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Report of an excursion to Henley and Watlington
HERBERT
L.
HAWKINS,
REPORT OF AN EXCURSION TO HENLEY AND WATLINGTON. SATURDAY, JULY 13TH,
1912.
By HERBERT L. HAWKINS, M.Sc., F.G.S., Director of the Excursion. PROBABLY owing to an unpropitious weather forecast(which was abundantly realised), only six members travelled by train toHenley for this excursion. Three others joined the party there, bringing the total number up to nine. The party drove up the Assendon Valley to Stonor, but the drive was considerably marred by persistent rain, accompanied by an abnormally high temperature. Leaving the brake at the foot of Pishill Bottom, the members followed the half-deserted ridge-road to Hollandridge. For the most part the road is in a cutting some 10 or more feet deep, affording an almost continuous section of Chalk for over a quarter of a mile. This first region (indicated on the section, Fig. 39, as I) is entirely in the zone of Terebratulina. The lower exposures show thin, flaky Chalk, very soft and free from flints, while higher up in the section the Chalk is seen to become more massive, though retaining its soft texture. The only fossils found were thin-shelled Inocerami, probably.l. mytiloides. This form is uniformly abundant throughout the length of the section. At about 440 ft O.D., the slope of the hill ceases, and a short but well-marked platform is developed. This feature coincides with the outcrop of the Chalk-rock, which was seen in the road-cutting and in a shallow excavation in the field to the south of the road (2 on the section). This exposure of the Rock is referred to by Mr. Osborne White in the" Geology of the Country around Henley," * and was discovered by him after the printing of the Survey map. The director pointed out how far away, both horizontally and vertically, from its position on the map the outcrop occurs in reality. The field-exposure showed characteristic Chalk-rock, yellowish and massive below, with greencoated nodules, often perforated by borings of about U in. diameter, in the upper part. The President found a good specimen of Terebratulina in the pit, but it was not in situ, although embedded in a hard matrix. The other fossils found were broken examples of a Terebratula near T. semzj;iobosa, and a large Rhynchonella. Above the platform caused by the Chalk-rock, the upward slope of the road is resumed, and the section becomes more interrupted, though still fairly continuous. At a point about 500 ft. above O.D., the first well-marked band of nodular flint appears.
* Mem.
Geo', Surv., p. 37, and Sketch-Map, p. vlti.
EXCURSION TO HENLEY AND WATLINGTON.
A little above that level a good exposure of Chalk occurs (3 on the section, Fig. 39). The only fossils found were sponges, chiefly branching forms encased in flint. The section is rendered important by the number and peculiarities of the tabular flints. A considerable time was spent in examining these, and some interesting features were noticed. For the most part these tabular flints seem to follow the bedding-planes, although, if the correspondence is exact, the Chalk must have been irregularly bedded in places. The comparatively horizontal layers of flint separate and converge irregularly, so that the amount of Chalk included between two "tabulars" may be over a foot in one place, and only a few inches in others. Other tabular flints are seen to pass obliquely, and even nearly vertically, along joints and sliding-planes. In a few cases it was noticed that a horizontal layer was continuous with one of the inclined "tabulars," but extended for a few inches horizontally beyond the point of intersection of the joint and bedding - plane. The whole appearance and relations of the tabular flints were wonderfully suggestive of the characters of intrusive igneous rocks. The horizontal and Inclined layers of flint are alike double In this section, the
251
I
,. 44 0
0 0
t: d
tin
HERBERT L. HAWKINS,
two halves enclosing a layer of wh~te powder of varying thickness. A very casual examination of this enclosed powder sufficed to show that it is largely composed of long sponge-spicules, and is very similar in appearance to the" flintmeal" enclosed in round flint-nodules. An interesting but difficult problem thus arose as to the means whereby these spicules could become aggregated along the very extensive lines of bedding and jointing. It was remarked that, in the case of nodular flint" meal," spicules of very different types of sponges are often found together, indicating a corresponding aggregation in that condition, but towards a point rather than on a plane. A suggestion was made by the writer that tabular flints might be analogous with mineral veins in other rocks, the "vein" being formed along planes of separation which, in this section, had as yet been only partially filled. The President added to this suggestion the further one that the abundant spicules might, for some reason at present unknown, become washed into the open joints, and so supply the silica for the formation of the flint. Apart from any theoretical suggestions, however, it was obvious to all present that the tabular flints were definitely subsequent to the consolidation of the Chalk in their formation, and in no part contemporaneous, in spite of the apparent interstratification. A short distance beyond the exposures of Chalk with tabular flint, the vegetation was seen to change to a very striking degree, elm and oak trees and bracken replacing the beeches and dog-mercury of the earlier part of the walk. This ecological change corresponds with the oncoming of the Clay-with-flints, although, from the fact that beech woods supervene later, on the same deposit, it seems possible that there may be a hidden outlier of tertiaries on this part of the ridge. A short halt was made at Hollandridge Farm, and some remarkable boulders of ferruginous conglomerate were examined. These are composed almost entirely of flint pebbles, not very well rounded, cemented by a brightly coloured matrix of sand with heematite and limonite. According to Mr. Osborne White (Mem. Geol. Survey, Sheet 254), the pebbles are probably derived from some part of the Reading Beds analogous with the shingles of Lane End and other localities. It was noted that here the pebbles are far less rounded than in the Lane End deposits. Most of the party agreed with the writer in regarding the cementation of the pebbles as having taken place in the Clay-with-flints, owing probably to the presence of an especially impervious layer at some place. All the surface deposits of the Chiltern ridge in this region are very ferruginous, and any conditions that prevented the free passage of water would give opportunities for the dissolved iron to be precipitated. The road from the farm to the crest of the escarpment at
EXCURSION TO HENLEY AND WATLINGTON.
253
Christmas Common passes through Queen Wood, and here and there the red colour of the Clay-with-flints shows through the vegetation. The climatic conditions at this stage were extremely trying, as warm mists were rising all around, with a tropical and exhausting result. However, the great beauty of the beech woods, which was somewhat enhanced by the mist, and the interest aroused by the primitive lathes used in the manufacture of chair legs, to some degree atoned for the discomfort involved in walking. After a halt for welcome refreshment at Christmas Common, the party descended the escarpment by the main road. The view was largely obscured by mist. The various small exposures by the roadside mentioned in the programme were almost overgrown, but Dr. Bassett succeeded in finding a typical example of Micraster c01-anguinum in the cutting marked 6 on the section, Fig. 39. This is the first certain record of this fossil from the immediate neighbourhood. The moderate inflation of the ambulacra seem to point to some region low down in the zone being represented at this point. The exposure of Chalk-rock near the 600 ft. contour (7 on the section) could not be found (it was clearly shown in May of this year), but some pieces of the rock were noticed. The large chalk-pit (8) near the foot of the hill (on the northern side) was next visited. There are two distinct faces exposed ID the pit, one well above the other. The members were soon able to find abundant specimens of Rhynchonella cuvieri (rather a small form), Terebratula, and almost globular examples of Conulus subrotundus, in the upper section. The pit is said, in the Survey Memoir, to be in the zone of Terebratulina, but the fauna seems to indicate the zone below. The lower section is chiefly remarkable for the enormous quantities of thinshelled Inocerami that occur there, their fragmentary shells often making up the bulk of the rock. Rhynchonella cuvieri, Terebratula, Conulus subrotundus (including one large globular form characteristic of the cuvieri zone), and a small Cardiaster were soon found here in considerable abundance. A short walk over the flat ground of the Lower Chalk brought the party to Watlington, where tea was taken at the" Hare and Hounds," opposite the quaint brick Market Hall. A vote of thanks to the Director was proposed by the President, and the iormer, in his reply, expressed his gratitude to Mr. Reader, who had undertaken the duties of Excursion Secretary at short notice. REFERENCES. Geological Survey Map, Sheet 3, r-inch scale; Sheet 254, New Series. 1906. JUKES-BROWNE, A. J.-"The Clay with Flints, its Origin and Distribution." Quart. yount. Geoi. Soc., vol, lxii, p. 132. 1908. JUKES-BROWNE, A. J., and WHITE, H. J. 0.-" Geology of Country around Henley-on-Thames," etc. Mem. CeDI. Surv.
L.
HAWKINS,
REPORT OF AN EXCURSION TO HENLEY AND WATLINGTON. SATURDAY, JULY 13TH,
1912.
By HERBERT L. HAWKINS, M.Sc., F.G.S., Director of the Excursion. PROBABLY owing to an unpropitious weather forecast(which was abundantly realised), only six members travelled by train toHenley for this excursion. Three others joined the party there, bringing the total number up to nine. The party drove up the Assendon Valley to Stonor, but the drive was considerably marred by persistent rain, accompanied by an abnormally high temperature. Leaving the brake at the foot of Pishill Bottom, the members followed the half-deserted ridge-road to Hollandridge. For the most part the road is in a cutting some 10 or more feet deep, affording an almost continuous section of Chalk for over a quarter of a mile. This first region (indicated on the section, Fig. 39, as I) is entirely in the zone of Terebratulina. The lower exposures show thin, flaky Chalk, very soft and free from flints, while higher up in the section the Chalk is seen to become more massive, though retaining its soft texture. The only fossils found were thin-shelled Inocerami, probably.l. mytiloides. This form is uniformly abundant throughout the length of the section. At about 440 ft O.D., the slope of the hill ceases, and a short but well-marked platform is developed. This feature coincides with the outcrop of the Chalk-rock, which was seen in the road-cutting and in a shallow excavation in the field to the south of the road (2 on the section). This exposure of the Rock is referred to by Mr. Osborne White in the" Geology of the Country around Henley," * and was discovered by him after the printing of the Survey map. The director pointed out how far away, both horizontally and vertically, from its position on the map the outcrop occurs in reality. The field-exposure showed characteristic Chalk-rock, yellowish and massive below, with greencoated nodules, often perforated by borings of about U in. diameter, in the upper part. The President found a good specimen of Terebratulina in the pit, but it was not in situ, although embedded in a hard matrix. The other fossils found were broken examples of a Terebratula near T. semzj;iobosa, and a large Rhynchonella. Above the platform caused by the Chalk-rock, the upward slope of the road is resumed, and the section becomes more interrupted, though still fairly continuous. At a point about 500 ft. above O.D., the first well-marked band of nodular flint appears.
* Mem.
Geo', Surv., p. 37, and Sketch-Map, p. vlti.
EXCURSION TO HENLEY AND WATLINGTON.
A little above that level a good exposure of Chalk occurs (3 on the section, Fig. 39). The only fossils found were sponges, chiefly branching forms encased in flint. The section is rendered important by the number and peculiarities of the tabular flints. A considerable time was spent in examining these, and some interesting features were noticed. For the most part these tabular flints seem to follow the bedding-planes, although, if the correspondence is exact, the Chalk must have been irregularly bedded in places. The comparatively horizontal layers of flint separate and converge irregularly, so that the amount of Chalk included between two "tabulars" may be over a foot in one place, and only a few inches in others. Other tabular flints are seen to pass obliquely, and even nearly vertically, along joints and sliding-planes. In a few cases it was noticed that a horizontal layer was continuous with one of the inclined "tabulars," but extended for a few inches horizontally beyond the point of intersection of the joint and bedding - plane. The whole appearance and relations of the tabular flints were wonderfully suggestive of the characters of intrusive igneous rocks. The horizontal and Inclined layers of flint are alike double In this section, the
251
I
,. 44 0
0 0
t: d
tin
HERBERT L. HAWKINS,
two halves enclosing a layer of wh~te powder of varying thickness. A very casual examination of this enclosed powder sufficed to show that it is largely composed of long sponge-spicules, and is very similar in appearance to the" flintmeal" enclosed in round flint-nodules. An interesting but difficult problem thus arose as to the means whereby these spicules could become aggregated along the very extensive lines of bedding and jointing. It was remarked that, in the case of nodular flint" meal," spicules of very different types of sponges are often found together, indicating a corresponding aggregation in that condition, but towards a point rather than on a plane. A suggestion was made by the writer that tabular flints might be analogous with mineral veins in other rocks, the "vein" being formed along planes of separation which, in this section, had as yet been only partially filled. The President added to this suggestion the further one that the abundant spicules might, for some reason at present unknown, become washed into the open joints, and so supply the silica for the formation of the flint. Apart from any theoretical suggestions, however, it was obvious to all present that the tabular flints were definitely subsequent to the consolidation of the Chalk in their formation, and in no part contemporaneous, in spite of the apparent interstratification. A short distance beyond the exposures of Chalk with tabular flint, the vegetation was seen to change to a very striking degree, elm and oak trees and bracken replacing the beeches and dog-mercury of the earlier part of the walk. This ecological change corresponds with the oncoming of the Clay-with-flints, although, from the fact that beech woods supervene later, on the same deposit, it seems possible that there may be a hidden outlier of tertiaries on this part of the ridge. A short halt was made at Hollandridge Farm, and some remarkable boulders of ferruginous conglomerate were examined. These are composed almost entirely of flint pebbles, not very well rounded, cemented by a brightly coloured matrix of sand with heematite and limonite. According to Mr. Osborne White (Mem. Geol. Survey, Sheet 254), the pebbles are probably derived from some part of the Reading Beds analogous with the shingles of Lane End and other localities. It was noted that here the pebbles are far less rounded than in the Lane End deposits. Most of the party agreed with the writer in regarding the cementation of the pebbles as having taken place in the Clay-with-flints, owing probably to the presence of an especially impervious layer at some place. All the surface deposits of the Chiltern ridge in this region are very ferruginous, and any conditions that prevented the free passage of water would give opportunities for the dissolved iron to be precipitated. The road from the farm to the crest of the escarpment at
EXCURSION TO HENLEY AND WATLINGTON.
253
Christmas Common passes through Queen Wood, and here and there the red colour of the Clay-with-flints shows through the vegetation. The climatic conditions at this stage were extremely trying, as warm mists were rising all around, with a tropical and exhausting result. However, the great beauty of the beech woods, which was somewhat enhanced by the mist, and the interest aroused by the primitive lathes used in the manufacture of chair legs, to some degree atoned for the discomfort involved in walking. After a halt for welcome refreshment at Christmas Common, the party descended the escarpment by the main road. The view was largely obscured by mist. The various small exposures by the roadside mentioned in the programme were almost overgrown, but Dr. Bassett succeeded in finding a typical example of Micraster c01-anguinum in the cutting marked 6 on the section, Fig. 39. This is the first certain record of this fossil from the immediate neighbourhood. The moderate inflation of the ambulacra seem to point to some region low down in the zone being represented at this point. The exposure of Chalk-rock near the 600 ft. contour (7 on the section) could not be found (it was clearly shown in May of this year), but some pieces of the rock were noticed. The large chalk-pit (8) near the foot of the hill (on the northern side) was next visited. There are two distinct faces exposed ID the pit, one well above the other. The members were soon able to find abundant specimens of Rhynchonella cuvieri (rather a small form), Terebratula, and almost globular examples of Conulus subrotundus, in the upper section. The pit is said, in the Survey Memoir, to be in the zone of Terebratulina, but the fauna seems to indicate the zone below. The lower section is chiefly remarkable for the enormous quantities of thinshelled Inocerami that occur there, their fragmentary shells often making up the bulk of the rock. Rhynchonella cuvieri, Terebratula, Conulus subrotundus (including one large globular form characteristic of the cuvieri zone), and a small Cardiaster were soon found here in considerable abundance. A short walk over the flat ground of the Lower Chalk brought the party to Watlington, where tea was taken at the" Hare and Hounds," opposite the quaint brick Market Hall. A vote of thanks to the Director was proposed by the President, and the iormer, in his reply, expressed his gratitude to Mr. Reader, who had undertaken the duties of Excursion Secretary at short notice. REFERENCES. Geological Survey Map, Sheet 3, r-inch scale; Sheet 254, New Series. 1906. JUKES-BROWNE, A. J.-"The Clay with Flints, its Origin and Distribution." Quart. yount. Geoi. Soc., vol, lxii, p. 132. 1908. JUKES-BROWNE, A. J., and WHITE, H. J. 0.-" Geology of Country around Henley-on-Thames," etc. Mem. CeDI. Surv.