6
379 NOTES ON THE WESTLETON BEDS NEAR HENLEY-aN-THAMES. By HAROLD ]. OSBORNE WHITE. [R ead Friday, July 1St, 1892 ] S INCE the publieation, in the forty-sixth volume of the Quarterly Journal 0/ the Geological Society, of Prof. Prest- wich's papers on the Westleton Beds, so much attention has been directed to these deposits that I thought a few short notes on some features they present at the three localities in which they occur in the neighbourhood of Henley-on-Thames would be of some interest to you; and more especially so as this Association visited one of those localities last summer, * and intends making an excursion to another during this one.] The three exposures of Westleton shingle that I shall speak of are those to be seen on the hills bearing the names of Bowsey, Ashley, and Nettlebed, of which the first and second are in Berkshire, and the third on the opposite side of the Thames Valley in Oxfordshire. t The higher portions of these hills are composed of Reading Beds and London Clay, forming outliers on the Chalk more or less distant from the northern edge of the Tertiary escarpment of the London Basin, the drift gravel with which this paper is concerned occurring either at or near their summits. Beside giving a short general description of each deposit I shall mention certain important differences which exist between them, both as regards their structure and the positions they occupy. The first locality to be spoken of is Bowsey. This hill is situated at a distance of about three miles south-east of Henley, and attains a height of 467 feet above Ordnance Datum. The London Clay which forms its higher part is capped in a few places by patches of light-coloured, ochreous, sandy gravel, from 3 to 8 feet in thickness, composed as follows :- r, Tertiary flint pebbles, 2. Numerous pebbles of white quartz, 3. Subangular flints, 4. Light-coloured quartzites and quartz-grits, 5. Pebbles and subangular pieces of chert, 6. Veinstone and Lydian stone pebbles, 7. Subangular and unworn Sarsenstones, 8. Fragments of various kinds of sandstones. Both the rounded and subangular flints are frequently weathered white and occasionally much decomposed. * See Prot. Geoi, Assoc., vol. xii, p. 204. t See page 406. t Geological Survey Map, Sheets 7 and '3. NOVEMBER, 1892.]

Notes on the Westleton Beds near Henley-on-Thames

Embed Size (px)

Citation preview

379

NOTES ON THE WESTLETON BEDSNEAR HENLEY-aN-THAMES.

By HAROLD ]. OSBORNE WHITE.

[R ead Friday, July 1St, 1892 ]

SI NC E the publieation, in the forty-sixth volume of theQuarterly Journal 0/ the Geological Society, of Prof. Prest­

wich's papers on the Westleton Beds, so much attention hasbeen directed to these deposits that I thought a few short noteson some features they present at the three localities in which theyoccur in the neighbourhood of Henley-on-Thames would be ofsome interest to you; and more especially so as this Associationvisited one of those localities last summer,* and intends makingan excursion to another during this one.]

The three exposures of Westleton shingle that I shall speakof are those to be seen on the hills bearing the names of Bowsey,Ashley, and Nettlebed, of which the first and second are inBerkshire, and the third on the opposite side of the ThamesValley in Oxfordshire. t The higher portions of these hills arecomposed of Reading Beds and London Clay, forming outlierson the Chalk more or less distant from the northern edge of theTertiary escarpment of the London Basin, the drift gravel withwhich this paper is concerned occurring either at or near theirsummits.

Beside giving a short general description of each deposit Ishall mention certain important differences which exist betweenthem, both as regards their structure and the positions theyoccupy.

The first locality to be spoken of is Bowsey. This hill issituated at a distance of about three miles south-east of Henley,and attains a height of 467 feet above Ordnance Datum. TheLondon Clay which forms its higher part is capped in a fewplaces by patches of light-coloured, ochreous, sandy gravel, from3 to 8 feet in thickness, composed as follows :-

r, Tertiary flint pebbles,2. Numerous pebbles of white quartz,3. Subangular flints,4. Light-coloured quartzites and quartz-grits,5. Pebbles and subangular pieces of chert,6. Veinstone and Lydian stone pebbles,7. Subangular and unworn Sarsenstones,8. Fragments of various kinds of sandstones.Both the rounded and subangular flints are frequently

weathered white and occasionally much decomposed.

* See Prot. Geoi, Assoc., vol. xii, p. 204.t See page 406.t Geological Survey Map, Sheets 7 and '3.

NOVEMBER, 1892.]

H. ]. OSBORNE WHITE ON

The quartz pebbles are often stained pink with iron oxide, andrange up to about 3 in. in diameter, though their average size• 1 •IS "4 in,

The quartzites are very compact and exhibit small veins ofquartz. Their edges are usually quite rounded; their shape isoval; and they range up to 2t in. in diameter. They are verydifferent from the larger and more coarse-grained red and greyTriassic quartzites which characterise the Glacial and RiverGravels occurring at much lower levels in this district.

Specimens of light and dark grey chert and the pebbles ofVein and Lydian stone are of fairly common occurrence.

Sarsenstones are rare, but attain considerable dimensions, oneblock about 3 feet in length, of which a fragment is exhibited,having been obtained from the gravel in Bear Grove. The largesize and unworn condition of these blocks point to their havingbeen derived from some older sands, possibly of Bagshvt age, atone time existing in the immediate neighbourhood.

Specimens of other rocks are rare.The pebbles and sand are occasionally cemented together by

iron oxide to form a friable conglomerate. The gravel is thickestnear the top of the hill on the eastern side, but is very sparinglyrepresented at the highest point. Sections are to be seen in somedisused pits in Park Wood and Bear Grove. At the formerplace the deposit is about 6 feet thick, with little sign of bedding;at the latter the gravel is more sandy, only about 3 feet in thick­ness, and is noticeably bedded.

The next locality is Ashley Hill, which is rather more than amile north-east of Bowsey and four miles east of Henley. It isabout 480 feet high; like Bowsey in being thickly wooded, butdiffering from it in being steeper and more regularly conical illshape. The Westleton gravel is here on the whole thicker, occu­pies a larger area, and, owing to the more regular shape of thehill, forms one large capping to the summit and higher part of theslopes, instead of a few separated patches as on Bowsey. Thoughexhibiting some minor differences, such as greater coarseness ofthe constituents, it is practically identical in composition withthat on Bowsey.

Two pits showing fairly good sections are to be seen: one ashort distance north-west of the farmhouse on the summit; theother some distance down on the eastern slope, though above the400 foot contour line. In the upper pit 8 feet or more of veryindistinctly bedded, yellow, sandy gravel is shown, containing, inaddition to the abundant light-coloured quartzites and grits, a fewvery rare pebbles of dark-red quartzite, which I do not believe tobe the red quartzites of the Glacial gravels, though bearing anunpleasantly close resemblance to them. In the lower pit asection of about 7 feet of distinctly bedded sands and gravel isexposed. There can be little doubt that the bedded structure,

S ECTI O:-; FIW~1 X ETT I.J:TIEll to Bo wsnv

(nislill/a 10 miltS.)

II ILL.

~I':

b. Reading Beds.

bC~

~['len>-3t'"['l>-3o:.::t:lr,<j

CIen

zM..;>:l

:r:M:zr­t'l--:

f OO'

~oa'

CD

sce7 00 '

bOO'

:50 0 '

4 00'

300 '

£:30" S.

!low,or lI i ll.

a

e. Posit ion of Glacial Sands.

I T hames Valle)' Gra vels.

Park Place.River T hames.

,i::::}~~~~4~~~?~j~*i':~'~Jt-'iMito.

c. London Clay.

d. Wesrlcton Beds.

nix.

~.

~

Xcttlebcd Hill.

a. Ch alk.

100

10 0"

40d

3 CO'

5Co'

Soo"\tJ.33°N.

1/

CoNCIO

H . ]. OSBOR NE WHITE ON

here so well marked, is not a consequence of the original deposi­tion of th e gravel, but has been brought ab out by the su bsequentwash of materials down the hill side.

It may be of int ere st to mention her e that towards the end ofDecember, 1891, I found in thi s pit an artificial flint-flake, closelyresembling tho se found in the impl ement -bearin g gravels of th eThames Valley. From its position I was led to conclude thatit had fallen out of the peaty soil that here occur s ab ove thegra vel ; and if th is was actually the case the flake may not beof any co nsiderable age. I have known this district for someyears but I have never heard of anything of the kind being foundon th is hill before.

Nett lebed Hill is five miles north-west of H enley, and risesto a he ight of 696 feet above sea-level. Those members of theAssociation who attended the excursion to this place ill July,'*'1891, wiil remember that the Westleton Shingle here occur s forthe most part in pipes and hollows in the Reading Beds on theflanks of the hills, and caps the adjoining Priest's Hill at aheight of about 660 feet. Although the gravel is now repre­sented on the summit of the Windmill Hill only by a fewscattered pebbles, it seems very probable that it originally form eda covering to the London Clay, from which it has been subse­qu ently denuded. It is evide nt that the London Clay itself hassuffered much denudation, for littl e more than its base mentbed now remains ; and although its thi ckn ess here was proba blyat no time so great as furth er so uth, there is no reason to doubtthat the existing bed is a mere remnant of a much more con­siderabl e mass.

The gravel is composed of:­I. T ertiary flint pebb les,2 . Subangular flints,3. Sm all pebbles of white qu art z,4. A few chert pebbles,5. Small pebbles of light -coloured quartzite.The flint speci mens are generally white-coated, and ran ge

up to ten or more inches in diameter. The qu art z pebbles areall quite small, from mere grains up to one inch in diameter.Chert specimens are rare , and pebbles of other rocks extremelyrar e.

The stones are, as a rule, imb edded in a light sand or slightlycalcareous sandy clay, and th e deposit shows no sign of definitearr angement, the pebbles being as often on end as not. Therecan be no doubt that th e gravel has suffered a good deal ofre-arrangement since its deposition. t

* F or account of this ex cu rsion see P rot . Geol, A ssoc. , vol. xii, part 5, p, 204. See a lsoII On Some Hill Gravels N orth of the Th ames," by H. W..Monckt on and R . S. H erries.Vol. x ii, PP. II 2 and II3.

t The remnant of a simila r deposit ex ists on a neighbou ring T ertia ry outlie r S. of S ton.»Par k , but is not of su fficie nt importance to me rit more th an passing a tte ntion here .

THE WESTLETON BEDS NEAR HENLEY. 383

Now to consider the two important points of distinctionbetween the Westleton Beds at Bowsey and at Nettlebed. Theseare:-

(1.) Their difference in height above sea-level.(2.) Their difference in composition.1. While at Bowsey the Shingle occupies the top of the hill

at an elevation of 460 feet, at Nettlebed it is found, mostly on itsflanks, at a height of 660 feet, or 200 feet higher. Thisdifference is evidently due to the south-easterly dip of the Chalkand Tertiary strata in this district. The distance from Nettlebedto Bowsey is eight miles, so that a very slight inclination of thestrata would account for the gravel occurring some 200 feethigher on the former hill than on the latter. The fact that theposition of the Westleton Beds depends so much upon that of theTertiary Beds on which they rest, while plainly showing that theyhave participated in the movements affecting those beds, goes farto prove the truth of the theory that they were formed prior to theelevation of the high ground constituting the northern limit of theLondon Basin.

Though this paper is not concerned with the relation of theWestleton to the newer gravels in this district, it may be noticedthat a considerable period must have elapsed between the forma­tion of the pebble gravels and the succeeding Glacial sands andgravels; for in this area the latter occur at levels from IS0 to200 feet lower than the former, and rest in most cases directly onthe much eroded surface of the Chalk. The accompanyingsection will render the relations of these deposits more clear.

2. The difference in the composition of the Shingle cannot,however, be quite so satisfactorily accounted for.

As has already been noticed, the Bowsey and Ashley gravels arealmost identical: but the Nettlebed deposit presents so marked adeparture from them and from the general Westleton type, thatProfessor Prestwich hesitates to class it with his Westleton Bedsat all. The appearance of the gravel is certainly different. Thepebbles of quartzite, of Lydian stone, and of chert and otherGreensand debris of such common occurrence on Bowsey, arethere small and very scarce, while the specimens of subangularflint are larger and more abundant. Professor Prestwich" thinksthat the less definite composition of this gravel is due to itsdistance from the main body. If, as he supposes, the pebblesof quartzite, chert, etc., have been derived from the south andeast, their absence or scarcity at Nettlehed and in other Westle­ton outliers, such as that of Tiler's Hill near Chesham on thenorth-west of the London Basin, is intelligible; for it is only tobe expected that they would become more and more rare as theyreceded from their source, while materials derived both directlyand indirectly from the Chalk should show a corresponding

* Quart. /ourn. Geoi. SOC,) vol. xlvi, p. 140.

384 H. J. OSBORNE WHITE ON THE WESTLETON BEDS NEAR HENLEY.

increase in proportion. Still, so great a change in the characterof the gravel within such a comparatively short distance as thatseparating Bowsey from Nettlebed is somewhat remarkable.

There are many interesting questions connected with thewhole subject which it is impossible to discuss within the limitsof a paper primarily intended to accompany the specimens la idbefore you this evening. Without expressing any opinion as tothe general probability or otherwise of Professor Prestwich'stheory concerning the age, relation, and mode of formation of hisWestleton Beds, I may say in conclusion that on the whole itseems to agree pretty well with the observed distribution of thepebble gravels in this particular neighbourhood.