925 



CHALK FORMATION. 



CHALK FORMATION. 



926 



Bedfordshire, is perfectly analogous in structure and appearance to 

 the Downs of Surrey and Sussex. It is interrupted by three or four 

 indentations or gulfs ; one of great width, opening towards the west 

 between Crewkerne and the heights about Stourhead, in South Wilt- 

 shire ; another expanding to the north-west, and terminating in the 

 defile where the Thames cuts through the chalk in its way to the 

 south-east from Buckinghamshire and Oxfordshire. The vales of 

 Pewsey and of Warminster are intermediate bays of the same general 

 structure, but of smaller dimensions ; and all these valleys are appa- 

 rently the result of denudation, aided by previous disturbance of the 

 strata, which has carried away the chalk, and laid bare to various 

 depths the strata beneath it." (' Geol. Tran.,' 2nd series, vol. iv. part 

 2nd., p. 243.) From the heights near Dunstable the Upper Chalk 

 range passes through the north-west of Hertfordshire, by Hitchiu 

 and Baldock, to Barkway and Royston Downs, and thence by Balsham 

 and Newmarket into Suffolk by Mildenhall, 2 miles west of Brandon, 

 4 miles west of Downham, by Narborough, and on to Snettisham. 

 The chalk-hills decline rapidly in height in the north-east of Bedford- 

 shire. Kensworth Hill is 904 feet above the level of the sea ; the 

 hills east of that town are 850 feet high. Lilleyhoe is 664 feet ; 

 Barkway Windmill, 513 feet; the station near Royston, 484 feet; 

 Balsham, on the east of Cambridge, 380 feet ; Newmarket station, 

 267 feet; Brandon, in Suffolk, 190 feet. The chalk-hills stretching 

 from Cambridgeshire to the north-west coast of Norfolk rise nowhere 

 probably above 600 feet in height, bounding " the fen country like 

 the low shore of a sea." 



The Chalk Inland Range is highest towards the central part between 

 Wiltshire and Hertfordshire. In departing from that central tract 

 the rise is comparatively small both towards Devonshire on the south, 

 as well as in the counties northwards. The Chalk nearly disappears 

 in Devonshire, a few insulated portions only appearing there, prin- 

 cipally upon the coast between Sidmouth and Lyme, and along a line 

 from Beaminster through Chard and White Stanton. The transition 

 from the Chalk to the Greensand is here distinctly seen, especially on 

 the south-west of Axmouth, where sections of the subjacent beds are 

 exposed in the cliffs. In many parts of the chalk-range the upper 

 and lower strata are well denned even by the outward features ; a 

 marked difference appearing in the vegetation and general aspect. 

 The Upper Chalk has usually layers of flint nodules occurring at 

 regular intervals, and is softer than the Lower Chalk. At Sandown 

 Bay, in the Isle of Wight, the latter stratum is defined by a layer of 

 distant and insulated flints which separates it from the Flinty Chalk 

 above ; it is sometimes of a grayish colour, as is also the bed of Marly 

 Chalk immediately beneath it. This Marly Chalk is of a tenacious 

 nature, and sustains the. water which descends through the Chalk; 

 in consequence, a line of ponds has been produced along the bottom 

 of the escarpment of the South Downs. The Malm-Land, remarkable 

 for its fertility, is the soil over the lowest beds of Marly Chalk. . The 

 Malm-Rock, immediately below the Marly Chalk, consists of stony beds 

 belonging to the Upper Greensand formation. Salisbury Plain, which 

 is more than 25 miles in extent from west to east, and 1 2 miles from 

 north to south, is occupied by the Upper Chalk. Though it has many 

 inequalities of surface, it would be .almost destitute of water but for 

 the Avon and its brunches, which traverse it. But in the tracts 

 occupied by the Lower Chalk, and still more in the Chalk Marl, there 

 are few valleys without streams ; hence, as well as owing to the dif- 

 ference of .soil, the vegetation differs also, and the luxuriance of the 

 lower regions affords a strong contrast to the barrenness of the higher 

 downa. The village of Dunstable stands on the Lower Chalk, which 

 may be distinctly traced from Totternhoe through Houghton Regis, 

 Upper .Sundon, and Streatley. 



Accurate measures of the entire thickness of the Chalk have rarely 

 been made in England. The following are however approximate 

 numbers taken from the best authorities. Sir Henry De La Beche's 

 ite of the average thickness of this stratum is 700 feet. Dr. 

 Conybeare considers it to range from 600 'to 1000 feet. The height 

 of the cliff at Beachy Head, which at the summit includes part of the 

 Flinty Chalk, and goes down very nearly to the Upper Greensand, is 

 only 535 feet ; 350 feet is the thickness of the Flinty Chalk at Dover. 

 If then 250 feet are added for the remainder of that division at 

 Beachy Head the aggregate thickness of the chalk on the Sussex 

 coast may be estimated at about 800 feet. At Wendover Hill the 

 total thickness of chalk is considered something more than 500 feet. 

 At Diss, in Norfolk, it was found by boring to be' 610 feet. The 

 great variation in thickness is ascribed in part to the unequal removal 

 and abrasion of the upper strata, and in part to the original inequality 

 in the thickness of the Chalk itself. 



Greensand. The general position of this stratum Has been suffi- 

 ciently indicated in describing the boundary of the Chalk, the former 

 outcropping round the latter in an uneven line, in some places much 

 wider than in others. 



The Upper Greensand commences immediately on the north of 

 Copt Point, beyond Folkstone, where the succession of the various 

 beds of the system is best seen. Here the Upper Greensand is of 

 comparatively small thickness, and occupies a very narrow belt round 

 the Chalk, running in a west-north-west and then in a westerly 

 direction, as already defined in describing the chalk boundary. It 

 assumes a new character near Godstone, and in there more distinctly 



marked. The firestone obtained in that part of the country is in the 

 Upper Greensand, occupying four beds separated by seams of stratifi- 

 cation ; the thickness of the first two beds is respectively 1 foot 

 9 inches and 1 foot 4 inches ; the two others are only 10 inches each. 

 This stone is extensively quarried between Godstone and Reigate. 

 Continuing still in a narrow belt to Guildford, the Upper Greensaud 

 forms a slight projection along the foot of the Hog's Back ; from 

 Farnham by Selborne and Petersfield to the south of Petworth, this 

 stratum runs out beyond the foot of the chalk-escarpment like a step 

 or terrace. Near Petersfield it is remarkable for its width, which is 

 there 2 miles, a much greater extent than at any other part. A little 

 east of Petersfield, for a short space, this stratum entirely disappears ; 

 it then continues in a narrow belt along the north escarpment of the 

 South Downs. This formation is but partially disclosed along the 

 base of the central ridge of Chalk in the Isle of Wight, but it is 

 distinctly seen along the escarpments of the Under-Cliff; its step-like 

 projection beyond the Chalk, as seen in Western Sussex, is likewise 

 observable here in several places. 



In the Isle of Purbeck the Greensand runs in a narrow band on the 

 south escarpment of the Chalk, but there the separation of the Upper 

 and Lower Greensands has in a great measure disappeared, and the 

 latter is greatly reduced in thickness or wholly united with the Upper 

 Greensand. The Black Down Hills, in Devonshire, are composed of 

 Greensand ; the two beds thus united wanting the intermediate 

 < iault. These hills are distinguished by the uniform level of their 

 summits ; and, when cursorily viewed, appear to be composed of hori- 

 zontal or nearly horizontal beds of Greensand, with here and there 

 an occasional patch of Chalk. It is fouud however that the rocks 

 composing these hills have been fractured subsequently to their 

 deposition, and that the valleys mostly are lines of faults having a 

 general northerly direction. At some parts there are faults which 



do not form valleys. In a section across a southern part of the 

 hills the Chalk and Greensand seem to form a continuance ; Greensand 

 being on one side of a vertical section, and Chalk on the other. 1 n 

 the yellowish sand, near the surface, at the Barnscombe side of the 

 Beacon Hill, brown iron ore is found in polished fragments of very 

 high lustre. Whetstones are obtained from the Black-Down Hills, 

 the manufacture of which occupies a great number of the neighbouring 

 inhabitants. The quarries are driven in direct lines into the hill, 

 almost horizontally, about 80 feet below the top of the hill. The 

 stones from which the whetstones are cut are irregular concrete 

 masses, imbedded in a looser sand, and more properly belong to the 

 Lower Greensand stratum ; but the different strata of the Greeusand 

 of these hills are not distinguished by Dr. Fitton, as the Gault 

 between the Upper and Lower Greensand is entirely wanting. The 

 Upper Greeusand is not distinctly marked in the great south-western 

 escarpment of the Chalk till beyond the Stour, from the north bank 

 of which it extends northward for about 10 miles to Shaftesbury, 

 and continues thence round the Vale of Wardour. On the south side 

 of the Vale of Wardour the upper beds of this stratum are concealed 

 at the foot of the chalk-hills, but the lower beds shoot out into 

 plateaus, which form the tops of the hills all the way from Shaftes- 

 bury along the south side of the Vale of Wardour. On the north 

 side of the vale, the whole scries of the Upper Greensand rises 

 abruptly and forms a narrow ridge of unequal height. At the north 

 of the valley this stratum appears to consist of two portions : that 

 which immediately succeeds the Chalk is the same as the firestone of 

 Surrey ; the other is equivalent to the Malm-Rock of Sussex, and 

 abounds iu chert. In a well sunk at Ridge the Upper Greensand is 

 distinguished into four different strata, their aggregate thickness 

 being 100 feet. The Upper Greeusand is not nearly so conspicuous 

 inland as near the coast; from the Vale of Wardour it does not occur 

 again till the Vale of Pewsey. The town of Devizes stands upon a 

 platform of the Upper Greensand, which is there about 430 feet above 

 the sea. It occurs again at Swindon, and then at Tetsworth, whence 

 it continues in a narrow belt and in a north-east direction beyond 

 Cambridge. 



The Upper Greensand consists in some places of a soft marly 

 sand, traversed in every direction by stem-like cylinders, having 

 within them cores of darker green matter ; it also contains some 

 irregular masses of a bright brown or orange hue, but the greater 

 part is composed of gray calcareous marl, resembling the lowest 

 chalk, but so thickly interspersed with green particles as to entirely 

 assume their colour. The green particles, according to analysis, are 

 found to consist of 



Silica 48-5 



Black Oxide of Iron 22 



Alumina 17 



Magnesia .... . . 3'8 



Water 7 



Potash tracer 



98-3 



