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Report of an excursion to Soulbury, Stewkley, and Wing
REPORT OF AN EXCURSION TO SOULBURY, STEWKLEY, AND WING. SATURDAY, JULY 18TH, 1914.
By A.
MORLEY
DAVIES, D.Se., F.G.S., Director ojthe Excursion.
ARRIVING at Leighton Buzzard at IO.45 a.m., the party drove out to Soulbury. On the way the small escarpment of the Aptian (Lower Greensand) was noticed, with a spring at its foot, near the entrance to Liscombe Park. In Soul bury village a large erratic of Carboniferous Limestone was seen, but the first important halt was at Mr. Robinson's gravel-pit, nearly half-way between SoulHere was a fine section of the glacialbury and Stewkley. outwash gravels, composed of all the materials which characterise the Chalky Boulder-Clay of the district. The finer beds, though physically" sand," were composed to a very great extent of grains of chalk, and surface-water percolating through them had produced numerous brown" pipes" by decalcification. Among the coarser materials flint was most abundant, but chalk, Jurassic limestones (oolitic and other), gryphites and belemnites, Carboniferous limestone, chert, dolerite and other igneous rocks, and pink and white quartz were also found. The Director explained that they were then, and would be throughout the day, on that part of the Oxford clay-vale which formed part of the Tharnes-Ouse watershed. This alone, however, would not suffice to account for the hilly character of the land, which was, in fact, a small plateau dissected by rejuvenated streams, with valleys almost gorge-like in places. The explanation was that here we were on the southern margin of the great sheet of Chalky Boulder-Clay and its associated gravels, and the head waters of the Thame and Ouse were cutting down valleys through this drift to the underlying Jurassic clays. Driving on to Stewkley, the party halted in the village for lunch and a hastv examination of the beautiful N orman church. Then two brickfields in Kimmeridge Clay were visited to the south-west of the village. The first of these (Mr. Hedges') showed the Kimmeridge Clay thrown into folds, with axes running west-north-west to east-south-east. These folds the Director could only explain as of glacial origin, they were too local for tectonic folds, and they were disposed at right angles to the most probable direction of ice-flow at this point. A little Boulder-Clay was seen on the north-eastern side of the exposure; on the higher ground, on which the village stood, there was typical Chalky Boulder-Clay. As to the Kimmeridge Clay itself, it appeared to belong to a
9r
EXCURSION TO SOULBURY, STEWKLEY, AND WING.
rather low zone, as Exogyra virgula was fairly common, but not abundant. The ammonites and lamellibranchs were too much crushed for identification. There were, however, well-preserved bones of Ichthyosaurs and Plesiosaurs, and, in the case of the latter, it seemed evident that a complete skeleton had been partially dug up, the rest still being in the shale. A move was then made to Mr. Bliss's pit. Although little more than IS0 yds. away, it was evident that different beds were exhibited here, probably a much higher zone, the director thought, but the fossils were again too much crushed for satisfactory identification. Orbiculoidea latissima, however, had certainly been found here. Reptilian bones were absent. There was a band of large septaria with good cone-in-cone structure on their surface, and beautiful nail-head spar in the fissures. The party then drove to the Warren Farm, where they were most hospitably entertained by Mrs. and Miss Hedges. The classic section here displayed-the most northerly exposure of typical Portland and Purbeck beds-is now as follows : SECTION AT WARREN FARM, STEWKLEY.
ft. 14. 13· 12.
11. 10.
9. 8. 7· 6. 5. 4· 3. 2. 1.
in.
Soil, with materials derived from boulder-clay. 9 in. to I 0 Marl, white above, grey below, up to " . 2 4Jointed, fissile marly limestone (" Pendle " of Hartwell) with Ostracods and small Gastropods . . . . I 0 Brown shaley marl, with Ostren of expansa type 0 3 White chalky limestone with casts of Trigunill of r:tbbosa type. • . . . • . I 3 . . . . . . . • 0 6 Grey shaley clay White and yellow shaley marl, very shelly: Ostrea fragments, small Gastropods . . . 0 10 Hard grey limestone with Trigonia casts. 0 3 Shelly fissile marl, like No.8. • .. . 0 6 White limestone, very irregulariy jointed, with Trigonia and other fossils. (About I ft. 0 in. from top, a band of small fossils weathered out conspicuously on the surface) 3 8 Hard grey limestone . . . . . . . 0 9 Calcareous sandstone with casts of fossils, one lydite 0 9 pebble found . . • Fine sand, slightly glauconitic. . • 9 0 Limestone with large ammonites (exposed only in winter) 22
= Beds I Z and r3 are of Purbeck age, the rest Portlandian. The joint-planes and other fissures of the latter, up to No. 7 at least, are filled with a mixture of dark brown clay and white marl; but at what period this material was introduced is not clear at present. Many large ammonites from the lower stone-bed were seen in the porch of the house, and Mrs. Hedges showed a very beautiful specimen of, probably, Olcostephanus triplieatus (Blake).
92
EXCURSION TO SOULBURY, STEWKLEY, AND WING.
Returning to the village, a long drive next revealed the plateau character of the country, until Wing was approached. At the brick field at Littleworth, Wing, the party was met by the manager, Mr. Jones, whose account of the lie of the beds enabled them to reconcile the section actually seen with that described by earlier observers. The following sequence was verified down to the Gault : ft.
Coarse morainic gravels Chalky Boulder-Clay . Sands and finer gravels, about Boulder-Clay Gault . . . • . . [Basement-bed with phosphatic nodules.] [Kimmeridge clay.]
8
2t
10 3° 17
To the north, Mr. Jones explained, the lowest beds rose up, and the lower Boulder-Clay disappeared altogether, while on the other side of the brook they apparently dipped down again. Whether this anticlinal structure was due to glacial disturbance or not, it explained the absence of any mention of the lower Boulder-Clay by Green or Jukes-Browne. The road to Leighton Buzzard was now taken. Time did not allow of the slight detour to Southcott, where would have been seen the first coming in between Gault and Jurassic of the Aptian sands, which attain such thickness and prominence immediately on the other side of Leighton. After tea at the " Swan" the return journey was made by the 7.2 p.m, train. Mr. Douglas Leighton acted as Excursion Secretary. REFERENCES. Ordnance Survey Map. Sheet 220 (Leighton Buzzard). Geological Survey Map. O.S., Sheet 46, S.W. 1836. FITTON, W. H.-" Strata between Chalk and Oxford Oolite." Trans. Geol. Soc., ser, 2, vol. iv, pp. 291, 292. 1895. WOODWARD, U. B.-" Jurassic Rocks of Britain." Vol. v, pp. 169, 228. Mem. Geol. Survey. 1900. JUKES-BROWNE, A. J,-" Cretaceous Rocks of Britain." Vol. i, p. 278, Mem. Geol. Suroe«, 190I. DAVIES, A. M.-" Excursion to Leighton Buzzard, Wing, and Stewkley." Proc. Geo], Assoc., vol, xvii, pp. 139-14I. 1912. .-" Buckinghamshire." Cambridge Coun.ty Geographies, pp. 46, 48.
By A.
MORLEY
DAVIES, D.Se., F.G.S., Director ojthe Excursion.
ARRIVING at Leighton Buzzard at IO.45 a.m., the party drove out to Soulbury. On the way the small escarpment of the Aptian (Lower Greensand) was noticed, with a spring at its foot, near the entrance to Liscombe Park. In Soul bury village a large erratic of Carboniferous Limestone was seen, but the first important halt was at Mr. Robinson's gravel-pit, nearly half-way between SoulHere was a fine section of the glacialbury and Stewkley. outwash gravels, composed of all the materials which characterise the Chalky Boulder-Clay of the district. The finer beds, though physically" sand," were composed to a very great extent of grains of chalk, and surface-water percolating through them had produced numerous brown" pipes" by decalcification. Among the coarser materials flint was most abundant, but chalk, Jurassic limestones (oolitic and other), gryphites and belemnites, Carboniferous limestone, chert, dolerite and other igneous rocks, and pink and white quartz were also found. The Director explained that they were then, and would be throughout the day, on that part of the Oxford clay-vale which formed part of the Tharnes-Ouse watershed. This alone, however, would not suffice to account for the hilly character of the land, which was, in fact, a small plateau dissected by rejuvenated streams, with valleys almost gorge-like in places. The explanation was that here we were on the southern margin of the great sheet of Chalky Boulder-Clay and its associated gravels, and the head waters of the Thame and Ouse were cutting down valleys through this drift to the underlying Jurassic clays. Driving on to Stewkley, the party halted in the village for lunch and a hastv examination of the beautiful N orman church. Then two brickfields in Kimmeridge Clay were visited to the south-west of the village. The first of these (Mr. Hedges') showed the Kimmeridge Clay thrown into folds, with axes running west-north-west to east-south-east. These folds the Director could only explain as of glacial origin, they were too local for tectonic folds, and they were disposed at right angles to the most probable direction of ice-flow at this point. A little Boulder-Clay was seen on the north-eastern side of the exposure; on the higher ground, on which the village stood, there was typical Chalky Boulder-Clay. As to the Kimmeridge Clay itself, it appeared to belong to a
9r
EXCURSION TO SOULBURY, STEWKLEY, AND WING.
rather low zone, as Exogyra virgula was fairly common, but not abundant. The ammonites and lamellibranchs were too much crushed for identification. There were, however, well-preserved bones of Ichthyosaurs and Plesiosaurs, and, in the case of the latter, it seemed evident that a complete skeleton had been partially dug up, the rest still being in the shale. A move was then made to Mr. Bliss's pit. Although little more than IS0 yds. away, it was evident that different beds were exhibited here, probably a much higher zone, the director thought, but the fossils were again too much crushed for satisfactory identification. Orbiculoidea latissima, however, had certainly been found here. Reptilian bones were absent. There was a band of large septaria with good cone-in-cone structure on their surface, and beautiful nail-head spar in the fissures. The party then drove to the Warren Farm, where they were most hospitably entertained by Mrs. and Miss Hedges. The classic section here displayed-the most northerly exposure of typical Portland and Purbeck beds-is now as follows : SECTION AT WARREN FARM, STEWKLEY.
ft. 14. 13· 12.
11. 10.
9. 8. 7· 6. 5. 4· 3. 2. 1.
in.
Soil, with materials derived from boulder-clay. 9 in. to I 0 Marl, white above, grey below, up to " . 2 4Jointed, fissile marly limestone (" Pendle " of Hartwell) with Ostracods and small Gastropods . . . . I 0 Brown shaley marl, with Ostren of expansa type 0 3 White chalky limestone with casts of Trigunill of r:tbbosa type. • . . . • . I 3 . . . . . . . • 0 6 Grey shaley clay White and yellow shaley marl, very shelly: Ostrea fragments, small Gastropods . . . 0 10 Hard grey limestone with Trigonia casts. 0 3 Shelly fissile marl, like No.8. • .. . 0 6 White limestone, very irregulariy jointed, with Trigonia and other fossils. (About I ft. 0 in. from top, a band of small fossils weathered out conspicuously on the surface) 3 8 Hard grey limestone . . . . . . . 0 9 Calcareous sandstone with casts of fossils, one lydite 0 9 pebble found . . • Fine sand, slightly glauconitic. . • 9 0 Limestone with large ammonites (exposed only in winter) 22
= Beds I Z and r3 are of Purbeck age, the rest Portlandian. The joint-planes and other fissures of the latter, up to No. 7 at least, are filled with a mixture of dark brown clay and white marl; but at what period this material was introduced is not clear at present. Many large ammonites from the lower stone-bed were seen in the porch of the house, and Mrs. Hedges showed a very beautiful specimen of, probably, Olcostephanus triplieatus (Blake).
92
EXCURSION TO SOULBURY, STEWKLEY, AND WING.
Returning to the village, a long drive next revealed the plateau character of the country, until Wing was approached. At the brick field at Littleworth, Wing, the party was met by the manager, Mr. Jones, whose account of the lie of the beds enabled them to reconcile the section actually seen with that described by earlier observers. The following sequence was verified down to the Gault : ft.
Coarse morainic gravels Chalky Boulder-Clay . Sands and finer gravels, about Boulder-Clay Gault . . . • . . [Basement-bed with phosphatic nodules.] [Kimmeridge clay.]
8
2t
10 3° 17
To the north, Mr. Jones explained, the lowest beds rose up, and the lower Boulder-Clay disappeared altogether, while on the other side of the brook they apparently dipped down again. Whether this anticlinal structure was due to glacial disturbance or not, it explained the absence of any mention of the lower Boulder-Clay by Green or Jukes-Browne. The road to Leighton Buzzard was now taken. Time did not allow of the slight detour to Southcott, where would have been seen the first coming in between Gault and Jurassic of the Aptian sands, which attain such thickness and prominence immediately on the other side of Leighton. After tea at the " Swan" the return journey was made by the 7.2 p.m, train. Mr. Douglas Leighton acted as Excursion Secretary. REFERENCES. Ordnance Survey Map. Sheet 220 (Leighton Buzzard). Geological Survey Map. O.S., Sheet 46, S.W. 1836. FITTON, W. H.-" Strata between Chalk and Oxford Oolite." Trans. Geol. Soc., ser, 2, vol. iv, pp. 291, 292. 1895. WOODWARD, U. B.-" Jurassic Rocks of Britain." Vol. v, pp. 169, 228. Mem. Geol. Survey. 1900. JUKES-BROWNE, A. J,-" Cretaceous Rocks of Britain." Vol. i, p. 278, Mem. Geol. Suroe«, 190I. DAVIES, A. M.-" Excursion to Leighton Buzzard, Wing, and Stewkley." Proc. Geo], Assoc., vol, xvii, pp. 139-14I. 1912. .-" Buckinghamshire." Cambridge Coun.ty Geographies, pp. 46, 48.