Alderney is the northernmost of the cluster of islands, islets and skerries that make up the Channel Islands lying off the north-west coast of the Normandy (Cotentin) Peninsula. Geologically,Footnote ¹ the north-western three-quarters of Alderney is underlain by a varied igneous Basement; in a coastal strip to the south-east, where the Roman fort, The Nunnery, overlooks the natural harbour of Longy Bay, these rocks are followed by the Cambro-Silurian Alderney Sandstone, a red-bed sequence composed of arkosic and some quartzitic arenites with a few shales. Correlative red-bed strata have coastal outcrops on Jersey to the south, on the nearby mainland at the northernmost tip of the Cotentin and, further afield in Britanny, north of St Brieuc and west of St Malo.Footnote ² The arkosic facies of the Alderney Sandstone has a distinctive appearance in hand-specimen and thin-section. The rocks are tough, massive and crudely laminated, varying chiefly from pale greenish grey to dark red in colour and from very coarse sand to granule in grade. Some of the sandstones, however, are by contrast dark grey in colour. Under the microscope the chief sandstones comprise an ill-sorted mixture of angular-subangular quartz (some polycrystalline) with abundant orthoclase feldspar (some with intergrowths), plentiful microcline and a little plagioclase. Also present are muscovite flakes and dark brown, strongly pleochroic biotites. Some of the micas are blackened along particular laminae. A variety of rock fragments form a subordinate component: finely to coarsely microcrystalline ‘cherts’, recrystallised acid lavas with scattered feldspar and occasional quartz phenocrysts, indurated quartzo-feldspathic crystal tuffs, feldspathic metaquartzites, phyllites and very rare granophyric intergrowths. These detrital grains are set in a continuous, plentiful groundmass of colourless to greenish, very finely crystalline sericite. Some of the feldspars are thinly and irregularly veined with this mineral.

The Alderney Sandstone was used to make a rotary quern recovered from the Romano-British villa at the village of Brading in the extreme east of the Isle of Wight,Footnote ³ where the River Yar flows north-eastward to the natural harbour at Bembridge. The origins of the villa lie in the mid-first century a.d., but the site subsequently was rapidly developed into a large and sumptuous masonry complex, with fine mosaics, built around a courtyard. The site was more or less continuously occupied up to the late fourth century.

The quern (unstratified, object 54, trench 7) is of the beehive typeFootnote ⁴ and was recovered as three joining fragments representing about one-quarter of an upper stone ( fig. 1). It measures 350–360 mm in diameter and stands 72 mm high. The steep-sided hopper is unusually wide, measuring about 165 mm, almost one-half of the diameter of the whole. The cross-section resembles an ill-shaped loaf, the grinding surface slopes outward at about 18 degrees, and for the turning handle there is a deep radial slot along which the stone broke. Externally the stone was ground smooth and there is no trace of any preliminary chiselling or pecking.

FIG. 1.

The Alderney Sandstone rotary quern (upper stone) from Brading Roman villa, viewed in cross-section (left) and from above (right).

The quern is made from a tough, pale greenish grey, pink-speckled, very coarse-grained sandstone with scattered granules and occasional small pebbles. In thin-section under the microscope the rock is indistinguishable petrographically from samples of Alderney Sandstone from Alderney. Angular-subangular quartz (some polycrystalline) is seen with abundant orthoclase, microcline and plagioclase feldspars, with some micas and rock fragments, all set in a brightly birefringent matrix of greenish sericite ( fig. 2). The rock fragments include ‘chert’, recrystallised acid lavas, phyllites and very rare granophyric intergrowths.

FIG. 2.

Photomicrograph of Alderney Sandstone viewed in doubly-polarised light.

The quern fragment from Silchester (object 5796) is much smaller, measuring 98 by 70 by 59 mm. It was recovered from a context described as an ‘accumulation deposit’ dated to the mid-first century a.d. during excavations for the ‘Town Life’ Project,Footnote ⁵ and has the shape of a piece off a tall upper stone. Although mid to dark grey, the rock in hand-specimen closely resembles Alderney Sandstone of other colours, as described above. It is an ill-sorted mixture of angular, partly granule-grade quartz and feldspar with a little mica set in a dense, finely crystalline groundmass. A thin-section displays under the microscope angular to subrounded grains of quartz (many polycrystalline) with subordinate feldspars (orthoclase, microperthite, microcline, some plagioclase), ‘chert’ and porphyritic acid lavas with a recrystallised groundmass. There are scattered micas. These detrital components are set in an abundant groundmass of brightly birefringent, very finely crystalline sericite. Some of the feldspars are irregularly veined with the latter.

Rotary querns made from Alderney Sandstone have not previously been recognised from Roman Britain. Of the five other querns recorded from the 2008–10 excavations at Brading,Footnote ⁶ four are of Lodsworth rock,Footnote ⁷ well within the geographical spread of this West Sussex product, and one is of the local Upper Greensand from perhaps no more than 10–20 km away. The Romano-British town of Silchester has afforded numerous rotary querns attributed to several geological sources, including the Rhineland volcanics.Footnote ⁸ It would now seem that querns from another overseas source — the Channel Islands — were coming by sea into southern Britain during the Roman period, just as black-burnished coarseware pottery from south-east Dorset was travelling in the opposite direction.Footnote ⁹ Have other querns of Alderney Sandstone been found in Roman Britain, perhaps masquerading as Old Red Sandstone or Millstone Grit?

Although no other querns have so far been attributed to the Alderney Sandstone, there is evidence that rock in another form was reaching Britain from the Channel Islands, and to a lesser extent from Britanny and Cornwall. Barry CunliffeFootnote ¹⁰ recorded from the earlier levels at Fishbourne Palace a ‘number of boulders of different kinds of foreign stone’ which ‘showed signs of being water worn’, but in one case had been ‘smoothed and cut into thin sheets’. These were interpreted as ballast and included a range of igneous and metamorphosed lithologies and an iron-stained, arkosic sandstone that resembled ‘rocks on the east side of Alderney’.