Sarsen #5

Blogging about what archaeologists can learn from toolmarks left by the sharp edges of metal tools – and how ephemeral those marks can be – reminded me of one of my all-time favourite archaeological features.

Last Saturday I was in Avebury to see what a pasture to the north of the Manor House would be like for a Young Archaeologists’ Club activity.   Lovely earthworks, probably medieval, quiet and secluded without being remote, and great views out to Windmill Hill and back to the church and manor – so it’s perfect for a surveying session.

The weather was holding up.   Heading east out of the village up Green Street, I was reminded just how enclosed, protected, encircled, claustrophobic, it feels inside the prehistoric earthworks of the huge henge monument.   Green Street – the herepath on the way to Marlborough – felt faintly like an Anglo-Saxon military way as runners participating in the Marlborough Downs Challenge plodded uphill and teenagers working on their Bronze Duke of Edinburgh Award splashed down.

I was heading for the Valley of Stones and the open-access area of Totterdown.   Crossing the Ridgeway, the rise is broached at 236m above sea level on Overton Down and three landmarks come into view; the remains of a barrow in a cluster of wind-blown hawthorns, the Experimental Earthwork hung with more stunted trees, and another barrow crowning the south-east facing spur of the hill.   The first hint of what’s to come appears in the light scattering of grey wethers on the slope below.

The view to the east from the Ridgeway, over Overton Down.

The view to the east from the Ridgeway, over Overton Down.

Having enjoyed the best part of three hours amongst the sarsen stones in a small area around Delling, taking photos and gathering data, it was time to head back down to Avebury.   Dark downpours of freezing rain had been interspersed with bright, blowsy sun.   Sometimes it felt heavenly up on the heights – sometimes it was impossible to imagine just how the prehistoric farmers had teased life out of their cold, wet arable fields now shadowed under sheep-run pasture.

But I could hardly take the short route back and not visit this:

The polissoir - a grinding stone for polishing stone axe heads - one of a number on the Downs but the only one with this variety of toolmarks.

The polissoir – a grinding stone for polishing stone axe heads – one of a number on the Downs but the only one with this variety of toolmarks.

This magnificent sarsen stone is a polissoir, just to the east of the Ridgeway at grid reference SU 1283 7150.   It’s a large version of the grindstones and honing blocks I use to sharpen my steel axe edges – but it was used in the Neolithic to put smooth surfaces onto stone axe heads.   Sarsen is a sandstone: not so finely grained that it won’t rip off material from an axe head; but nicely regular and evenly textured to work down a flaked surface.

Not every Neolithic axe head was ground and polished – and some were only partially worked –  but those that were have beautifully finished surfaces.   You get a hint of this if you touch the dished corner of the polissoir, run a finger along the grooves, and stroke the glossy surface which you can see shining in the sunlight.

This polissoir has probably lost part of its prehistoric grinding surface.   It was split by the stone cutters who used to work this area, and who removed the whole of the west side of the stone, taking some of the worn area with it.   Before being used as a grinding bench it had perhaps been a standing stone.   Excavations in 1963 revealed the remains of a pit at the northern end of the stone, interpreted as a socket in which the sarsen had once stood (Fowler 2000:66-8).

This is not the only polissoir in this area to have been one thing, then another.   For example, one of the sarsens used to construct the chambers of the West Kennet long barrow (no more than about 4km away to the south-west) bears a similar dished, glossy grinding area.   There is a portion of a polissoir, with a shallower but still glossy surface, incorporated as a block in the farmyard wall at North Farm, West Overton (just over 2km to the south).

The stone tools ground and polished on this sarsen have left their marks.   Knapped axe heads, like the flint one below made recently by Karl Lee, lost their flake scars and ridges; but in the process of becoming smooth, cut and marked the sarsen.   For such a hard, tough stone, these cuts are deep.

Flint axe head knapped by Karl Lee - Neolithic form, 20cm long.

Flint axe head knapped by Karl Lee – Neolithic form, 20cm long.

Fowler, P.J. (2000)  Landscape Plotted and Pieced   London: The Society of Antiquaries

Sarsen #2

Before I write more about sarsen objects, sarsen tools and sarsen working, I thought I would summarise sarsen geology.

Sarsen is a silcrete sandstone, formed through the surface or near-surface silicification of other deposits.   In Wiltshire, sarsen stones probably represent locally-silicified zones in the 4m to 5m thick Paleocene deposits overlying Cretaceous chalk; perhaps the Reading Beds or upper Bagshot Beds.   The sand, silt and mud of these layers are thought to have been cemented by water-borne silica, aided by silica-rich plants growing at the time.  It is possible that this silicification happened rapidly, over some 30,000 years or more.   The cementation was, however, uneven.   As the Paleocene deposits eroded, the scattered silicified patches were left on the surface.   You can see these most clearly on the Marlborough Downs, in the Fyfield National Nature Reserve, where sarsen stones lie sleeping across the beautiful landscape.

Archaeologists tend to talk about two types of sarsen in Wiltshire.   The most common – and the one of which the standing stones at Avebury and Stonehenge are comprised – is “saccaroid” sarsen.   The less common is “quartzite” sarsen, which appears in the archaeological record as the principal hammerstone material at Stonehenge.

Saccaroid sarsen

Saccaroid sarsen

Saccaroid sarsen’s fresh break is white to grey in colour.   The name is derived from the similarity of the fresh break’s surface to broken sugar loaf.   It is made up of quartz sand grains and is usually found in large boulders.

quartzite sarsen

Quartzite sarsen

Quartzite sarsen is usually a darker grey-brown colour, comprising much finer grained, clayey silts.   It is usually found as nodules up to c. 60cm diameter.   Fossils are rarely found in sarsens; but root voids and silicified roots are common, part of the characteristically  gnarled look of much sarsen.

These two distinctions are useful although they mask sarsen’s subtle variations.   For example, pebbles can be found cemented into the mix; varying amounts of iron oxide stain the stone red-brown; the cementation varies, leaving some grains of sand more-or-less well “glued” together.   The “case-hardening” effect brought about by atmospheric weathering also affects texture and hardness.   These variations can be found between stones and within individual boulders.   And the shapes and sizes of sarsen stones vary immensely; from smooth, rounded boulders to irregular, contorted, pitted blocks.

Geddes, I. (2000, 2003) Hidden Depths: Wiltshire’s Geology and Landscapes   Bradford on Avon: Ex Libris Press

Summerfield, M.A. and Goudie, A.S. (1980) “The sarsens of southern England: their palaeoenvironmental interpretation with reference to other silcretes”   In Jones, D.K.C. (ed) The Shaping of Southern England   Institute of British Geographers Special Publication 11   London: Academic Press