§ 5. The Chalk. 54. FROM the large extent of country occupied by the Chalk formation in the counties around London, from the breadth of its denuded surface, its great porosity and absorbent power, and the number and abundance of its springs, peculiar interest attaches to it as a source of water supply. The question presents so many conflicting facts that it has given rise to much diversity of opinion upon some of the more theoretical points.* * It has been made the subject of inquiry and research on the part both of engineers and geologists ;— With regard to the large supplies unquestionably often obtained from the Chalk where it forms the surface of the country, Mr. Robert Stephenson, in his "Reports to the Directors of the London, Westminster, and Metropolitan Water Company," in 1840 and 1841, mentions amongst a number of other localities, at which the quantity of water has been distinctly ascertained-the Tring cutting of the North Western Railway, where, notwithstanding the height of the ground, the yield of water amounted to upwards of 1,000,000 of gallons per day; Winchester, where a well in the chalk supplies 31,680 gallons per 12 hours; Arundel, where a shallow well in the chalk in the lower part of the town, and a spring in the chalk, in the upper part, supply nearly the whole town. At Brighton, from a well at the water-works, 97 feet deep, an engine of twenty-nine horse power lifts 231,840 gallons per 12 hours, and no complaints of want of water in the older wells in the town have arisen. At Dover the supply is nearly as large. At Deal, Ramsgate, Gravesend, and St. Albans, the supply is also large and abundant. Twelve hours' pumping reduces the level of the water in these different towns, from two to ten feet, but it is speedily restored. The report also expresses views favourable to deriving, at Watford, large supplies of water from the chalk- —as much or more than 8,000,000 gallons per diem, by means of works to proper depths, and sufficient driftways. With reference to underground springs Mr. Stephenson considered that the "plastic clay sands," are supplied with water chiefly through the medium of the chalk, which latter is the "great waterbearing stratum,' ," "from which all the Artesian wells beneath London, directly or indirectly, draw their supplies." In a discussion, however, in 1849, on the subject of the Artesian wells in London, he stated his opinion that these sources (from the sands and the chalk in their subterranean position) would not be at all adequate to the water-supply of the Metropolis. (Meeting of Inst. Brit. Architects, Nov. 1849.) Mr. S. C. Homersham, in his "London and Watford Spring-water Company Report" (1850), is of opinion, " that from the deep springs of the Chalk formation an enormous amount of water may be easily and cheaply obtained ;" and states that the As this formation cannot, however, like one composed of arenaceous strata, be considered as truly permeable, an exact experimental well in the Bushey meadows actually yielded 1,800,000 gallons per day. He gives a number of calculations relative to the superficial area of the chalk, and estimates the quantity of rain falling upon it, in the South of England, to amount to 1,595,000,000 gallons per day. The area of the chalk sloping towards Watford he calculates at 1200 square miles, receiving a daily mean supply of rain-water, equal to 408,000,000 gallons, and that at present the water finds a vent, and is discharged along the sea coast. He states that there are large fissures or cavities in the chalk, from one to twelve feet in depth, and containing large quantities of water, and instances the powerful springs of Amwell and Chadwell, as issuing from such interstices and fissures. The greater part of the rain-water falling on the chalk, he thinks, however, descends through fissures, "until arrested by the bed of gault clay lying beneath the chalk." He is further of opinion that even a much larger quantity than 8,000,000 gallons per day, "might with facility be procured by a well or wells with adits, in the chalk under Bushey meadows, from a very small area," and that the source from which this water is obtained will not diminish in its yield. He observes also "that this water is remarkably free from organic matter, at all times perfectly pellucid,—and when collected, and allowed to remain for a short time in reservoirs suitable for distribution, is as soft or softer than the Thames water." He gives in an appendix, a variety of information connected with the quality and analysis of different waters. A plan has lately been suggested by Mr. P. W. Barlow for collecting the water which he proves to abound in many of the chalk valleys in Kent; to intercept, in fact, by means which cannot be described here, a portion of the large quantity of water, -received by the exposed surface of the chalk in Kent,-as it passes down to the lower levels along the Thames. The quantity of water in some of these springs he has shown to be very considerable (Report to the Directors of the South-Eastern Railway, 1850). The preceding observations have reference chiefly to wells at the exposed surface of the chalk; those which follow relate to the Artesian wells which are sunk, through the overlying Tertiary strata, into the chalk. Dr. Buckland has frequently pointed out that the height of the water in the Artesian wells at London diminishes as the number of wells increases, to prove that the limits of the supply are already exceeded. (Bridgewater Treatise, 1836, p. 564, and elsewhere subsequently.) The Rev. J. C. Clutterbuck states, that the Artesian wells in the chalk around London, are being exhausted more rapidly than supplied. From a series of very careful experiments on the level of the springs in the chalk, he estimates the general permanent depression of the water-level beneath London to be from 50 to 60 feet below Trinity high-water-mark (Proc. Inst. Civ. Eng. for 1842-3 and 1850). Mr. R. Mylne, in the third volume of the Trans. Civ. Eng. 1840, p. 229, makes some general observations on the sand and chalk wells in London, with reference, however, more particularly to the former. Mr. J. Simpson, in his observations on this paper, observed upon the number of Artesian wells of London in the chalk, and considered that large supplies might be derived from this source, by means of properly constructed works, and that its capabilities had not yet been fully developed. Mr. Robert Davison thinks the water to be obtained from the chalk more precarious comparison between it and the Lower Tertiaries and Greensands, which afford more definite and positive data for calculating their water value, cannot be established. I will therefore examine its conditions of water supply by themselves, and as briefly as the nature of the subject will allow. 55. The chalk not only forms a broad belt at a short distance around London, but also passes under the city at a depth not exceeding 150 to 250 feet; it might, therefore, if all the conditions were favourable, transmit a very large quantity of water from its exposed surfaces to these deep-seated portions underlying the Tertiary strata,-to be there available by means of Artesian wells. Taking the whole of the chalk district surrounding the Tertiary tracts of Kent and Surrey, along with that portion in Hampshire and Wilts which lies north of a line drawn from Alton to Devizes,-of Berkshire, Oxfordshire, Buckinghamshire, Hertfordshire, Essex, Bedfordshire, Cambridgeshire, and of Suffolk, as far north as a line drawn from Newmarket to Woodbridge,—it forms an area of about 3794 square miles, than from other springs. Still, when crevices exist, it flows in large quantities (Min. of Proc. Inst. Civ. Engineers, p. 192, 1842). For various other opinions on this subject I would further refer to the discussions at the Institutes of Civil Engineers and British Architects, on the papers of Dr. Buckland, in 1842 and 1849, as well as to those on the papers abovementioned of the Rev. Mr. Clutterbuck. The capabilities of the chalk as a source of deep water-supply have been maintained by Mr. J. L. Tabberner, who states that the quantity already supplied by the different deep wells (almost all in the chalk) in and around London amounts to 10,000,000 gallons daily, and considers that it might be increased to 50,000,000 gallons daily. He recommends that the wells should be sunk deeper into the chalk. (Letters to the Daily News, 13th and 15th March, 1850, and since republished by Renshaw). Sir W. Clay considers the objections to Artesian wells (those in the chalk and Tertiaries) as insuperable, on the grounds of insufficiency of supply, and uncertainty of quality, "Remarks on the Water supply of London, 1849." Second Edition, p. 39-a pamphlet containing much important information respecting the existing Water Companies. Mr. Seaward also objects strongly to Artesian wells for public supplies (Trans. Civ. Eng. Vol. I. p. 145. Since these pages were written the question of the supply derivable from the chalk, has been discussed at length by Prof. Ansted in his Treatise on Geology (see p. 74). on which the mean daily fall of rain is probably not less than 3800 to 3900 million gallons (see Map). 56. It is evident from the absence both of streams and also of standing waters on the surface, that a very large portion of the rain falling on a bare chalk district (as on the Downs of Epsom, Banstead, and the South Downs in general), infiltrates at once into the ground; but what proportion of it is absorbed by the surface chalk or is returned again to the atmosphere by evaporation and by vegetation, and what proportion passes into the interior of the deposit to add to the springs, has yet to be determined. Over large districts in Essex, Hertfordshire, Buckinghamshire, Oxfordshire, Berkshire, and to a lesser extent over Surrey and Kent, the chalk, however, is not bare, but is covered by an impermeable bed of a ferruginous drift clay, 10 to 20 feet thick, impeding almost invariably the passage of the surface water into the chalk below. This clay, however, rarely exists in any extent except on the higher grounds.* The sides and bottoms of the valleys, and a large portion of the hills, are generally free from impermeable drift; and the former receive not only the rain-water falling immediately upon their surface, but also a portion of that which is thrown off by the clay drift, where it covers the adjacent hills. It is to be observed also that there are several outliers of Tertiary strata of some extent (omitted generally in the Map) which further reduce the area of bare chalk. 57. That the chalk is porous, and will imbibe a large quantity of water is certain (see note p. 74); but its texture is too fine and close to allow water to pass freely through it. I have found by experiment that a piece of chalk+ two inches thick, and containing 63 cubic inches, absorbed 12 cubic inches of water in one minute, 20 cubic inches in five minutes, and became *The gravel in the valleys in these districts is far more permeable. 10 10 fully saturated with 26 inches in 15 minutes. Nevertheless when left to drain for 12 hours, this specimen yielded only 1th cubic inch of water; while the same piece of chalk thus saturated transmitted water so slowly, that in 12 hours, and with 8 square inches of its surface kept covered with half an inch of water, only 6th of an inch filtered through it. Through a mass of siliceous sands* of the same dimensions, and under the same conditions, on the contrary, 320 cubic inches of water passed through in one hour, being equal to 3840 inches in the 12 hours; whilst 63 cubic inches of sand saturated with 22 cubic inches of water, gave off by drainage about four cubic inches in the 12 hours (p. 114). The surface of the chalk to the depth of several feet beneath the ground is commonly very much broken and fissured in all directions, and into these fissures the rain-water rapidly passes. They decrease in number as the distance from the surface increases, but the larger fissures being continued to greater depths, a certain quantity of free water can pass to the lower portions of the chalk. In these deep-seated beds it is along the unadhering surfaces formed by the planes of stratificationt especially that water is transmitted. Looking, however, at the remarkable rapidity with which chalk imbibes water, the excessively crevised condition of its surface, and its strong retentive power, I apprehend that by far the greater proportion of the rain-fall is arrested in the few feet of chalk immediately beneath the vegetable mould; and that it is only in heavy and long-continued rains that any water finds its way to those low levels, where, from the constant contact of water, the mass of the chalk must be fully saturated, and where, consequently, the water passes through the fissures without any further loss. The mere contact of dry chalk, for a few minutes, with water, sufficing for the *Specimen No. 11 in table, p. 114. The parallel horizontal surfaces dividing the deposit into beds. |