1839.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



207 



ON CHALK EXCAVATIONS. 



BY SAMUEL HUGHES, C. E. 



On Chalk E .icavations, and the means adopted tinder different circum- 

 stances of intersecting the Great Chalk Ridyes of England, for the 



purposes of Railway and Canal communication. 



The extent :incl position of the chalk formation in this country are 

 so well known as scarcely to require description. The ridge of 

 chalk surrounding the London clay, and forming the basin in wtiich 

 this latter has been deposited, extends through the counties of Nor- 

 folk, Suffolk, Cambridge, Hertford, Bedford, Buckingham, Oxford, 

 Wiltshire, Hampshire, Surrey, and Kent. The entire length of the 

 outer boundary of this ridge from Himstanton, in Norfolk, to Folk- 

 stone, in Kent, is about 350 miles. 



At Salisbury two branches diverge from the main formation, the 

 one extending in a south-easterly direction by Winchester, Chiches- 

 ter, Brighton, and Newhaven, as far as Beachy-head, a length of 

 about 92 miles ; and the other extending in a south westerly direc- 

 tion to a short distance beyond Dorchester, a length of about 50 

 miles. 



The chalk of Lincolnshire and Yorkshire Commences on the 

 north side of the Wash, opposite to Himstanton, and extends by 

 Scamblesby and Caistor across the Humber, by Market Weighton 

 and Nortli Grimston, to the sea at Hamboro' Head, a length of 

 about 100 miles. 



Around the outer side of the London Basin, the chalk, except 

 where broken through by the passage of water, is elevated with a 

 bold precipitous escarpment, whilst from the summit of the range 

 towards the centre of the basin the dip is more gradual. In tlie 

 county of Norfolk, however, the range is s* much depressed that no 

 escarjimcnt is observable. 



Without entering into any geological theory to explain the ori- 

 ginal deposit and formation of the calcareous substance so univer- 

 sally known under the name of chalk, it can hardly fail to attract 

 the observation of the most superficial, that the escarpment of the 

 whole chalk range has formed at some time the margin of an im- 

 mense lake. It would seem that the water thus bounded has over- 

 llowed its banks, carrying witii it vast quantities of the material 

 composing the upper part of the range. Thus we find the flint 

 gravel which has been washed from the top of tlie chalk hills dis- 

 tributed entirely over the London Clay Basin, and along the southern 

 side of the Soutli Downs throughout their whole extent. 



The chalk formation is divided by geologists into two groups, the 

 upper chalk with flints, and the lower or grey chalk, which contains 

 no flints. The firestone, although appearing from its position imme- 

 diately under the lower chalk, to form a part of the same group, is 

 ranked by geologists as the upper bed of the green sand formation, and 

 the propriety of this arrangement will be evident o:i examining the 

 structure of the firestone : it is evidently composed of an arena- 

 ceous grit, and appears not unlike some varieties of the Oolite or 

 Bath stone. 



The principal railways which intersect the chalk range of the Lon- 

 don Basin, are the Northern and Eastern, the London and Birmingham, 

 the Great Western, the London and Southampton, the London and 

 Brighton, and the South Eastern. Amongst these various lines, my 

 attention has been more particularly directed to those laid out into 

 the south of England, on which a greater extent of chalk country is 

 passed through than on any of the lines in the direction of north 

 or west. The lines to the south enter the chalk at the distance of 

 about 10 miles from London, and continue in this stratification as 

 far as the escarpment of the Surrey Chalk Downs. The line of tnis 

 escarpment may be accurately traced by the towns of Eainham, 

 Guildford, Dorking, Reigate, Westerham, &c., the lowest passes in 

 the range between these points being at Dorking, the Valley of the 

 Mole, otherwise known as the Vale of Mickleham ; at Mersiham, in 

 the line of one of the present coach roads to Brighton ; and at 

 Oxted, where the road from London to Lewes crosses the chalk. 



In addition to parsing over from eight to ten miles of the chalk 

 district, composing the Surrey clialk range, each of the lines of 

 r.iilway proposed between London and Brighton, intersected the 

 South Down range, in which the lowest passes near Brighton, are at 

 Shoreham, through the Valley of the Adur ; at Saddlescombe, a 

 little east of the Devil's Dyke ; and at Clayton, where the present 

 mail road from London crosses the chalk. The lines of Brighton 

 railway intersecting in their course of about 50 miles the two most 

 important chalk districts in the country, and passing over not less 

 than IS or 20 miles of this stratification, will naturally furnish some 

 important information as to tlie facilities of overcoming, for the 

 purposes of internal communication, the great natural barrier pre- 

 sented by the chalk ridges of Surrey and Sussex. 



Commencing at the northern end of these lines, it has been 



already stated that the principal passes in Surrey are at Dorking, 

 Merstham and Oxted. These three passes, with the particular com- 

 parative merits of each, have been very prominently brought before 

 the notice of Parliament in the Session of 1837. 



The Dorking pass was the one adopted for Mr. Stephenson's 

 Brighton line, in its course from the Southampton Railway by Epsom 

 and Leathcrhcad to the Holnnvood Common, beyond Dorking. 



The Merstham pass was fixed on by three engineers, each engaged 

 for a separate line, as the most eligible for the required objects. 

 Thus the several lines proposed by Mr. Vignoles, by Sir John 

 Rennie, and Mr. Rastrick, and by Mi'. Gibbs, all passed Ihrough 

 Merstham, with very trifling deviation from each other in any part 

 of their course, through the chalk district. 



The Oxted pass was adopted lor the South Eastern Railway, but 

 this part of their line has been abandoned, and this railway is now 

 to commence by a junclion with the London and Brighton at a point 

 between Merstham and Redhill. 



It may now be advisable to view separately each of these passes, 

 the first in order being the Dorking. At this place the river Mole 

 forms a complete outlet ihrough the chalk ; and as the river rises 

 on the south side of Dorking, and flows over the chalk as far as 

 Leatherhead in a northerly direction, it would seem natural that a 

 line adopting the valley of this stream through the chalk range 

 should encounter no considerable summit in its passage through the 

 chalk. Accordingly we find that Mr. Stephenson's line passed over 

 the London clay as far as the south side of Epsom Common, and 

 then entirely over the chalk to Dorking, without rising more than 

 1 GO feet above high-water* mark; this summit occurs at about 15^ 

 miles from the commencement at Vauxhall-bridge. The works 

 through the chalk are very inconsiderable, with the exception of the 

 cutting on Leatherhead Common ; the chalk passed through by this 

 line is remarkably dry, and the firestone and gault brickearth are 

 entirely wanting as they have been broken through by the river 

 Mole, or rather by those waters which on bursting through the chalk 

 originally formed the Vale of Mickleliam. The greatest height to 

 which the chalk rises on this line is 230 feet above high water; 

 namely, in the cutting through Norbury park, where the surface of 

 the ground rises from the river Mole to'the height of about 120 feet 

 above the same, with a slope of not less than 2 to 1. 



Of the three lines proposed through the Merstham pass, Mr. 

 Vignoles" rises to a summit of ,304 feet above high-water; the 

 summit on Sir John Rennie's was 324 feet, and on Mr. Gibbs' 2>^1 

 feet. The highest summit of chalk on Mr. Vignoles' line was 4()4 

 feet, on Sir John Rennie's 435 feet, and on Mr. Gibbs' 522 feet. 

 The greater height of the summit on Mr. Gibbs' line was 

 occasioned by the direction taken in order to preserve a straight line 

 through the tunnel. The line of Sir John Rennie and Mr. Rastrick, 

 commonly called the direct line, and which, as well known, is now 

 in course of execution under the direction of Mr. Rastrick, Nvas laid 

 out through the lowest ground that could be followed consistently 

 with the nature of the curves, which were limited to a radius of not 

 less than one mile, the intention at the time the bill was before Par- 

 liament being to open cut entirely through the chalk district. On 

 Mr. Gibbs' line, however, a tunnel three-quarters of a mile long was 

 proposed, and it being obviously of little consequence how high the 

 summit might be above the tunnel, it happened that the ground 

 through which this was carried rose to a height of nearly ninety feet 

 above the highest point on the direct line. 



The Merstham pass has been very minutely investigated, in order 

 to discover the nature of the chalk," and the subordinate beds of fire- 

 stone and gault. The ereatest height to which the gault rises m the 

 neighbourhood of MersTtham is 290 feet above high water mark, and 

 the firestone which rests upon it is about seventy feet higher. 



The following are selected from a variety of borings and shafts 

 which have been sunk immediately upon the chalk :— - 



Shafts sunk close to the Turnpike-road at the Jolifl'e Arms Inn, 

 Merstham, commenced at surface 3f;4 feet above high water 

 mark — 



ft. in. 



7 Chalkwash and loam. 

 2 G Stifi" chalkmarl. 

 5 6 Stifl'er do. 



9 Chalk marl, a little shattered. 

 16 G Very fine firm rock chalk. 



40 G 



The rock chalk continues — 



• Tlie expression "high-water," wherever used in thi^ imiicr, refers to the Trinity 

 hi''h-water mark in London. 



Q 



