FIELD MEETING IN THE HITCHIN DISTRICT.

Saturday, 18th June, 1938.

Report by the Directors: E. F. D. BLOOM, M.A., B.Sc. and J. C.HARPER, A.R.C.S., B.Sc., F.G.S.

A PARTY of twenty members assembled at Hitchin station. at 10.15 a.m. on Saturday, 18th June, where they weremet by the leaders. The object of the meeting was (a) to givea general view of the solid geology of the district in the regionof the Hitchin-Stevenage gap, from the Chalk scarp to theOxford Clay plain, (b) to study the drift deposits in the path ofthe gap and (c) to examine in more detail some recent exposures{see Fig. 70).

A halt was first made at White Hill (300' O.D.) a chalk spuron the east side of the Hitchin gap, where the topographicalfeatures of the town and the site of the' Lake Bed' excavatedhy Clement Reid in 1897[1J* were pointed out.

Near the village of St. Ippolyts, two miles south of Hitchin,a large gravel pit (PI. 28, B) recently opened by Messrs. Innsand Co., adjacent to the rather similar pit visited by the Geologists'Association in 1930 [2J was examined in some detail. Thisaccumulation of glacial gravel, more than 40 ft. in thickenssflanks the small Ashbrook, an obsequent stream whose waterultimately reaches the Great Ouse. The dip of the gravel towardsthe valley suggests that the brook occupies a channel left betweenoutwash fans and not that it has itself excavated a valley in thedrift. The source of this brook lies in a consequent valley(usually dry in its upper part) between Graveley and Stevenage,but at Corey's Mill I mile N.\V. of Stevenage it takes a right­angled turn and becomes an "obsequent ' stream. In pre­glacial days it probably continued its south-easterly coursethrough Stevenage and formed a tributary of the R. Beane.Deposits of drift around Stevenage may have turned its watersinto the Purwell-Hiz valley draining northwards.

A feature of the gravel in the S1. Ippolyts pit is the presenceof a large proportion of Jurassic limestone boulders. Triassicsandstones are also fairly frequent, but there are few Bunterpebbles. Specimens of Carboniferous Limestone and sandstonewere also obtained and a few igneous rocks. Other rocks ofinterest included an iron-stained conglomerate of glacial pebbles,and a sandstone formed of glacial sand cemented with silica.The bedding of the gravel varies from placid to tumultuousand there are sometimes alternate layers of coarse and finematerial, with blackish bands (manganese oxide) at intervals.The upper stratum contains more clay, with boulders of moderate

• For List of References see p. 418.

E. F. D. BLOO:>r A~V J. C. HARPER,

size and more angular flints, while the floor appears to be clayey.It is not easy to visualise the conditions under which depositionmight have taken place.

Arlesey Brickworks was next visited, where there is exposeda section showing 10 feet of Lower Chalk resting on a foot or twoof Cambridge Greensand, which in turn rests on the Gault forwhich the pit is worked. The director pointed out that verylittle was known about the Gault of eastern Bedfordshire, as fewfossils had been recorded and from Arlesev itself no fossils wereknown. Towards the end of Albian times' there was a consider­able shallowing of the sea over the British area, causing denuda­tion of the Gault in an area extending from Buckinghamshireto Ely. Many of the fossils found in the Cambridge Greensandwere derived from this eroded Gault. Derived fossils foundhere included Inoceramus concentricus and Belemnites minimus.Indigenous fossils included Dimyodon nilssoni which was foundattached to some of the phosphatic nodules which are numerousin the Cambridge Greensand. The Upper Greensand is absenthere and appears to die out at Charlton one mile to the west ofHitchin.

From Arlesey the party took the road to Henlow and Clifton,passing workings in the river gravel of the Ivel valley, and thenext point of call was the Meppershall Hoo Brickyard, situatednear the junction of the main Hitchin-Bedford road and theClifton-Meppershall bye road. This brickyard, on a much smallerscale than Arlesey, has been worked for many years and theexcavation of the Gault is being gradually carried northwardsuntil it has almost reached an adjoining gravel pit (PI. 28, A).The Gault here is a pale calcareous clay. No fossils had pre­viously been recorded, but members of the party found severalammonites. Dr. Spath has kindly identified these as Hysterocerasorbignyi and H. bucklandi. These are characteristic of thelower part of the Upper Albian. Other fossils included ostracodsand fragments of a large Inoceramus.

The gravel is clearly of glacial origin, with typical beddingand the usual assortment of erratics, although it is shown onthe old I-inch geological map as River Gravel. Gault appearsin the floor of the pit and the drift occupies an oval-shapeddepression for which it is difficult to account unless it had beenexcavated by a sub-glacial stream. A similar deposit of graveloccurs in another pit, now disused, about half a mile to the east,but this is surrounded by Boulder Clay.

Time did not permit of the intended visit to Pollard's Nur­series at Holwellbury, It miles further south, but an accountwas given of borings for water made there in 1935 and 1936.These encountered the buried channel described by the lateWilliam Hill [3 and 4]. The first borings at the Nurseries were

PROC. GEOL. Assoc., VOL. XLIX. (1938). PU.TE 28.

- '-B.---ST. IpPOLYTS GRAVEL PIT.

[To fare p. 416.

made about 300 yards west of themain road and penetrated about400 feet of drift restin g on OxfordClay. This is comparable with the" Bird in Hand," Henlow bore, halfa mile to the north, as described byHill, where 376 feet of undoubteddrift and a further 80 feet of possibled rift were passed through ; appa­rently the site was near the centreo f the buried channel. As no waterwas obtained a second six-inch borewas then made about 380 ya rds tothe N.W. of the first , and agai nBoulder clay and sand were enCOUIl­t ered to a depth of abou t zoo feet,where Gault was struck. The boringwas continued through I II ft. ofGault , to the Lower Greensand atthe base of which a hard ban d, threefeet in thickness, slowed down therate of borin g to 2 or 3 inches a day.This hard ban d proved to be theE lsworth Rock (Cora llian) and belowit Oxford Clav was encountered . Aplentiful supply of water (7000 gal­lons per hour) was yielded fromthe Lower Greensan d and anotherbore of 1 2 " diamet er was made ashort distance away. Here verys imilar conditions were found an dthe yield of water was increase d to1600 gallons per hour in the testpumping. Some t rouble was occa­sioned by sand; blockage occurredwhich had to be ' shelled out,' butt he difficulty was finally overco meby putting ab out half a ton ofba llast (1" screened) into the 1 2 inchbore for 20 feet. The rest level ofth e water is about 30 feet belows urface and after pumping it sinksto 98 feet.

" I G . 69.-Ho LW E I.LBURY BORE . 1\0. 2 .

Legend as in m ap. The t hin band abovet he Oxford Clay is E lswo rth Hock.

170

OD

-230

o

--

200

---- .- - - -

1--_-43 83386400

E. F. D. BLOOM AND J. C. HARPER,

sulcatus,mmsmus:

InoceramusBelemnites

Brancoceras uaricosum,Inoceramus concentricus,

Lower Greensand: Phosphatised wood.

The following fossils were identified from the cores of theboring :­Upper Gault :

A chart of the boring is shown in Fig. 69.On the way to Old Warden pleasantly wooded Lower Green­

sand country was traversed and the flat country of the OxfordClay was seen beyond Southill. At the west end of the villagethe feature made by the unconformable Lower Greensand waspointed out. A Lower Greensand pit at Clophill was examined,which showed coarse glauconitic sand with occasional layers ofcarstone. There were also many vertical branching tubularstructures of limonitic material which the directors thoughtwere of inorganic origin. These led to some discussion andsome members of the party thought that they might representplant roots, which would mean that there were terrestrial horizonspresent.

Another section in Lower Greensand was examined at CainhoeCastle where carstone, locally used for building at one time, wasseen to be resting on a dark grey clay.

The last pit visited was Harlington Brickworks where there isa deep section in the Gault. Fossils were rare, but terebratulids,ostracods, foraminifera and fish scales were noted and a poorexample of a Hysteroceras sp. was found by one member of theparty. An outlier of the Chalk protected by boulder clay waspointed out. The party then motored back to Hitchin along thebase of the Chalk escarpment. Tea was taken at the SunHotel, after which a number of fine Acheulian implements fromthe Hitchin Lake Bed were exhibited. Mr. J. F. Kirkaldyproposed a vote of thanks to the directors, after which most ofthe party returned to town.

REFERENCES.r , CLEMENT REID. 1897. Proc. Roy. Soc., vol. lxi, p. 40.~ BLOOM, E. F. D. 1930. Proc. Geol, Assoc., vol. xli, pp. 441-442.3. HILL, W. 1908. Quart. Journ. Geol, Soc., vol. lxiv, pp. 8-40.4. 1912. Proc, Geol, Assoc., vol. xxiii, pp. 2l7-224.

EXPLANATION OF PLATE 28.A.-Meppershall Hoo Gravel Pit. Fine sand overlying current-bedded

shelly gravel and overlain by boulder clay.B.-St. Ippolyts Gravel Pit showing Sands and gravels of varying coarse­

ness, with a boulder bed at the base, dipping towards the Ashbrookvalley.

FIELD )IEE TI XG I N TH E HITCHI N DI STRICT. 419

EJ Dri ft dep osit s

o Glacial deposit s

ES3 Chalk

1=- -= Gault

CJ Lower Green san d_ Oxford Clay

GEOLOGICAL SKETCH

MAP OF THE

HITCHIN DISTRICT.

o

FIG. 70-GEOLOGICAL SKETCH M AP OF THE HlTCHlN DISTRICT.