26G 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL, 



[August, 



feet, of wliich about 70 are sands. In that part of the tertiary 

 district in wliich London is jjlaced, a chanpe tal<es ])lace in the 

 condition of tlie lower tertiary strata. The thickness of tlie 

 sands diniinishcs, wliilst the number anil thickness of the subordi- 

 nate beds of clay rapidly increase. It results, that beneath London 

 the total tliickness averages about 7.5 feet, of which iO only are 

 sands, and 3j clays. At Isleworth and Twickenham the thickness 

 of the sands beneath the London clay do not exceed Ij feet, whilst 

 that of the associated clays amounts 'to 60 feet. At Claremont the 

 sands are 10 feet, and the clays 50 feet thick. Westward from 

 these places to the end of the tertiary district at Hungerford, the 

 mean of a number of sections pives only U feet of sand, whereas 

 the clays are 38 feet thick. This reduces the water capacity of 

 these strata within very narrow limits. The effect also of these 

 changes, which are jiarticularly ra])id in the district a few miles 

 westward from London, is materially to impede the subterranean 

 Jlow of water, and it is probable that little, or if any, of the water 

 absorbed at the outcrop of the lower tertiary strata 'in the portions 

 of the tertiary district, from Guildford and'iMaidenhead westward 

 to Hungerford, reaches London. i 



The second point of inquiry refers to the superficial area occu- I 

 pied by the strata between the London clay and the chalk, which i 

 has apparently been generally represented as much larger than it 

 really is. From an examination of our best geological maps, this I 

 outcrop might be estimated at about SCO square miles, whereas, it 

 IS certain that the area actually occupied by the exposed surface 

 of these strata is under 100 square miles, and of this, a portion of ' 

 nearly 200 square miles is in Kent, and is, therefore, for reasons 

 given hereafter, useless with regard to London Artesian wells. 

 1 he dimension of their exposed surface and outcrops being known, 

 the quantity of rain received on any deposit can be readily calcu- 

 lated; but the quantity that would be absorbed would depend upon 

 conditions named in the third point for inquiry. It will easily be ' 

 conceived, that if a stratum crojis out at the bottom of a valley, 

 the rain fulling upon it, as well as any waters derived from the 

 drainage of the adjacent hills, will remain on the surface of its 

 outcroj), until either absorbed by it, or else removed by the surface 

 channels, or by e\ aporation. This is a common form 'of structure 

 in the lower tertiaries from Croydon to Hungerford, along which 

 line they very generally form a small and narrow valley, running 

 parallel to the higher range of the chalk. But if the'outcrop of 

 the stratum should take place midway on the slope of a hill, then 

 much of the rain water falling on its' surface will naturally drain ' 

 to the lower levels, and little or no supplies from the adjacent 

 surfaces will be received. This is the condition which holds good, ! 

 to a great extent, along the northern outcrop of the lower tertiary 

 strata from \\ afford, or e\en from Newbury, to Hertford, and 

 also, as far as can be judged, from Hertford 'fo Ipswich. There 

 are other positions of outcrop; these, however, are the two principal 

 ones. '^ I 



The fourth point is one which exercises great influence on the 

 supply of water to the water-bearing strata. If the surface of 

 outcrop of any deposit were always bare (the mere \egetable 

 mould excepted) as the London clay at Primrose Hill, or the chalk 

 downs around Brighton, then, necessarily, there would be no im- 

 pediment to the passage of the surface waters into any absorbent 

 stratum; but if the stratum should be covered by any form of drift 

 in the shape of sand, clay, or gravel, then the passage of the sur- 

 face waters would be more or less impeded, according to the 

 tenacity and thickness of the overlying mass. In geological maps 

 this drift is not laid down, and therefore it is sometimes concei\ed 

 that the underlying strata came to or near to the surface; such is, 

 however, not at all invariably the case. Beds of drift are very 

 generally, but at the same time, \ery irregularly, dispersed all 

 over the surface of the country. In the neighbourhood of London 

 they are largely developed, but do not much affect the southern 

 outcrop of the lower tertiaries. On the northern line, tlie outcrop 

 being commonly on the slope of the hills, the covering of drift is 

 very partial. In Essex and Suffolk, where the lower tertiaries 

 frequently crop out on the summit and on the brow of the hills, a 

 thick nia-is of jierfectly impermeable drift clay (Boulder clay 'of 

 Sir Charles Lyell) overlies them, and entirely excludes at places 

 the surface vvaters. 



In the fifth place, whatever may be the value of any deposit with 

 regard to its thickness or its area of outcrop, its e'ffective power 

 will depend upon these conditions of thickness and area acting 

 without interruption from the circumference to the centre; for it; 

 from any cause, the continuity of the strata should be broken or 

 in any way interfered with, t'hen the other conditions, howsoever 

 favourable, are rendered more or less inoperative, according to 



the amount of the disturliance. In the neighbourhood of London 

 these causes have materially impaired the efficiency of the lower 

 tertiary strata, as a source of water supply to London. The ter- 

 tiary district is traversed by two main lines of disturbance, dividing 

 it into four unequal areas, each of which is more or less independent 

 one of the other. One line runs nearly east and west, and forms 

 an irregular ridge, or small anticlinal line, passing from Cliff by 

 Gravesend and U'oolwich to New Cross, and bringing up the chalk 

 to the surface along that portion of its range. It then proceeds, 

 apparently, to Windsor, and thence towards .Maidenhead. By the 

 operation of this line of disturbance, the drainage of the lower 

 tertiary strata in the north-west of Kent, where they are largely 

 developed, is prevented from passing under the large mass of 

 London clay spread over Essex. Suuthwaid and westward of 

 London, as the lower tertiaries do not come to the surface, their 

 continuity is not so completely broken by this disturbance. 

 Another line of disturbance runs nearly due north and south, and 

 intersects the first line at Deptford, passing apparently down the 

 valley of the Lea, crossing the Thames, and then running up the 

 valley of the Ravenshourne. Its effects with regard to the supply 

 of water to London are important: it intercepts — in conjunction 

 with the first fault — almost, if not all, the drainage %vater of the 

 lower tertiaries in Kent from passing to the strata below London, 

 and in the same n'ay, it separates to some extent the Essex district, 

 and prevents it from contributing any large supply of water to 

 the division in which London is placed. There are other smaller 

 lines of disturbance, which cannot now be noticed. From all 

 these conditions it must be apparent that, so far from the larger 

 portion of the outcrop of the strata between the London clay and 

 the chalk contributing to the supply of the Artesian wells at 

 London, the contributing surface is confined to a very smaU section 

 of the whole area. The whole of Kent (except possibly for a 

 short distance betw een Beckenham and Croydon) must be excluded; 

 Essex is of but slight assistance, and the district of country from 

 Hungerford eastward to at least about Guildford and Alaideuhead, 

 contributes probably little or nothing to the water supply beneath 

 London. 



Of the remaining portion of the area, the part from Maidenhead 

 to Hertford is generally placed under conditions unfavour- 

 able for the absorption of the rain-water, whilst the physical 

 structure of the district between the southern line of the outcrop 

 and London places difficulties in the way to the free passage of 

 water. These restrictions render it probable that, of the — say 

 400 square miles occupied by the lower tertiary strata, probably 

 not more than an area of 30 square miles, if so much, can be con- 

 sidered as contributing to the water supply of London, which is 

 placed at the south-east corner of the north-west division, in such 

 u position, that, instead of its being a matter of surprise that the 

 water value of the lower tertiary strata is not greater, it is, on 

 the contrary, an indication of what that value would be, if a 

 similar series of arenacious strata were placed under more favour- 

 able conditions. 



As evidence of the water value of the lower tertiaries, a few 

 cases may be briefly mentioned. In Essex, where the area of out- 

 crop is both small and very unfavourably situated, nevertheless, 

 from the large subterranean mass and the thickness of the strata, 

 the supply of water is general and steady. Wells of 100 to 200 

 feet in depth are common, and there are many from 300 to 100 

 feet. The water rises in most cases to above the level of the 

 Thames, and in quantities varying from two quarts to eight 

 gallons per minute. It must be observed, however, that, unlike 

 the wells in London, the Artesian wells in Essex almost invariably 

 end in the sands below the London clay, and do not often reach 

 the chalk. In the low marshy islands at the mouth of the 

 Thames, these wells have of late become numerous, and have 

 proved of the greatest value. Formerly, in dry seasons, great 

 distress used to be experienced in these districts for want of fresh 

 water; now, there are wells in Wallisea Island -tOO feet, and in 

 Foulness Island 150 feet deep, the water in all of which rises 

 abo\e the surface, and furnishes a good and steady supply in all 

 seasons. 



In Kent, the .Artesian wells of Sheppy may be instanced as cases 

 of a larger supply. 



To the Soulli of London the number of Artesian wells ending 

 in the tertiary sands is not inconsiderable, and the sup)ily of water 

 is large. To allude only to one set of them, we will take tliose 

 which are sunk in the Valley of the ^^ andle, as at Garrett, near 

 A\'andsworth, and at Tooting, where there are several such vvelis, 

 130 to ItiO feet deep. Some of them have been in operation for 

 several (10 to 25) years, and they continue to overflow at the 



