where exposed in cuttings or pits, that they dip gently towards the 

 central line of the valley of the Thames, disappearing beneath 

 clays and sands in the strata immediately above, which, as we 

 proceed from the chalk district towards the Thames, very soon 

 give way to that huge sheet of London clay which forms the 

 surface rock of almost the whole of Middlesex and much of the 

 adjoining counties. This suggests that the chalk everywhere 

 exists beneath this sheet of clay, and in fact forms a basin (or 

 rather saucer, a very shallow one), in which the clay lies, a 

 suggestion which is made a certainty by the wells which, like the 

 ideal one we have been considering, have been sunk into the 

 chalk in all parts of the district. 



These features are shown accurately to scale in the horizontal 

 section published by the Ordnance Survey, extending for 36 miles 

 from the downs of Beddington northwards through Mitcham, 

 Clapham Common, Battersea Park, across the Thames to Hyde 

 Park Corner, through Regent's Park by the Zoological Gardens 

 to the London and North Western Railway, when it turns to 

 the north-west and passes through Hampstead (showing a section 

 of the hill), Hendon, Elstree, Aldenham, across the Colne, and 

 on over the open chalk to Hemel Hempstead. 



Now, let us consider the bearing of these facts of geology on 

 the question of the source of the water in our deep wells. 



Since compact clay, such as we find in the widespread sheet 

 of the London clay, is impermeable by water, it is plain that the 

 150 or 200 feet of thickness of this clay effectually ])revents any 

 of the rain that falls on its surface from penetrating to deep wells. 

 The breadth of the water-bearing sands between the London clay 

 and the chalk is small at their outcrop, and consequently the 

 water which enters into and saturates these beds represents only 

 the rainfall and drainage of a comi)aratively small area. It is 

 very different, however, when we come to the outcrop of the 



