How Do You Cross a Stream With No Tools? Lay a Slab on Two Piles

The form consists of large flat slabs of stone laid horizontally across piers of stacked stone, with each slab spanning from one pier to the next, and nothing holding any of it together but weight. A short crossing needs one slab, and a longer one uses several in series with piers between them. The slabs weigh tonnes, which is what makes the structure stable, since a flood strong enough to move them is rare. The piers are dry stone, built without mortar, which lets water pass through rather than pushing against a solid face.

The virtues are all about what the method does not require:

• •No mortar, so no lime burning and no skilled mason
• •No arch, so no centring frame and no wedge-shaped stones
• •No timber, so nothing to rot or be washed away
• •Materials taken from the immediate surroundings
• •Repairable by replacing one slab or rebuilding one pier
• •Low enough that flood water passes over without destroying it

Dating these structures is close to impossible and the reasons are instructive about the limits of archaeology. There is no mortar to sample, no timber for radiocarbon and no datable artefacts in the fabric. The stone itself dates from the geology and says nothing about when it was moved. Repairs are indistinguishable from original work, since a replacement slab looks exactly like the one it replaced, and the structures have been maintained continuously. Documentary references establish only that a crossing existed by a certain date, not that this one did. Claims of prehistoric origin are common locally and unsupported, and most examined examples appear to be medieval or later.

Placing a stone of several tonnes without machinery is the part that impresses visitors, and the methods are understood from experiment and from ethnographic records rather than from any documentation of these particular structures. Levers and packing are the foundation of all of it, since a slab can be raised a few centimetres at a time by levering one end and packing under it, building a timber crib beneath as it rises. Rollers and sledges move it horizontally, and a slope of packed earth built up to the pier lets it be dragged into position and the earth then removed. Water helps, since floating a slab on a raft at high water and grounding it is easier than dragging it.

The distribution follows the availability of the right stone rather than any cultural tradition, which is why the form appears in unconnected places. Dartmoor and Exmoor in south west England hold the best known concentration, built from the local granite, which splits naturally into large flat slabs. Similar structures occur in the Yorkshire Dales, in Cornwall, in Spain, in Italy, in China and in several parts of Africa and Asia. Wherever a bedded or jointed stone produces slabs of the right size and a stream needs crossing, somebody arrived at the same answer, which makes the type an example of a solution determined by material rather than transmitted between cultures.

Flat slabs weighing tonnes laid on dry stone piers make a crossing needing no mortar, no arch, no timber and no skilled trade, repairable slab by slab and low enough that floods pass over it. Nothing in the fabric can be dated, since there is no mortar, no timber and no artefacts, and repairs are indistinguishable from original work, so local claims of great antiquity are unsupported.