The Victoria History of the County of Buckinghamshire/Geology

GEOLOGY

IN Buckinghamshire from the valley of the Great Ouse on the north to that of the Thames on the south there are outcrops of a series of geological formations, which are tilted gently towards the south-south-east. Through the northern belt range portions of Jurassic strata which serve to connect the county with midland regions. There we find traces of the Lias, with uplands of Oolitic limestone or stone-brash, and a broad vale of Oxford and Kimeridge Clays, modified towards the south by outlying hills of limestone capped by Cretaceous sands and clays. Still further south we come to the vale of Aylesbury which is largely formed of Gault clay; and this is succeeded in the central part of Buckinghamshire by the bold Chalk range of the Chiltern Hills, which, forming part of the London Basin, slope gently south-eastwards, and are eventually covered by Eocene deposits and by the gravels of the Thames valley.

Throughout the entire area there are various superficial deposits on hill and in vale, which help to diversify the soils of this essentially agricultural county.

Of the industries directly connected with the geology, those of lime-burning and brick-making are the principal; but changes here, as elsewhere are in progress. The smaller brickyards cannot compete with the larger, while the local road-mending materials have given place to more durable stone obtained from a distance; hence in remoter regions there are fewer pits or 'geological sections' than was formerly the case, and except as lime-works many small quarries have been abandoned. A striking instance of the changes is noted by Mr. A. Morley Davies, who, in writing of the Thame valley in 1898, says: 'Now however the stone-pits seem all abandoned and mostly levelled over; the only one I saw was being used to store Leicestershire road-metal.'^[1]

Phosphatic deposits were formerly worked, but owing to foreign competition the industry is being gradually extinguished in this country. Elsewhere the market and nursery gardens of the Thames valley, the dairy lands of Aylesbury, and the beech woods used for chair-making at High Wycombe, give rise to occupations due more or less directly to the nature of the soil.

The county is one which is by no means devoid of interest to geologists, and our knowledge is especially associated with the names of Fitton, John Morris and A. H. Green in the northern part ; and with those of Prestwich and William Whitaker in the southern part. ^[2]

The following is a table of the formations met with in Buckinghamshire :

Period	Formation	Character of the Strata	Approximate thickness in feet
Recent to Neolithic	Alluvium	Silt,peat,clay	up to 20
Pleistocine,Paleolithic, and Glacial	Valley Brick Earth	Loam	10
	Valley Gravel	Stones of flint, quartzite etc.	30
	Boulder Clay	Chalky Clay, with flints and erratics	up to 40
	Glacial Gravel and Sand	Gravel made up of flints, quartzite,etc; and sans	25
	Clay-with flints and loam	Red clay and loam with unworn chalk flints and other materil in 'pipes' of the Chalk	up to 50
Eocene	London Clay	Brown and blue clay with septaria	up to 200
	Reading Beds	Motled clay,sand, and flint pebble beds	35 to 80
Cretaceous	Upper Chalk	Chalk with flints	400
	Middle Chalk	Chalk with new flints	175
	Lower Chalk	Grey chalk and chalk marl	150
	Upper Greensand	Green Sand and calcareous sandy rock	20
	Gault	Pale marly clay	200 to 250
	Lower Greensand	White and coloured sands,sandstone,ironstone, and fuller's earth	up 250
Jurassic	Purbeck Beds	Thin limestones and clays	20 to 30
	Portland beds	Shelly limestone, sands and clay	60
	Kimeridge Clay	Dark Clay and shale	100
	Corallian	Clay with slenite.	40 to 50
	Oxford Clay	Clay with septaria; sandy beds at base	400
	Cornbrash	Rubbly limestone	5
	Great Oolite series	Oolitic and shelly limestones, marls and clays	50 to 60
	Inferioe Oolite series	Clays, sand and sandstone	5 to 10
	Upper liss	Blue clay	55 to 120
	Middle and Lower Lias (not exposed)	Stone beds,clays,etc	not proved

Nowhere in Buckinghamshire have any very deep borings at present been made consequently we have no information with regard to the character of the older (Paleozoic) strata, which probably occur in some parts of the county within a thousand feet of the surface.

At Bletchley fragments of granitic rock were obtained in a boring from depths of 400 and 407 feet from the surface. The evidence as to their precise mode of occurrence was by no means clear, but probably the rock was present as boulders in the Kellaways Beds which form the basement portion of the Oxford Clay and consist of hard calcareous sandstones and clays.^[3] Water which was obtained proved to be very saline.

A boring at Stone near Aylesbury was carried to a depth of 570 feet through Portland Beds, Kimeridge and Oxford Clays into the Great Oolite. It was made in search of water and was unsuccessful. ^[4]

LIAS

The oldest formation exposed in Buckinghamshire is the Upper Lias, which comes to the surface in the northern part of the county. There can be no doubt that both Middle and Lower Lias occur also in that region underground, but we have no definite particulars regarding them. It is probable that the Middle Lias (Marlstone) was reached at a depth of about 100 feet at Stony Stratford, and at a depth of about 200 feet beneath Drift and other deposits at Brickkiln farm to the south- east of that town, where a rock-bed yielded a scanty supply of brackish water. ^[5]

Even the Upper Lias is nowhere well exposed. It appears in the higher part of the Ouse valley south-west of Turweston, where it is faulted on the east against the Great Oolite Limestone. It occurs also in the Tove valley above Castlethorpe. Both these tracts are meadow- land, and the sub-strata are in a measure concealed by alluvium. No sections of the strata have been recorded, but in adjoining parts of Northamptonshire we know that the Upper Lias consists mainly of a mass of bluish-grey clay, which is locally worked for brickmaking. It contains small nodules of limestone or cement-stone, selenite and pyrites, and it yields Ammonites communis, A. fibulatus, many Belemnites, Leda ovum, Inoceramus dubius, and other fossils. The lowermost portion comprises alternations of clay or shale with limestones, and these yield remains of fishes and insects, Ammonites serpentinus, etc. They indicate marine con- ditions and water shallower than that in which the thick series of over- lying clays was deposited.

INFERIOR OOLITE SERIES

Again we find representatives of an important division which is but poorly exhibited in the county, and in this case poorly developed. The Inferior Oolite Series, so prominent in the Cotteswold Hills, undergoes such considerable changes as we pass into the midland counties that distinct stratigraphical divisions are needful. In place of a great series of marine sands, oolitic freestones and rag beds yielding numerous fossils, we find in Buckinghamshire a marine and estuarine group divided as follows :——

Lower Esturaine Series
Northampton Sands

In parts of Northamptonshire and in Lincolnshire this group is overlain by an important division known as the Lincolnshire Limestone, which furnishes many a valuable freestone. No portion of it is repre- sented in Buckinghamshire, and consequently there is a considerable break between the representatives of the Inferior Oolite and Great Oolite Series in the county. Locally also there are evidences of erosion between the Upper Lias Clay and the succeeding Northampton Sands.

Just beyond the borders of the county, in a brickyard north-east ot Brackley, resting on the blue pyritic clays of the Upper Lias, there were to be seen green and dark grey sands and hard ferruginous sandstone, together 3 feet 6 inches thick. The stone contained Avicula braamburiensis and some other fossils, and also pebbles of hardened Upper Lias shale. These sandy beds, representing the Northampton Sands, were overlaid by 8 feet of purplish loam, clay and white and brown sand, perhaps belonging to the Lower Estuarine Series.

In this neighbourhood however the lower beds of the Great Oolite comprise an Upper Estuarine Series, and where the two Estuarine series come together it is most difficult to distinguish between them, for in characters they are alike, and it is only where one group is seen to rest with marked unconformity on the other that any division can be made. Further north in Northamptonshire the two are separated by the Lin- colnshire Limestone, but the Upper Estuarine Series may be regarded as the more persistent as it stretches unconformably across the eroded faces of the subdivisions in the Inferior Oolite Series.

The Northampton Sands are exposed in Buckinghamshire only along the Ouse valley at Whitfield Mill below Biddlesden. Hard bands such as occur near Brackley have been met with in borings on the north-east of Stowe Park near Akeley and along the borders of the Ouse valley near Stony Stratford ; but the evidence of their age is indecisive.

There is no doubt that both Northampton Sands and Lower Estuarine Series die out in a south-easterly direction from Northampton- shire towards Olney and Stony Stratford. Together they appear to represent in places the higher portion of the zone of Ammonites jurensis, but they consist mainly of the zones of A. opalinus and A. Muchisonae; or in other words they are equivalent to the higher part of the Midford Sands and the lower part of the Inferior Oolite of the west and south- west of England.^[6]

GREAT OOLITE SERIES

This series, which occupies a considerable area in the northern part of the county, is locally divided as follows :——

	Approximate equivalents
	in the west of England
Great Oolite Clay	Forest Marble
Great Oolite Limestone	Great Oolite
Upper Esturine Series	Fullonian or Fuller's Earth

The Upper Estuarine Series which enters but little into the surface geology, consists of black, grey, reddish, greenish and bluish clays, with white and brown sands, much like the Lower Estuarine Series. Nail-head spar, a form of calcite, is frequently present. These beds occur at Stoke Goldington and along the borders of the Ouse from Weston Underwood to near Olney, where they are from 15 to 20 feet in thickness. Formerly they were regarded as Upper Lias.

They have been proved in borings in Salcey Forest, and at Deanshanger on the Northamptonshire borders, and also at Stony Stratford. To the south-west of that town the Upper Estuarine Series rests directly on the Upper Lias clays, and it may do so near Olney.

The Great Oolite Limestone consists of white limestones, marls and compact grey limestone. Some layers contain scattered grains of oolite, others are largely oolitic and false-bedded, while some are sandy and minutely current-bedded. The upper beds comprise compact shelly limestone with the gasteropod Nerinæa, and with many lamellibranchs such as Cyprina, Astarte, Gervillia, etc. Other bands yield corals, Lima cardiiformis, Terebratula maxillata and Clypeus.

The Great Oolite Limestone which extends over much of the northern part of the county forms an undulating well-wooded district. The higher tracts are however largely covered by Boulder Clay: hence there is a mixed soil of chalky clay on which beans and wheat are cultivated, amid other arable tracts of stonebrash and much dairy land.

Numerous quarries are to be met with from Turweston and Biddlesden to Shalstone, in Stowe Park, along the Ouse valley at Water Stratford and Buckingham, at Leckhampstead and onwards by Calverton, Bradwell, Great Linford, Stony Stratford and Wolverton. Some of these quarries are but 10 or 15 feet in depth and many are now disused.

The stone has been employed for building purposes, but even when well-seasoned before use it is by no means durable, and it is not to be compared with the Great Oolite (Bath stone) of the west of England. The argillaceous nature of the limestone causes the lime to be strong and better adapted for mortar than for agricultural purposes.

In a pit at Bradwell near Newport Pagnell, beneath the Great Oolite Clay, about 16 feet of Great Oolite Limestone has been exposed, comprising pale earthy, shelly, and oolitic limestones, the lower layers false-bedded and containing veins of selenite. This mineral occurs also in thin seams an inch or two thick between the bands of stone. Most probably it is due to the decomposition of pyrites in the clay above, and the consequent formation of sulphate of lime, which was deposited along the open planes of bedding and in crevices of the fractured rock. ^[7]

Beyond Wolverton the Great Oolite extends northwards from Castlethorpe to Hansthorpe and along the borders of Salcey Forest; it appears in the Ouse valley at Gayhurst, Ravenstone, Warrington, Lavendon and Olney, also at Clifton Reynes, Emberton and Sherington. At Gayhurst many fossils were formerly collected by J. H. Macalister.^[8] In this region the stone layers are much jointed and fissured and the walls of the fissures are seen to be water-worn. Under favourable circumstances the strata would hold a good deal of water, although free circulation is liable to be arrested by the partings of marl.

In the south-west of England and northwards as far as Bicester in Oxfordshire the Great Oolite is surmounted by beds of clay, sands and fissile oolitic and shelly limestone grouped as the Forest Marble, so named from the occurrence of the strata in the forest of Wychwood east of Burford in Oxfordshire, where in old times the stone was employed for chimney pieces.

On Blackthorn Hill south-east of Bicester we find the last definite representative of Forest Marble type, about 18 feet in thickness and comprising clays with a band of tough blue shelly oolite, with masses of lignite and greenish marly galls. Here one of the characteristic fossils Waldheimia digona is to be found, together with Ostrea and Acrosalenia. The upper beds comprise pale greenish grey clays which indicate the incoming of the estuarine conditions which prevailed in the area to the north-east where the term Great Oolite Clay is usually applied.

East of Tingewick and again at Thornton Prof. A. H. Green noted a hard limestone similar to that above mentioned. It is overlain by blue and white marly clays and these occasionally contain calcareous bands and concretions.^[9] It was mentioned by Buckland that shelly limestone obtained at Buckingham had been used for ornamental purposes under the name of Buckingham Marble^[10].; but the stone may have been obtained from one of the shelly bands at top of the Great Oolite Limestone. At Buckingham the Great Oolite Clay is about 1 5 feet in thickness, but northwards at Akeley it is less developed. Here we find grey and black clay and marl, while in some localities there is greenish clay with Ostrea sowerbyi and O. subrugulosa. Thus at Bradwell beneath the Cornbrash and above the Great Oolite Limestone there is about 13 feet of marly clays, variegated in colour, with sand and marly limestone crowded with Ostrea, and at the base much ferruginous matter. Here we have the type of the Great Oolite Clay of the midland counties, a formation of uncertain thickness and varying character.

Where exposed this clayey series forms a wet tenacious soil, noticeable along the gentle scarp to the south of the Ouse valley between Buckingham and Newport Pagnell.

CORNBRASH

This formation consists of earthy and shelly limestone with marly or clayey bands, altogether from 5 to 8 feet in thickness. Among fossils Terebratula intermedia, Waldheimia obovata, Avicula ecbinata, Pholadomya and Ostrea flabelloides may usually be found. Although insignificant in thickness and of no economic importance in Buckinghamshire, the formation is of interest as being one of the more persistent bands in the Jurassic system.

North of Buckingham a large outlying mass of Cornbrash almost concealed beneath Oxford Clay and Glacial Drift occurs between Akeley and Thornborough. On the south side of the Ouse the formation extends from Barton Hartshorn to Tingewick, and eastwards from Beachampton, Bradwell, Great Linford and Newport Pagnell to near Newton Blossomville. Over much of the area the Cornbrash is concealed by Drift, and the broad outcrop is indefinite ; moreover it was extended more than it should have been on the geological survey maps by the inclusion of the Great Oolite Clays.^[11] The formation is brought to the surface in inliers at Marsh Gibbon and West Stan Hill by an anticlinal structure which has disturbed and faulted the beds from Islip in Oxfordshire in a north-easterly direction. At Akeley north of Buckingham we again meet with an anticline with a northerly trend where the Cornbrash, Great Oolite Clay and Great Oolite Limestone have been bent into an arch, locally eroded, and exposed beneath the Kellaways Beds. ^[12]

OXFORD CLAY

As its name implies this is a great clay formation ; it occupies a vale chiefly of grass land with many dairy farms, and it forms part of a famous hunting country. It extends from Gawcott, Steeple Claydon, Grendon Underwood and Ludgarshall to Winslow, Whaddon Chase, Bletchley and. Fenny Stratford in the Ouzel valley, and thence to Chicheley and Astwood. The vale is an undulating one, rising at Knowl Hill between Edgcot and Middle Claydon into a conspicuous elevation, but the surface is modified by coverings of Boulder Clay and Drift Gravel, to the presence of which the scattered villages are to be attributed, as the gravels yield springs and furnish limited supplies of water to shallow wells. Many of these however have become polluted owing to defective sanitary arrangements, and deeper or distant supplies of water have to be looked for. The clay is locally dug for the manufacture of bricks, tiles and drain-pipes.

Resting on the Cornbrash there is usually about 10 feet of clay which is overlain by the yellow sands, sandstones and loams, all belonging to the Kellaways division, and about 20 or 30 feet in thickness. The lower beds are much better exhibited in the adjoining county of Bedford, but they have been exposed at Akeley, Padbury and Little Woolston. Among the fossils which they yield are Ammonites calloviensis, A. gowerianus, A. koenigi and Grypboea bilobata. The middle division, which comprises shales and clays with much pyrites, is characterized by Ammonites Jason, A. lamberti, Belemnites oweni and Cerithium muricatum ; the strata have been opened up along the Great Central Railway near Charndon, also near Winslow and at Fenny Stratford. The highest division, consisting of clays, contains Ammonites cordatus, Belemnites hastatus and great numbers of Grypbaea dilatata ; and it is well exposed in a brickyard by Quainton Road station.

CORALLIAN

Between the Oxford and Kimeridge Clays there is usually developed a series of sands and calcareous sandstones, oolitic limestones and coral beds, grouped as the Corallian formation. In Buckinghamshire these rock beds are not present in any conspicuous form ; they terminate north-east of Wheatley in Oxfordshire, and thence until we reach Upware in Cambridgeshire the formation is represented almost wholly by clay to which the name Ampthill Clay was given by Professor H. G. Seeley.

In the absence of the Corallian stone beds the probable equivalents have been shown on the geological survey map as extending through Shabbington, Ickford, Worminghall, Oakley and Boarstall, and thence from Dorton by Wescot to Quainton. Further on the Ampthill Clay outcrops between North Marston and Granborough, at Stewkley and onwards to the south of Linslade church.

The beds comprise dark clay and shale with selenite, and they contain Ammonites cordatus and var. excavates, A. plicatilis, A. vertebralis, Belemnites abbreviates, Gryphaea dilatata, Ostrea discoidea, and also Ostrea deltoidea.

There is a mingling of forms elsewhere belonging to the Oxford and Kimeridge Clays, and this is natural, as the conditions of deposition, in the absence of the Corallian rock beds, were more uniform ; but it is only by attention to the fossils that the division can be recognized. West of Boarstall a hard cherty band has been observed, which helps to form the gentle escarpment of Pans Hill. It has yielded Ammonites cordatus and A. vertebralis.^[13]

KIMERIDGE CLAY

This clay formation extends from the Thame valley near Thame to the neighbourhood of Brill, Nether Winchendon and Waddesdon, and eastwards to Aylesbury, Quarrington, Hardwick, North Marston, Dunton and Stewkley. It occupies the lower grounds in a part of the celebrated vale of Aylesbury, which is diversified by numerous outlying hills of Portland and Purbeck Beds, with here and there coverings of Lower Greensand and Gault. It consists mainly of dark shale with occasional bands of septaria. In the lower beds Ostrea deltoidea is to be found, higher up Exogyra virgula is characteristic, and the upper beds contain Discina latissima. In Buckinghamshire the upper beds pass into some- what sandy grey clay, well adapted for brickmaking, and dug for the purpose at Brill and Whitchurch. These beds indeed merge into Lower Portland strata, and they contain numerous iridescent fossils, including Ammonites biplex (see p. 10).

PORTLAND BEDS

Resting comfortably upon the Kimeridge Clay are the Portland Beds, which appear from beneath the great covering of Cretaceous strata at Haddenham and Cuddington, Dinton and Hartwell. It may be that the portions here exposed are but parts of a large outlier, as we have no evidence of their occurrence underground far to the south. North- wards we find a group of outliers wholly or in part formed of Portland Beds as at Long Crendon, Brill and Muswell Hill, Ashendon, Nether and Over Winchendon, Quainton, Oving and Whitchurch, Weedon and Aylesbury ; while an inlying mass appears beneath the Gault between Cublington and Wing. ^[14]

The general characters of the strata may be best observed in the sections in the neighbourhood of Aylesbury, where the succession is as follows: ^[15] ——

Upper Portland Beds	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$	Earthy and shelly limestones	about 10 feet
		Sands	" 5 "
		Rubbly limestone with many fossils	" 8 "
		Sands with Conglomerate at base yielding many pebbels of lydite	" 10 "
Lower Portland Beds		Hartwell Clay	upwards of 20 feet

The upper beds of limestone have been quarried for building stone and lime at the Bugle Pit, Hartwell. Large ammonites known as Ammonites giganteus and A. boloniensis, Natica ceres, Cardium dissimile, Pecten lamellosus, Lucina portlandica, Ostrea expansa and species of Perna and Trigonia occur. Examples of the large ammonites, which are sometimes 3 feet in diameter, were built in the walls bounding Hartwell Park by the former proprietor, Dr. John Lee.^[16]

The lower rubbly limestones, known as Aylesbury stone, occur at Aylesbury and yield Myoconcha portlandica, Umcardium, and many fossils which occur in the higher beds. The pebbly layer has been exposed at Bierton and in a brickyard where the Hartwell Clay is worked between Aylesbury and Hartwell. It is in some places cemented into a hard rock. At Brill there are chalky limestones and green glauconitic beds, below which the pebbly layer is well seen overlying a few feet of brown and greenish sand, which passes gradually down into the dark grey Hartwell Clay, which again merges downwards into the Kimeridge Clay. The clay is used for brickmaking, and in Roman times there was a pottery at this locality.

The Hartwell Clay which represents the Lower Portland Beds of other localities contains Belemnites souicbi, Ammonites biplex, Area longipunctata, Astarte hartwellensis, Thracia tenera, Perna mytiloides, etc. It has been dug for brickmaking also at Whitchurch.

At Long Crendon there are several exposures of the Portland Beds, and in one pit near the southern windmill four formations were shown in succession :——

		ft.	in.
Gault	Clay	8	0
Lower Greensand	Sand, clay and ironstone	3	6
Purbeck ....	Limestone and clay with Cyprides .	6	0
Portland Beds . .	Limestone with Trigonia gibbosa, etc.	-	-

Mr. A. Morley Davies estimates the thickness of the limestones of the Portland Beds hereabouts at 32 feet, beneath which is about 2 feet of sand and the pebble bed with lydites, as near Aylesbury. Still lower there is about 30 feet of light-coloured sandy beds with clayey sands, and with a bright green sand at the base. These are the Lower Portland Beds equivalent to the Hartwell Clay of Aylesbury and to the Portland Sands in the south-west of England.^[17]

The Upper Portland Beds of Buckinghamshire form dry brashy soil, which is largely under arable cultivation. Springs are thrown at the junction with the Hartwell Clay, and good supplies of water are locally met with. At Dorton below Brill there is a famous chalybeate spring.

PURBECK BEDS

Several of the more prominent of the outlying hills of Portland Beds in the Vale of Aylesbury are capped by Purbeck strata, as at Oving and Whitchurch, Quainton, Coney Hill, Brill and Long Crendon ; other outlying patches occur at Haddenham and Cuddington, Stone and Hartwell, and at Bishopstone, while their presence has been noted by Mr. Morley Davies between Towersey and Kingsey, and by Fitton at the Warren south of Stewkley, as well as at other localities. Of Purbeck as well as of Portland Beds we have but isolated remnants of formations which may formerly have extended a good deal further north ; but while the record of the Portland Beds is complete, nowhere in this region have we the full thickness of Purbeck Beds. They comprise a variable series of marls, compact and fissile limestones known as ' Pendle,' and calcareous sands, with here and there a cherty layer. The organic remains betoken their freshwater and estuarine origin ; there are plant remains, abundant cyprides (ostracoda), insect remains, mollusca such as Cyrena, Paludina and Mytilus, also fish remains and bones of turtles. Evidence of the changing conditions which fol- lowed the deposition of the marine Portland Beds is shown by an admixture of marine and freshwater ostracods in the Lower Purbeck Beds, and likewise in the uppermost Portland Beds.^[18] No doubt during the Purbeck period there were occasional irruptions of the sea over the area in which freshwater beds were for the most part accumulated.

In the building up of what is now Buckinghamshire the Jurassic strata form the immediate foundation, but the Lias and Inferior Oolite Series so far as we know occur only in the northern part of the county, while the Great Oolite Series probably extends from north to south of the county being connected underground with beds of this age proved in deep borings in Middlesex and Surrey.

The succeeding Jurassic strata occupy a lesser area in the central portion of the county, owing to disturbance and erosion. They were spread over the entire area ; at any rate such was the case with Oxford Clay, Corallian and Kimeridge Clay, and possibly with the Portland Beds, which initiate changes that ultimately led to the estuarine and freshwater Purbeck Beds. The land must then have been to some extent upraised and the strata bent into broad folds, and during the closely connected Wealden epoch there may have been much erosion and possibly deposition of freshwater strata.

The exposed areas of Oxford Clay and newer Jurassic strata evidently form part of a broad synclinal structure, the anticlinal portions north and south having been worn away, and this erosion took place to some extent prior to, and to some extent during, the deposition of the Lower Greensand. Thus the Lower Greensand rests indifferently on any of the Jurassic formations from the Purbeck Beds at Stone to the Oxford Clay at Brickhill. During these periods of erosion the Portland and Purbeck Beds were to some extent separated into outlying masses, the shapes of which have been modified during later epochs.

LOWER GREENSAND

The Lower Greensand comprises a variable group of sands and sandstones, with ochre, clays and fuller's earth, and it forms the charm- ing and salubrious region of Woburn, on the borders of which in Buckinghamshire are the heaths of Wavendon and Bow Brickhill, the pleasant uplands of Little and Great Brickhill, and the wooded valley at Linslade. The Lower Greensand appears again at Bishopstone, and near Towersey from beneath the main mass of Gault, and it occurs in outliers near Bierton, Hartwell and Stone, at Brill (603 feet) and Muswell Hill (649 feet), and in other eminences resting directly on Purbeck or Portland Beds, and overlapping their margins in places. Evidently it lies unconformably on the Purbeck and older strata, and yet curiously enough in some of the lower bands of ferruginous sandstone there have been found remains of freshwater shells, Unio, Cyrena and Paludina, which led Fitton, John Phillips and also Prestwich to regard these beds as of Wealden age.^[19] Now elsewhere, where Wealden Beds occur, they are comformable with the Purbeck Beds, and the explanation given by other geologists, that these fossiliferous layers are freshwater beds of Lower Greensand age seems most reason- able. ^[20]

It may be remarked that John Morris noted the occurrence of freshwater mollusca at Hartwell, and he mentioned the finding of the Wealden plant, Endogenites erosa, near Stone church, probably at the base of the sands.^[21] (1867). At Muswell Hill Unio porrectus, a Wealden species, has been found. We have however no grounds for concluding that Wealden species of freshwater mollusca died out at the close of the Wealden period, and the stratigraphical evidence is in favour of group- ing with the Lower Greensand all the strata about to be described which occur between Shotover and Woburn. It may be useful to group, as Mr. A. M. Davies has done, the beds at Muswell Hill, Brill, Long Crendon, Oving and Quainton under the old name of Shotover Beds. These beds attain a thickness of about 50 feet.

Elsewhere near Aylesbury, at Bishopstone, Haddenham and Stone the sands and hard ferruginous layers have yielded impressions of marine Lower Greensand fossils, Exogyra sinuata, Lima and Pecten, and these beds, which may belong to a newer stage than the Shotover Beds, are grouped as Bishopstone Beds by Mr. Davies.^[22]

The precise relation of these two divisions to the thick mass of Woburn Sands may well be left an open question——probably both are represented in that thick series.

Much of the Lower Greensand in the outlying hills consists of coarse and fine sands 20 feet or more thick with hard concretionary masses, the sand being in places clean and white, and adapted as near Stone for glass-making, for which purpose it was formerly sent to Birmingham. At Stone it is cemented in places into hard and irregular siliceous concretions of such grotesque forms that they are known as ' bowel stones.' Elsewhere the sands are of various tints, some red and orange-coloured, the coarser sands containing pebbles of quartz, quartzite and lydian stone. Thin seams of ironstone occur as well as beds of clay, while fuller's earth and ochre were formerly dug at Brill, and whitish pipeclay is met with at Oving.

The soil in general is a reddish brown sandy loam, and the strata are usually water-bearing, having at Stone furnished a good supply. Further on towards Woburn, where there is a greater thickness of the sands, larger permanent supplies of water may be expected, but that district is partially covered by Boulder Clay, which occupies hollows and thus modifies the extent and flow of the underground water.

Important beds of fuller's earth occur in the Lower Greensand at Brickhill and Wavendon. So long ago as 1723 the working of fuller's earth at Wavendon Heath was described by the Rev. B. Holloway, who noted the succession of strata as follows :^[23]——

		feet		feet
Reddish sands	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	about 18	Fuller's Earth	about 8
Red Sandstone			White rough stone . . .	-
Sand		" 22	Sand	-

The section at Woburn, afterwards published by Fitton, ^[24] closely agrees with the above, except that the thickness of the sands overlying the fuller's earth is there estimated at 130 feet. He notes that the fuller's earth is of a very light olive green colour, in which particular it agrees with the fuller's earth in the Lower Greensand at Nutfield, and with that in the Oolitic series at Midford near Bath.

At Great Brickhill, to the north-east of Brickhill Manor, resting on the upper beds of the Oxford Clay there was discovered in 1873 by Mr. J. J. H. Teall a bed with phosphatic nodules of somewhat similar character to that which was formerly worked at Potton in Bedfordshire. The nodules or so-called ' coprolites ' were scattered through about 30 feet of sands, but more abundantly in the lower part.

For some time the beds at Brickhill were worked, the coprolites being separated by sifting, and the quartz, chert, lydites and other stones being picked out. Among the coprolites were phosphatized remains of saurians and fishes, worn casts of ammonites and other mollusca, also brachiopoda and other fossils from the Portland Beds and Kimeridge Clay, together with a few that may have been derived from the Corallian. Some fossils from the Oxford Clay occurred, the ammonites in this case ' being preserved in oxide of iron (Limonite) and never phosphatized.'^[25] This is noteworthy, but many of the Oxford Clay fossils are preserved in pyrites, and on this account they may have been proof against phos- phatization. Evidently during the overspread of the Lower Greensand fossils were derived from many of the underlying formations.

Such beds of phosphatic nodules usually indicate a pause in deposi- tion, and may sometimes represent one or more zones. The minerali- zation of the fossils was due to decomposing animal matter, the carbonate of lime being replaced by phosphate of lime ; and even wood was thus mineralized, as well as bones and nodules of limestone.^[26] The nodules, which are dark brown or yellow, yield from 30 to 50 per cent of phosphate of lime.

In the Gault other bands of phosphatic nodules occur, generally black in appearance but usually pale grey or buff in the interior. During the formation of the Gault the Lower Greensand was extensively eroded, as well as the various Jurassic rocks. The Gault was spread over their worn surfaces, resting in places in the Vale of Aylesbury on Lower Greensand, Purbeck and Portland Beds and Kime- ridge Clay ; and between Wing and Soulbury on Corallian and Oxford Clay.

GAULT

The Gault, a stiff dark blue and pale calcareous clay with concretions of carbonate of lime or ' race,' occupies a considerable tract, and is more calcareous in the northern part of the area. It occurs at Wing, Cubling- ton, Mentmore, and thence from Cheddington it comes to the surface over the Vale of Aylesbury by Stoke Mandeville and Ilmer to Towersey.

The Lower Gault, which is characterized by Ammonites interruptus and A. lautus, together with Belemnites minimus, attains a thickness of from 140 to 150 feet. About 20 or 30 feet from the base there is a band of phosphatic nodules, and other nodules are found at the junction with the Upper Gault. The Upper Gault characterized by Ammonites rostratus is 70 or 80 feet thick.

The lower band of phosphatic nodules was formerly worked at Towersey, and between Ford and Moreton, south-east of Dinton, and near Bishopstone. The seam, which is but 3 or 4 inches thick, is made up of buff and black nodules, comprising coprolites and phosphatized shells of the Gault fossils A. rostratus, A. varicosus, Inoceramus sulcatus, etc. The upper band of nodules, about 1 8 inches thick, was at one time worked at Puttenham, Cheddington and Slapton.^[27]

The soil of the Gault is naturally thin, so that the land is often heavy, cold and tenacious, and best adapted for pasture. From infor- mation communicated by the Rev. F. W. Ragg, it appears that as late as the fifteenth century, before the district was drained, there were many swampy tracts and two or three lakelets in the vale north of Marsworth. The soil is however much modified in places by the scattered Drifts and by downwashes from the neighbouring hills of Upper Greensand, Lower Greensand and Portland Beds. Hence it is that the celebrated Vale of Aylesbury, ' the pastoral garden of the county,' which extends from Mentmore and Cheddington to Ilmer and Waddesdon, while mainly a clay country of Gault and Kimeridge Clay, has a soil improved by the waste of the bordering and outlying hills, as well as by the superficial drifts; and thus it ranks high as grazing and dairy land.^[28] The Gault is utilized in many places for brickmaking.

UPPER GREENSAND

This formation, which enters largely into the scenery of many southern counties, is thin and impersistent in Buckinghamshire, being in fact largely replaced by the Gault clay. It comprises greenish (glau- conitic) sands and marls, with layers of fine-grained clayey calcareous rock known as malmstone, and is nowhere more than 25 feet thick in the district. Among the few fossils recorded, Avicula gryphæides is noteworthy. ^[29]

The Upper Greensand is exposed at Bledlow Cross, Horsendon, near the railway stations of Princes Risborough, Monks Risborough, and Kimble, near Aston Clinton and Buckland ; and it has been proved in a boring near Marsworth. It has thinned out at Ivinghoe, but reappears at Eddlesborough.

Owing to its limited extent it is locally of no great importance, although the soil is fertile. A certain amount of water is held in the formation, and springs are thrown out where the Gault clay appears beneath the gentle scarp of Upper Greensand.

CHALK

The Chalk comes to the surface over a large area in Buckingham- shire and forms the main foundation of the southern part of the county.

There is a passage upwards from the glauconitic sands and marls of the Upper Greensand into the Chalk Marl which forms the lowest division of the Chalk. So gradual indeed is the passage that as Mr. Jukes-Browne remarks, ' one cannot say where the Greensand ends and the Chalk begins ' at Bushey Leys near Eddlesborough and in Aston Clinton Park.^[29]

The Chalk Marl consists of clayey and slightly sandy chalk, which gives rise to a somewhat tenacious soil, suitable for the growth of wheat and beans.

At the top of this division there is a band or two of sandy lime- stone which has been extensively quarried at Totternhoe in Bedfordshire. It hardens on exposure and has been much used as a building stone, being well adapted for inside work. Hence it is known as the Tot- ternhoe stone. It yields Ammonites varians and Inoceramus, and outcrops below the main scarp of the Chiltern Hills above Bledlow and Princes Risborough, below Wendover, and at Ivinghoe. Locally it is but 2 or 3 feet thick. A mass of hard grey and white chalk and a band of softer marly chalk occur above the Totternhoe Stone completing what is known as the Lower Chalk. The Middle Chalk commences with a band of hard yellowish nodular chalk, known as the Melbourn Rock from its occurrence at Melbourn in Cambridgeshire. It is from 8 to 10 feet thick, and has been observed at Chalkshire, two miles west of Wen- dover.^[30] Above we find a mass of white chalk with few flints, which stands up boldly in the Chiltern range and may be seen at White Cliff Cross (813 feet), Combe Hill (852 feet) and Haddington Hill, and in the fine escarpment above Ivinghoe (8 1 1 feet) and Eddlesborough. It extends along the valleys to Great Missenden and West Wycombe.

Another rocky band occurs at the base of the Upper Chalk. This is a hard jointed chalk known as the Chalk Rock ; it is from 1 to 8 feet thick, and contains green-coated nodules of cream-coloured and slightly phosphatic limestone. It has been observed in the Loudwater valley at Wycombe Marsh above High Wycombe, in the Misbourn valley near Amersham, and in the Chess valley near Chesham.

The mass of the Upper Chalk with its many bands of flint nodules extends over the greater part of the Chalk area in Buckinghamshire, occupying the high grounds above Chesham, Amersham, High Wycombe and Great Marlow, where it is largely covered with gravel, brickearth and clay-with-flints. It is not far below the surface at Eton and Datchet, for it appears above ground at Windsor Castle owing to an anticlinal structure which has locally upraised the Chalk.

As a whole the Chalk is one of the most uniform of geological formations : its lower portion is argillaceous and an occasional compact and nodular band occurs, but it represents a great and continuous deposit of calcareous mud, slowly accumulated in the deep ocean and due mainly to the decay of calcareous organisms and partly (in its flint bands) to the siliceous matter derived from organisms with siliceous structures.

Remains of marine saurians and fishes occur, but the more abun- dant fossils are those of mollusca, brachiopoda, echinodermata, and sponges ; and yet despite the absence of any great changes in sedimen- tary condition, such as would be likely to affect the forms of life, there is a gradual change in the assemblages of organic remains in the suc- cessive groups of strata. Owing to the slowness of deposition and uniformity over wide areas in Britain it is convenient to divide the life history into certain zones or assemblages of fossils, characterized by particular genera and species which had a wide distribution in space and a more restricted distribution in time. These zones, though purely zoological, afford useful indices of stratigraphical position, and conven- tional limits are assigned to them in different localities, the order of succession being maintained. These zones in Buckinghamshire are as follows :——^[31]

Upper Chalk $\scriptstyle {\left\{{\begin{matrix}\ \end{matrix}}\right.}$	Chalk with flints	Actinocamax quadratus	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	100
		Marsupites
		Micraster		300
Middle Chalk $\scriptstyle {\left\{{\begin{matrix}\ \end{matrix}}\right.}$	Chalk Rock	Holaster Planus		50
	Chalk with few flints and	Terebratulina		50 to 100
	Melbourn Rock	Rhynchonella cuvieri		50 to 60
Lower Chalk $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Grey marly Chalk,	Actinocatnax (Belemnitella) plenus		4 to 6
	Hard grey and white Chalk,	Holaster subglobosus		60 to 80
	Totternhoe Stone and Chalk Marl	Ammonites varians		80

In 1891 Mr. A. Strahan drew attention to the occurrence of two bands of phosphatic chalk in the neighbourhood of Taplow Court.^[32]

These bands consist of brown friable chalk, the colour being due to a multitude of brown grains in a white chalky paste. The grains are almost entirely of organic origin, foraminifera, fragments of Inoceramus and of teeth and bones of fishes, together with small oval pellets which are evidently coprolites of small fishes forming the bulk, all being more or less phosphatic. Two bands occur, the higher is from 8 to 1 1 feet thick, and occurs at about 20 feet from the base of the Eocene strata as proved in a shaft. The lower band is 4 feet thick and occurs from 1 2 to 19 feet lower, according to measurements made in the shaft section and in a pit near the lodge of Taplow Court. Mr. Strahan observes that there can be little doubt that this phosphatic chalk underlies a con- siderable part, if not the whole, of the outlier of Tertiary strata on which Taplow stands, but there are no other sections to prove its exten- sion. Hence it appears to be strictly local. Analysis showed from 18 to 35 per cent of phosphate of lime.

The Taplow phosphatic chalk bears a strong resemblance to a bed, approximately on the same horizon, which has been worked in the north of France.

At Taplow the Chalk has yielded Actinocamax (Belemnitella) quadratus, Ostrea acutirostris and other fossils, rather above the zone of Marsupites.

The sloping chalk plateaus, formed mainly of Upper Chalk, are mostly under arable cultivation, but owing to their coverings of loam and gravel the soils are often deficient in lime. Hence it has been the custom to sink wells or pits in the fields to a depth of 15 or 20 feet to obtain chalk for the land. The steeper slopes of Upper and Middle Chalk form down land with a herbage adapted for sheep walks. Here the soil is thin, and although the ploughed fields may show a brown clayey or loamy soil, adjacent pits often exhibit but a trace of soil.

Plantations of beech trees occupy many tracts in this area on the borders of the plateaus and along the deep and ramifying valleys. These beech woods furnish material for the important chair manufactory at High Wycombe, and for sundry wooden articles made at Chesham and elsewhere. The celebrated Burnham Beeches are situated partly on gravel and partly on Reading Beds, but no doubt in places they are rooted into the underlying Chalk which appears at the surface to the north-west of Burnham Common.

Old terraces of cultivation or lynchets occur in places, as on the hillsides near Chesham, and notably on the Chalk outlier of Southend and of Westend Hill, near Cheddington, where, as I am informed by the Rev. F. W. Ragg, some of the best examples (locally called ' lynces ') may be seen.

The Chalk is a famous water-bearing formation, and the upper and middle divisions are practically undivided by any impervious layers. The marly chalk at the base of the Melbourn Rock sometimes throws out springs, but the more copious outflows of water are at the base of the Totternhoe Stone which rests on marl. Here strong springs are thrown out as at Wendover. As the strata dip towards the south-east the underground flow follows that course, and the streams which have cut down to the plane of saturation carry away the overflow in that direc- tion, and they issue at higher places in the valleys when the plane of saturation rises after long continued rain. The Colne, which cuts into the saturated Chalk carries away much water and receives springs along its bed.

Water is naturally scarce on the higher Chalk uplands, and on some of the downs ' dew ponds ' have been constructed. These are made from 30 to 40 feet in diameter and from 4 to 6 feet deep in the chalk, and they are clayed at the base. A high and exposed situation is selected, and in the first instance water or snow is introduced, and the supply is then maintained by the condensation of moisture from the atmosphere as well as by rain, the condensation exceeding the evaporation. ^[33]

READING BEDS

There is evidence of a considerable change in conditions between the Chalk, a deep sea deposit, and the next succeeding deposits of Eocene age which exhibit shallow-water conditions, both estuarine and fresh- water. There are no traces of Thanet Sands which occur directly above the Chalk under parts of London and eastwards. During these early Eocene times the Chalk was upraised over large areas and much eroded. Evidence of this is furnished by the pebble beds made of rolled flints, which occur in the Reading Beds and other Eocene strata, also by the fact that higher stages of the Chalk characterized by Belemnitella mucronata occur elsewhere in England, and are not known in Bucking- hamshire owing to the erosion to which the Chalk has been subjected.

The Reading Beds comprise dark grey and mottled clays with crimson, green and other tints and white and coloured sands, together with pebble beds made up of black flints. Occasional seams of iron- stone occur, and there is often a layer containing unworn green-coated flints at the base. The strata rest on the Chalk and are overlain by the London Clay to the south-east, but their outcrop is largely concealed by gravel both in the Thames valley and on the uplands to the north of it.

Usually Ostrea bellovuacina is found in the lower beds, and higher up remains of leaves of plants have been observed. The flora indicates conditions approaching to tropical in character.

Portions of the main outcrop of the Reading Beds may be seen here and there at East Burnham, Cliefden, Hedsor, Hedgerley and Ful- mer, while outlying masses occur at Taplow, between Little Marlow and Loudwater, at Lane End and Cadmore End, Turville and Ibstone, at Beaconsfield and Penn, between Chalfont St. Giles and Amersham, and to the east of Chesham. ^[34]

Many of these outliers form picturesque wooded tracts, diversified with commons, as at Lane End and Cadmore End Commons, where the strata are evidently faulted. In some dark sands at this locality traces of nickel and cobalt have been detected.^[35]

The most distant outlier is that noticed by Mr. Whitaker at Ring- sail to the north of Ashridge Park and not far from the Ivinghoe hills, where a pit showed fine white sand. In such situations relics of Eocene strata are preserved in deep pipes in the Chalk, far away from the parent source, as the mass of the strata above had been removed by erosion. ^[36]

The mottled clays are dug in many places for brickmaking, the sands for mortar making, and the pebble beds for road mending. Both sands and pebble beds yield a fair amount of water under favourable circumstances.

LONDON CLAY

This great clay formation forms the substratum over much of south-eastern Buckinghamshire, but like the Reading Beds it is almost wholly concealed by gravels.

It has been observed at the surface at Upton and along the borders of Stoke and Fulmer Commons, at Iver, and to the east and north-east of Fulmer. Outliers appear at Lane End, Penn and Tyler's Hill east of Chesham. In many places the clay is dug for brickmaking.

Nowhere in the county have we the full thickness of the formation, but the greatest thickness is probably on the borders of the Colne, south- east of Wraysbury. In mass it is a bluish-grey clay with septaria, brown at the surface. The basement bed, 6 or 8 feet thick, is a brown loam, which contains flint pebbles and green sand. The sand is sometimes cemented into tabular masses of rock, and these are in places crowded with fossils, such as Cardium, Cytherea, Panopaea, Pectunculus, Nucula, Natica, Rostellaria and Ditrupa plana. Many have been found at Hedgerley.

Fossils are by no means common in the mass of the London Clay. We may occasionally meet with a Nautilus, but near the surface the shells have been almost wholly destroyed. The fauna and flora are of tropical aspect.

There is a great gap between the London Clay and succeeding deposits in Buckinghamshire. Of the interval we have no actual records in the county. The Bagshot Beds were no doubt spread over large areas, for they occur in Middlesex and on the Berkshire and Surrey side of the Thames valley. During Oligocene, Miocene and Pliocene times the area must have undergone great waste by subaërial agents. The London Basin took form by the upraising of the bordering Chalk tracts and the removal from the higher grounds of great masses of Eocene strata. The relics of this denudation have mostly been worked up into the Drifts which were deposited at various periods during Pleistocene and recent times, but it is possible that some of the irregular accumulations known as Clay-with-flints may date back to the Miocene and Pliocene periods.

CLAY-WITH-FLINTS AND LOAM

On the higher Chalk tracts, especially about Chesham, Little Missenden, Little Hampden and High Wycombe, there are thin but widespread accumulations of unworn, little worn and broken chalk flints and reddish-brown clay. On some ploughed fields there appears to be such a mass of these flints that it is difficult to believe that any crop could be grown, yet turnips and other roots flourish.

When we see a cutting through this accumulation of Clay-with- flints we find the Chalk to be irregularly eroded in great hollows or ' pipes,' some of which may be 50 feet deep and 20 feet or more across. These hollows are due to the dissolution of the Chalk, and the dark brown Clay-with-flints which lines these pipes and occurs as a thin covering on the irregular surface of the Chalk is the residue. Some, if not all, of these pipes may be regarded as swallet holes, formed on the margin of Eocene Clay areas, before the Eocene strata were wasted away. ^[37]

In practice we have to include with the Clay-with-flints a very variable accumulation. Preserved in some of the pipes and sometimes intermixed with the Clay-with-flints are relics of Reading Beds, such as mottled clays and sands and pebble beds ; and thus since some of the pipes were formed there is evidence to prove that the Tertiary strata have been so eroded that only small outliers, or the contents of pipes, remain here and there.

Large areas of loam or brickearth, much of it bright and mottled in colour owing to its derivation from the mottled clays of the Reading Beds, occur on the higher Chalk tracts at St. Leonards, Lee and Hyde Heath east of Great Missenden, again to the south of Hampden, on Priestwood Common and Wycombe Heath, on Bledlow Ridge, on the hills above Bradenham, on Radnage Common and near Lane End.

Numerous unworn flints occur in the loam, also irregular masses of greywether, a hard sandstone, almost a quartzite, of which materials have been extensively dug, broken up and squared for paving at Aylesbury and other places.

Professor Morris mentioned that blocks of the stone known as Hampden Stone have been extracted 5 or 6 feet in length, and used as ornamental stones or rude pillars as at Hartwell Park.^[38] Masses of Hertfordshire puddingstone also occur. This is but a pebbly modification of the local greywethers, which are indurated masses of sands

THE VICTORIA HISTORY OF THE COUNTIES OF ENGLAND

}}

OROGRAPHICAL MAP.

or pebble beds derived from the Reading Beds. Other greywethers

have been derived from the Bagshot Sands, which are locally solidified. In these accumulations we see the waste of an old land-surface in the true Clay-with-flints, but it is so intermingled with the extraneous loamy and gravelly deposits that we can only look upon these wide- spread Drifts as in the main a wreck of Eocene deposits.

GLACIAL DRIFT

Extensive sheets of gravel occur on the lower dip slopes of the Chalk tracts from Amersham and Cheneys, southwards over the uplands east of Chalfont St. Peter. They occur also at Chalfont St. Giles, south of Penn, at Flackwell Heath and west of Great Marlow, and they extend over the Eocene tracts from Beaconsfield to Gerrard's Cross, and over Burnham, Stoke and Fulmer Commons to near Iver.

To what extent these Drifts were connected with certain stages in the development of the Thames Basin is a question difficult to decide, but the subject has been ably discussed by Mr. H. J. O. White ^[39] ; and there is little doubt that the Thames belongs to a very early system of drainage, modified from time to time by various physical changes.

In the northern part of the county we find Drift gravels and sands and Chalky Boulder Clay, all distinctly connected with the Glacial period. The Boulder Clay contains many fragments of glaciated chalk, much flint, large boulders of Oolitic rocks and fossils derived mainly from the Oxford and Kimeridge Clays, such as Ammonites, Belemnites and Gryphaea, It is spread over the Great Oolite Series and over portions of the vale of Oxford and Kimeridge Clays——districts formerly more richly wooded than they now are, but of which traces remain in Whittlewood Forest, Stowe Park and Salcey Forest, on the borders of Northamptonshire and Buckinghamshire, and in Whaddon Chase. Patches of Boulder Clay occur on the western end of the Woburn Hills, and thence southwards from near Leighton Buzzard across the Vale of Aylesbury to Long Crendon, and through the region of the Claydons to the north of Bicester.

Here and there beneath the main mass of Boulder Clay, as at Shalstone, we find beds of sand and gravel of irregular thickness and extent ; and both Boulder Clay and gravel contain much chalk and fragments of limestone, which are dissolved away at the surface, giving rise to irregular furrows or pipes like the Chalk itself. Fine sections of Boulder Clay were exposed along the Great Central railway at Chetwode, and at Rosehill farm, where it rests on buff sands. ^[40]

The ice sheet to which this Boulder Clay owes its origin may have covered the northern portions of the county, but it did not overspread the main escarpment of the Chiltern Hills, although it extended east- wards into Hertfordshire and the northern parts of Middlesex.^[41] Near Buckingham, at Tingewick, Radcliff and elsewhere coarse boulder gravel with large blocks of Oolitic rocks and finer gravel and sand occur with intercalations of Boulder Clay.

If we believe that the main mass of Boulder Clay was formed on the land, that much of it was overridden and pressed down into the tough material which it usually is, and that it was left on the melting back of the ice sheet, we might expect, along the borders of the glaciated area, to find alternations of gravel, sand and Boulder Clay. The ice during fluctuations in climatic conditions extended and receded for some distance more than once before it finally retreated, and the melting of the basal portions gave rise to such diverse sedimentary accumula- tions, largely torrential, as in fact we find near Buckingham.^[42]

It may be therefore that over the Chalk tracts which were not glaciated the brickearths and gravels of the plateaus belong to the marginal area of the ice sheet, whence more or less mixed deposits were spread out by the flood waters.

VALLEY GRAVEL AND BRICKEARTH

In the higher courses of the Chalk valleys, where no streams now flow at the surface, or only occasionally in times of excessive rain, we find accumulations which have been termed ' Dry Valley Gravel '—— largely made up of loam with unworn and broken flints, a waste from the clay-with-flints and brickearth and occasional gravel beds of the bordering hills. These merge in the lower courses into the ordinary valley gravel, as between Wendover and Great Missenden.

The gravels of the Thames valley extend over broad tracts near Great Marlow, Burnham, Dorney, Eton, Wraysbury and Colnbrook, thence merging into the gravels of the Colne valley by Denham. They consist of angular, subangular and rounded flints, with pebbles of quartz and quartzite, and are exposed in pits to a depth of 1 2 or 15 feet or more. Over considerable areas there are sheets of brickearth which at Slough and Langley have been worked for brickmaking.

We have thus in these lower lands the same kinds of deposit as occur on the Chalk and Tertiary plateaus, but the valley deposits yield the mammalia characteristic of the Pleistocene deposits, and palaeolithic implements. They are distinctly river deposits, although in composition the gravels naturally do not differ from the higher beds from which they were mainly derived ; and near Great Marlow it is difficult in places to separate the higher terraces of river gravel from the plateau Drifts.^[43]

In the Ouse valley we find gravels at Buckingham, Stony Stratford, Stanton, Lathbury, Tyrington and Filgrave, Emberton, Olney and Cold Brayfield ; and in the Ouzel valley there is gravel at Linslade, Fenny Stratford, Woughton-on-the-Green and Newport Pagnell. Along the borders of this valley there are extensive tracts of grazing land. Although the main features of the country appear to have been sculptured prior to the Glacial epoch, yet during that epoch and in subsequent times considerable modifications were brought about by the accumulation of material as well as by erosion.

The valley gravels which border the present rivers lie in hollows cut through the plateau Drifts. They are evidently newer, but whether they are wholly Post Glacial is difficult to decide, as in both North and South Wales there is evidence that Glacial action continued later than some of the deposits which contain mammalia, like those found in the older Thames valley deposits.

If following Ramsay ^[44] we believe that the Thames drainage com- menced when the Chalk and Eocene strata extended much further to the west and north-west, and flowed across a gently inclined plane towards the south-east, its general course was marked out perhaps in Miocene times ; but there are no deposits along its valley which date back beyond the Pleistocene period. Subsequent physical changes may have removed such deposits, while the wasting away of the Chalk escarpment must have modified the extent and direction of the drain- age. The Bedfordshire Ouse, according to Professor W. M. Davis, ^[45] was a subsequent stream which beheaded certain northern streams originally connected with the Thames drainage. Among these is the Tove, which now joins the Ouse near Stony Stratford ; while the Ouzel may have been formed later on as an obsequent stream, as it flows northwards into the Ouse at Newport Pagnell. The Thame is regarded as a subsequent stream, and likewise the Colne which cuts off the waters of the Misbourn and Chess.

If however we judge by the mammalian and other remains found in the gravels of these valleys, it is difficult to make any distinction in point of age. They all belong to the Pleistocene period. Thus in the Thame valley, not far from the borders of Buckinghamshire, between Shabbington and Rycote, remains of elephant have been found. ^[46]

At Taplow remains of the musk ox have been discovered,^[47] and in the lower parts of the Thames valley the gravels and brickearths have yielded many mammalian remains and palaeolithic implements. Again in the Ouse valley of Bedfordshire there have been found numerous palaeolithic implements ; and in the Ouzel tributary remains of elephant (mammoth) have been recorded from Linslade.

The Alluvium, which is the tract of level ground bordering the streams and liable to be flooded when they overflow their banks, occupies but small areas in the county. Wider tracts are seen along the Colne valley than along the Thames or the Ouse. These are mostly meadow land, and should always be avoided as sites for human habitations. Even the low-lying valley gravels bordering the Thames are liable in places to be inundated, and elsewhere they may prove damp as sites for dwellings, hence all living rooms should be well above the level of the highest flood waters in the river. The gravels hold a considerable amount of water, which rises according to the rain- fall, or in other words, according to the amount of water in the river, and thus basements of houses may be rendered damp.

Looking generally to the relation between the geological structure and the early settlements, we find in Buckinghamshire as in other counties that the question of water supply from river, spring, or well was the natural guide in the fixing of sites. In the deeper Chalk valleys where springs break out and streams flow, along the base of the Chalk escarpment and of the Upper Greensand, on outliers of Lower Greensand and Portland Beds or along their margin, and on the Great Oolite Series, there we find the principal villages and towns. On the clay areas the settlements were fewer and less important, for only where patches of gravelly Drift occur could local supplies of water be readily obtained.

1. Proc. Geol. Assoc. xvi. 19.
2. A list of works on the geology of Buckinghamshire, up to 1873, by W. Whitaker, was printed in the Report Brit. Assoc. for 1882, p. 344.
3. Jukes-Browne, Geol. Mag. (1889), p. 356.
4. H. B. Woodward, ' Middle and Upper Oolitic Rocks of England,' Geol. Survey, p. 337.
5. Ibid., 'Lower Oolitic Rocks of England,' Geol. Survey, p. 391.
6. 'Lower Oolitic Rocks of England,' Geol. Survey, pp. 33. 38, etc.
7. 'Lower Oolitic Rocks of England,' Geol. Survey, p. 393.
8. Geologist iv 215, 486
9. Geology of the country around Banbury, Woodstock, Bicester and Buckingham, p. 28 (1864).
10. Ann. Phil. ser. 2, i. 464 (1821)
11. Green, Geology of Banbury, etc. p. 30.
12. 'Lower Oolitic Rocks of England,Geol. Survey, p. 450.
13. Green, Geology of Banbury, etc. p. 44.
14. See Fitton, Trans. Geol. Soc. ser. 2, iv. 279-92 ; and J. F. Blake, Quart. Joun. Geol. Soc. xxxvi. 215.
15. W. H. Hudleston, Prof. Geol. dssoc. x. 166 ; and H. B. Woodward, 'Middle and Upper Oolitic Rocks of Britain,' p. 225.
16. See Admiral W. H. Smyth, Ædes Hartweliana (1851-64).
17. See J. F. Blake, Proc. Geol. Assoc. xiii. 74 ; and A. M. Davies, ibid. xvi. 21, 22.
18. T. R. Jones, Quart. Journ. Geol. Soc. xli. 328 ; and H. B. Woodward, ' Middle and Upper Oolitic Rocks of England,' Geol. Survey, p. 280.
19. Phillips, Geology of Oxford and the Valley of the Thames, pp. 410, 412, 418 (1871) ; Prestwich, Geology, ii. 264.
20. E. Hull and W. Whitaker, Geology of parts of Oxfordshire and Berkshire, p. 15 ; A. H. Green,Geology of Banbury, etc. p. 50 (1864).
21. Geol. Mag. pp. 458, 459
22. Proc. Geol. Assoc. xvi. 45, 50.
23. Phil. Trans, xxxii. 419 ; reprinted in Conybeare and Phillips' Geol. England and Wales, p. 138.
24. Trans. Geol. Sac. ser. 2, iv. 294 ; see also A. C. G. Cameron, Proc. Geol. Assoc. xii. 395.
25. W. Keeping, Geol. Mag. (1875), p. 372 ; and The Fossils, etc., of the Neocomian Deposits of Upware and Brickhill, pp. 44-6 (1883).
26. See Teall, 'The Natural History of Phosphatic Deposits,' Proc. Geol. Assoc. xvi. 379.
27. Jukes-Browne, Cretaceous Rocks of Britain, i. 275, 277, 280 ; and Quart. Joun. Geol. Soc. xxxi. 264.
28. 2 See C. S. Read, Joum. Roy. Agiic. Soc. xvi. 281.
29. See A. J. Jukes-Browne, Cretaceous Rocks of Britain, i. 283.
30. W. Hill and A. J. Jukes-Browne, Quart. Journ. Geol. Soc. xlii. 224.
31. W. Whitaker, Quart. Joum. Geol. Soc. xxi. 398, and Geology of London, i. 58 ; C. Barrois, Recherches sur le Terrain Cretace Superieur de Angleterre et de l'Irlande (1876), p. 150.
32. Quart. Journ. Geol. Soc. xlvii. 356. For account of the Microzoa see F. Chapman, ibid. xlviii. 514.
33. Rev. J. C. Clutterbuck, Journ. Roy. Agric. Soc. ser. 2, i. 273.
34. Prestwich, Quart. Journ. Geol. Soc. x. 90.
35. Summary of Progress of Geol. Survey for 1900, p. 123.
36. Mem. Geol. Survey, iv. 234.
37. See Prestwich, Quart. Journ. Geol. Soc. xi. 73, and pl. 6.
38. Geol. Mag. p. 457 (1867) ; H. B. Woodward, ibid. p. 120 (1891).
39. Proc. Geol. Assoc. xv. 157.
40. H. B. Woodward, Geol. Mag. p. 105 (1897).
41. See S. V. Wood, jun. Quart. Journ. Geol. Soc. vol. xxxviii. map 4, pi. 26.
42. H. B. Woodward, Geol. Mag. pp. 485, 496 (1897).
43. * See also H. J. O. White, Proc. Geol. Assoc. xv. 158.
44. Quart. Journ. Geol. Soc. xxviii. 152 ; see also A. Strahan, Proc. Geol. Assoc. xiv. 405.
45. Geograpb. Journ. v. 127 (1895).
46. T. Codrington, Quart. Journ. Geol. Soc. xx. 374.
47. Owen and Prestwich, Quart. Journ. Geol. Soc. xii. 124, 133.