How to build a dolmen

Exploring Neolithic  construction  at Garn Turne

Well known on the Continent and scattered  along the coasts of Wales, Cornwall, and  Ireland, dolmens are an immediately  recognisable form of chambered tomb.  They represent remarkable achievements  for their Neolithic builders, crowned  with stones weighing as much as 160  tonnes. Vicki Cummings and Colin  Richards investigate how these distinctive  monuments were constructed — and what  happened when a project did not go to plan.

Of all of the kinds of chambered  tomb that are found in Britain  and Ireland, dolmens are perhaps  the most iconic — and the  least understood. Yet their composition  is very simple: to create a  dolmen, you simply place a large slab or ‘capstone’  on top of three or more upright stones, creating an  open, box-like chamber. So far, so straightforward  — except for the small matter of raising the capstone. In order to get a better idea of  how these dolmens were engineered, we began a  campaign of excavations at Garn Turne Major in  Pembrokeshire, home to the largest surviving capstone  from a British megalithic monument.

Weighing in at around 80 tonnes, the Garn  Turne Major capstone is a truly massive slab of  rock. To put this in perspective, the great sarsen  stones at Stonehenge weigh on average 25 tonnes.  But even Garn Turne cannot compare with the  absolutely colossal capstone at Brownshill, Co.  Carlow (Kernanstown), which at 160 tonnes ranks  as one of the largest known anywhere in Europe.

Tomb with a view

Building these mighty monuments using prehistoric  technology was an incredible feat —  something their proud creators were apparently  eager to highlight. The capstones often balance  precariously on the tips of the supporting slabs  beneath. Was this showboating designed to draw  attention to the prowess of those assembling  sites like these? To us, this delicate arrangement  indicates that the cairn material sometimes  found around such monuments served a radically  different purpose to the traditional view:  these are not the vestiges of a lost burial mound,  but packing to keep the uprights in place.

It has long been assumed that all chambered  monuments were encased within a mound or  cairn. In the case of dolmens this would mean  that the stone frame was only visible because the  mound had weathered, or been robbed, away.  There is very little supporting evidence for this,  however; sometimes you find a small platform  of cairn material around the base of the dolmen,  but often there is nothing at all. Rather than the  material having been removed at a later date, it  seems much more likely that these fantastic constructions  with their impressive capstones were  meant to be admired, not obscured. Dolmens are  visually spectacular sites, with a design geared  far more towards ostentatious display than the  practicalities of a burial space.

Previous studies of dolmens have tended to  focus on architecture and burial chamber contents  — both complex subjects, not least because  extensive local variation makes it difficult to slot these monuments into any neat typological scheme. Their design has also frustrated generations of archaeologists attempting excavation; dolmens are effectively open boxes that have been exposed to the elements — and curious fingers — for millennia. Few dolmens produce the quantity of material that is typically found in other forms of Neolithic chambered tomb. When material is recovered, analysis is complicated by the fact that these sites clearly saw depositions over very long timescales, often thousands of years.

Our project took a different approach, exploring how dolmens were constructed. We think that, first, prehistoric builders identified a suitable rocky outcrop and prised off the upper surface to create their capstone — hence most of them have a natural, weathered upper surface, while their underside is flatter. Sometimes glacial boulders were used instead, but the underneath was still carefully shaped, to help them balance on the upright supports. These uprights were also quarried stones, with their inner sides smoothed and the outer left natural.

This deliberate working of the surfaces makes dolmens the earliest form of monument in Britain and Ireland to incorporate quarried and shaped stones. While excavating at Garn Turne Major we found debitage created when its huge capstone was flaked into shape using massive hammerstones. Dating back to 3790-3640 cal BC, this represents the oldest-known evidence of stone-working in Britain, pre-dating Stonehenge by around 1,000 years.

Deconstructing Garn Turne

In the summers of 2011 and 2012 we opened  a large trench at Garn Turne, which revealed  multiple phases of activity, including the remains of at least two dolmens. When we first arrived on site, the monument’s forecourt — an open area perhaps for assembly — appeared to have a natural outcrop in the middle of it, an arrangement unparalleled at other dolmen sites in Britain or Ireland.

Our investigation revealed that this ‘outcrop’ was in fact a quarried stone, sitting on the edge of a pit — probably its source — that contained traces of intense burning. Charcoal from this area was radiocarbon dated to 3702-3639 BC. The stone, which we named the Floss Stone after a dog belonging to one of the diggers, was partly set on a rammed-stone platform that had been cut by another large pit, which lies directly in front of Garn Turne Major and probably once housed its capstone. Radiocarbon analysis of burnt hazel found in this second pit suggests the dolmen was constructed in c.3787-3656 BC or 3761-3643 BC. Stratigraphically there is evidence that the Floss Stone was quarried and moved before Garn Turne Major’s construction — perhaps it was initially intended to be part of the monument, or played a role in the building process, but it could have been a special, sacred stone in its own right.

This was not the only activity pre-dating Garn Turne Major, however: we also found evidence of a smaller dolmen, dubbed ‘Garn Turne Minor’, directly to the north-west. Prior to excavation, only its capstone was visible, but our trenches revealed a number of collapsed orthostats alongside this. It seems that the monument had once stood in a large pit, much like Arthur’s Stone on the Gower, but at a later date, after it had collapsed, the dolmen was surrounded with a platform of stones and soil, hiding the pit and toppled uprights. A series of smaller standing  stones were erected around the fallen capstone — perhaps commemorating the monument, or marking its destruction.

Our site was not isolated in the historic environment. We also identified a series of standing stones in the immediate vicinity, placing the dolmens in a broader monumental landscape. Garn Turne Major’s forecourt, constructed partly in the remains of the massive quarry pit in front of the monument, was apparently added at a later date. Analysis of this layer of the pit yielded dates in the Late Neolithic or Early Bronze Age (2464-2210 BC, and 2618-2470 BC). Another radiocarbon date from higher up in the pit is from the Iron Age (800-547 BC), which — together with the discovery of iron slag — suggests that the dolmen had a long and eventful use in prehistory.

Dolmen disasters

One thing that became clear during the course  of our investigation is that things did not  always go to plan when building these monuments.  There are many surviving examples of  dolmens where problems in the construction  process can be seen. Garn Turne Major was  apparently never completed after its capstone  collapsed. At Brownshill the builders seem to  have had even greater difficulty getting their  project off the ground: perhaps overambitious  in estimating the weight they would be able  to lift, ultimately the workmen had to make  do with only elevating the front part of their  record-breaking capstone, while to this day the  rear remains firmly rooted to the ground.

It is not surprising that disaster struck when  working with such massive and unwieldy materials.  At other sites there is clear evidence of capstones  breaking, collapsing, or slipping off their  supports. The Neolithic builders do not seem to  have helped themselves, however. Many times  we have observed monuments where prepatory  groundwork was shoddy at best: upright slabs  had not been firmly bedded in the ground, but  were effectively propped against one another for support, like a house of cards. It is no shock, then, that once a massive capstone was placed on top these unstable constructions often went awry.

Chocks away: raising the capstone
How did people manage to shift such enormous stones 6,000 years ago? During  the Neolithic period, building materials for monuments were most likely moved  using a combination of ropes, levers, wooden rollers, and grease, with people and  possibly animals such as oxen providing the muscle. When constructing a dolmen,  however, no such shifting was required.

Our excavations have shed some light on how the feat was achieved. After  the capstone had been quarried from the bedrock, the monument could be built  directly above it, allowing the stone to be raised in situ using levers. Once one end  had been raised even a few inches, chocks (probably a combination of timbers and  stones) could be placed underneath. The builders could then lever up the other  side and put more chocks under it. Repeating this process would slowly elevate  the capstone until it was in the right position.

It was then time to remove carefully small portions of this material in order to  put in the upright supporting stones. Finally, when all the uprights are in place, any  remaining chocks could be removed and — voila! — you have a finished dolmen.  This method is much easier than  dragging the capstone up a ramp  and onto the uprights — though  it was during this final phase of  construction that everything went  wrong at Garn Turne Major: when  the final support was removed,  the uprights could not hold  the capstone’s weight, and the  monument collapsed.

This feature appeared in issue 286 of  Current Archaeology.