PROTON MAGNETOMETER SURVEYING ON SOME BRITISH HILL-FORTS

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  • PROTON MAGNETOMETER SURVEYING ON SOME BRITISH HILL-FORTS

    BY M. J. AlTKEN and M. S. TITE

    HE ramparts of Iron-Age Hill-forts often enclose interiors of several acres or more which are devoid of surface indications. Aerial photography T seems to be less powerful here than in other circumstances, the archaeolo-

    gical features-pits, hearths and huts-are not of an easily recognisable geometric design, the land tends to be continuously under pasture rather than crop, and even when features are revealed from the air the frequent absence of trees, telegraph poles, etc. handicaps accurate location on the ground. The important role that the proton magnetometer might play in exploring hill-fort interiors was pointed out by P. J. Fowler in 1958 and its first successful trial in this context was reported in an earlier issue of this journal (Fowler, 1959). Since then the magnetic surveys have been made on many sites of this type and the notes following give an account of the results obtained on four. It should be borne in mind that hill-fort explora- tion and pottery-kiln detection are the two roles in which in Britain the proton magnetometer has proved, in general, most successful.

    MADMARSTON, OXON (Surveyed June-July 1958) This Iron Age hill-fort was excavated by the Oxford University Archaeological

    Society in 1957 and 1958 (Fowler, 1960). The sub-soil on the hill-top is a micaceous clay containing thin bands of ironstone and the hill lies within the marlstone region around Banbury. No abnormal magnetometer readings were obviously attributable to the presence of ironstone, and indeed, the associated high iron content* of the top-soil (averaging 10%) resulted in an above average sensitivity for the method on this site. This was evidenced by the dimensiofis of the first prehistoric pit ever detected in this way: it was oval in plan, maximum diameter 3 ft., but only 15 inches deep and overlaid by 18 inches of plough-soil. The magnetic anomaly, 9 inches above the ground level, was 55 gamma. The magnetic susceptibility** of samples of plough-soil taken at random on site averaged 3x10-4 emu per gm (to be compared with an average figure of 1x10-3 emu per gm elsewhere) while samples from the excavated pits themselves ranged between 9x10-1 and 22x104 emu per gm. The pit from which the latter value was obtained produced an anomaly of 200 gamma at a bottle-height of 9 inches-the pit was approximately 48 inches in diameter, 36 inches deep and overlaid by 18 inches of plough-soil.

    The magnetic survey covered nearly two acres of the 5-acre interior. Fifty equally-spaced readings were taken in each 50 x 50 square, mostly at a bottle- height of 9 inches. About fifteen isolated anomalies exceeding 50 gamma were detected and nine of these were excavated. In all cases but one (for which no

    * The iron contents quoted refer to roral iron and were kindly determined by P. M. Reid using the laboratorys X-ray Fluorescent Spectrometer.

    ** Here and elsewhere the susceptibility quoted refers to measurement with an instru- mental time constant of a few seconds. On the average this has been found to be twice the value obtained by measurement at 2000 cps, the field strength in both cases being less than 1 oemted. Sometimes the value quoted is deduced from 2000 cps. measurement using this empirical factor. The effective susceptibility in the ground is probably about four times the 2000 cps value, due to magnetic viscosity.

  • A R C H A E O M E T R Y 127

    apparent cause was discovered) an archaeological feature was discovered : five pits, two gullies and one iron object.

    In one region the survey extended across the inner defence ditch and an anomaly of 50 gamma was noted along its length. Within the rampart on the S.W. side of the camp a 75 gamma linear anomaly was recorded. This could be attributed both to thermoremanent magnetism in the sun-baked clay capping on the inward facing slope of the rampart and to the one-foot thick occupation layer on top of it.

    DANES CAMP, WORCESTERSHIRE (Surveyed July 1958 and July 1959) This small 2-acre Iron Age hill-fort, a subsidiary of Bredon Hill, was excavated

    in 1958 and 1959 jointly by the City Museum and the Archaeological Society of Birmingham, under the direction of Mr. Nicholas Thomas. The sub-soil is lime- stone; geologically it is Inferior Oolite, of the bottom part of the Lower Freestone subdivision and may be described as white. The magnetic susceptibility of 15 samples of soil taken at random within the camp, from just below the turf-layer. ranged between 7 and 11x104 emulgm. That this is somewhat higher than at Madmarston for a lower iron content (about 6%) is presumably due to a greater proportion of the ferrimagnetic maghemite (gamma-Fe,O,) which has a much higher susceptibility than the weakly ferrimagnetic haematite (alpha-Fe,O,) Studies on sedimentary formation in France by Le Borgne (1955, 1960) indicate that ground clearance by burning is the most likely cause of this enhancement of maghemite, with alternating periods of wetness and dryness as a subsidiary

    FIG. 1. General plan of Danes Camp.

  • 128 A R C H A E O M E T R Y

    mechanism, Certainly the characteristic blackness which one has come to associate with high magnetic susceptibility was evident in the soil of Dane's Camp.

    The survey in 1958 covered the major part of the main camp and the annexe, at a spacing of 50 readings for each 50' x 50' square, and a bottle height of 1 ft. Several dozen anomalies ranging from 10 to 150 gamma were noted and of these twelve were pin-pointed. Nine were excavated, yielding eight pits and one cauldron support. The annexe contained only two significant anomalies (30 gamma and 35 gamma-both confirmed as pits on excavation), supporting the idea that occupa- tion had been restricted to the main camp-already adumbrated by aerial photo- graphy. In the main camp there were one or two areas 50 ft. across which were free from significant anomalies but no obvious pattern of occupation.

    The survey in 1959 was mainly a more detailed investigation of an anomaly- rich half-acre of the 1958 survey (plus a subsidiary foray on to the main camp of Bredon Hill which located two currency bars between the inner and outer ramparts). The anomalies from two of the pits which had been completely exca- vated (and then back-filled) in 1958 werc measured again. These were pit F (dia. 52 ins., depth 41 ins., 12 ins. of top-soil) and pit G (dia. 48 ins., depth 60 ins., 6 ins. of top-soil). The anomalies before excavation had been 125 gamma and 90 gamma; on remeasurement these were only 60 gamma and 30 gamma respec- tively. This reduction is presumed due to re-arrangement of the soil layers during back-filling, so that the soil of highly enhanced susceptibility is randomly distri- buted throughout the pit instead of being concentrated near the surface. On the other hand the anomaly due to pit E I1 (42 ins. across, 31 ins. deep, 6 ins. of top- soil) was within 10 gamma of 130 gamma both in 1958 and 1959 before its excava- tion in the latter year. This suggests that the dry conditions of 1959 compared to 1958 did not significantly affect the anomaly strength.

    Pit E I1 was one of a pair located close to grid peg 47, and it is in the area covered by fig. 2 where the results of surveying at bottle heights of 1 ft. and 3 ft. are compared. The closer resolution of the lower height is to be expected for a top-soil thickness averaging only 6 ins., and also the enhancement of anomaly strength by a factor of roughly three. Another point of interest in fig. 2 is the normal* anomaly, running from midway between 3 8 and 4~ to near 48 and showing a radius of curvature of the order of 40 ft. On excavation this was found to be due to a thickening of the topsoil, due to accretion against the remains of a circular hut-wall. This wall lay to the left of the normal anomaly and was too diffuse to produce its own reverse anomaly; the reverse anomaly observed to the right was interpreted as return flux from the normal anomaly. Elsewhere on this site somewhat less well-defined variations had been noted, but of the same order of horizontal gradient (20 to 30 gamma per 5 ft.) and excavation in one such region showed a top-soil depth variation as required to explain the anomaly, but of no immediate archaeological interpretation. Possibly these variations did result from ancient activities but the need for wide-scale stripping to clarify how, detracts from the value of a magnetic survey, arid illustrates how a proton magnetometer should be most properly regarded on a hill-fort-as a finder of pits, ditches, cauldron supports and currency bars. All these are distinguished by the sharpness

    Following a convention put forward during the VIth International Prehistoric Congress in Rome in 1962, a normal anomaly means an intensification of magnetic field strength, a reverse anomaly means a reduction, and return flux is ussd to indicate the subsidiary anomaly sometimes found displaced (to the North in the Northern hemisphere) from the main anomaly.

  • 6.-

    FIG. 2 ~ . Extract from magnetic survey at 3ft. bottle height (of area shown dotted on general plan).

    FIG. 213. Exract from magnetic survey at Ift. bottle height (of area shown dotted on general plan).

    I / I / # / / * . 0 00:I

    * /*

    ~ n / n / . . o . . / I / O / . / I # 0 0 0 I O I I I # . I I

    . O . . , . / I . O O O I # I I I I I O . . * ' I . O O O . , / , ,, . 0 0 0 , I . . P O . / 0 ,/ . 0 0 0 . . . 0 0 0 . . . I , - 0 0 0 /

    O " 1 . 0 0 0 . . . 0 ,, . o o o o . . . . . . 0 . . . . t o o 0 . 0 . . 0 . . . / . 0 0 0 . O O . 0 0 0 . . 0 0 0 0 0 . O O / 0 0 . . 0 0 0 . 0 0 0 0 0 . 0 . 0 . / , . O

    . 0 0 0 0 0 . . 0 0 0 0 0 0 . . 0 . 0 # I

    . 0 0 0 0 . . 0 . . . 0 . 0 0 . 0 ) I / . 0 0 0 . . O O / I 0

    10-ma

    i =loor I 0 / / /

  • 130 A R C HA E 0 M E T R Y

    of the associated magnetic gradient, and these considerations led historically to the development of the gradiometer (whether proton- or fluxgate-) as the more discriminating instrument for use in this context (and also when looking for kilns).

    PIMPERNE, DORSET (Surveyed April and July 1961) This 113 acre Iron Agc hill-fort on Pimperne Down was excavated in 1960

    and 1961 under the direction of Mr. Dennis Harding (Harding and Blake, 1963). The subsoil is chalk with patches of clay with flints and the magnetic susceptibility of samples of top-soil taken from the site averaged 0.5 x 10-4 emu/gm. The average depth of topsoil was 9 ins.

    The survey of the interior, undertaken with the proton magnetometer, was confined to the north-east sector and covered nearly 1 acre, readings being taken at intervals of 5 ft. at a bottle height of 1 ft. Of the magnetic anomalies detected the four which were archaeologically significant were grouped together in a small area. Excavation revealed a roundhouse over 50 ft. in overall diameter and consisting basically of two concentric circles of post-holes. The features which had shown up magnetically were two post-holes 3 ft . in diameter which formed part of the entrance porch and had each produced 20 gamma anomalies, a small pit adjacent to the inner ring of post-holes which provided a 10 gamma anomaly and a shallow depression just outside the house whose resultant magnetic anomaly was 5 gamma in strength and 5 ft . in diameter. The clay hearth and the two circles of post-holes were not detected with the magnetometer probably because their associated magnetic disturbances were not sufficiently widespread.

    Using the proton gradiometer (Aitken and Tite, 1962) the enclosure ditch was followed for a distance of 400 ft. in the southern sector and the southern gateway was located. The use of this instrument instead of the single magnetometer was made necessary by the occurrence of a magnetic storm which resulted in changes in the magnetic field strength of 60 gamma in less than 10 minutes. The ditch which had been cut through the chalk to a depth of approximstely 5 ft., produced a 20 gamma magnetic anomaly.

    A second smaller enclosure, 200 ft. to the south, was visible on air photographs of the site and the gradiometer was used in an attempt to establish its position on the ground. In a survey covering six 50 x 50 squares a series of short disconnected linear anomalies ranging in strength from 5 to 10 gamma was detected but it was not possible to interpret these satisfactorily as the second enclosure, However, a cutting made across one of the linear features showed it to be a gully 18 ins. deep and 2 ft. wide.

    RAINSBOROUGH CAMP, NORTHAMPTONSHIRE (Surveyed June 1961 and June 1962)

    This 64 acre Iron Age hill-fort was excavated by the Oxford University Archaeological Society in 1961 and 1962. It is situated on a continuation of the Cotswolds. on limestone (Inferior Oolite) and underlying this are clay and sand (Upper Lias) which come to the surface on the north-western side of the camp. The magnetic susceptibility for samples of top-soil averaged 1.5 x 10-4 emu/gm, for weathered limestone taken from the vicinity of an excavated pit it was 0.2 x 10-4 emu/gm while samples from the pits themselves ranged from 2 x 10-4 to 8.5 x 10-4 emu/gm.

    An almost complete survey of the interior was carried out with the proton magnetometer, readings being taken at 5 ft . intervals with the normal 50 ft. x 50 ft.

  • A R C H A E O M E T R Y 131

    FIG. 3. Results of magnetometer survey at Rainsborough Camp: shading indicates those areas that have been excavated.

  • 132 A R C H A E O M E T R Y

    grid at a bottle level of 1 ft. In addition two areas (50 ft. x 50 ft.) which appeared, from the general survey to be rich in magnetic anomalies were selected and here readings were taken at intervals of 2 ft. 6 ins., in order to try to detect smaller archaeological features such as gullies and post-holes which had previously been missed (Tite, 1961). Figure 3* shows the results of the overall magnetometer survey; approximately 100 isolated anomalies of 25 to 50 gamma in magnitude and 5 to 10 ft. in diameter were detected and these were distributed fairly uniformly over the entire site.

    Excavation of the interior was limited essentially to the two areas surveyed in detail. In the first of these, three well-defined anomalies of 30 gamma, correspond- ing to small rubbish pits, and a hearth of hardened clay, were detected together with an irregular anomaly of 10 gamma which corresponded to a natural hollow filled with red clay of magnetic susceptibility 0.8 x 10-4 emu/gm. In addition excavation revealed a gully 18 ins. deep and 12 ins. wide which had only become apparent magnetically when the area was surveyed at intervals of 2 ft. 6ins. and six post-holes for which no corresponding magnetic anomalies had been recorded. The second area in which four magnetic disturbances, each of approximately 50 gamma, were detected, produced on excavation two shallow working hollows, 18 ins. deep and 14 ft . across which wer...