1850."! 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



267 



surface at the rate of from 50 to 100 gallons per minute. At the 

 new almshouses at Garrett, the supply from the Artesian well is 

 60 gallons per minute, and the water is laid on to the ground-floor 

 of the forty-two houses, and supplies beside a small fountain in 

 front of them. 



At Kingston, Richmond, and Twickenham, the supply of water 

 from these strata is also good. It then diminishes in proceeding 

 westward, apparently from the thinning out of the beds of sand, 

 and the preponderance of mottled clays in tlie lower tertiary 

 strata. At Sandgate, near Chertsey, a well was sunk to the depth 

 of 600 feet thi'ough the London clay, and ended in tlie mottled 

 clays. No water was obtained. At Cobham and at Knapp-hill, 

 near Woking, the same result was experienced. At Cohham-place, 

 near Cobham, a well was sunk some years since through the whole 

 thickness of the tertiary strata, commencing with the Bagshot 

 sands, to a depth of 64-9 feet. The lower tertiary strata here 

 were about 50 feet thick, of which 47 were of clay, and only about 

 3 to 4. feet of sand. The supply of water being small, the works 

 were continued down further to a depth of 150 feet into the chalk; 

 but after all, the quantity obtained was not large. 



In the north-west division of the map the supply "o water in 

 the lower tertiary strata is very uncertain, and at all times small. 

 At Norwood, in Middlesex-, these strata vi'ere traversed, together 

 with 50 feet of chalk, without finding any water; and at Hanwell, 

 although a supply of 20 gallons of water per minute was at first 

 obtained, j'et, at the end of six years, the quantity had diminished 

 by more than three-fourths, and other sources of supply had to be 

 sought. 



In the valley of the Lea, the Artesian wells are numerous, and 

 tolerably well supplied. There are some at Broxbourne, se\eral 

 at Waltham Abbey; also at Enfield, Edmonton, and Tottenham 

 The water rises above or near to the surface in all of them; their 

 depth varies from 70 to 120 feet. 



In London, the great number of Artesian wells has rendered it 

 necessary to extend a large proportion of them down to the chalk, 

 in order to obtain a better supply of water than can now be pro- 

 cured from the lower tertiary strata. 



^Vith regard to the much debated question as to the probable 

 supplies of water to be expected from the chalk, there can be 

 little doubt that a very large portion of the rain falling on any bare 

 chalk district is absorbed at once. This is generally admitted, and 

 is evident from the absence both of streams and also of standing 

 waters on the surface — whether the water so absorbed passes to, 

 and percolates freely at great depths in the chalk, or whether it 

 remains near the surface in the upper beds, is to be determined. 

 It is evident from the recent experiment of Professor Ansted, that 

 the absorbent power of the chalk is very great — as much as two 

 gallons of water per cubic foot of chalk. But so far from this 

 property being of value as a source of free water supply, it 

 probably favours a contrary result. For this absorbent power is, I 

 consider, owing to a strong capillary attraction arising from the 

 extremely fine texture of the clialk; and if such be the case, there 

 will be a natural tendency to a rapid absorption by the upper beds 

 of the chalk of the rain-water which falls upon its surface, but 

 the very strength of this tendency must cause these upper beds of 

 chalk to part with difiiculty with the water so absorbed. 



It will follow that it is only when the upper beds of the chalk 

 are in a state of saturation, or when fissures allow of gravity to 

 act on the water with a force stronger than that which solicits it 

 by capillary attraction, that water passes deeper into the mass of the 

 chalk. Notwithstanding these counteracting causes, as the surface 

 of the chalk is frequently much broken and fissured, the quantity of 

 water in its upper beds is, in many valleys, often very great. As 

 the depth of the chalk increases, these small fissures rapidly 

 decrease in number; but they are intersected at intervals by larger 

 ones, which conduct part of the water to greater depths. The 

 planes in which the flints are deposited also present unadhering 

 surfaces and joints through which water can pass; and this, rather 

 than a general difl'usion of water in the mass, will account for the 

 phenomenon presented in hilly chalk districts where the level of 

 the water in wells follows nearly the surface level of the intersect- 

 ing valleys; for the base of these vallies being fissured and satu- 

 rated with water, this water finds probably a readier passage late- 

 rally along the planes of stratification in which the flints occur, 

 than downwards through irregular fissures. Consequently, in 

 sinking wells on the hills, the water is frequently found on reach- 

 ing the strata which are on a level with the base of the adjacent 

 valleys. At a certain depth in the chalk the passage of water is 

 usually obstructed by the lower beds, known as the chalk marl, 

 which form au almost perfectly impermeable mass, holding up the 



water from the upper and middle chalk, and throwing it ofl" in 

 numerous springs at the base of the chalk escarpment, where the 

 angle of inward dip is not too rapid. 



Unlike, therefore, strata of sand, through which water can per- 

 meate with facility in all directions, and where it will tend to take 

 the form of large slieets co-extensive with the strata themselves, 

 the percolation of water in the chalk is partly in the seams of 

 bedding, and partly through fissures irregularly distributed, the 

 direction of which can only be determined by experience. It may 

 be compared to a mineral occurring in veins traversing a rock in- 

 dependently in a great measure of its stratification, and the volume 

 and permanence of which is very uncertain; whilst, in arenaceous 

 strata, it may be represented by the same mineral occurring in 

 beds in any stratified deposit, of which the range is persistent and 

 uniform, and the dimensions can be tolerably well determined 

 beforehand. 



It is also to be observed that the chalk is far from presenting a 

 generally bare surface. On the contrary, a large portion of it in 

 Hertfordshire and Buckinghamshire is covered by beds of a red- 

 dish drift clay, generally very tenacious and impermeable. It i.< 

 from 10 to 20 feet thick, and prevents to a great extent the passage 

 of the surface waters into the chalk. It is confined almost entirely 

 to the summit of the hills. The valleys usually present nearly 

 bare chalk slopes. This drift is of much less extent in Kent and 

 Surrey. 



The deepest well in the chalk is at Safi'ron AValden. It was 

 bored to the depth of 1001 feet, all in chalk, and was abandoned 

 for want of a sufficient supply of water. There are also many 

 wells from 200 to 400 feet deep in the chalk district south of 

 London. The very depth of these wells shows the mass not to be 

 so water-charged as it has been frequently supposed. Water, in 

 fact, rather percolates than permeates through the chalk. That 

 that portion of water which finds its way through the mass of the 

 chalk is kept up by the gault at its base, is therefore seemingly 

 incorrect. It is more probable that it is held up almost entirely 

 by tlie chalk marl. 



Immediately below these latter beds is the formation called the 

 upper greensand, which exhibits to the north and south-east of 

 London a type so insignificant, that it would be likely to be 

 regarded, with reference to this question, merely as a few feet of 

 unimportant sandy beds at the base of the chalk formation itself. 

 It must, however, be viewed over a longer range, and then it will 

 be found to possess an importance of which the narrower limits 

 give no indications, and to which I would call attention with 

 regard to its value as a water bearing deposit. 



At Folkstoue it is only 15 feet thick, but expands to 40 or 50 

 feet at Merstham. At the first place it is very argillaceous, and 

 of little value as a water-bearing stratum. It is the same at Cam- 

 bridge, where it is only two or three feet thick. In Bedfordshire 

 it is rather thicker. Taken on a line passing from Bedfordshire 

 through London to Godstone, the lower greensand may be about 

 20 to 30 feet thick. Westward, however, from this line it gra- 

 dually expands, slowly at first, and more rapidly afterwards, and 

 at the same time it assumes a more distinct type, and becomes 

 much more arenaceous and permeable. At Farnham it has attained 

 a thickness of nearly 100 feet; near Watlington of 70 feet ; at 

 ^\'antage of above 100 feet; at Burbage, in the vale of Pewsey, 

 apparently of more than 1 40 feet, whilst at its extremity at Devizes 

 it is also about 140 feet thick. It thei'efore represents a long 

 wedge, of which the thinner edge is beneath London, and the 

 thicker one rises to the surface at Devizes and near Calne. 



Unlike the lower tertiaries, which present such rapid changes in 

 their lithological structure, the upper greensand presents, notwith- 

 standing its various development, a remarkable uniformity in its 

 structure throughout its range from the meridian of London to 

 Devizes. It may be considered on the whole as formed of two 

 divisions — the upper one of sands, occasionally slightly mixed with 

 clay, and of various shades of green, generally light — the lower 

 one of soft thin bedded or fissile, calcareous sandstone. The upper 

 division expands more than the lower one, and, as it expands, it 

 becomes more purely sandy and very permeable ; whilst the lower 

 division is so fissile and fissured that water can generally percolate 

 through it with facility. 



The area occupied by the outcrop of the upper greensand west- 

 ward of the meridian of London is apparently about 160 square 

 miles, of which about 110 maybe ett'ective as a source of water 

 supply, whereas, as before mentioned, the lower tertiary possess 

 less than 30 square miles of such surface. In their subterranean 

 range the difference is still greater — the tertiaries spreading over 

 au extent of about 500 square miles, and the upper greensand of 



36* 



