Excursion to Dundry Hill

WHlTSUNTlDE EXCURSION, 1901. The last spot visited was the old long-disused Middlemill or Horsley quarry, which is also opened in the lower trap-band. Here a central mass of much weathered and shattered trap has been left by the quarrymen, while the more compact material to the right and left has been quarried away. Overlying the trap along the western border of the quarry is a limestone, containing lapilli and fossils of Upper Llandovery age, especially corals. So that here, immediately overlying the lower trap-band, just as at Cullimore's quarry, overlying the upper trap-band, th ere are undoubted calcareous tuffs, whose presence strongly suggests, if it does not prove, that the igneous rocks with which they are associated are lava flows. From Middlemill quarry the party drove back to Charfield, and returned to Bristol by the 6.0 o'clo ck train. REFERENCES. Geologi cal Survey Map, Sheet 35. (One-inch.) 1898. MORGAN, C. LLOYD.-British Association Bristol Meeting, Excursion to Tortworth.

EXCURSION TO DUNDRY HILL. THURSDA V, MAY 30TH. D irector: S. S BUCKMAN, F.G.S. (Report by TH E DIR ECTOR.)

ON Thursday morning (May 30th, 1901) a party of sixteen drove at 9.30 to the west end of Dundry Hill. The carriages were left where the slope became steep, and the party ascended the hill, inspecting a section in the road to Castle Farm. Here the Director pointed out (I) that the part of the section near the field gate was presumably the Middle Lias below the Marlstone, but no fossils had been found; (2) that the position of a definite horizon, the Ammonites turneri-zone, of th e Lower Lias was known at Barrow R eservoir, about 200 feet below ; (3) that the Marlstone itself was absent from the particular section under observation, though found in other parts of Dundry Hill, a result due to penecontemporaneous erosion": (4) that in the bank in the field the lower part of the Upper Lias might be found as a thin band of rock composed of beds of different lithic structurea band similar to that shown to the Association on their visit to Down Cliffs, near Bridport-not an "aggregate deposit" any more than that at Down Cliffs because both here and there each bed had its own distinct lithic character and its peculiar fauna, so that the band of rock was polyzonal in each case; (5) that above this rock band were some 50 feet of clay-the Dumortieria-beds-form• See S . S. Bu ckman, " Baj ocian of th e North Cott eswold s: th e Mat n Hill Mass." Quart. [ ourn, Geol, Soc., vol. lvi!, p. 126.

WHITSUNTIDE EXCURSION, 190 I.

153

ing truly the upper part of the Upper Lias; (6) that above the clay were the various limestone deposits known as Inferior Oolite. The party then set to work to find the polyzonal band of Upper Lias, and after a little trouble were able to confirm the Director's statement by knocking out Dacty!ioceras commune, some fragments of H i!doceras bifrolls, a nd portions of a Harp oceras allied to H. exaratum. The party then crossed over the Dumortieria-beds to the well-known quarry at Castle Farm. Here, with a good outlook over the surrounding country, the Director was able to practically illustrate his remarks on the geological history of Dundry, and to give the following address, after pointing out the geography of the scene before the party. Dundry is the most interesting of all Inferior Oolite localities; because by position it is an outlier of the Cotteswold Hills, but so far as its Inferior Oolite rocks are concerned, both in lithic structure and faunal contents, it belongs to the Anglo-Norman basin. Nothing is more remarkable than that the common Brachiopoda of the little-distant Cotteswolds are not found at Dundry, and the common Brachiopoda of Dundry are not found in the Cotteswolds, though the beds are strictly contemporaneous. For a definite period of Inferior Oolite time there must have been som e barrier which parted the sea of Dundry from the sea of the Cotteswolds. The genesis of Dundry may be said to commence at the close of the Carboniferous period. Then was formed to the south of it the west-to-east anticline of the Mendips ; roughly parallel thereto a minor anticline of Wrin gton, evident in the carboniferous mass which bounds Dundry on the west; and this anticline probably dips beneath Dundry Hill (see Fig. 7) . There was another rou ghly east-and-west ; a nticline across the Avon, west of Br istol; and probably an eastward bounding anticline running north and south about the line of Bath. T his much-folded region suffered for some time from subaerial denudation; and during th is peri od the Dolomitic Conglomerate collected as scree on the hillsides, to be afterward cemented by deposits from the waters of the Keuper, which oc cupied the irregular synclines of the Dundry area . Then followed the deposits of Rhsetic and Lias; and there is evidence that the latter at any rat e was laid down on a constantly moving surface. For the Mendip axis sank low enough to be nearly submerged by the Liassic sea (Fig. 7); and, according to the map of the Geological Survey, the Jun ction line of Lias and Rhsetic is some 300 feet higher on the south side of Dundry than on the north side, whereas the Inferior Oolite is nearly level (F ig. 7). This means that during Liassic time there was elevation both of the Mendip axis and of the Wrington anticline, with consequent penecontemporaneous erosion of Lia ssic rocks before the deposi-

154

WHITSUNTIDE EXCURSION, Igor.

tion of Inferior Oolite beds, causing considerably greater removal of Liassic beds on the south side of Dundry than on the north side. There was penecontemporaneous removal, too, of Inferior Oolite rocks. The removal was not great; but the evidence is so clear, and the result is so disastrous to the palseontologist, in having robbed him of his most fossiliferous strata, that the fact bulks largely in his eyes. In order to make the point clear, it is necessary to give the following table of the sequence of the Inferior Oolite and some Liassic deposits of Dundry: DATES

Ages. Z

STRATA.

Hemerre.

;:S

z { Po," Garantianre

0

V>

The Coralline Beds The Freestone

Z

;;;

~

-<:

c,

z

;S

z

zz 0

if]

z

<

G §; 0

~

.....l

The Conglomerate Bed

Sauzii

The Iron shot

j l

•

0:: c,

The Upper White Tronshot

Sonniniee

The Lower White Ironshot

Discitre

-<:

15..0

•

0::J

'1 -0 ·J~~ ·

,2

~3 ~' ....

l'

-~ ~

I

rom'd

Limestone and ;\larl Beds

~

.-

"=0

~O

:\1urchisonze

Hard irony beds of Rackledown •

:< ....

Aalensis .

Bottom beds (clayey stone)

·

.....l '""

I

Dispansi to Bifrontis )

>

Clay Beds The Blue Tronshot Beds

~

tn

-'"" .2" c.~

".~

Witchellire

z

'" u 0

.s

.... 0

Garantianro

-c { Dumortierire !-

-c

r

··

Falciferi

The Pink Beds

Spir.ati

The Marlstone (Xliddle Lias)

•

-"

o.~

}~

OJ

00-

;:J

Between the Ironshot and the Conglomerate Bed strata of two hemeree, Niortensis and Blagdeni (Humphresiani), are missing; and the Ironshot itself shows signs of erosion. But the Ironshot is only found in the middle of the hill. It does not extend to the west end, or even to the church; and eastwards the planing away of rocks is still more remarkable-all between the Conglomerate Bed and the Dumortierie beds has been removed at Maes Knoll. What these facts, together with the absence of strata, seem to point to is this: that some time before the deposition of the

WHITSUNTIDE EXCURSION, 1901.

s.

155

Conglomerate Bed the Dundry area was synclinated, the west and east ends of the hill being raised and then denuded, and on the level area so formed the Conglomerate Bed, or its equivalent, was laid down. The annexed diagram (Fig. 8), drawn in accordance with the above ideas, will show the relative position of the strata, and will indicate the small syncline by which the Ironshot was preserved in the middle of the hill. Now a word as to the separation of Dundry from the Cotteswolds during a part of Inferior Oolite time. From the hemera

N.

S. side of A \'0 11 Gorge.

Wr iugton

:ll elldi ps.

Anticli,ne. I ) u n~ rr.

.,

,

,, ,,

-- - -_ ..-

t40R IZ ONTAl. SCAL f. L-_"--_-=-_~_-.:Tt

..,..~ .

,·. ······.·:·.··1 j ! I M OUN TA IN L I M f, 5 T O NC,. f '500 . "': 1 FIG. 7.-DIAGRAM SHOWING THE RELATION OF DUNDRY TO THE MEN DIPS AND TO THE PAUEOZOIC ROCKS (S. S. Buckman).

I

j

Murchisonce until the commencement of the hemera GarantianiX, Dundry must have been cut off from the Cotteswolds. With the evidence of earth movements at Dundry in Liassic and Inferior Oolite times, it is not difficult to understand how. After the Dumortieria-beds were laid down there was an earth movement with denudation: we have good evidence thereof in the Corteswolds. Such a movement bringing to the surface Upper Lias or earlier rocks to the east of Dundry, and continuing in the line of the Malvern axis, might easily raise the necessary barrier. And there is some evidence for such a movement, because Fig. 8 shows that at a later date the east end of Dundry is more raised than the west end. At that time there must have been an anticline east of Dundry, denuded so as to bring, at Maes Knoll, the upper beds of Inferior Oolite in contact with the Upper Lias.

WHITSUNTIDE EXCURSION, IgOr.

But if we cut off connection with the Cotteswold sea, it is necessary to have connection with the Dorset sea. That there was such connection the identity of fossil remains shows. Therefore the Mendip anticline must have been breached; and it was presumably broken somewhere in the line of the present Bristol Channel where the Holmes are. This breach of Jurassic date had perhaps much to do with the river system of the present day. On that subject I would say a few words. We have built up Dundry to the top of the Inferior Oolite. No doubt there were Mai n R oad.

Church.

E a st D undry,

Ma es

Knoll.

'01 . .. . .... . . .

. ' , ... .. .. : . ...< ~ . ..'\I '-t

t _ _ ,-

' -

-

' -

-'-

, _~-- . _ ,

. • -

I

:.. ', ,,

:~,, ~

._

. -- •

__ . '

-- ~ .

. ~

__ '

1 -- ---:; ,

",..........

~

G I ~

-- ' ~.-- '- ', =:= :=..:.::-~, ---:~.::::: ~ ~-" --~' -' --' -- .-- ' ~' »>

'_'~---' --I 0- -._ -. -.- - , "__ ' , -' --' ::---:

'---._,-_.-

~ --l

__ ,

t_

'

'

FIG. 8,-DIAGRAM OF STRATA AT DUNDRY J. Coralline Beds 2. Freestone . 2A. Conglomerate

3. 4. 5. 6. 7. 8.

Ironshot Oolite White fronshot . Lower Inferior Oolite Dumortieria Beds Bifrons Bed Marlstone

9. Clay

I

(S. S. Buckman).

Upper Inferior Oolite. } (Parkinsonian Age).

I Middle Inferior Oolite. f (Sonninian Age). (Ludwigian Age).

I Upper Lias f (Harpoceratan Age).

I Middle

Lias J (Deroceratan Age.)

many more rocks deposited on it; but we have no definite evidence. Our next episode is not deposition, but denudationthe carving of Dundry to its present shape. In this matter the Mendip and Wrington anticlines play a part; and between them was the Congressbury (pronounced Combsbury) syncline, This syncline would become the bed of a trough stream draining towards Keynsham, and the resultant river erosion would separate Dundry from the Mendips. On the north, excavation was effected by the predecessor of the Avon-a stream which flowed from Mid-Wales in a south-easterly direction, until coming

WHITSUNTIDE EXCURSION, 1901.

157

under the influence of the Mendip anticline its further southeasterly progress would be checked, and it would be given a more easterly course. The next episode is the tapping of this preAvon river-accomplished by a stream which flowed south-westward from the Mendip anticline into the syncline between South Wales and North Devon (now the Bristol Channel, which in this part is a drowned out river valley). This stream working back, probably in the line where the Mendips had been breached in Jurassic days, and hence in a line of soft Jurassic rocks, got to the north of the Mendip-South-Wales anticline, and captured the pre-Avon river just north-west of Bristol. As a result, this stream would next start a stream in the pre-Avon valley, which, working back, would become the present Avon, capturing in turn

FIG. g.-EASTERLY COURSES OF

TRIBUTARIES ON SOUTH SIDE OF THE

BRISTOL AVON, WHICH IS A WEST-FLOWING RIVER.

the tributaries of its predecessor. Of this we have striking evidence. Fig. 9 shows how the streams flowing from the north side of the Mendip anticline have a genera I north-easterly course, quite opposed to the course of the present Avon, which is generally north-westward. Like the remains of a Norman arch left in a church wall beside a perpendicular window, these streams remain as relics of a past age: they indicate the time when the main stream which they joined flowed in a direction opposite to that of the present Avon. A word in conclusion concerning the suggestion that the Avon once travelled to the sea along the Nailsea Valley. Such an idea cannot be entertained for a minute. It is as if a man, having a direct way from his house to the road through a gate, should make for himself a longer way to the road, and break down a massive stone wall for the purpose. A stream having obtained a direct way to the sea through easily eroded rocks, is not going to

WHITSUNTIDE EXCURSION, 1901.

leave it and take a longer course, involving the cutting down of a great barrier of hard rocks. The difficult may be abandoned for the easy, but not the easy for the difficult. The stream would not have cut the Clifton gorge unless compelled. It was compelled because the Nailsea Valley was then high ground filled with Mesozoic rocks. Therefore the lowering of the Nailsea Valley, by a process of differential subaerial erosion, has been accomplished since the time when the Avon was settled in its task of cutting the Clifton gorge. A little more lowering of the Nailsea Valley, and then the Avon will naturally make its course to the sea vid that valley, leaving the Clifton gorge dry. Some discussion arose on the points of this address; and then, the Director having made a few remarks on the rocks exposed in the quarry at Castle Farm, the party set to work to find fossils. Their labours were successful; but rain came on. When it cleared, and the party could leave the shelter of the quarry it was time to seek lunch at Dundry Inn. After lunch the large Freestone Quarry by the church was inspected, and fossils were procured from the Coralline beds. Then the North Main Road Quarry was visited, but it proved to be out of work. However, the Director pointed out the Ironshot in the bank by the roadside; and part of a large ammonite of the genus Sonninia was obtained from the bed. Rain again threatened; it was decided to drive to Maes Knoll, where, in spite of the wet and a cold wind, some of the members investigated a small opening, showing the Conglomerate Bed of the upper part of the Inferior Oolite resting on a bored and eroded surface of the Upper Lias, as illustrated in Fig. 8. REFERENCES. Geological Survey Map, Sheet 19. 1859. R. ETHERIDGE.-" Notes on Dundry Hill," in paper on" Subdivisions of Inferior Oolite," by T. Wright. Quart. J'ourn. Geol. Soc., vol. xvi. 1879. "V. W. STODDART.-'· Geology of the Bristol Coalfield." Proc. Bristol Nat. Soc., n. ser., vol. ii, part iii, p. 279. 1889. S. S. BUCKMAN. "Tt.e Relations of Dundry with the DorsetSomerset and Cotteswold Areas." Proc. Cottesiooid C!ub, vol. ix, 1896. 1897. 1899.

P·374·

S. S. BUCKMAN AND E. WILSON.-" Dundry Hill." Quart. Journ. Ceo!. Soc., vol. Iii, p. 669. S. S. BUCKMAN AND E. WILSON.-" Geological Structure of Dundry Hill," with coloured map. Proc. Bristol Nat. Soc., vol. viii, p. 188. S. S. BUCKMAN.- H Development of Rivers; Natural Science," vol. xiv, No. 86.