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Easter Excursion, 1889. Excursion to Weymouth
PRELIMINARY EXCURSION TO LYME REGIS.
xlix
A hearty vote of thanks was accorded to Mr. Grover, and also to Sir H. Peek and Mr. C. E. Peek for their courtesy in permitting the Members to examine the Landslip on a day when the public are nsnally excluded. April 22nd.-On Easter Monday the Members took leave of Lyme Regis, and, after being driven to Bridport, took train to Upway Station, where they were met by Mr. Hudleston, who now took the direction of the party. REFERENCES. Geol. Survey Map, Sheets 22 and 17; and Horizontal Sections, Sheet 21. DAY, E. C. H.--" On the Middle and Upper Lias of the Dorsetshire Coast." , Quart. Journ. Geol. Soe.,' Vol. xix, pp. 278.297. WRIGHT, DR. T.-" The White Lias of Dorsetshire." , Geol. Mag.,' Vol. i, 1864, pp. 290-292. ,VRIGHT. DII. T.-" On the Zone of Aoicula. contorta; and the Lower Lias of the South of England." 'Quart. Journ. Geol. Soc.,' Vol. xvi, pp. 374411; and" Lias Ammonites" (' Palseontograph. Soc.'). DOWNES, HEV W.-" The Cretaceous Beds at Black Veil, neal' Lyme Regis." 'Quart. J ourn, Geol. Soc.,' Vol. xli, pp. 23-26. ROBERTS, GEORGh~.-' An Account of the Mig-hty Landslip at Dowlands and Bindon, near Lyme Regis, December 25, 1839.' 8vo. Lyme, 1840. WOODWARD, H. B.-' Geology of England and Wales.' Ed. 2, 1887, pp. 262-264, 271, 272, 391, 418, and 598.
l
EXCURSION,
1889.
EXCURSION TO WEYMOU'l'H. EASTER MONDAY AND TUESDAY, APRIL 22ND AND 23RD. Director: W. H. HUDLESTON, ]'.R.S., Sec.G.S. (Report by THE DIREcTon.)
ApTil 22nd, Easter frlonday.-The principal portion of the party, viz., that from Lyme Regis, reached Upway Station about 11.RO a.m. Contingents from London and Bath joined subsequently. A move was made in the direction of Elwell, at the foot of the Ridgway Rise. The Director had to announce that the clay-pit in the railway-cutting at the foot of the hill had been closed since the last visit of the Association ten years ago. Moreover, there had been very little fresh working in the stonequarries on the hill lately. The lower division of Portland Stone, known as the chalky or flinty series, has been here quarried for lime, but there is no section showing the actual base. A
EXCURSION TO WEYMOUTH.
line of fossils with large ammonites (? A. boZoniensis) is conspicuous towards the base of the quarry, and it was somewhere in this portion of the series that a Member of the Association obtained the specimen of Ischyodus townsendii, described by Mr. E. T. Newton in 'Proc. Geol. Assoc.,' Vol. vii, p. 116. 'I'he upper division of the Portland Stone, which contains the building-stone series, is but poorly developed in this quarry. There is also another point of difference, viz., that the Lower Dirt-bed of Portland Isle seems to have no representative here, since the Portland and Purbeck Limestones are welded into one block, just as is the' case at Chicksgrove in the Vale of Wardour. This is, however, no proof of conformity. As time was short, the party did not visit the Purbeck portions of these quarrIes. The prospect from this slope is extensive, and the weather being unusually clear, the interesting physical features in connection with the Weymouth anticlinal were very distinct. The Director pointed out some of the localities where the sharp dips on the north side of the axis and the gentle slopes on the south side seem almost to tell their own story. In this remarkable triangle, which has the Chalk escarpment for its base and the Bill of Portland for its apex, the whole of the Middle and Upper Oolites (or Upper Jurassic) are exhibited in duplicate, and he observed that it would be one of the objects of the present Excursion to compare the different developments of the several series on either side of the anticlinal. As a rule, it would be noted that the deposits were thicker on the south. 'I'his is especially noticeable in the Corallian and Portlandian rocks. Further remarks on the age of the anticlinal and its correlation with similar phenomena in other districts the Director would postpone till later in the day. 'I'he party now moved a little higher up the Ridgway Hill, so as to command a thoroughly good view of the cutting, which has formed the subject of some interesting papers by Mr. C. H. Weston in the' Quarterly Journal of the Geological Society (Vol. iv, pp. 245-256; Vol. v, pp. 317-319; and Vol. viii, pp. 110-120). Previous to the making of this cutting no one suspected such a large development of the Wealden or Hastings series so far west. Since the beds dip at a very high angle towarrls the fault the thickness must be very considerable. There is no trace now of the curious dyke-like development of
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FIG. I.-The Weymouth Anticlinal'-9 R and 9 9, Forest Marble and Cornbrash. 9 10, Oxford Clay. 9 11, Coralllan, gl2, Kimer idge Clay. g13l, Portland Sand. gU, Purbecks, k', "Upper Greensand." k', Chalk. i 2, Lower Tertiaries. ta, Quaternary Deposits. .d, Ridgway Fault. (Reduced from Prof. Prestwich's section' Q. J. G. S.,' Vol. xxxi.)
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Iii
EXCURSION TO WEYMOUTH.
Oxford Clay depicted and described by Mr. Weston, which abuts against the actual fault, unless it be in the shape of a long stone drain, which would seem to indicate argillaceous material. One of the party remembered that GTypha'a dilatata had been picked up at the spot ten years ago. '1'0 account for the pl'esence of this Oxford Clay in such a peculiar position has somewhat exercised geologists. The most probable supposition is that there exists a double fault here, and that one of the faults is of pre-Cretaceous age, pointing to extensive pre-Cretaceous denudation. Thus the base of the Upper Cretaceous (here the Upper Gt-oonsand) at Ridgway may really rest upon Oxford Clay, as is represented to be the case at the inlier of East Compton, a few miles to the north-west. In this case the \Vealden-Purbecks would not pass under the Chalk, as they are bound to do, .if this were a simple fault heading to the downthrow in the ordinary manner. Thus the old line of disturbance seems to have become a line of weakness in post-Cretaceous times, although the direction of the throw must have been reversed. Seen from a distance, the enormous deposits of bleached flint, gravel on the top of Binoombe Downs have very much the appeamnce of the Chalk on which they rest. Hereabouts, also, the boring-rod disclosed some sixty feet of Lower Tertiaries, doubtless an extension of the much larger patch on Blackdown, which constitutes the most conspicuous summit of the great escarpment. The route of the party, however, was quite in an opposite direction. After effecting a junction with the contingents which had lately arrived at Upway Station, the entire party proceeded in carriages to the neighbourhood of Osmington Mill, a pleasant drive of about five miles. The route for part of the distance lies in t.hc Kimeridge Clay hollow, and there are good views of the Poi-tland ian and Chalk escarpments on the left. Above thevilJage of Sutton Pointzthegreatfigure cut in the chalk slope representing George Ill, on horseback had evidently been lately cleaned out, and became a very conspicuous object. On arriving at Osmingt on Mill the party made at once for the seashore. The path at the edge of the cliff very nearly coincides with the position of the great fault, which, after running parallel to the coast, passes through Lodrnoors, and
liii
EXCURSION TO WEYMOUTH.
ultimately merges in the axis of the Weymouth anticlinal. This is probably the most important disturbance in this part of the South of England, becoming much more intense as we pass eastwards both in the Isle of Purbeck and at the back of the Isle of Wight. Its effects may be traced in the well-known section at the termination of Ballard Down, which is so graphically depicted in Sir Henry Englefield's work. Before the party star-ted on their arduous walk the Director took the opportunity of making a few remarks as to the age and relations of this great fault. Few persons now believe that the Weymouth anticlinal is of so recent a date as was supposed at one time by Prof. Prestwich. If there are difficulties with respect to the age and mode of formation of the gravels on the Isle of Portland, surely such difficulties are small compared with the acceptance of a hypothesis which requires such an immense amount of meteoric denudation and sculpture within so comparatively recent a period. He failed to see why these movements should be separated in time from the general system known as the" Folding of the Downs." This probably occurred during the Oligocene period, when the principal flexures and fractures in the South of England having an approximately east and west direction came into existence. Since that period an enormous quantity of land in the direction of the Channel has been planed away by marine denudation, and of this land the Island of Portland, preserved by a series of physical accidents not yet explained, is a remnant. The Weymouth anticlinal possesses an important physical feature in common with the three great ant.icliuals of the South of England, viz., that the north wing is the steepest and consequently the shortest-a feature particularly marked in the Vale of Wardour. Thus has come about the idea that in all these movements, which originate in the shrinkage of the earth's crust, the beds have been thrust from the south towards the north.* The general effects of the dislocation along the shore are somewhat masked by the sliding forwards of the Kimeridge Clay from the higher parts of the cliff; but tho confused and tilted position of the Corallian beds bear testimony to its it For further remarks on this subject generally, see Vol. vii, p. 178, et seq.
I
Proc. Geol. Assoc."
Jiv
EXCURSION '1'0 WEYMOUTH.
force. The Corallian section was estimated by Blake and Hudleston* to be about 180 feet thick on this coast. The equivalents of the N othe Grits and Clays, and of the Bencliff Grit, may be taken as constituting the Lower Calcareous Grit of this section, and they are visible at the Cascade, where the large" cheese" doggers, which originate about the base of the Bencliff Calc Grit, sliding over the Nothe Clay, are scattered all over the scaur at this point like so many stranded whales. The Osmington Oolites, which succeed, constitute an important and fossiliferous member of the Corallian series, and above these come the Main Limestones, capped by the 'l'1·igonia-beds. It was along the highly-inclined outcrops of these beds that the party had to scramble on their way to Jordan Hill, but as the afternoon was fine, the task, though slow, was not very arduous. About miles, straight, were accomplished in three hours, and the carriages having been regained at the Coastguard Station, the party proceeded to Weymouth. April 23Td, Easter Tttesday.-The party, after crossing the ferry, made for the summit of the Nothe ridge, where the Director briefly indicated the plan for the day. It was proposed to spend a couple of hours in examining the Corallian series from the N othe to Sandsfoot. Here the beds, dipping towards the south at a moderate angle, and quite free from dislocation, afford an excellent opportunity for studying the sequence. 'l'he series is estimated (op. cit.) at 230 feet; the two Lower Grits and intervening clay being more than twice as th ick as the corresponding part of the series at Osmington, whilst the equivalents of the Osmington Oolites suffer a corresponding diminution. The Trigonia-bed series, or Main Corallian Limestone, affords a rough, though nearly level, platform for several hundred yards (dip 8'5°) in the little bay between Bencliff Point and Sandsfoot Castle. Above this platform succeed what may be termed the Upper Corallian beds, commencing with the Sandsfoot Clay and continued by the Sandsfoot Grits, with occasional clays. There was no time to take more than a passing look at these, but the Director pointed out the sudden and immediate change from the Corallian Limestones (represented in other districts by Coral Rag and Coralline Oolite) to clays which were III some
2t
* . Q. J.
G. S.,' Vol. xxxiii,
gXCURSION TO WEYMOUTH.
Iv
respects Kimeridgian in character, these being succeeded by red grits full of large Monomyaria and with a tendency to ferrugination in their upper parts. 'I'he Abbotsbury Ironstone is probably on the horizon of the clays and sandy beds of the upper part of the Sandsfoot Grits, and may be regarded as on the borders of the Corallian and Kimeridge. The party now doubled back towards Weymouth, and after inspecting the Rodwell cutting took train for Portland. Here they were joined by Mr. Wallis, who acted very efficiently as local guide. The slope of the escarpment on the west side of the Vern Fort was ascended to a point where a new incline had been constructed, leading to the Admiralty quarries on the plateau. No better section of Portland Stone, or one more easily examined, bas ever been made. 'l'he cutting of the incline commences at the top of the indurated Portland Sand, and displays several feet of Portland Clay, though this deposit is said to be variable in thickness." As is well known, the Portland Stone of the island is divisible into two portions-the Lower or Cherty series and the Upper or Building-Stone series. The lower series is a sort of impure Chalk full of brown and black chert, which possesses no economic value, beyond that of filling up a hole where necessary, but there are several lines of fossils. Each of these can be examined inch by inch, and thus a fine opportunity must have been afforded to obtain the numerous varieties of Triqonia gibbosa, for beautiful specimens of which this rock is so remarkable. The BuildingStone series here consists of two thick beds of freestone (Base bed and Whit bed), separated by a coarse fossiliferous and flinty series, part of which is termed the Curf. 'I'he " Roach," which terminates the Portland Stone upwards, contains a fine and varied series of fossils, which on the whole are of a very different character to those of the Cherty series. Numerous minute Gastropoda, often associated with small Conchifcra and t.riradiate sponge-spicules, the shells cha.Icedonized, characterize the topmost layer of the Roach.
'*' This clay is represented in the section appended to Mr. W. Gray's paper "On the Geology of the Isle of Portland," 'Proc. Geol. Assoc.' (Vol. i, p. 132), and is duly recorded by I'rof, Blake in the" Portland Rocks of England" (' Q. J. G. S.,' Vol. xxxvi), but appears to have escaped the notice of the author of the rection given by Mr. Damon (' Geology of Weymouth,' ete., 1884, p. 81).
Ivi
EXCUR S ION TO WEY MOUT H.
'l'h e p ar ty were next conducte d throug-h th e V ern ditch to t he east side of the island , wh er e an inter esti ng view can be ob ta ined looking down up on th e Breakwater, and an adv ance was made for a shor t time in the direction of the con vict pri son. Ultimately, hav ing now gain ed t he sum mit of t he plateau , t he par ty proceeded to ins pect some of thc p r ivate build ing-stonc quarries, where t he var ious p henomena in connection with t he R oach , t he Lower Dirt-bed , th e Skull Cap , and th e Main Dir tbed with its "Burr" Trees were obser ved and dul y discu ssed. Although t here is no t mu ch left of t he L ower Purbeck s on P or tland I sland , dou btl ess t hat little is of ext-reme in terest. A novel fea ture in connection with th e ca lca reo us r ings w hich en viron the " B ur r" Trees was mentioned by Mr. H. B. Woodward. A tree occur r ing hori zontally in the Dirt-bed at P or tisham had a ca lca reous envelope of su ch larg e dimen sions and sing ula r shape t hat it was pronounced by t he local authorities to be a fossil elep ha nt, and peopl e were in vit ed from far and near to in sp ect it. Th e topmost g ro up of t he P urbecks on P or tl and I sle cons ists of a kin d of calcareous slate, often bearing t he marks of mu ch d istu r ban ce. A few y ears ago some excavations were di scovered in t hese sla tes whi ch attracted the atten t ion of Mr. W alli s, wh o, for tunately, betlJought himself of applyi ng to the Asso ciati on for assist an ce in in terpreting the d iscover ies. The r esult was the ver y interesting paper " On some em-io ns excav ati ons in th e I sle of Portl and, " by the present P res ide nt of t he Associa tion, Mr T. V . H olm es, 1<'.G.S. (' P roe. Geo l. Assoc.,' Vo l. vii i, p. 404, et seq.) . Up to 1884 fro m twelve t o fourteen beehive-sh ap ed holes had been discovered . 'l'h e au th or poin t s out t he affinities bet ween t hese" P or tl and Pit s " and t he ordinary K ent and E ssex D en eh oles. The holes ap pear t o have bee n used as granaries, As t he basin g is being cons tantl y clea red away in order t o r each fr esh buil din g-stone, these pit s are doome d to disa pp ear shor t ly after their discovery. 'I'hose figured in Mr. Holmes' pap er are in one respect m or e for tuna te than the people wh o made t hem, in asmuch as a monum ent has bee n erec te d to t heir memory in t he P roceedings of t he Association. But, as good lu ck would have it, Mr. W all is h ad la tel y discover ed anothe r pi t or Den eh ole on t he summit of t he pl ateau , t o which the
EXCUHSION TO WEYMOU1'H.
lvii
Members were conducted. The increasing rain had been troublesome for some time, but here it succeeded in dissolving the party, and the remainder of the programme was not fully carried out. There was a gathering towards three o'clock of most of the Members at the Victoria Hotel, near the Station. Here Mr. Wallis displayed some of his fossils, incl uding a very fine Ammonites g1:gantells, said to have been the second biggest everfound. After dinner t.he President proposed a vote of thanks to the two Directors, Mr. H. B. Woodward and ~h. W. H. Hudleston. This was duly acknowledged, and the party left Portland by train at 4.15 p.m. REFERENCES. Geological Survey Map, Sheet 17; and Horizontal Section, Sheet 20. BUCKLAND, REV. PROF. W., and DE LA BECHE [SIR] H. T.-" On the Geology of the Neighbourhood of Weymouth:' 'Trans. Geol. Soc.,' Ser. 2, Vol. iv, pp. 1-46. WESTON, C. H.-" On the Geology of Ridgway, Weymouth." 'Quart. Journ.' Geol. Soc.,' Vol. iv, pp, 245-256; Vol. v, pp. 317-319; and Vol, viii, pp. 110-120. GRAY, W.-" On the Geology of the Isle of Portland." 'Proc. Geol. Assoc., Vol. i, pp. 128-147. BLAKE, PROF. J. F.-" On the Kimeridge Clay of England." 'Quart. Journ. Geol. Soc.,' Vol. xxxi, pp. 19fi-232. BLAKE, PROF. J. F., and HUDLESTON, W. H.-' On the Corallian Rocks of England.' Ibid., Vol. xxxiii, pp. 260-404. BLAKE, PROF. J. F.-' On the Portland Rocks of England.' Ibid., Vol. xxxvi, pp. 189-235. BLAKJ!:, PROF. J. F., and HUDLESl'ON, W. H.-" Excursion to Weymouth and Portland." 'Proc. Geol. Assoo.,' Vol. vi, 1879, pp. 172-174. DAMON, ROBERT.-' Geology of Weymouth, Portland, etc.' 81'0. Weymouth, 1884. With Supplement (Plates of Fossils), 1888. WOODWARD, H. B.-' Geology of England and Wales.' Ed. 2. Jurassic section. PRESTWICH, PROF. J.-" Notes on the Phenomena of the Quaternary Period in the Isle of Portland and around Weymouth." 'Quart. Jonrn. Gool. Soc.,' Vol. xxxi, pp. 29-53.
xlix
A hearty vote of thanks was accorded to Mr. Grover, and also to Sir H. Peek and Mr. C. E. Peek for their courtesy in permitting the Members to examine the Landslip on a day when the public are nsnally excluded. April 22nd.-On Easter Monday the Members took leave of Lyme Regis, and, after being driven to Bridport, took train to Upway Station, where they were met by Mr. Hudleston, who now took the direction of the party. REFERENCES. Geol. Survey Map, Sheets 22 and 17; and Horizontal Sections, Sheet 21. DAY, E. C. H.--" On the Middle and Upper Lias of the Dorsetshire Coast." , Quart. Journ. Geol. Soe.,' Vol. xix, pp. 278.297. WRIGHT, DR. T.-" The White Lias of Dorsetshire." , Geol. Mag.,' Vol. i, 1864, pp. 290-292. ,VRIGHT. DII. T.-" On the Zone of Aoicula. contorta; and the Lower Lias of the South of England." 'Quart. Journ. Geol. Soc.,' Vol. xvi, pp. 374411; and" Lias Ammonites" (' Palseontograph. Soc.'). DOWNES, HEV W.-" The Cretaceous Beds at Black Veil, neal' Lyme Regis." 'Quart. J ourn, Geol. Soc.,' Vol. xli, pp. 23-26. ROBERTS, GEORGh~.-' An Account of the Mig-hty Landslip at Dowlands and Bindon, near Lyme Regis, December 25, 1839.' 8vo. Lyme, 1840. WOODWARD, H. B.-' Geology of England and Wales.' Ed. 2, 1887, pp. 262-264, 271, 272, 391, 418, and 598.
l
EXCURSION,
1889.
EXCURSION TO WEYMOU'l'H. EASTER MONDAY AND TUESDAY, APRIL 22ND AND 23RD. Director: W. H. HUDLESTON, ]'.R.S., Sec.G.S. (Report by THE DIREcTon.)
ApTil 22nd, Easter frlonday.-The principal portion of the party, viz., that from Lyme Regis, reached Upway Station about 11.RO a.m. Contingents from London and Bath joined subsequently. A move was made in the direction of Elwell, at the foot of the Ridgway Rise. The Director had to announce that the clay-pit in the railway-cutting at the foot of the hill had been closed since the last visit of the Association ten years ago. Moreover, there had been very little fresh working in the stonequarries on the hill lately. The lower division of Portland Stone, known as the chalky or flinty series, has been here quarried for lime, but there is no section showing the actual base. A
EXCURSION TO WEYMOUTH.
line of fossils with large ammonites (? A. boZoniensis) is conspicuous towards the base of the quarry, and it was somewhere in this portion of the series that a Member of the Association obtained the specimen of Ischyodus townsendii, described by Mr. E. T. Newton in 'Proc. Geol. Assoc.,' Vol. vii, p. 116. 'I'he upper division of the Portland Stone, which contains the building-stone series, is but poorly developed in this quarry. There is also another point of difference, viz., that the Lower Dirt-bed of Portland Isle seems to have no representative here, since the Portland and Purbeck Limestones are welded into one block, just as is the' case at Chicksgrove in the Vale of Wardour. This is, however, no proof of conformity. As time was short, the party did not visit the Purbeck portions of these quarrIes. The prospect from this slope is extensive, and the weather being unusually clear, the interesting physical features in connection with the Weymouth anticlinal were very distinct. The Director pointed out some of the localities where the sharp dips on the north side of the axis and the gentle slopes on the south side seem almost to tell their own story. In this remarkable triangle, which has the Chalk escarpment for its base and the Bill of Portland for its apex, the whole of the Middle and Upper Oolites (or Upper Jurassic) are exhibited in duplicate, and he observed that it would be one of the objects of the present Excursion to compare the different developments of the several series on either side of the anticlinal. As a rule, it would be noted that the deposits were thicker on the south. 'I'his is especially noticeable in the Corallian and Portlandian rocks. Further remarks on the age of the anticlinal and its correlation with similar phenomena in other districts the Director would postpone till later in the day. 'I'he party now moved a little higher up the Ridgway Hill, so as to command a thoroughly good view of the cutting, which has formed the subject of some interesting papers by Mr. C. H. Weston in the' Quarterly Journal of the Geological Society (Vol. iv, pp. 245-256; Vol. v, pp. 317-319; and Vol. viii, pp. 110-120). Previous to the making of this cutting no one suspected such a large development of the Wealden or Hastings series so far west. Since the beds dip at a very high angle towarrls the fault the thickness must be very considerable. There is no trace now of the curious dyke-like development of
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FIG. I.-The Weymouth Anticlinal'-9 R and 9 9, Forest Marble and Cornbrash. 9 10, Oxford Clay. 9 11, Coralllan, gl2, Kimer idge Clay. g13l, Portland Sand. gU, Purbecks, k', "Upper Greensand." k', Chalk. i 2, Lower Tertiaries. ta, Quaternary Deposits. .d, Ridgway Fault. (Reduced from Prof. Prestwich's section' Q. J. G. S.,' Vol. xxxi.)
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Iii
EXCURSION TO WEYMOUTH.
Oxford Clay depicted and described by Mr. Weston, which abuts against the actual fault, unless it be in the shape of a long stone drain, which would seem to indicate argillaceous material. One of the party remembered that GTypha'a dilatata had been picked up at the spot ten years ago. '1'0 account for the pl'esence of this Oxford Clay in such a peculiar position has somewhat exercised geologists. The most probable supposition is that there exists a double fault here, and that one of the faults is of pre-Cretaceous age, pointing to extensive pre-Cretaceous denudation. Thus the base of the Upper Cretaceous (here the Upper Gt-oonsand) at Ridgway may really rest upon Oxford Clay, as is represented to be the case at the inlier of East Compton, a few miles to the north-west. In this case the \Vealden-Purbecks would not pass under the Chalk, as they are bound to do, .if this were a simple fault heading to the downthrow in the ordinary manner. Thus the old line of disturbance seems to have become a line of weakness in post-Cretaceous times, although the direction of the throw must have been reversed. Seen from a distance, the enormous deposits of bleached flint, gravel on the top of Binoombe Downs have very much the appeamnce of the Chalk on which they rest. Hereabouts, also, the boring-rod disclosed some sixty feet of Lower Tertiaries, doubtless an extension of the much larger patch on Blackdown, which constitutes the most conspicuous summit of the great escarpment. The route of the party, however, was quite in an opposite direction. After effecting a junction with the contingents which had lately arrived at Upway Station, the entire party proceeded in carriages to the neighbourhood of Osmington Mill, a pleasant drive of about five miles. The route for part of the distance lies in t.hc Kimeridge Clay hollow, and there are good views of the Poi-tland ian and Chalk escarpments on the left. Above thevilJage of Sutton Pointzthegreatfigure cut in the chalk slope representing George Ill, on horseback had evidently been lately cleaned out, and became a very conspicuous object. On arriving at Osmingt on Mill the party made at once for the seashore. The path at the edge of the cliff very nearly coincides with the position of the great fault, which, after running parallel to the coast, passes through Lodrnoors, and
liii
EXCURSION TO WEYMOUTH.
ultimately merges in the axis of the Weymouth anticlinal. This is probably the most important disturbance in this part of the South of England, becoming much more intense as we pass eastwards both in the Isle of Purbeck and at the back of the Isle of Wight. Its effects may be traced in the well-known section at the termination of Ballard Down, which is so graphically depicted in Sir Henry Englefield's work. Before the party star-ted on their arduous walk the Director took the opportunity of making a few remarks as to the age and relations of this great fault. Few persons now believe that the Weymouth anticlinal is of so recent a date as was supposed at one time by Prof. Prestwich. If there are difficulties with respect to the age and mode of formation of the gravels on the Isle of Portland, surely such difficulties are small compared with the acceptance of a hypothesis which requires such an immense amount of meteoric denudation and sculpture within so comparatively recent a period. He failed to see why these movements should be separated in time from the general system known as the" Folding of the Downs." This probably occurred during the Oligocene period, when the principal flexures and fractures in the South of England having an approximately east and west direction came into existence. Since that period an enormous quantity of land in the direction of the Channel has been planed away by marine denudation, and of this land the Island of Portland, preserved by a series of physical accidents not yet explained, is a remnant. The Weymouth anticlinal possesses an important physical feature in common with the three great ant.icliuals of the South of England, viz., that the north wing is the steepest and consequently the shortest-a feature particularly marked in the Vale of Wardour. Thus has come about the idea that in all these movements, which originate in the shrinkage of the earth's crust, the beds have been thrust from the south towards the north.* The general effects of the dislocation along the shore are somewhat masked by the sliding forwards of the Kimeridge Clay from the higher parts of the cliff; but tho confused and tilted position of the Corallian beds bear testimony to its it For further remarks on this subject generally, see Vol. vii, p. 178, et seq.
I
Proc. Geol. Assoc."
Jiv
EXCURSION '1'0 WEYMOUTH.
force. The Corallian section was estimated by Blake and Hudleston* to be about 180 feet thick on this coast. The equivalents of the N othe Grits and Clays, and of the Bencliff Grit, may be taken as constituting the Lower Calcareous Grit of this section, and they are visible at the Cascade, where the large" cheese" doggers, which originate about the base of the Bencliff Calc Grit, sliding over the Nothe Clay, are scattered all over the scaur at this point like so many stranded whales. The Osmington Oolites, which succeed, constitute an important and fossiliferous member of the Corallian series, and above these come the Main Limestones, capped by the 'l'1·igonia-beds. It was along the highly-inclined outcrops of these beds that the party had to scramble on their way to Jordan Hill, but as the afternoon was fine, the task, though slow, was not very arduous. About miles, straight, were accomplished in three hours, and the carriages having been regained at the Coastguard Station, the party proceeded to Weymouth. April 23Td, Easter Tttesday.-The party, after crossing the ferry, made for the summit of the Nothe ridge, where the Director briefly indicated the plan for the day. It was proposed to spend a couple of hours in examining the Corallian series from the N othe to Sandsfoot. Here the beds, dipping towards the south at a moderate angle, and quite free from dislocation, afford an excellent opportunity for studying the sequence. 'l'he series is estimated (op. cit.) at 230 feet; the two Lower Grits and intervening clay being more than twice as th ick as the corresponding part of the series at Osmington, whilst the equivalents of the Osmington Oolites suffer a corresponding diminution. The Trigonia-bed series, or Main Corallian Limestone, affords a rough, though nearly level, platform for several hundred yards (dip 8'5°) in the little bay between Bencliff Point and Sandsfoot Castle. Above this platform succeed what may be termed the Upper Corallian beds, commencing with the Sandsfoot Clay and continued by the Sandsfoot Grits, with occasional clays. There was no time to take more than a passing look at these, but the Director pointed out the sudden and immediate change from the Corallian Limestones (represented in other districts by Coral Rag and Coralline Oolite) to clays which were III some
2t
* . Q. J.
G. S.,' Vol. xxxiii,
gXCURSION TO WEYMOUTH.
Iv
respects Kimeridgian in character, these being succeeded by red grits full of large Monomyaria and with a tendency to ferrugination in their upper parts. 'I'he Abbotsbury Ironstone is probably on the horizon of the clays and sandy beds of the upper part of the Sandsfoot Grits, and may be regarded as on the borders of the Corallian and Kimeridge. The party now doubled back towards Weymouth, and after inspecting the Rodwell cutting took train for Portland. Here they were joined by Mr. Wallis, who acted very efficiently as local guide. The slope of the escarpment on the west side of the Vern Fort was ascended to a point where a new incline had been constructed, leading to the Admiralty quarries on the plateau. No better section of Portland Stone, or one more easily examined, bas ever been made. 'l'he cutting of the incline commences at the top of the indurated Portland Sand, and displays several feet of Portland Clay, though this deposit is said to be variable in thickness." As is well known, the Portland Stone of the island is divisible into two portions-the Lower or Cherty series and the Upper or Building-Stone series. The lower series is a sort of impure Chalk full of brown and black chert, which possesses no economic value, beyond that of filling up a hole where necessary, but there are several lines of fossils. Each of these can be examined inch by inch, and thus a fine opportunity must have been afforded to obtain the numerous varieties of Triqonia gibbosa, for beautiful specimens of which this rock is so remarkable. The BuildingStone series here consists of two thick beds of freestone (Base bed and Whit bed), separated by a coarse fossiliferous and flinty series, part of which is termed the Curf. 'I'he " Roach," which terminates the Portland Stone upwards, contains a fine and varied series of fossils, which on the whole are of a very different character to those of the Cherty series. Numerous minute Gastropoda, often associated with small Conchifcra and t.riradiate sponge-spicules, the shells cha.Icedonized, characterize the topmost layer of the Roach.
'*' This clay is represented in the section appended to Mr. W. Gray's paper "On the Geology of the Isle of Portland," 'Proc. Geol. Assoc.' (Vol. i, p. 132), and is duly recorded by I'rof, Blake in the" Portland Rocks of England" (' Q. J. G. S.,' Vol. xxxvi), but appears to have escaped the notice of the author of the rection given by Mr. Damon (' Geology of Weymouth,' ete., 1884, p. 81).
Ivi
EXCUR S ION TO WEY MOUT H.
'l'h e p ar ty were next conducte d throug-h th e V ern ditch to t he east side of the island , wh er e an inter esti ng view can be ob ta ined looking down up on th e Breakwater, and an adv ance was made for a shor t time in the direction of the con vict pri son. Ultimately, hav ing now gain ed t he sum mit of t he plateau , t he par ty proceeded to ins pect some of thc p r ivate build ing-stonc quarries, where t he var ious p henomena in connection with t he R oach , t he Lower Dirt-bed , th e Skull Cap , and th e Main Dir tbed with its "Burr" Trees were obser ved and dul y discu ssed. Although t here is no t mu ch left of t he L ower Purbeck s on P or tland I sland , dou btl ess t hat little is of ext-reme in terest. A novel fea ture in connection with th e ca lca reo us r ings w hich en viron the " B ur r" Trees was mentioned by Mr. H. B. Woodward. A tree occur r ing hori zontally in the Dirt-bed at P or tisham had a ca lca reous envelope of su ch larg e dimen sions and sing ula r shape t hat it was pronounced by t he local authorities to be a fossil elep ha nt, and peopl e were in vit ed from far and near to in sp ect it. Th e topmost g ro up of t he P urbecks on P or tl and I sle cons ists of a kin d of calcareous slate, often bearing t he marks of mu ch d istu r ban ce. A few y ears ago some excavations were di scovered in t hese sla tes whi ch attracted the atten t ion of Mr. W alli s, wh o, for tunately, betlJought himself of applyi ng to the Asso ciati on for assist an ce in in terpreting the d iscover ies. The r esult was the ver y interesting paper " On some em-io ns excav ati ons in th e I sle of Portl and, " by the present P res ide nt of t he Associa tion, Mr T. V . H olm es, 1<'.G.S. (' P roe. Geo l. Assoc.,' Vo l. vii i, p. 404, et seq.) . Up to 1884 fro m twelve t o fourteen beehive-sh ap ed holes had been discovered . 'l'h e au th or poin t s out t he affinities bet ween t hese" P or tl and Pit s " and t he ordinary K ent and E ssex D en eh oles. The holes ap pear t o have bee n used as granaries, As t he basin g is being cons tantl y clea red away in order t o r each fr esh buil din g-stone, these pit s are doome d to disa pp ear shor t ly after their discovery. 'I'hose figured in Mr. Holmes' pap er are in one respect m or e for tuna te than the people wh o made t hem, in asmuch as a monum ent has bee n erec te d to t heir memory in t he P roceedings of t he Association. But, as good lu ck would have it, Mr. W all is h ad la tel y discover ed anothe r pi t or Den eh ole on t he summit of t he pl ateau , t o which the
EXCUHSION TO WEYMOU1'H.
lvii
Members were conducted. The increasing rain had been troublesome for some time, but here it succeeded in dissolving the party, and the remainder of the programme was not fully carried out. There was a gathering towards three o'clock of most of the Members at the Victoria Hotel, near the Station. Here Mr. Wallis displayed some of his fossils, incl uding a very fine Ammonites g1:gantells, said to have been the second biggest everfound. After dinner t.he President proposed a vote of thanks to the two Directors, Mr. H. B. Woodward and ~h. W. H. Hudleston. This was duly acknowledged, and the party left Portland by train at 4.15 p.m. REFERENCES. Geological Survey Map, Sheet 17; and Horizontal Section, Sheet 20. BUCKLAND, REV. PROF. W., and DE LA BECHE [SIR] H. T.-" On the Geology of the Neighbourhood of Weymouth:' 'Trans. Geol. Soc.,' Ser. 2, Vol. iv, pp. 1-46. WESTON, C. H.-" On the Geology of Ridgway, Weymouth." 'Quart. Journ.' Geol. Soc.,' Vol. iv, pp, 245-256; Vol. v, pp. 317-319; and Vol, viii, pp. 110-120. GRAY, W.-" On the Geology of the Isle of Portland." 'Proc. Geol. Assoc., Vol. i, pp. 128-147. BLAKE, PROF. J. F.-" On the Kimeridge Clay of England." 'Quart. Journ. Geol. Soc.,' Vol. xxxi, pp. 19fi-232. BLAKE, PROF. J. F., and HUDLESTON, W. H.-' On the Corallian Rocks of England.' Ibid., Vol. xxxiii, pp. 260-404. BLAKE, PROF. J. F.-' On the Portland Rocks of England.' Ibid., Vol. xxxvi, pp. 189-235. BLAKJ!:, PROF. J. F., and HUDLESl'ON, W. H.-" Excursion to Weymouth and Portland." 'Proc. Geol. Assoo.,' Vol. vi, 1879, pp. 172-174. DAMON, ROBERT.-' Geology of Weymouth, Portland, etc.' 81'0. Weymouth, 1884. With Supplement (Plates of Fossils), 1888. WOODWARD, H. B.-' Geology of England and Wales.' Ed. 2. Jurassic section. PRESTWICH, PROF. J.-" Notes on the Phenomena of the Quaternary Period in the Isle of Portland and around Weymouth." 'Quart. Jonrn. Gool. Soc.,' Vol. xxxi, pp. 29-53.