Books, Prehistory, Report Of Diggings In Silbury Hill by Flinders Petrie

Report Of Diggings In Silbury Hill by Flinders Petrie is in Prehistory.

Aug 1922. Report Of Diggings In Silbury Hill [Map] by Prof. W. M. Flinders Petrie (age 69), F.R.S. From the 14th to the 30th of August, 1922, I made some examination of Silbury Hill, with the kind co-operation of Mr. A. D. Passmore.

The two necks of land connecting it with the Bath Road are well known. The eastern neck proved to be solid chalk levelled down, and subsequently piled with chalk rubble, to form a smooth gradient from the road down to the neck. Accepting the nearest corner of the nieadow as 491 O.D. the chalk of the neck is at 496ft. 10in. Opposite to the two sides of the eastern neck, trenches were cut in to the hill down to unmoved chalk, which was at 496 in the east trench, and at 497.2 further in. The west trench has chalk at 500, where the cutting was carried forward 40ft. into the hill, as far as 520 level contour, or 60 feet from the middle of the neck. From the end of this cutting a tunnel was cut 20ft. eastward, past the end of the east trench, so as to intercept any possible line of passage in continuation of the neck. No break was found in the chalk surface. Similarly a trench was cut from the outer end of the east trench towards the west, but without meeting any difference in the chalk base. In no case was a turf-band left, which shows the surface to have been cut down.

In the middle of the south face, equidistant from the two necks, a trench was cut up the side of the hill on solid chalk up to 503ft. 6in., where the top of the chalk was found without any turf-band. The untouched down surface opposite the notice board is at 526ft. On this ground fifteen flowering plants were identified, twelve of which occur also on the shifted ground of the hill; but eleven other plants are far commoner on the bill, and give it an entirely different aspect from the untouched down.

In these various cuttings it was notable how little trace there was of rubble slipping down, contrary to what seems to be general impression. In no case was there any proof that the rubble face was not as originally laid ; in one case a level clay band ran along to the base of the turf: in other cases the clay bands came within a few inches of the turf. From the hardening by showers, on the face of the loose rubble that we threw, it seegls unlikely that any face at the angle of rest would subsequently slip. Moreover the angle of rest of our tip was 33°50,' while the hill slope is 27½ to 34°.

Around the hill Dean Merewether records seeing eight sarsen stones, seven of which we identified, five earth fast and two loose. He states that they were 18ft. apart in some places, but there is no such interval between these. What we noted were in the following positions From the N, W. post of the E. end of the ditch, the first was at 997 inches, and from that others at 94 (loose), 399, 3769, 5709, 6327 (loose), and 8261 inches. The mean diameter of the hill from the Ordnance Survey is 6240 inches. The question arises whether these stones are remains of a regular circle (there are about thirty re-used for a cartway across a ditch), if they were, the numbers of the stones counting from the first would be 1, 5, 48, 73, 81, 106, and thece would have been 250 intended for the whole. On this scale the radius of the mound would be 40, Thus the proportion of diameter to circle would be I to 3⅜, instead of 3ꞏ141. The unit of this scale would be 78 inches (= the French architectural canne), which might be the fathom of the northern foot, usually 79. This was the base of our land measures, 10 to the chain, 100 to the furlong, to the old mile. This unit had a long history, the foot being the most usual measure in medieval England, the Roman standard in the Decunuxtes agri on the German frontier, and having a long history before that.

A section of the whole hill was measured, where the shelf around the top is best preserved. The form of the sides was noted by offsets at each 10ft. , from sight lines sloping from top to bottom. Before excavating, levels were taken from the "491" O.S. datum, up to 500, 510, and 520ft., and each level marked out along the hill side by a row of pegs. The positions of the pegs were taped and plumed, to show the contours above the neck, where the hill is distinctly concave. These pegs served as reference marks for plan and level in all the excavating.

The tunnel cut in 1849 was also examined, and the old turf surface was connected with the external levels. From the external level of 520ft. at 103ft. inward from the face of the mound (where the turf is first seen clear of roof-falls), the top of the clay on the turf is at 522ft. 5in., or the base of the turf is 520ft. 9in. At 168ft. inward the top of the clay is 518ft. 2in. As the old down outside is at 526 opposite the tunnel, it appears that there was only 4ft. fall in 180ft, and 4ft, again in 65ft. further. That is to say, the mound was centred on a long almost level spur of down, which feli away sharply on the east, 18ft. in 110 distance to the middle of the south face.

A cut was made on the east side at 1750 from the beginning of the railing, at about level, and about 3ft. inward ; but only rubble was found, It would be well to try on the north face for the tail of the original spur.

At the head of the west trench there was a pocket of larger blocks, limited sharply along a S.S.W.—N.N.E. line by dense rubble. This was searched 6ft. further into the hill; the floor of it, and the top of the loose blocks rose on going inward. It was concluded to be only an accident of the original piling.

Pieces of deer-horn picks and a few flint flakes were found in the rubble; mostly about 8ft. to 10ft. beneath the surface. These are mostly labelled with the levels and placed in the Devizes Museum.

After drawing the section, with the chalk levels, an approximate estimate was made of the voltune of the piled work, at 8.7 million cubic feet (or cubes 100ft. each way). An estimate of the amount of material removed, above the meadow level, gives 2.6 million feet; and the fosse appears to have been 150ft. long, 20ft. deep, and not over 100ft. wide. or three million feet. There is thus a deficit of three million feet, which must have been supplied by the wider fosse on the west, perhaps two millions, and by general lowering of the hill to the south.

The direction of the digging was carried on by tenting on the spot with my son, from first to last ; Mr. Passmore was also generally on the ground during the working hours.


1. The strata of chalk and yellow clay being usually horizontal, or else slightly tilted either way, show that the mound was heaped in level layers, and not added to on the sloping face. This points to the size being originally so designed, and not casually accreted.

2. The large diameter of the fosse (löft. to 22ft. deep), leaving only a narrow berm around the foot of the mound, also shows that the size was thus designed.

3. The absence of any slipping, or sloped piling, shows that the work was regulated with care, probably by a level cord stretched from the central tree found in the shaft of 1777. The angle averages 30 flatter than the angle of rest ; but this may be partly due to consolidation.

The sarsens around the base suggest that two hundred and fifty were to be placed a filthom apart, in a circle 80 fathoms across: the fathom being short form (78in.) of the usual northern fathom (79in.)

5. The chillk surface about the S.E. was all stripped of turf before any rubble was thrown on it, and cut down to between 497 and 500 0.1). The neck left across the fosse was cut to the same level.

6. For a gangway at the S.E. a rubble bank was thrown up on the neck of 497ft„ to join the road at 512ft., while the field on the opposite side is 508. This shows that access for heavy work was needed on this side. The slope of the gangway is one in 4 (12Ht. in and the flat width loft. The present road has doubtless largely degraded, being on a slope, and much used in all ages.

7. The slope of the outer side of the fosse on the south, being in line with the slope outside of the fosse, east and west of that, points to an intention of completing the fosse by removing the necks across it. This suggests that the work was never completed.

8. The trenches and tunnel now cut, prove that there is no access to chamber near the eastern neck.

9. The mound was based on a long, almost level, spur of down, running N. from the present spur of old down which forms the western neck. This spur fell away on the eastern side at a slope of at least 1 in 5.

10. The position of Silbury, so low down that it is hidden in most directions by the nearest hills, would be most unlikely for a great monument, as barrows are usually in prominent positions. The low situation can only be due to the need of making a water fosse round it. Such a feature strongly supports the view that the fosse of Avebury was likewise intended to be flooded. A promising line of enquiry now would be to seek on the Continent for great earthworks which are not defensive, but which have wet fosse around. Any such works would indicate a direction of origin for the constructors of these great monuments.

I have to thank Lord Avebury and H.M Office of Works for the ready permission to make this examination. From the digging of the shaft to the cutting of the tunnel was seventy-two years, from the tunnelling to my cutting was seventy three years ; are we to wait seventy-two years more for further exploration?

For earlier work see the Salisbury volume of the Royal Archæological Institute, 1851 ; papers by Dean Merewether, p. 73, and by C. Tucker, p. 297. Also Wilts Arch. Mag., 1887, vol. xxiii„ p. 245, on the pits sunk in the fosse by A. C. Pass.

Note By Mr. A. D. Passmore.

During the excavation many fragments of deer horn picks were turned up, all of which bear signs of very rough usage, the tines being broken away from the shafts probably in digging the rubble from the great ditch below. There were a few bones; these have been kindly examined by Dr. C. W. Andrews, F.R.S., who definitely determined them as red deer and • pig. A few flint flakes—like the bones and horns—occurred at all depths; they are very rough waste chippings with no secondary workr stained grey by contact with the chalk, but dull and lustreless. In the top soil of the east neck was one piece of coarse pottery containing much broken shell and flint, probably native of the Roman period. The difference in herbage mentioned above by Prof. Petrie is probably explained by the fact that nothing bigger than a rabbit depastures the hill. In all the' cuttings there was a remarkable absence of silting, the horizontal layers or rubble coming right out to the edge. This suggests that the hill was turfed over as made, thus any tendency of the loose rubble to roll down or to be, washed down was effectually prevented.

I have sent photos to the Devizes Museum which illustrate the latter remark and show the work Of excavation at different stages.