Friday, November 9, 2012

What's a Wattle Bridge or Causeway?

If you cruise around the Republic of Ireland's newest “M” roads, you'll see the name of Dublin in Irish: Baile Átha Cliath.

Baile stands for “town” and this word in the name is an understatement considering Dublin is the island's largest city.

But what do the other two words stand for?

Átha means “ford” which is a low point in a stream often used for crossing by people or livestock.

Cliath means “hurdle” or “wattle.”

The Oxford Dictionary of Celtic Mythology relates:

The usual Irish language name for the city, Baile Átha Cliath [Irish, settlement/town of the hurdle ford], denotes the narrowest point on the Liffey, forded in pre-Norse times by the road between Tara and Wicklow, near the Wood Quay area, west of the modern commercial centre. It was a ‘hurdle ford’ because of a causeway built of woven wicker, boughs, or hurdles. The ford was known by different names in Irish tradition, including Áth Liag Mairgene [Ford of Margenn's Sling Stone], after the killer of Dub(h).

A hurdle, besides being a barrier over which racers must leap, is a portable panel usually of wattled withes and stakes used especially for enclosing land or livestock.

A wattle is a fabrication of poles interwoven with slender branches, reeds, or withes used formerly in building.

A withe is a slender flexible branch or twig, more particularly one used as a band or line.

Here is my personal speculation without any research beyond the above:

I visualize branches bundled together in a manner similar to a sheaf of corn stalks or wheat stems, but larger diameter stems. Probably they used material held in a manner similar to thatched roofs. I'm speculating, but willow is a common brush size plant that grows along stream banks and many have straight stems. If sheaves of willow were laid down with the mass of stems parallel to the current, they would be permeable to the flowing water.

If the sheaves were laid down thick enough to create the necessary height, it would create a structure looking like a permeable dam.

Then more tightly bound stems or reeds could be woven into tight mats and laid on the top. These might support feet and hooves, although it would seem the relatively high loads would tend to wear out the mats and allow penetration. Soil or sod might be supported by the top layer, especially if the top layer was formed tight and if it was above the water. The soil would tend to prolong the life of the mats or stems and would keep the hooves and legs of livestock from penetrating the wattle.

Larger diameter wood could be used atop the wattle to make for firmer footing. Corduroy roads of tree trunk diameter pieces have been laid down around the world to make soft, wet soil passable.

This must have been a civil engineering structure of great utility at the time and was laudable for being made of sustainable materials. Also, it appears to be appropriately scaled in that everything in it can be accomplished by humans working with relatively primitive tools. Willows and other branches can be cut. Reeds can be cut. All these materials can be collected and transported. Tying materials exist and can be manipulated by human hands. Sheaves can be made light enough to be transported and placed by a few workers.

Something fun, at least for me, is that I found a brief note about the investigation of an old wattle.

The source is: Oxford Journals, Life Sciences, Annals of Botany, Volume os-45, Issue 1, Pp. 207-210 dated 1931. The title is, “Note on some plant remains from an old causeway in essex.” The wattle layer in the causeway found in the Thames estuary in Southchurch, Essex, England was made up of hawthorn, alder, and willow branches. The causeway was determined to have been built in 800 to 500 BC. The branches were approximately 2 ½ inches or 6 centimeters in diameter and smaller. The wattle layers were approximately 6 to 8 inches or 15 to 20 centimeters thick and were separated by about 12 inch or 30 centimeter thick layers of soil. From one to five wattle and soil layers existed. The branches in the wattles were perpendicular to the length of the causeway.

At the top of the wattle layers was a layer consisting of oak saplings approximately 4 to 5 inches or 10 to 13 centimeters in diameter placed approximately 16 inches or 41 centimeters apart placed parallel with the length of the causeway. These saplings were embedded in and overlain by black mud.

I had some ideas that really were used, apparently.

I'd love to see one of these wattle bridges built and tested. Maybe some Dublin folk would like to demonstrate the basis for the Irish name of their "town"- Baile Átha Cliath.

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