C. EVANS ON THE GEOLOGY OF HAMPSTEAD.

ORDINARY MEETING, MARCH 1ST, 1873.·

21

Professor MORRIS, F.G.S., Vice-President, in the Chair.

The following Donations were announced :-" Archives of Science and Transactions of the Orleans County

Society," Nos. 1, 2, and 3, from the Smithsonian Institution ofAmerica.

"Volcanos," by G. Poulett Scrope, F.R.S., F.G.S. (newedition), from the Author.

"Quarterly Journal of the Geological Society," from thatSociety.

" Abstract of the Proceedings of the Geological Society," fromthat Society.

"Journal of the London Institution," from that Institution."Journal of the Quekett Microscopical Club," No. 17, from that

Club."South Africa and its Diamonds," by Professor T. Rupert

Jones, F.G.S., from the Author.

The following were elected Members of the Association :­Phineas S. Abraham, Esq., B.A., B. Sc., &c.; Thomas

Randolph Mellor, Esq., C.E.; and William Paton Sutherland,Esq.

The following Papers were read:-

1. ON THE GEOLOGY OF HAMPSTEAD, MIDDLESEX.

By CALEB EVANS, ESQ., F.G.S.

The highest part of the ridge on the slope of which Hampsteadis situated, is 443 feet above the mean level of the sea at Liverpool(the Ordnance datum), and, as is well known, the hill is cappedwith sand, which is exposed in several pits on the upper part ofHampstead Heath.

This sand is about 80 feet thick, and is an outlying portion ofthe Lower Bagshot Sands of Surrey. It extends to the north-eastas far as a depression in the road beyond Caen Wood, and it isperhaps united to the similar but smaller outlier which caps High­gate Hill. The present outline of the sand on the west and south­west sides of the hill is very irregular as several ridges radiate invarious directions from the central plateau.

22 C. EVA~S ON THE GEOLOGY OF HAMPSTEAD.

The sand is frequently coarse, and of a yellow colour, but occa­sionally fine and light coloured, and in places thin seams of lightcoloured, fine, sandy clay or loam are interstratified with the sand.Angular masses of a ferruginous grit are seen at a few spots nearthe surface, and there appears to be a considerable amount of ironin the sand, as several of the springs that issue from the lowerportion of the deposit are strongly chalybeate. No fossils havebeen recorded from the sand of Hampstead Heath.

The lower part of Hampstead Hill is occupied by the LondonClay, which is about 400 feet in thickness. This fact has been as­certained by means of a well, sunk some years since on the LowerHeath, '" in which 284 feet of the clay were traversed. At the baseof the clay was a bed of rock, about 5 feet thick, containingpebbles and several species of shells, and below this rock were bedsof mottled clays and sands 89 feet thick, representing the W 001­

wich and Reading Series and the Thanet Sands. This well pene­trated into the Chalk to the depth of 72 feet, tho total depth ofthe well being 450 feet.

At Kentish Town, about II mile to the east of Hampstead, theartesian well, sunk by the New River Company to the depth of1,302 feet,t passed through the following beds;-

Ft. In.Yellow and Blue Clay,' with Septaria 236 0Sands and Mottled Clays 61 6Thanet Sands 25 0Green Coated Flints 2 0Chalk with Flints . 244 6Chalk without Flints 294 0Chalk Marl 47 6Upper Greensand, represented by bluish

grey marl and green sand 72 6Gault, consisting of bluish micaceous day

with two seams of green sand 130 6A Series of Clays, Sands, and Conglome-

rates, mostly of a red colour 188 6

The true age of the last mentioned beds is doubtful. Thesearch for water at this depth was unsuccessful, and the workswere, therefore, suspended.

#> Prestwich, Quart. Jonrn. Gaol. Soc., vol, x.t Prestwich, Quart. Joum. Geol. Soc., vol. xii.

C. EVAKS OY THE GE OI.OGY OF R.~MPSTEAD. 23

Our knowledge of the London Clayis derived chiefly from the variousexcavations that have been, from time to time, made in the vicinityof the metropolis, and I now proceed to notice the principal onesthat I have observed in and near Hamp stead during th e last tenyears.

In th e early part of June, 1871, some excavations wcre com­menced in th c Fin chley Road, near Child's Hill, for thc purpose ofconuecting the houses at Burgess Hill with the main lin e of sewer.Th e excavations were sunk at the southern end of the works t o adepth of 31 feet , and at the north end of the drain to about 16feet below th e surface of the road. The abundance and the excel­lent state of preservation of the fossil shells, which were present inthe lower beds exposed, made this section very interesting. Duringthe few weeks that th e works were in progress I collected nearlysixty species.s

Th e deposit nearest to the surface of the hill, extending to thedepth of about 12 feet , consisted of a clayey sand of a yellowishor ochreous tint. Thi s superficial deposit contain ed few fossilsbut casts of shells of the same species as tho se common in the bedbelow were occasionally found. It passed downward s graduallyinto a dark grey argill aceous sand, and the latter into a sandy clay,whi ch, in the deepest part of th e excavations passed to a stiff dark­coloured clay, pres enting the ordinary characters of the Lond onClay. Nodules of septaria and of iron pyrites were not verynumerous in the sandy clay j but it was in this bed that the fossilsh ells were in great abundance. Of these a few species were pre­sent in gr eat numbers, and were highly characteristi c, belonging toforms rar e in th e lower part of the London Clay.

By far the most abundant shells were an univalve, Voluta nodosa;and a small bivalve, P ectunculus decussatus. Among the oth ercommon forms may be menti oned N atica labellata.Fusus complanatus,F. ,tl'ilineatus, Rostellaria lucida, several species of Pleurotoma,Cardium nitens, Cultellu» aJfinis, Pecten cO/'neus, Modiola elegans,]J[. subcarinata, Cythel'ea tenuistria, A vicula media, &c.

An undescribed species of Fusus of the slender elongatedform, of which th ere are several examples in the Middle Eoceneof Hampshire, and of which Fus us colus is the recent typ e,was occasionally found j also a Cassidaria, resembling Cassidaria

• Proc. Geol, Assoe., vol. ii., p. 283.

24 O. EVANS ON THE GEOLOGY OF HAMPSTEAD.

carinata of the Middle Eocene of Hampshire. A small and verydelicate echinoderm was not uncommon. The whole fauna of thissandy clay indicates a deposit formed not very far from land undera depth of water of about 50 fathoms.

The relation of this fossil-bearing bed to the other deposits oc­cupying this part of the London Tertiary area has been shown bymeans of the various excavations that have, from time to time,been made in the neighbourhood of Hampstead for drainage andrailway purposes. The upper limit of the more argillaceous por­tion of the bed is denoted by the copious springs which appear atthe surface around the hill at nearly the same level. Of thesesprings the best known are the chalybeate spring in Well Walk,still occasionally used for medicinal purposes, and the spring in theOonduit Fields, between Hampstead and Belsize Park, which, pre­viously to the falling in of the North London Railway Tunnel,supplied an excellent drinking water to the neighbourhood. Atabout the same level there is a well at North End. A spring onthe West Heath, belowthe flagstaff, flows over marshy ground to the" Leg of Mutton Pond." Another spring, on the west side of thehill near Ohurch Row, gives rise to the Bayswater rivulet, whichflows by Kilburn to the Serpentine; and similar springs at theVale of Health form one of the sources of the Hampstead Pondsand of the Fleet River. The series of ponds to the west of High­gate are fed by springs rising in Oaen Wood. There are alsoother springs and small ponds at this level in the neighbourhood.

In the year 1866 a sewer was constructed for the purpose ofconnecting the northern and western sides of Hampstead with theMiddle Level System of the Metropolitan Main Drainage System.This sewer, commencing at North End, was formed across the north­west side of Hampstead Heath, and along Platt's Lane and theFinchley Road to West End and thence to Kilburn.

On the high ground, at the hill overlooking Child's Hill theexcavations passed through the yellow Bagshot Sand, but in de­scending the hill towards the Finchley Road this sand becameslightly more argillaceous, and contained a great amount of water.The upper part of tJiis water-bearing sand was yellowish brown incolour, the lower portion being dark grey. Lower down the hillthe dark grey sand passed into a sandy clay, which was drier andcontained fossils in great numbers, especially Voluta nodosa andPectunculus decussatus. This sandy clay extended along Platt's

C. EVANS ON TIlE GEOLOGY o~' IIAMPSTEAD. 25

Lane and the Finchley Road nearly to New West End, and it wasa portion of this bed that was exposed at Burgess Hill. Acrossthe Heath, between Child's Hill and North End, the sewers tra­versed the grey water-bearing stratum, and it was only at one spotthat the fossiliferous bed was reached. This was in the swampyground by the" Leg of Mutton Pond."

A similar succession of strata was seen, in 1862, in drainageworks in Frognal Lane. The upper part of this exposure showedthe yellow Bsgshot Sand at Frognal House. Lower down thelane, near the entrance gate to Oakhill Park, the dark grey sandwas seen, and at the corner of the lane leading to the parish churchand near the Priory the sandy clay with Pectunculus decussatus,Cm'dium nitens, Modiola elegans, &c., was exposed.

Mr. Whitaker, in his memoir- on the geology of this district,states that he " saw some bluish green clayey sand lying at once onthe London Clay in Well Walk, on the south of the Vale ofHealth." The deposit to which this gentleman alludes was exposedin some drains, and these drains were subsequently extended acrossthe Heath to the Vale of Health. Here, as elsewhere, the darkclayey sand was seen below the Bagshot Sand. f-did not observeany fossils in this excavation.

There have been, within the last few years, other exposures ofthis sandy clay in and near Hampstead, which, with the sectionsalready noticed, furnish evidence of the thickness of the fossil­bearing stratum. At the permanent bricked shaft of the MidlandRailway Tunnel, which is nearly opposite the" Belsize Arms," a fewfeet of this bed were traversed, but the other shafts being onslightly lower ground, passed only through the stiff London Clay.There is at that shaft, therefore, the lowest portion of the bedunder consideration. From this spot the ground rises slightly tothe level of the Conduit Spring, above which the Bagshot Sandsets in. A comparison of these two levels gives a thickness of about50 feet to the sandy clay with Voluta and Pectunculus, and it ap­pears to be about the same thickness near Burgess Hill.

The foregoing details show that this sandy clay is present at adefinite level on the south, the west, and the north-west sides ofHampstead,'] and there can be no doubt that it forms a continuousband uuderlying the Bagshot Sand as far as and also to the east of

II Mem. Geol, Surv., No.7, p. 62. t See Ma.p.

26 C. E VAYS OY THE G EOr,O OY OF HA!tIPSTE AD.

Highgate, since a pre cisely similar deposit was exposed at the timeof th e construction of tho Archway Road, whcn large collecti onsof fossil s were made by Mr. Wetherell and others. Th e sam e bedwas also seen in 1864 at High gate Wood, during the formation ofth e Edgware and H igh gate Railway Tunnels.

Th is fossiliferous st ratum well illustrates the denudation to whichth e Thames Vall ey has been subjected. In the lower lying dis­tricts of the metropolis th is bed is absent, and tho characteristi cshells of this zone are r arely, if ever, pr esent in any excavat ionsexposing the lower part of th e L ondon Clay, but on the highground around the Crystal P alace a similar sandy clay ha s bcenseen, both at the Palace Railway Station" and in the tunnels underDulwich Wood on th e high level line- of Railway. At the lastmentioned locality I obtained Pectunculus decussatus, Cardiumnitens, and other fossils similar to th ose of Burgess Hill and Hi gh­gat e in great abundance,

Mr Prestwich records a similar group of fossils from Margaret­t ing Street ncar Chclmsford, Wandsworth- Common, and N ewn­ham and Clewett's Green, near Ba singstoke ; and t his stratumprohably formerly extended over th e whole of th e London Tertiarydistrict, aIt-hough i t is now nearl y restricted to those limi t ed ar easwhcre th e Bag shot Sands cap the Lond on Clay, as around BagshotH eath , and at Kin gston Hill , Harrow, Hampst ead, H igb ga te andEpp ing. The Norwood hill s, whe re thi s st ratum remains, ar c nothigh enoug h to retain th c Bagshot Beds.

The nature of the deposit occupying the lower parts of tho dis­trict around H ampstead has been shown by th e excavations for th etunnels and cuttings of the North London and th e Midland Ra il­ways.

The tunnel on t he latter line, betwecn Haverstock Hill and th eF inchl cy Road, was excavated in very stiff and compact dark g reyclay, with many zones of sept aria and much iron pyrites. Thi sclay contained a group of fossils very distinct from th ose of thesandy clay above. In place of Voluta nodosa, Pectunculus, Cy thel'el/,&c., the most abundant species belong to such gcnera as Nautilus,L eda, Nucula, Oorbula, Pholadomya, Cyprina, &c_ Fusus, Pleuro­toma, and other gast cropods arc present, but the species arc mostlydistinct from tho se of th e Burgess Hill and Highgate clay.

• Prestwich. " Ground beneath as," p. 86.

C. EVANS ON THE GE OLOGY OF HAMPSTEAD. 27

Thecharucteristic species of the lower bed indicat e, apparently,a gr eater depth of water than that und er which the Burgess Hilland Highgate clay was deposited, and a similar group of deep seashells has been obtained, on the north side of London, from th etunnel on th e Great North ern Railway, near th e Islington CattleMarket (formerly known as Copenhagen Fields) , and from thatncar Primrose Hill , and on the south side from th e tu nnel underSydenham Hill .

In th e deposit s forming H ampstead Hill we have indications ofthe extensive changes of condition and physical geography thatt ook place while the depositi on of a portion only of th e strata ofthe Lower Eocene period was in progress. The Wo olwich Beds,containing estuarine and freshwat er fossils with occasional remainsof terrestrial animal s and plants, must have been deposit ed in areassubordinate to an extensive tract of dry land; but th e highestst rata of that series (the " Oldhaven Beds" of Whitaker) show anapproach to marin e conditions, the result of either the depressionor of th e denudation of the land . A period of depression is thenindicat ed by th e band of pebbles forming the" Basement Bed" of theLondon Clay, at which time the old land of th e W oolwich period haddisappeared, and th e fine mud dy sediment derived from more dis­t ant lands was spread over th e bed of a deep and open sea. Theseconditions continued for a period long enough to admi t of the de­positi on of more than 300 feet of London Clay ; but towards theclose of that epoch the condi tion of th e sea bed was changed toone of shallower water, and the sediment became coarser and moresandy , th e result probably of th e proximity of land. St ill coarsersediment mark s th e period of the Bagshot Sands, but owing to thegeneral absence of fossils in th ese sands we are unable to ascertainthe pr ecise conditi ons und er which th ey were deposit ed.

The coarseness of th e material indicates th e strength of th ecurrents under which th e sands were deposited, and that th e landfrom which they were derived was not far distant, and th is is con­firmed by the fact that th e only organic remains found in th e LowerBagshot Beds in England arc leaves, which have been observed ncarChert sey, in Alum Bay, and at a few other places.

The London Clay appears to thin out in its range to th e south­ward, and is perhaps represented by th e upper porti on only of theBogn or Beds of Sussex and Hamp shire, and by a bed of clay cap­ping the cliffs on the north coast of France near Dieppe, It ie

28 C. EVANS ON THE GE OLOGY OF HAMPSTEAD.

wanting in th e neighbourhood of Paris ; but in Belgium andNorth ern France it is of considerable thickness. These facts in­dicate that the land, during the period of the deposition of th eLondon Clay, was situated to th e southward, and the open seato the north or north-east.

Although the shells of this deposit belong, in many instan ces,to genera now found in the seas of warm latitud es, the small size ofthe extinct species, when compared with their reeent congeners,renders it doubtful if any conclusions as to the climate of theperiod can be formed from the presence of these forms, Cassidariais a genus living at the present time in 'the Mediterranean Sea, andRostellaria, Pieurotoma, and Cancellaria prevail in th e seas ofwarm, regions, but nearly all the species of these genera, found inth e London Clay, are of small size. Until lately Pholadomya, avery characteristic genus of the lower beds of the London Clayat Hampstead and Sydenham, was only known as surviving in theseas of the West Indi es; but I understand tha t Mr. Gwyn Jeffr eyshas recently obtained a living species of this shell from deep seadrcdgings off the Coast of Portugal. Th e abundance of N autilusand the local presence near Sheppey of crocodiles, turtles, &c.,show that during a par t at least of the period represented by th eLondon Clay, thi s distri ct must have possessed a higher tempera­ture than that which now prevails in th ese latitudes.

The range of the Lower Bagshot Sand is very different fromthat of the London Clay. The sands are seen in th e cliffs ofWhitecliff Bay, with mineral characters very like those of th e sandat Hampstead, while to the eastward, in Belgium, the equivalentbeds become fossiliferous, and they are represented in the Parisbasin by beds known by the name of the" Lits Coquilliers." Th esefacts clearly indicate an extension of the sea to the southward , andpr obably a land surface to the north and west. We may thereforeconclude that vast changes of physical geography must bave tak enplace during the interval between the time of the deposition of theLondon Clay with Pholadompa and the formation of tho LowerBagshot Sand.

In one of Mr. Prestwich's valuable papers on the Eocene beds,that gentleman has divided the London Clay into four zones, cha­racterised by distinct groups of fossils. Of the se zones the firstor highest consists of the clay of South end and the I sle of Sheppey,the second zone of the H ighgate Clay, the third zone includes the

C. EVANS ON THE GEOLOGY OF HAMPSTEAD. 29

clay of Primrose Hill, Copenhagen Fields, and other places nearLondon; and the fourth and lowest zone comprises beds containinga fauna similar to that of the Bogner Beds of Sussex and Hamp­shire.

The sections around Hampstead show that the second or IIigh­gate zone passes gradually into the Lower Bagshot Sand, andthat the Sheppey zone is not there represented. The interminglingof organic remains of terrestrial origin with marine forms, whichis a characteristic feature of the Sheppey beds, in itself shows thatthe influences under which that zone was deposited were very local.

The fossils of the fourth or lowest zone are by no means commonin the London Clay itself near London, but some of the speciesare met with in the Basement-bed.

There are, however, in the neighbourhood of the metropolis, atleast two well defined zones, the second and the third, which, beingbased on changes in the condition of the sea bcd, may be taken asnatural subdivisions of the London Clay.

The Lower Bagshot Sand is the most recent of the Eocene bedsseen in the immediate neighbourhood of the metropolis. It is,however, probable that higher strata of the Bagshot Group for­merly existed on the Hampstead outlier. These higher strata con­sist chiefly of green and ferruginous sands with subordinate clayeybeds, and are seen around Chertsey and Bagshot. The outliers ofHarrow, Hampstead, and Epping have been separated from themain mass by the extensive denudation to which the district hasbeen subjected, and the traces of sandy gravel and of rounded flintpebbles which arc seen at a few spots at the top and on the higherslopes of Hampstead Hill, may perhaps be the remains of thesehigher deposita."

Near the level of the junction of the sand and clay, the super­ficial deposit frequently consists of a sandy clay forming a goodbrick-earth, which has for several years been worked in the fieldsbetween Hampstead Heath and Highgate, and more recently in thefields between South Hill Park and Parliament Hill. The subsoilin the lower parts of Hampstead consists apparently of the LondonClay, somewhat modified by atmospheric action .

• Mr. Searles Wood, jun., considers that the pebbly gravel on which the fir treesnear the "Spaniards" grow, is distinct from the other superficial deposits onHampstead Heath, and is of Postglacial age.-Quart. Journ. Geol, Soc" vol.uiv., p. 466.

30 C. EVANS ON TIlE GEO L OGY OF HAMPSTEAD.

It is possible that within a short distance from the northern slopeof the hill, traces of th e Glacial Beds may be found. The sectionsof these remarkable beds lately exposed at Finchl ey, three milesto the north of Hampstead, have been described by Mr. H enryWalker, F.G.S.'" These deposit s consist of clays full of suban­gular masses of chalk and of other rocks, and containing fossilsderived from tho Oxford Olay and the Lias, and associated withthe chalky clays are beds of gravel and sand.

These Glacial deposits occupy a plateau varying in height from230 to 300 feot above the sea, and extending from the northernslope of Highgate Hill as far as Whetstone in a north erly direc­tion, and to Church End, Finchl ey, to the north-west. At FortuneHill, in the Finehley Road, one mile south of Finehley and twomiles from Hampstead, there is a portion of these beds remaining.

The position in which these Glacial Beds are now found showsthat the district has suffered extensive denudation at two distinctperiods, the one being pre- and the other post-Glacial. The GlacialBeds, both at Finchley and Whetston e, rest on London Clay con­taining Nautilus , but there is in that locality no trace of the sandyclay ~th Pectunculus which constitutes the upper member of thelast-named formati on. A well section at the Hyde,t about twomiles west of Finchley, shows that at that spot th ere is not morethan 66 feet of London Olay, and if it is assumed that the depositis nearly horizontal (as is probably th e case), it appears from thedifference in height between the Hyde and Finehley that the thick­ness of the Lond on Clay at the latter place does not exceed 170feet. Near the Barn et Railway Station , to the east OfWhetstone,:tthe London Olay is 115 feet tbick. These facts show th at prior tothe deposition of the Glacial clays and gravels, the Eocene beds badbeen extensiv ely denuded, and since the formation of the Glacialdeposits further denudation has taken place, and the streams havedeepened their valleys in some instances as much as 60 feet; in fact,some of the streams take their rise in the Glacial gravels. Whetherthe extensive denudation to the west lind south of London (whichhas separated the sands of Hampstead from those of Harrow andChertscy, and the Burgess Hill sandy clay from that of the Oryst alP alace) took place prior or subsequently to the Glacial Epoch re­mains to be proved.

• Proc- Ocol. Assoc., vol. ii ., p.289. t ~hitHker. Mem. Owl . Surv ., sheet 7 ·:t Op. CIt.

C. EVANS ON THE GEOLOGY OF HAMPSTEAD. 31

The country around and to the north of Hampstead is muchmore diversified by hills and valleys than the other parts of Mid­dlesex are, and the superior character of the scenery of NorthMiddlesex is owing, I am inclined to think, to the circumstancethat the Lower Bagshot Sand, and the Glacial Beds have there beenpreserved.

As has been already observed, the outline of the sand at Hamp­stead is very irregular on all sides except the one to the north­west." On the West Heath the upper part of a deep and narrowvalley extends nearly to the flagstaff by" Jack Straw's Castle." Theridge flanking tbis valley on the south side extends with anirregular outline nearly to Child's Hill; another spur extends toKidderpore Hall, south-east of which a deep valley separates thatspur from the ridge on which Oakhill Park and Frognal aresituated. Auother broad inlet faces the south, and the higher partof this valley extends to Church Row, and to the south of tbisvalley the sand projects as far as the Conduit Spring and RoslynRank. On the eastern side of the hill a valley descends fromFlask Walk to the Lower Heath. This valley unites, at its lowertermination, with the valley forming the eastern boundary of Hamp­stead Heath, in which the well-known Hampstead Ponds aresituated. The highest part of this last valley is situated between"Jack Straw's Castle" and the Vale of Health, and the ridge separat­ing it from that on the West Heath is the narrowest part of theoutlier. The ridge to the east of the lastly described valley ex­tends to Parliament Hill (which perhaps still retains a very smallportion of the Bag shot Saud), and between this ridge and High­gate there is a similar valley, in which the Highgate Ponds aresituated.

The bottoms of most of these valleys are occupied by smullstreams, among which may be noticed the branch of the Brentrising on the West Heath, the Bayswater rising near Church Row,and the branches of the Fleet rising respectively at Flask Walk,the Vale of Health, and Caon Wood , and it appears to me notimprobable that these valleys have all been excavated since thedistrict was raised above the level of the sea by means of thestreams that now flow through them.

All these streams take their rise from the springs that issue from

• See Map.

32 O. EVANS ON THE GEOLOGY OF HAMPSTEAD.

the lower portion of the Bagshot Sand. This sand, resting on theimpervious London Clay, must always have been (whatever was itsextent) a water-bearing stratum. On the first emergence of thedistrict from below the level of the sea the whole of the sand musthave been saturated with water, and the immediate effect of thetendency of that water to reach the lowest level would have beenthe formation of chines, or ravines, around the coast line downwhich the water would flow. Time would then only be requiredto enabl~ the streams thus formed to cut back their valleys to anyextent, until the water-bearing bed in which they had originatedhad becn entirely removed. Slowly and imperceptibly this processhas probably been continued from the time of the emergence of theland to the present day; and by these means not only have thediverging valleys around Hampstead and Highgate been formed,but the outlier itself may also, in course of time, have beenseparated from the sands of Harrow and Epping, and even fromthe extensive tract around Bagshot. The Norwood Hills, whichpresent an outline somewhat similar to the contour of the districtaround Hampstead, have resulted from the same cause; but in thatinstance the denuding action has been continued to a greater extent,and nearly the whole of the water-bearing deposit has been removed.

The same kind of action appears to have operated in the countryto the north of Hampstead, where the Glacial gravel, resting onthe London Clay, forms a similar water-bearing stratum, in whichstreams take their rise, and these streams have also excavated theirvalleys to a lower level.

That much denudation has resulted from the action of the seaon land slowly rising above its level is highly probable, and manyhills and valleys now existing, are the result of the upheaval andthe fracture of the strata; but in a district like Middlesex, wherethe stratification is nearly horizontal, it is in the subaerial actionconstantly in operation that the cause must be sought of the pre­sent contour of the country.