159

E XCU RS ION TO AVO N GORGE AND .

F RIDAY, MAY 31ST.

Under the Direction of P rof. C. L LOYD M ORGAN, F.R.S., and Rev. H. H. Wl:-lWOOlJ, :'>1..'\., F.G.S. AVON GORGE. (Report by C. L LOYD MORGAN.) A party of eighteen members and friends met Prof. Lloyd Morgan at 9.30 at Observatory Hill, Clifton Downs. Fr om this point a general view was obtained of the surround­ ing country, and the connection ·of the broader featur es of the landscape with the geological stru cture of th e district was br iefly indicated. The hill itself is composed of , the massive beds of which are here thrust up by the Cl ifton fault, strata of the same horizon being repeated on Durd­ ham Down, farther north. Dipping S.S.E. at about 26°, the Lower Carboniferous rocks form the southern limb of an anticline, of which the northern limb is seen in the middle distance, the outstanding limestone form ing the tree-clad ridge of Kin g's Western Down. The arch of th e ant icline was denuded in pre­ Mesozoic times, and on th e Old R ed Sandstone thu s exposed, beds of Dolomitic Conglomera te were laid down unconformably. Beyond the northern limb lies the Severn, on the farther side of which are seen the Old Red Sandston e uplands of Monmouthshire and South Wales. In the valley south of Observator y Hill lie the Coal Measures of part of the Coalfield, overlain by Triassic Marls. Beyond this is seen the Carboniferous Limestone anticline of Broadfield Down, th e eastern end of which bears th e Jurassic outlier of Dundry Hill. Further south rises the hroad summit of th e anticlinal arch of Mendip. Be fore leaving th e hill atte ntion was drawn to a so-called " T riassic Dyke " (that sho wn by Buckland and Con ybea re in the ir figure). A fissure in the limestone has here been filled in with fragmen ta l materials, du e to pre-Mesozoic denudation, and the whole cemented in a finer marly or calcareous material intro­ du ced in Keuper times. By the removal, partly through quarry­ ing, of the limestone on either side, this now sta nds out as a vertical wall. Further evidence of the Keuper which once over­ lay the limestone was seen near the vallum of the British camp which crowns the hill. Descending somewhat and proceeding northwards the line of the Clifton fault was crossed, and in a few paces the Carboni­ ferou s Limestone was seen to give place to Millstone Grit, which shows abundant evidence of the effects of the neighbouring dis­ location in its slick ensided surface. .... FIG. Io.-D IAGRA:\l SECT IO:"[ ACROSS TH E BRI STOL COALF IELD. 0\ Ri ver Wapl ey o \Vi nterbourne. Fr ome. Church. Tormarton.

3 2 w 8 9 '0 '0 ..-.....:.~..::., . ~ ~ "---..--.. ... f --: I. Great Oo lite. 6. Marlstone, ::c 2 . Fu ller's Earth. 7. Lower Lias Sha le and Limestone. 3. Inferior Oolite. R. Rh ::Clic Bed s, ac 4. Sand . I) . Keuper Ma rl. ,..,z 5. U pper Lias Sha le. 10 . Coal Measures. 8 t'l t'l oX c: ;

.... N.W. 'D o~

Q>1.rnado ~~

I. :\1illstone Grit. b. Middle Li meston e. T. T rias Dyke. 2. l\liddle Gr it. Co Great Limestone. T. b. T ria s Co ng lomerate. WHITSUNTIDE EXCURSION, 1901. 161

The Millstone Grit is overlain by Upper Limestone Shales, a transitional series showing alternations of grit and limestone with some shaly beds. These strata occupy the space which intervenes between the upthrust limestone of Observatory Hill and that of Durdham Down. Their surface was in pre-Mesozoic times denuded so as to form a valley in which a fine tumbled mass of Dolomitic Conglomerate (the local basement bed of the Keuper) has been exposed in the Bridge Valley Road. The rounded or sub-angular fragments included in this deposit are all of local origin derived from the limestones and grits close at hand; and some of them are of large size, weighing as much as 2t tons. The valley depression in which they lie deepened eastwards towards Cotham, and shelved up westwards across the present (but then non-existent) Avon Gorge. On the opposite or side of the river, some of the beds near the upper end of the old erosion valley may be seen lying uncon­ formably on the upturned and denuded edges of the Lower Car­ boniferous rocks. The pre-Triassic valley, or creek, occupied by the Dolomitic Conglomerate is seen on the accompanying section beneath the P. of Promenade (Fig. 12). Passing down the road so as to cut the fault-line at a lower level, some of the grit-beds of the Upper Limestone Shales were examined, and their lithological similarity to the local Millstone Grit was noticed. The overlying limestone beds were seen to contain Lithostrotion irregulare, while some of them were found to be markedly oolitic in structure. According to Buckland and Conybeare (Trans. Geol. Soc., and series, Vol. I, pt. ii, p. 240). the Upper Limestone Shales in the Avon Section may be thus sub-divided: I. Upper Limestone having the usual character of Mountain Limestone. 2. Middle grit, consisting of shale and grit alternating, and having a thin coal-seam. 3. Middle limestone, consisting of several beds having the character of Mountain Limestone. 4. Lower grit, consisting of shale and grit alternating, and having two thin coal-seams. Near the foot of the road the lower grit of the above classification was seen near the point where the line of the Clifton fault was again crossed. The massive limestone of Observatory Hill is here thrust up over the contorted and torn beds of the limestone and upper grit which underlie the millstone grit already seen at a higher level. The throw of this reversed fault is between 1,100 and 1,200 feet. The direction in which the fault line courses westward forms an angle of about I S" with the strike of the beds, and cuts the strata in such a manner as to cause the Upper Limestone Shales to wedge out in that direction. 162 WHITSUNTIDE EXCURSION, 190I.

The line of section shown in the accompanying figure (Fig. 12) was then followed, the strata being traversed in descending order. Below the Upper Limestone Shales comes the upper part of the Carboniferous Limestone (U.L.). There was not, however, time "'.i~" ~ to. examine the beds, which con- 'i 6-o/ 'd'&- ~ /' tam. P~oductus Kzgallteus and P. \.; 0+" ~ ,'.: semireticuiatus, Terebratula has- '4f tata, and in parts abundant remains of Lithostrotioll. The limestone is often foraminiferal, and contains Calcispho:ra. Crinoid remains are relatively rare. Beneath this Upper Limestone lie more fissile limestones and shaly beds containing Michel­ deania, and hence termed by Mr. Edward Wethered the" Michel­ deania Beds" (M.S.) j and below this occurs a massive band of oolitic structure about 100 feet thick, locally termed by the Director the Gully Oolite (G.O.). This may be traced throughout the whole of the Bristol District. Near its base StreptorhYllcus is often abundant. It is underlain by the Lower Limestone (L.L.) which has been worked in the Black Rock Quarry beneath the Sea Walls. This part of the limestone shows abundant crinoi­ dal ossicles, and near the base where it passes into the Lower Limestone Shales it is largely made up of the remains of SjJi­ rifera striata. It is from this Lower Limestone that the greater number of the teeth and spines of Elasmobranch fishes have been o~<9 6-. found (Orodus, Oracanthus, Coch- e5 o?, liodus, Psammodus, etc.). r,;:: ~ 0.(6: V> Beneath the Lower Lime- ~ Q "'-..; ~ c::: stone lie the Lower Limestone <, : 0 Shales (L.L.S.) composed of thin- bedded limestones and marly strata. In its lower part, about 80 feet above the Old Red Sandstone, is a conspicuous massive red band, at first sight resembling a red sandstone, but mainly calcareous. This is locally known as the Bryozoa Bed. On WHITSUNTIDE EXCURSION, 190r , solution in dilute acid it leaves a residue largely made up of minute fragments of organisms converted into peroxide of iron. Crinoidal ossicles preponderate, but Polyzoa (chiefly Rhombopora) occur in considerable numbers. Some 3 feet above the Bryozoa Bed is the so-called Palate Bed, of which some members obtained satisfactory specimens. It is conglomeratic, contains rounded pebbles of vein-, and yields the teeth of many elasmo­ branch fishes, of which the commoner genera are He!odus and Psammodus. Below the Bryozoa Bed are thin-bedded limestones and marls, in which Mr. W. W. Stoddart records Modiola iuacadami, a form interesting as suggesting a correlation of these beds with the Marwood Beds of Devon, and the Coomhold Beds of Ireland. There is no sign of unconformability between the Lower LIme­ stone Shales and the underlying Old Red Sandstone. A conglomeratic bed with small pebbles of milky quartz is locally taken as the uppermost bed of the older system. A few feet below this, fissile beds, with abundant mica-flakes, occur j but many of the beds in the upper part of the Old Red system are here hard and conglomeratic. Near Sea Mills Station the Dolomitic Conglomerate again occurs overlying unconformably the Old Red Sandstone. In the railway cutting the irregular cavities contained in the rock, some partially filled with calcite, others with quartz (Bristol diamonds) were seen; and a distant view was obtained of the relation of the older to the newer strata on the Somerset side of the river. The following table gives the approximate thickness of the Lower Carboniferous rocks exposed in the Avon Gorge :- Feet. Upper Limestone Shales.. 400 Upper Limestone (including Micheldeania Beds) 1,000 Lower Limestone (including Gully Oolite) 650 Lower Limestone Shales 320

AUST CLIFF.

(RepOl'f by H. H. WINWOOD.)

The members having finished their inspection of the Avon section, under the guidance of Prof. Lloyd Morgan parted from him at Sea Mills Station, and at 12.45 began their drive to Aust Cliff. Leaving the now ruined tidal dock, during the making of which, in 17 I 2, many Roman remains were found, they followed the course of the Trym through Combe Dingle, left Shirehampton Park on the left and followed the road, skirting the north side of the Carboniferous Limestone ridge on which Blaize Castle is built. Henbury was passed on the right, a glimpse of the pretty Hen­ bury cottages on the left and the hamlet of Hallen then came WHITSUNTIDE EXCURSION, 1901. into view. Here the first halt was made to see a section of New Red Marl, with whitish streaks of some decomposed mineral traversing it from top to bottom (allophane?) The drive was then resumed through tortuous lanes across the alluvial flats of the Severn, and after passing through Northwich the members soon after alighted at what was formerly called" Passage House." The coaches formerly unloaded their passengers and mails here for what was then a dangerous crossing in a sailing boat to the opposite shore. The party descended by the old Causeway to the shore. Taking shelter from the breeze, always fresh at this place, the Director remarked that the section before them was one of those exposed in the valley of the Severn, so often visited by former geologists. It had become classical, and was one of the earliest des­ cribed exposures of Keuper Marls and Rheetic Beds in this district. Without entering into any details respecting these marls, he said that the whole section from the bottom to the Ostrea-beds of the Lower Lias measured about 120 feet. The base of the marls resting upon the upturned edges of the Carboniferous Limestone was sandy and gypsiferous. They would notice the sudden change near the top from red to grey. These grey beds, called by some" Tea Green Marls," indicated the horizon where the New Red series ended, and the ushering in of the Rhsetic Series, with their dark grey or black shales. The bone-bed, whence the fine collection of palates and teeth of Ceratodus, now in the Bristol Museum was obtained, they would see projecting from the Black Shales, with the Cotham Marble and White Lias above, succeeded to the top by thin representatives of the Limagigantea- and Ostrea­ beds of the Lower Lias. After this an examination of the fallen blocks was made during the walk along the base of the cliff, and many of the characteristic , Pecten valoniensis, &c., were found. The search for these was much facili­ tated by Mr. Wickes, who had preceded the party in the morning and collected large portions of the bone-bed for their inspection. Some examples of this were very thick (quite 18 in.), of dark grey crystalline-limestone, crowded with bones and coprolites much rolled. Some good specimens of pseudomorphs of salt crystals rewarded a search in the grey-streaked marls near the base. The gypsum appeared at the east end of the section near the bottom in strings and small pockets. During the walk the President directed attention to the projecting masses of the cliff standing out like miniature headlands, explaining this by the fact that in every instance where the capping of Lower Lias Limestones existed the softer shales and marls below were pre­ served from denudation, and where these had fallen down, exposing the softer beds, the cliff had been weathered and furrowed back. This process was going on continually. Time and the rapid incoming tide warned the members to retrace their WHITSUNTIDE EXCURSION, IgOr. 165

.~ JJa.s e cff~ 21

["i~ Zon.ecf.A~ p1arwrbf4 Zoru.of.AvicriltL contorta:

-~- - B~':a ..---;-- 11 _.. ~ -

~ - - Teaqreen.marl»

--- - 3

-_..::::;::--

----' ),. ~T I

· --"~~--~~~...... r~-,--~- -..;;-1.-- l Z0 Carb~

FIG. 1,3. SECTION OF AUST CLIFF.

Paoc, GEOr.. Assoc., VOL. XVII, PART +, AUGUST, 1901.] U 166 WHITSUNTIDE EXCURSION, 1901. steps, and after tea in a primitive arbour, overlooking the" Severn sea," the return drive was made to Bristol through Almonsbury and Henbury. REFERENCES. Geol. Survey Map, Sheet 35. 1875. STODDART, W. W.-" Geology of the Bristol Coalfield." Proc. Bristol Nat. Soc. 1882. SOLLAS, W. J.-" On the Geology of the Bristol District." Proc, eeol. Assoc., vol. vi, p. 375. 1898. MORGAN, C. LLOYD.-'· Geological History of the Neighbourhood of Bristol." Handbook to Brit. Assoc., Bristol Meeting. 1898. British Association, Bristol Meeting. "Excursion to Aust and Over Court,"

EXCURSION TO WARMINSTER AND MAIDEN BRADLEY.

SATURDAY, JUNE 1ST.

Director" J. SCANES. (Report by THE DIRECTOR.) A party of ten members was met at Warminster Station, and proceeded by brake to Maiden Bradley. The drive through Sheerwater was much admired, notwithstanding that the azaleas and rhododendrons were not at their best. Arriving at Maiden Bradley, a brief halt was made at the Somerset Arms for luncheon. [A short time was devoted to the inspection of Mr. Scanes's very fine collection of Cretaceous fossils collected from this district, which were displayed in the schoolroom, and well merited a much more detailed study, before walking to the quarry, west of the school.-A. C. Y.] After describing the section the Director drew attention to the Cornstone bed and brown fossiliferous sand, which presented the difficulty mentioned in the recent joint paper (see References at end of Report). From here the party was driven to Baycliffe Quarry, a few fossils collected from the spiculiferous beds, and attention drawn by the President to the peculiarly shaped valley. which might possibly have been made for irrigation purposes. The two small pits at the back of Rye Hill Farm were next visited, but unfortunately very few fossils were obtained. The Black Hill Quarry, east of Rye Hill Farm, was visited, and a short time devoted to hunting for fossils before driving to the Crockerton Clay Pit, which is opened in the inlier of Lower Gault which comes to the surface at this spot. Owing to recent rain much water had collected, and explorations were practically confined to the heap of stuff that had been taken out for brick­ making.