Fig. 16.
There are also, in some valleys, river deposits of silt, mud, and gravel. These are, however, of little importance to the subject before us. Under ordinary conditions they are generally sufficiently impervious to prevent the water from passing through the beds beneath.
Height of Water-bearing Strata above Surface of Country.
The height of the districts, wherein the water-bearing strata crop out, above that of the surface of the country in which the wells are placed, should be made the subject of careful consideration, as upon this point depends the level to which the water in Artesian wells may ascend.
Again, taking the London district as an example, Prestwich remarks that, as the country rises on both sides of the Thames to the edge of the chalk escarpments, and as the outcrop of the Lower Tertiary strata is intermediate between these escarpments and the Thames, it follows that the outcrop of these lower beds must, in all cases, be on a higher level than the Thames itself, where it flows through the centre of the Tertiary district. Its altitude is, of course, very variable, as shown in the following list of its approximate height above Trinity high water-mark at London. These heights are taken where the Tertiaries are at their lowest level in the several localities mentioned.
| South of London. | North of London. | ||||||
| Croydon | about | 130 | feet. | Thetford | about | 200 | feet. |
| Leatherhead | „ | 90 | „ | Watford | „ | 170 | „ |
| Guildford | „ | 96 | „ | Slough | „ | 60 | „ |
| Old Basing | „ | 250 | „ | Reading | „ | 120 | „ |
| Near Hungerford | „ | 360 | „ | Newbury | „ | 236 | „ |
Eastward of London these strata crop out at a gradually decreasing level. In consequence, therefore, of the outcrop of the water-bearing strata being thus much above the surface of the central Tertiary district bordering the Thames, the water in these strata beneath London tended originally to rise above that surface.
As, however, these beds crop out on a level with the Thames immediately east of the city between Deptford, Blackwall, and Bow, the water, having this natural issue so near, could never have risen in London much above the level of the river.
Rainfall in the District where the Water-bearing Strata Crop out.
When inquiring into the probable relative value of any water-bearing strata, it is necessary to compare the rainfall in their respective districts.