Position and General Conditions of the Outcrop.
This involves some considerations to which an exact value cannot at present be given, yet which require notice, as they to a great extent determine the proportion of water which can pass from the surface into the mass of the water-bearing strata. In the first place, when the outcrop of these strata occurs in a valley, as represented in [Fig. 11], it is evident that b may not only retain all the water which might fall on its surface, but also would receive a proportion of that draining off from the strata of a and c. This form of the surface generally prevails wherever the water-bearing strata are softer and less coherent than the strata above and below them.
Fig. 11.
It may be observed in the Lower Tertiary series at Sutton, Carshalton, and Croydon, where a small and shallow valley, excavated in these sands and mottled clays, ranges parallel with the chalk hills.
It is apparent again between Epsom and Leatherhead, and also in some places between Guildford and Farnham, as well as between Odiham and Kingsclere. The Southampton Railway crosses this small valley on an embankment at Old Basing.
This may be considered as the prevailing, but not exclusive, form of structure from Croydon to near Hungerford. The advantage, however, to be gained from it in point of water supply is much limited by the rather high angle at which the strata are inclined, as well as by their small development, which greatly restrict the breadth of the surface occupied by the outcrop. It rarely exceeds a quarter of a mile, and is generally very much less, often not more than 100 to 200 feet. The next modification of outcrop, represented in [Fig. 12], is one not uncommon on the south side of the Tertiary district. The strata b here crop out on the slope of the chalk hills, and the rain falling upon them, unless rapidly absorbed, tends to drain at once from their surface into the adjacent valleys. V, L, shows the line of valley level.
Fig. 12.
This arrangement is not unfrequent between Kingsclere and Inkpen, and also between Guildford and Leatherhead. Eastward of London it is exhibited on a larger scale at the base of the chalk hills, in places between Chatham and Faversham, a line along which the sands of the Lower Tertiary strata, b, are more fully developed than elsewhere. As, however, the surface of b is there usually more coincident with the valley level, V, L, of the district, it is in a better position for retaining more of the rainfall.
Fig. 13.
A third position of outcrop, much more unfavourable for the water-bearing strata, prevails generally along the greater part of the northern boundary of the Tertiary strata. Instead of forming a valley, or outcropping at the base of the chalk hills, almost the whole length of this outcrop lies on the slope of the hills, as in [Fig. 13], where the chalk c forms the base of the hill and the lower ground at its foot, whilst the London clay, a, caps the summit, thus restricting the outcrop of b to a very narrow zone and a sloping surface. This form of structure is exhibited in the hills round Sonning, Reading, Hedgerley, Rickmansworth, and Watford; thence by Shenley Hill, Hatfield, Hertford, Sudbury; and also at Hadleigh this position of outcrop is continued. If, as on the southern side of the Tertiary district, the outcrop were continued in a nearly unbroken line, then these unfavourable conditions would prevail uninterruptedly; but the hills are in broken groups, and intersected at short distances by transverse valleys, as that of the Kennet at Reading, of the Loddon at Twyford, of the Colne at Uxbridge, and so on. Between Watford and Hatfield there is a constant succession of small valleys running back for short distances from the Lower district of the chalk, through the hills of the Tertiary district. The Valley of the Lea at Roydon and Hoddesdon is a similar and stronger case in point. The effect of these transverse valleys is to open out a larger surface of the strata b than would otherwise be exposed, for if the horizontal line, V, L, [Fig. 13], were carried back beyond the point x, to meet the prolongation of b, then these Lower Tertiary strata would not only be intersected by the line of valley level, but would form a much smaller angle with the plane V, L, and therefore spread over a larger area than where they crop out on the side of the hills.
The foregoing are the three most general forms of outcrop, but occasionally the outcrop takes place wholly or partly on the summit of a hill, as, near the Reculvers in the neighbourhood of Canterbury, of Sittingbourne, and at the Addington Hills, near Croydon, in which cases the area of the Lower Tertiary is expanded. When the dip is very slight, and the beds nearly horizontal, the Lower Tertiary sands occasionally spread over a still larger extent of surface, as between Stoke Pogis, Burnham Common, and Beaconsfield, and in the case of the flat-topped hill, forming Blackheath and Bexley Heath, as in [Fig. 14]. Favourable as such districts might at first appear to be from the extent of their exposed surface, nevertheless they rarely contribute to the water supply of the wells sunk into the Lower Tertiary sands under London, the continuity of the strata being broken by intersecting valleys; thus the district last mentioned is bounded on the north by the valley of the Thames, on the west by that of Ravensbourne, and on the east by the valley of the Cray; consequently the rain-water, which has been absorbed by the very permeable strata on the intermediate higher ground, passes out on the sides of the hills, into the surface channels in the valleys, or into the chalk. Almost all the wells at Bexley Heath, for their supply of water, have, in fact, to be sunk into the chalk through the overlying 100 to 133 feet of sand and pebble beds, b.
Fig. 14.
Thus far we have considered this question, as if, in each instance, the outcropping edges of the water-bearing strata, b, were laid bare, and presented no impediment to the absorption of the rain-water falling immediately upon their surface, or passing on to it from some more impermeable deposits. But there is another consideration which influences materially the extent of the water supply.
If the strata b were always bare, we should have to consider their outcrop as an absorbent surface, of power varying according to the lithological character and dip of the strata only. But the outcropping edges of the strata do not commonly present bare and denuded surfaces. Thus a large extent of the country round London is more or less covered by beds of drift, which protect the outcropping beds of b, and turn off a portion of the water falling upon them.
The drift differs considerably in its power of interference with the passage of the rain-water into the strata beneath. The ochreous sandy flint gravel, forming so generally the subsoil of London, admits of the passage of water. All the shallow surface springs, from 10 to 20 feet deep, are produced by water which has fallen on, and passed through, this gravel, g, [Fig. 15], down to the top of the London clay, a, on the irregular surface of which it is held up.
Fig. 15.
When the London clay is wanting, this gravel lies immediately upon the Lower Tertiary strata, as in the valley between Windsor and Maidenhead, and in that of the Kennet between Newbury and Thatcham, transmitting to the underlying strata part of the surface water. Where beds of brick earth occur in the drift, as between West Drayton and Uxbridge, the passage of the surface water into the underlying strata is intercepted.
Sometimes the drift is composed of gravel mixed very irregularly with broken up London clay, and although commonly not more than 3 to 8 feet thick, it is generally impermeable.
Over a considerable portion of Suffolk and part of Essex, a drift, composed of coarse and usually light-coloured sand with fine gravel, occurs. Water percolates through it with extreme facility, but it is generally covered by a thick mass of stiff tenacious bluish grey clay, perfectly impervious. This clay drift, or boulder clay, caps, to a depth of from 10 to 50 feet or more, almost all the hills in the northern division of Essex, and a large portion of Suffolk and Norfolk. It so conceals the underlying strata that it is difficult to trace the course of the outcrop of the Lower Tertiary sands between Ware and Ipswich; and often, as in [Fig. 16], notwithstanding the breadth, apart from this cause of the outcrop of the Tertiary sands, b, and of the drift of sand and gravel, 2, they are both so covered by the boulder clay, 1, that the small surface exposed can be of comparatively little value.
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.