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An examination of Otmoor and the adjacent areas of the Oxford Clay Lowland
An Examination of Otmoor and the Adjacent Areas of the Oxford Clay Lowland by M. E. MARKER and A. D. COOPER Received 23 September 1958 CONTENTS 1. 2. 3. 4. 5. 6.
INTRODUCTION HISTORICAL EVIDENCE LEVELLING .•• AUGERING ..• MECHANICAL ANALYSIS OF SOIL SAMPLES CONCLUSIONS ACKNOWLEDGMENTS REFERENCES
page 41
43 44 44 45 46
47 47
ABSTRACT: The Oxford Clay lowland, east of Oxford, is drained by the River Ray, a tributary of the Cherwell. The river flows across two distinct basins, an upper basin near Marsh Gibbon and a lower basin known as Otmoor. The paper describes the results of some examinations of the basins, conducted in an attempt to find and map the alluvium which is generally thought to exist on Otmoor. Various methods were used, historical, topographical and geological, in the study. The principal conclusion reached is that Otmoor is not an alluvial basin but a normal clay lowland that suffers from poor drainage. The Ray has not always flowed through Otmoor and its development has probably gone on independently of events affecting the river.
1. INTRODUCTION NORTH-EAST of Oxford, the Oxford Clay lowland extends for some twelve miles before reaching the edge of the glacial drift deposits, which forms a marked boundary. It is drained by the River Ray, a Cherwell tributary. The Ray flows across two distinct basins, separated by a constriction caused by the northward outlier of the Corallian; Arncott Hill, and the small ridge at Ambrosden. The lower basin, Otmoor, is two miles above the RayCherwell confluence at Islip; the upper basin is around Marsh Gibbon. The research behind this paper has been carried out principally on Otmoor but reference has also been made to the upper basin. Otmoor lies some seven and a half miles from Oxford but, despite its proximity to the city, it has been a blank in the otherwise fairly welldocumented geology and geography of the Oxford region. This may be attributed to the wet conditions which persist there through much of the year, and to its use from 1933 to 1958 as a bombing range by the Royal Air Force. Otmoor is regarded locally as being rather remote, primitive 41
42
M. E. MARKER AND A. D. COOPER
and mysterious, contrasting with the life of its surrounding area. The late Dr. W. J. Arkell (1947, 247) described it as an outstanding occurrence of old alluvial deposits, peats and humus which offered a rich field for future research. Otmoor has an area of some four square miles and lies across the lower course of the Ray about a mile upstream from the gorge at Islip, where the stream cuts through the Cornbrash ridge to join the Cherwell. The moor is developed on Oxford Clay and lies between 200 and 193 feet O.D. It is surrounded on three sides by higher ground but is open on the east and north-east, where it is entered by the River Ray. The hills to the south and west are composed of Calcareous Grit, Coral Rag and Arngrove Stone, with patches of Kimmeridge Clay. These western hills belong to the Wheatley fault-zone and their structure has been described by Arkell (1942, 199-203). The northern boundary is formed by low periclines of Cornbrash and Kellaways beds. These periclines rise to only slightly over 210 feet O.D. but form a distinct boundary to the moor. They are continued in the upper Ray basin but do not form the limit of the area. The upper Ray basin is bounded by the Oolitic rocks to the north and upper Jurassic beds to the south. The eastern boundary is the Boulder Clay. The earlier Geological Survey maps showed Otmoor as an area of Oxford Clay, but the later maps, which include downwash and local material as 'alluvium', show it as a considerable expanse of this material. Arkell accepted this but modified the Survey's boundaries (Arkell, 1944, map). The principal object of our work has been to find this alluvium, its sources, and to try to map its extent; to examine the adjacent parts of the Oxford Clay vale and the drainage system in general. By so doing we hoped to be able to link our results with those of workers in the upper Thames area. This paper is not the summary of a completed task so much as a progress report. The River Ray has a total length of some fourteen miles. The overall gradient of the stream varies with its tributaries. The main stream from Grendon Underwood has a fall of some sixty feet. The tributary from Stratton Audley, which joins the river at Fencott, falls eighty feet, but those coming from the edge of the boulder clay fall only some forty feet from their source to the junction of the Ray with the Cherwell. In all cases the major part of the fall occurs in the upper part of the course. The majority of the Ray tributaries converge in the Marsh Gibbon basin. This basin is similar to Otmoor, but has more pronounced relief. The Ray leaves its upper basin at 203 feet O.D. and enters Otmoor, six miles downstream, at 198 feet O.D. Erosion is concentrated in the upper part of the system. Below Three Points most of the stream's power is absorbed in the transportation of material and the mechanical factors of flow. Any alluvial material encountered in the basin, except for local washdown, may be
KEY CARLI ER ••
RO"'AN
COURSE
22
LI N E 5
SA"'PLE
- - CONTOURS
To face p. 43J
R.R,AY
ROAO
s u FlVEYEO SOIL
OF
5 IT E S 50 FT.
AT
INTERVAL
THE OXFORD CLAY LOWLAND, OTMOOR
43
expected to be derived from the Oolites, the Corallian, the Oxford Clay, or from drift-derived flints, quartzites and other erratics. The passage of meltwater through the Ray system has not yet been proved (Larnpugh, 1920, lxxiv). Various theories have been advanced as to the origin of Otmoor, some more plausible than others. One heard , but not read, was that it was a meteorite crater! The general theory suggests that it has developed by the ponding back of the Ray, possibly as a lake, above its little gorge at Islip, and the gap at Oddington (Arkell, 1947, 225). This ponding back has been attributed to Cherwell flood-waters and their deposits (Beckinsale, 1954, 31). We have approached the problem in four ways. Firstly, historical evidence and aerial photographs were stud ied. Secondly, some levelling was carried out in conjunction with a study of bench-marks, both on and around Otmoor. Thirdly, soil augers were used on the moor and in other parts of the basin to determine the nature of the soil and subsoil. Fourthly, mechanical analysis of selected soil samples was made. 2. mSTORICAL EVIDENCE
The Roman Road from Alchester to Dorchester (Oxon) crosses Otmoor between Beckley and Fencott (Fig. 1). It is built up on a causeway of Corallian rubble with a surface dressing of finer material. It is in a fair state of preservation and has not been extensively rebuilt. That it is still clearly defined contrasts strongly with the Saxon causeway built at Binsey on Thames Alluvium which now lies six feet below the general level of the land (Hussey, 1841). North of Fencott the line of the road is less distinct, particularly where it crosses the area of alluvium derived from the Oolites, near Merton. The drainage of Otmoor was much modified in the 1820s by attempts to improve the moor (Marshall, 1944,215). The work was carried out following the reports by ]. Davis (1795) and Arthur Young (1809) on the state of agriculture in the county . Both drew attention to the need to drain Otmoor, which, they thought, would provide good pasture land, if it were properly enclosed. Young said: 'I searched and inquired for bog and peat, but have reason to believe that there is no such thing upon it [Otmoor), or if there be, in very small proportion. To the eye it is level, but there is inequality enough to make waters draw down to one part more than the rest, so that it is very wet in October' (Young, 1805,227). Despite the drainage works which give the moor its striking appearance on the topographical map, the wet conditions persist today, probably because the drainage plan was never completed. The New Ray, which was dug along the north-west edge of the moor, carries water collected by the
44
M. E. MARKER AND A. D. COOPER
upper stream. The moor itself is drained by the remnants of the Old Ray and a system of artificial ditches. Aerial photographs suggest that Otmoor was drained by a fan-like system converging on the gap in the Cornbrash at Oddington and linking with the Old Ray.
3. LEVELLING Representative sections of the slopes bounding the moor were levelled and profiles drawn. They showed no traces of a bench or series of benches which might have been associated with a lake. The composite section is shown in Fig. 2. The study of the distribution of Ordnance Survey bench-marks gives the impression that Otmoor may be lowest in the south-west. The majority of these bench-marks are on gate-posts leading off roads which surround the moor and which are embanked above the general level. The normal drainage slope is towards the line of the Old Ray.
4. AUGERING Systematic examination of the moor surface and other parts of the clay basin was carried out by augering. Three-foot and five-foot corkscrew-type augers were used; these were adequate to penetrate the gley horizon and to determine if alluvium were present. The majority of auger profiles on Otmoor have been very similar. The top four to nine inches, below a humic surface horizon, is a brown loam; this is underlain by brown or yellow mottled clay which passes down into grey clay. In some cases the clay has a platy structure and there are occasional bands of calcium carbonate. TABLE
I. A Typical Otmoor Soil Profile
Depth in inches 0--6
6-15 15-24 24-30+
HORTON (42{579142) Description Dark brown peaty loam Yellow brown clay Yellow-grey mottled clay Dark grey clay.
Generally, soil colour changes occur within a few inches and in some cases there is distinct banding. Horizon differentiation was not possible owing to the compaction which occurs with a corkscrew auger. A certain amount of downwashed material from the Calcareous Grit was encountered near Noke. Some sand and grit was found in an old stream channel but another similar channel was choked by a clay with a
, 220
N.A.
NO I( E
'~
LIN E
N.L.
NO K E
'a'
LIN £
O.C.
0 DOl N G TON
O.L.
0 D DIN G TON
O.R.
ODDINGTON
C H U Po C H
LIN E
O.R.:---_:::·---_~
210'
W.G.
WHI
LIN E LINE
TEC~OSS
(REVERSE)
GREEN
LINE
, 200
N.L.
O.L.
O.C.
190'
180'
0 0 0
"
0 0
0 0
Il)
"'
HORIZONTAL VERTICAL
DRAWl NG
0 0
'"
SCALE
SCALE DATU~
0 0
/-
N
100' OR
I "_ I 0 ' O R
180
FEET
0 0
0
0 0
0 0
0 0
0 0
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.$urvcycd
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0
lit
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0 0
.0
P I 0 tt c
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0 0
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J. C. A L ~ GIL L
I: I 2 0 Fig. 2. Composite section of the edge of the Otmoor basin
[To face p. 44
THE OXFORD CLAY LOWLAND, OTMOOR
4S
high vegetable matter content. A sample of this was ignited and the material lost represented 40 % of the sample dry weight. Otmoor is almost entirely covered by clay-derived soils. The high vegetable matter content may be attributed to the damp conditions which stimulate plant growth and retard its decomposition. We have found no peat as such, but we have seen substances which, if dried, might provide fuel. The Woodeaton and Holton Brook gaps leading out of Otmoor have been examined. They have been regarded as the result of spring sapping (Beckinsale, 1954,32), and theories that they were overflow channels discounted (Sandford, 1926, 144-5). This method of formation is acceptable for the Holton Brook gap, but auger holes and a trench in the Woodeaton gap revealed water-worn gravels containing flints and quartzite pebbles, with some material apparently slumped from the valley sides. This would suggest that the gap was used by a stream heading in the east of the clay vale. Between Fencott and Merton there are extensive areas of more sandy material which is probably Ray alluvium. The hamlets of Fencott and Murcott stand on this better drained material overlying the Oxford Clay. Alluvium is also encountered in the broad valley to the north of Charlton and Oddington, between the periclines and the main Combrash outcrop, and links with the Gallows Brook gravels near Islip (Sandford, 1927, 142). Associated with this alluvium is a discontinuous gravel terrace at about 205 feet which runs back from Oddington to Merton. The possibility of the diversion of the Ray near Merton, by the more strongly flowing Stratton Audley tributary stream, southwards between the Charlton and Street Hill periclines along the line of a transverse fault or other weakness, has been considered. The small terrace is only slightly above flood-plain level, but diversion appears to have occurred subsequently to its formation. Thus it is probable that the diversion of the main Ray drainage through Otmoor is a comparatively recent feature. Some parts of the upper Ray basin have been examined. The auger holes show that there is a considerable amount of silt and sand intermixed with yellow clay. The soil profiles differ noticeably from those of Otmoor in colouring, texture and vegetable matter content. The upper and lower Ray basins appear to differ importantly. 5. MECHANICAL ANALYSIS OF SOIL SAMPLES
The mechanical analyses of soil samples from Otmoor and Marsh Gibbon were done by the Piper method. The results were compared with two samples of Oxford Clay derived soils (Table II). The location of the sample sites is shown in Fig. 1. The results show a close similarity between the size range of Otmoor and Oxford Clay soils, allowingfor the differences
M. E. MARKER AND A. D. COOPER
46
TABLE
II
Otmoor 1.
HORTON
2.
BECKLEY
3.
4.
(42/579142) Per cent sample Greater than 0.104 mm. 0.53 0.104 mm.--o.053 mm. 0.33 0.053 mm.--o.020 mm. 5.53 0.020 mm.--o.002 mm. 4.49 Less than 0.002 mm. 85.94 Vegetable matter 3.18%
(42/573131) Greater than 0.25 mm. 0.25 mm.--o.104 mm. 0.104 mm.--o.053 mm. 0.053 mm.--o.020 mm. 0.020 mm.--o.OO2 mm. Less than 0.002 mm.
Marsh Gibbon Basin HEET FARM (42/652212) Greater than 0.25 mm. 0.25 mm.--o.104 mm. 0.104 mm.--o.053 mm. 0.053 mm.--o.020 mm. 0.020 mm.--o.002 mm. Less than 0.002 mm. (42/634215) Greater than 0.25 mm. 0.25 mm.--o.104 mm. 0.104 mm.--o.053 mm, 0.053 mm.--o.020 mm. 0.020 mm.--o.OO2 mm. Less than 0.002 mm.
1.00 This sample may be slightly biased by its 2.00 proximity to Roman Road 0.80 3.50 2.75 80.00 Vegetable matter 9.75 %
4.50 3.70 5.60 18.65 20.32 47.15 Vegetable matter 10%
CUTTERS BRIDGE
13.7 11.6 7.5 1.7 1.2 64.6
in vegetable matter content. They also show the differences between the composition of the soils in the upper and lower Ray basins. The clay components in all samples were built up similarly. 6. CONCLUSIONS Otmoor has been considered an alluvial basin. The results given above do not confirm this conclusion. They show that Otmoor soils are more akin to Oxford Clay than those derived from alluvial matter. If it were an alluvial basin it would be reasonable to expect a higher proportion of coarse material. Otmoor appears to be part of the Oxford Clay lowland which has developed, under periglacial and fluvial conditions, as a normal clay plain. It had its own fan-like system of drainage until the nineteenth century. This system appears to have linked with the earlier Ray between the addington gap and the Islip gorge. The actual level of Otmoor may have been slightly affected by earth movements along the Wheatley fault which have gone on
THE OXFORD CLAY LOWLAND, OTMOOR
47
during recent historic times (e.g. Wallis, 1665-6, Phil. Trans. No. 10, p.166). Any ponding back of the Ray drainage by Cherwell flood-waters would be most likely to affect the river immediately above the gorge at Islip. Here lies the expanse of Gallows Brook gravel (Sandford, 1926, 142) and the patch of 'Ray gravel' (ArkelI, 1944, 63). The Ray terrace and the associated alluvium lying to the north of the periclines appear to indicate that Otmoor was probably never a lake basin, or similar deposits could be expected on it. It is probable that Otmoor was never more of a marsh than other parts of the Oxford Clay lowland. The present backward state is the result of poor drainage by an incomplete system of dykes. Young was right in thinking that if it were properly drained it would be more productive. Parts of the moor are cropped for hay but much of it is in rough grazing. ACKNOWLEDGMENTS We would like to thank all those who have assisted our work, in particular the local farmers and landowners, also Prof. E. W. Gilbert, Dr. K. S. Sandford, Dr. M. M. Sweeting, Mr. D. F. Baden-Powell, Mr. R. J. Chorley, Mr. P. H. T. Beckett and Mr. J. C. Almgill. REFERENCES ARKELL, W. J. 1942, 4. Stratigraphy and Structures East of Oxford. Quart. J. geol. Soc. Lond.,98, 187; 100,45,61. ---,.1947. Geology of Oxford, Oxford. BECKINSALE, R. P. 1954. In the Oxford Region. Oxford. DAVIS, J. 1795. View of the Agriculture of Oxfordshire, HAMMOND, J. L. & B. The Village Labourer (deals with the period of enclosure and drainage of Otmoor). HUSSEY, T. 1841. The Roman Road/rom Alchester to Dorchester. LAMPUGH, G. W. 1920. Some Features of the Pleistone Glaciation. Quart. J. geol. Soc. Lond.,76. MARSHALL, M. 1944. Lond 0/ Britain. Part 56. PIPER, C. S. 1938. Soil and Plant Analysis. 47-79. SANDFORD, K. S. 1926. In the Memoir of the Oxford Special Sheet. Ed. T. I. Pocock. Mem. geol. Surv. U.K. YOUNG, A. 1809. The Agriculture of Oxford.
Miss M. E. Marker D. Cooper School of Geography Oxford
INTRODUCTION HISTORICAL EVIDENCE LEVELLING .•• AUGERING ..• MECHANICAL ANALYSIS OF SOIL SAMPLES CONCLUSIONS ACKNOWLEDGMENTS REFERENCES
page 41
43 44 44 45 46
47 47
ABSTRACT: The Oxford Clay lowland, east of Oxford, is drained by the River Ray, a tributary of the Cherwell. The river flows across two distinct basins, an upper basin near Marsh Gibbon and a lower basin known as Otmoor. The paper describes the results of some examinations of the basins, conducted in an attempt to find and map the alluvium which is generally thought to exist on Otmoor. Various methods were used, historical, topographical and geological, in the study. The principal conclusion reached is that Otmoor is not an alluvial basin but a normal clay lowland that suffers from poor drainage. The Ray has not always flowed through Otmoor and its development has probably gone on independently of events affecting the river.
1. INTRODUCTION NORTH-EAST of Oxford, the Oxford Clay lowland extends for some twelve miles before reaching the edge of the glacial drift deposits, which forms a marked boundary. It is drained by the River Ray, a Cherwell tributary. The Ray flows across two distinct basins, separated by a constriction caused by the northward outlier of the Corallian; Arncott Hill, and the small ridge at Ambrosden. The lower basin, Otmoor, is two miles above the RayCherwell confluence at Islip; the upper basin is around Marsh Gibbon. The research behind this paper has been carried out principally on Otmoor but reference has also been made to the upper basin. Otmoor lies some seven and a half miles from Oxford but, despite its proximity to the city, it has been a blank in the otherwise fairly welldocumented geology and geography of the Oxford region. This may be attributed to the wet conditions which persist there through much of the year, and to its use from 1933 to 1958 as a bombing range by the Royal Air Force. Otmoor is regarded locally as being rather remote, primitive 41
42
M. E. MARKER AND A. D. COOPER
and mysterious, contrasting with the life of its surrounding area. The late Dr. W. J. Arkell (1947, 247) described it as an outstanding occurrence of old alluvial deposits, peats and humus which offered a rich field for future research. Otmoor has an area of some four square miles and lies across the lower course of the Ray about a mile upstream from the gorge at Islip, where the stream cuts through the Cornbrash ridge to join the Cherwell. The moor is developed on Oxford Clay and lies between 200 and 193 feet O.D. It is surrounded on three sides by higher ground but is open on the east and north-east, where it is entered by the River Ray. The hills to the south and west are composed of Calcareous Grit, Coral Rag and Arngrove Stone, with patches of Kimmeridge Clay. These western hills belong to the Wheatley fault-zone and their structure has been described by Arkell (1942, 199-203). The northern boundary is formed by low periclines of Cornbrash and Kellaways beds. These periclines rise to only slightly over 210 feet O.D. but form a distinct boundary to the moor. They are continued in the upper Ray basin but do not form the limit of the area. The upper Ray basin is bounded by the Oolitic rocks to the north and upper Jurassic beds to the south. The eastern boundary is the Boulder Clay. The earlier Geological Survey maps showed Otmoor as an area of Oxford Clay, but the later maps, which include downwash and local material as 'alluvium', show it as a considerable expanse of this material. Arkell accepted this but modified the Survey's boundaries (Arkell, 1944, map). The principal object of our work has been to find this alluvium, its sources, and to try to map its extent; to examine the adjacent parts of the Oxford Clay vale and the drainage system in general. By so doing we hoped to be able to link our results with those of workers in the upper Thames area. This paper is not the summary of a completed task so much as a progress report. The River Ray has a total length of some fourteen miles. The overall gradient of the stream varies with its tributaries. The main stream from Grendon Underwood has a fall of some sixty feet. The tributary from Stratton Audley, which joins the river at Fencott, falls eighty feet, but those coming from the edge of the boulder clay fall only some forty feet from their source to the junction of the Ray with the Cherwell. In all cases the major part of the fall occurs in the upper part of the course. The majority of the Ray tributaries converge in the Marsh Gibbon basin. This basin is similar to Otmoor, but has more pronounced relief. The Ray leaves its upper basin at 203 feet O.D. and enters Otmoor, six miles downstream, at 198 feet O.D. Erosion is concentrated in the upper part of the system. Below Three Points most of the stream's power is absorbed in the transportation of material and the mechanical factors of flow. Any alluvial material encountered in the basin, except for local washdown, may be
KEY CARLI ER ••
RO"'AN
COURSE
22
LI N E 5
SA"'PLE
- - CONTOURS
To face p. 43J
R.R,AY
ROAO
s u FlVEYEO SOIL
OF
5 IT E S 50 FT.
AT
INTERVAL
THE OXFORD CLAY LOWLAND, OTMOOR
43
expected to be derived from the Oolites, the Corallian, the Oxford Clay, or from drift-derived flints, quartzites and other erratics. The passage of meltwater through the Ray system has not yet been proved (Larnpugh, 1920, lxxiv). Various theories have been advanced as to the origin of Otmoor, some more plausible than others. One heard , but not read, was that it was a meteorite crater! The general theory suggests that it has developed by the ponding back of the Ray, possibly as a lake, above its little gorge at Islip, and the gap at Oddington (Arkell, 1947, 225). This ponding back has been attributed to Cherwell flood-waters and their deposits (Beckinsale, 1954, 31). We have approached the problem in four ways. Firstly, historical evidence and aerial photographs were stud ied. Secondly, some levelling was carried out in conjunction with a study of bench-marks, both on and around Otmoor. Thirdly, soil augers were used on the moor and in other parts of the basin to determine the nature of the soil and subsoil. Fourthly, mechanical analysis of selected soil samples was made. 2. mSTORICAL EVIDENCE
The Roman Road from Alchester to Dorchester (Oxon) crosses Otmoor between Beckley and Fencott (Fig. 1). It is built up on a causeway of Corallian rubble with a surface dressing of finer material. It is in a fair state of preservation and has not been extensively rebuilt. That it is still clearly defined contrasts strongly with the Saxon causeway built at Binsey on Thames Alluvium which now lies six feet below the general level of the land (Hussey, 1841). North of Fencott the line of the road is less distinct, particularly where it crosses the area of alluvium derived from the Oolites, near Merton. The drainage of Otmoor was much modified in the 1820s by attempts to improve the moor (Marshall, 1944,215). The work was carried out following the reports by ]. Davis (1795) and Arthur Young (1809) on the state of agriculture in the county . Both drew attention to the need to drain Otmoor, which, they thought, would provide good pasture land, if it were properly enclosed. Young said: 'I searched and inquired for bog and peat, but have reason to believe that there is no such thing upon it [Otmoor), or if there be, in very small proportion. To the eye it is level, but there is inequality enough to make waters draw down to one part more than the rest, so that it is very wet in October' (Young, 1805,227). Despite the drainage works which give the moor its striking appearance on the topographical map, the wet conditions persist today, probably because the drainage plan was never completed. The New Ray, which was dug along the north-west edge of the moor, carries water collected by the
44
M. E. MARKER AND A. D. COOPER
upper stream. The moor itself is drained by the remnants of the Old Ray and a system of artificial ditches. Aerial photographs suggest that Otmoor was drained by a fan-like system converging on the gap in the Cornbrash at Oddington and linking with the Old Ray.
3. LEVELLING Representative sections of the slopes bounding the moor were levelled and profiles drawn. They showed no traces of a bench or series of benches which might have been associated with a lake. The composite section is shown in Fig. 2. The study of the distribution of Ordnance Survey bench-marks gives the impression that Otmoor may be lowest in the south-west. The majority of these bench-marks are on gate-posts leading off roads which surround the moor and which are embanked above the general level. The normal drainage slope is towards the line of the Old Ray.
4. AUGERING Systematic examination of the moor surface and other parts of the clay basin was carried out by augering. Three-foot and five-foot corkscrew-type augers were used; these were adequate to penetrate the gley horizon and to determine if alluvium were present. The majority of auger profiles on Otmoor have been very similar. The top four to nine inches, below a humic surface horizon, is a brown loam; this is underlain by brown or yellow mottled clay which passes down into grey clay. In some cases the clay has a platy structure and there are occasional bands of calcium carbonate. TABLE
I. A Typical Otmoor Soil Profile
Depth in inches 0--6
6-15 15-24 24-30+
HORTON (42{579142) Description Dark brown peaty loam Yellow brown clay Yellow-grey mottled clay Dark grey clay.
Generally, soil colour changes occur within a few inches and in some cases there is distinct banding. Horizon differentiation was not possible owing to the compaction which occurs with a corkscrew auger. A certain amount of downwashed material from the Calcareous Grit was encountered near Noke. Some sand and grit was found in an old stream channel but another similar channel was choked by a clay with a
, 220
N.A.
NO I( E
'~
LIN E
N.L.
NO K E
'a'
LIN £
O.C.
0 DOl N G TON
O.L.
0 D DIN G TON
O.R.
ODDINGTON
C H U Po C H
LIN E
O.R.:---_:::·---_~
210'
W.G.
WHI
LIN E LINE
TEC~OSS
(REVERSE)
GREEN
LINE
, 200
N.L.
O.L.
O.C.
190'
180'
0 0 0
"
0 0
0 0
Il)
"'
HORIZONTAL VERTICAL
DRAWl NG
0 0
'"
SCALE
SCALE DATU~
0 0
/-
N
100' OR
I "_ I 0 ' O R
180
FEET
0 0
0
0 0
0 0
0 0
0 0
"
III
'l
I: 1200
.$urvcycd
0
0
lit
anet
0 0 r-
0 0
.0
P I 0 tt c
a .. y
0 0
'"
0 0 0-
0 1tI
0-
J. C. A L ~ GIL L
I: I 2 0 Fig. 2. Composite section of the edge of the Otmoor basin
[To face p. 44
THE OXFORD CLAY LOWLAND, OTMOOR
4S
high vegetable matter content. A sample of this was ignited and the material lost represented 40 % of the sample dry weight. Otmoor is almost entirely covered by clay-derived soils. The high vegetable matter content may be attributed to the damp conditions which stimulate plant growth and retard its decomposition. We have found no peat as such, but we have seen substances which, if dried, might provide fuel. The Woodeaton and Holton Brook gaps leading out of Otmoor have been examined. They have been regarded as the result of spring sapping (Beckinsale, 1954,32), and theories that they were overflow channels discounted (Sandford, 1926, 144-5). This method of formation is acceptable for the Holton Brook gap, but auger holes and a trench in the Woodeaton gap revealed water-worn gravels containing flints and quartzite pebbles, with some material apparently slumped from the valley sides. This would suggest that the gap was used by a stream heading in the east of the clay vale. Between Fencott and Merton there are extensive areas of more sandy material which is probably Ray alluvium. The hamlets of Fencott and Murcott stand on this better drained material overlying the Oxford Clay. Alluvium is also encountered in the broad valley to the north of Charlton and Oddington, between the periclines and the main Combrash outcrop, and links with the Gallows Brook gravels near Islip (Sandford, 1927, 142). Associated with this alluvium is a discontinuous gravel terrace at about 205 feet which runs back from Oddington to Merton. The possibility of the diversion of the Ray near Merton, by the more strongly flowing Stratton Audley tributary stream, southwards between the Charlton and Street Hill periclines along the line of a transverse fault or other weakness, has been considered. The small terrace is only slightly above flood-plain level, but diversion appears to have occurred subsequently to its formation. Thus it is probable that the diversion of the main Ray drainage through Otmoor is a comparatively recent feature. Some parts of the upper Ray basin have been examined. The auger holes show that there is a considerable amount of silt and sand intermixed with yellow clay. The soil profiles differ noticeably from those of Otmoor in colouring, texture and vegetable matter content. The upper and lower Ray basins appear to differ importantly. 5. MECHANICAL ANALYSIS OF SOIL SAMPLES
The mechanical analyses of soil samples from Otmoor and Marsh Gibbon were done by the Piper method. The results were compared with two samples of Oxford Clay derived soils (Table II). The location of the sample sites is shown in Fig. 1. The results show a close similarity between the size range of Otmoor and Oxford Clay soils, allowingfor the differences
M. E. MARKER AND A. D. COOPER
46
TABLE
II
Otmoor 1.
HORTON
2.
BECKLEY
3.
4.
(42/579142) Per cent sample Greater than 0.104 mm. 0.53 0.104 mm.--o.053 mm. 0.33 0.053 mm.--o.020 mm. 5.53 0.020 mm.--o.002 mm. 4.49 Less than 0.002 mm. 85.94 Vegetable matter 3.18%
(42/573131) Greater than 0.25 mm. 0.25 mm.--o.104 mm. 0.104 mm.--o.053 mm. 0.053 mm.--o.020 mm. 0.020 mm.--o.OO2 mm. Less than 0.002 mm.
Marsh Gibbon Basin HEET FARM (42/652212) Greater than 0.25 mm. 0.25 mm.--o.104 mm. 0.104 mm.--o.053 mm. 0.053 mm.--o.020 mm. 0.020 mm.--o.002 mm. Less than 0.002 mm. (42/634215) Greater than 0.25 mm. 0.25 mm.--o.104 mm. 0.104 mm.--o.053 mm, 0.053 mm.--o.020 mm. 0.020 mm.--o.OO2 mm. Less than 0.002 mm.
1.00 This sample may be slightly biased by its 2.00 proximity to Roman Road 0.80 3.50 2.75 80.00 Vegetable matter 9.75 %
4.50 3.70 5.60 18.65 20.32 47.15 Vegetable matter 10%
CUTTERS BRIDGE
13.7 11.6 7.5 1.7 1.2 64.6
in vegetable matter content. They also show the differences between the composition of the soils in the upper and lower Ray basins. The clay components in all samples were built up similarly. 6. CONCLUSIONS Otmoor has been considered an alluvial basin. The results given above do not confirm this conclusion. They show that Otmoor soils are more akin to Oxford Clay than those derived from alluvial matter. If it were an alluvial basin it would be reasonable to expect a higher proportion of coarse material. Otmoor appears to be part of the Oxford Clay lowland which has developed, under periglacial and fluvial conditions, as a normal clay plain. It had its own fan-like system of drainage until the nineteenth century. This system appears to have linked with the earlier Ray between the addington gap and the Islip gorge. The actual level of Otmoor may have been slightly affected by earth movements along the Wheatley fault which have gone on
THE OXFORD CLAY LOWLAND, OTMOOR
47
during recent historic times (e.g. Wallis, 1665-6, Phil. Trans. No. 10, p.166). Any ponding back of the Ray drainage by Cherwell flood-waters would be most likely to affect the river immediately above the gorge at Islip. Here lies the expanse of Gallows Brook gravel (Sandford, 1926, 142) and the patch of 'Ray gravel' (ArkelI, 1944, 63). The Ray terrace and the associated alluvium lying to the north of the periclines appear to indicate that Otmoor was probably never a lake basin, or similar deposits could be expected on it. It is probable that Otmoor was never more of a marsh than other parts of the Oxford Clay lowland. The present backward state is the result of poor drainage by an incomplete system of dykes. Young was right in thinking that if it were properly drained it would be more productive. Parts of the moor are cropped for hay but much of it is in rough grazing. ACKNOWLEDGMENTS We would like to thank all those who have assisted our work, in particular the local farmers and landowners, also Prof. E. W. Gilbert, Dr. K. S. Sandford, Dr. M. M. Sweeting, Mr. D. F. Baden-Powell, Mr. R. J. Chorley, Mr. P. H. T. Beckett and Mr. J. C. Almgill. REFERENCES ARKELL, W. J. 1942, 4. Stratigraphy and Structures East of Oxford. Quart. J. geol. Soc. Lond.,98, 187; 100,45,61. ---,.1947. Geology of Oxford, Oxford. BECKINSALE, R. P. 1954. In the Oxford Region. Oxford. DAVIS, J. 1795. View of the Agriculture of Oxfordshire, HAMMOND, J. L. & B. The Village Labourer (deals with the period of enclosure and drainage of Otmoor). HUSSEY, T. 1841. The Roman Road/rom Alchester to Dorchester. LAMPUGH, G. W. 1920. Some Features of the Pleistone Glaciation. Quart. J. geol. Soc. Lond.,76. MARSHALL, M. 1944. Lond 0/ Britain. Part 56. PIPER, C. S. 1938. Soil and Plant Analysis. 47-79. SANDFORD, K. S. 1926. In the Memoir of the Oxford Special Sheet. Ed. T. I. Pocock. Mem. geol. Surv. U.K. YOUNG, A. 1809. The Agriculture of Oxford.
Miss M. E. Marker D. Cooper School of Geography Oxford