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Many Scottish lochs are littered with small, tree-filled islands, poking their heads up just above the waterline. What most people don't realise is that they are often man-made islands, and once contained spectacular Iron Age dwellings. Thatched, timber roundhouses were supported on huge, wooden piles driven deep into the bed of the loch. The surrounding water was the inhabitants' defence.

There are over 600 recognised crannogs in Scotland. Some, such as Eilean Dòmhnuill in Loch Olabhat on North Uist, are believed to date back to Neolithic times. More commonly crannogs typically date to the Iron Age.

Crannogs were probably the centres of prosperous Iron Age farms, where people lived in an easily-defended location to protect themselves and their livestock from passing raiders.

The settlement would have consisted of a farm house, with cattle and crops being tended in nearby fields, and sheep on hill pastures. Local woodlands would have serviced the home with fruit, hazelnuts, wild cabbage and medicines, as well as with wild boar and other woodland animals suitable for hunting.

Since 1980 archaeologists have explored the waters of Loch Tay for crannogs and have excavated one at Oakbank, just off the village of Fearnan. It was a remarkable discovery. Preserved in the loch's cold, peaty waters were structural timbers, food, utensils and 2600 year old clothing. They even discovered a butter dish with butter still clinging to the inside of it - a bit past its sell-by date though.

In 1994 experimental archaeologists from the Scottish Trust for Underwater Archaeology began to discover the secrets of the Iron Age crannog builders when they set about reconstructing a crannog on Loch Tay using exactly the same material as the original at Oakbank.

The reconstruction at Kenmore involved building a thatched roundhouse on a timber platform 15m across, which was connected to the shore by a timber causeway 20m long. The whole structure is supported on 168 timber piles.

Timber was obviously the most important building material for the crannog builders. To provide all the varieties of timber required, the Iron Age farmers must have carefully managed their woodlands. They would have been skilled woodworkers and would have understood the differing properties of tree species for everything from structural supports to making cooking bowls.

Considerable skill is required to build a crannog. To begin with rafts and boats must have been used, as well as wooden scaffolding built out in the loch. The first stage was to create an artificial island out of timber piles. Alder trees, 8-10m long, were used for the piles. Buoyant on water, the alder piles would have been easy to manoeuvre on the water's surface but very difficult and tiresome to pull vertical.

One of the most challenging tasks the archaeologists faced was how to drive the alder piles up to 2m into the lochbed using only Iron Age technology.

A crosspole would have been lashed to the upright pile and twisted back and forth to create enough momentum to drive it two metres into the loch bed. It has been estimated that it would have taken 12 days for the Iron Age builders to erect and secure the 168 piles needed.

Once the piles were driven, the platform and roundhouse of the crannog could be jointed, pegged and lashed onto the structure. Round timber poles were used for the flooring and to form the structure of the roundhouse. Its roof was thatched with reeds from the loch, with its enclosing walls made from hundreds of flexible hazel stems woven together.

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