Chapter XX: Section III: Structure of the Wooden Islands

In my Introductory Chapter I have remarked that none of the Irish writers appear to have paid much attention to the mechanical principles on which the wooden islands were constructed. A similar remark is equally applicable to the writers on Scottish crannogs. Dr. Stuart had got hold of the general idea that the mortised transverses were used for the purpose of steadying the uprights, and that the outer structures were adapted to resist the action of the surrounding water. The following are his words: "Of the first class, or the crannog proper, the ordinary construction was by logs of wood in the bed of the lake supporting a structure of earth or stones, or of a mixture of both, the mass being surrounded by piles of young oak-trees in the bed of the lake, the inner row of which kept the island in shape, and the external rows acted as defences and breakwaters."[43] But these views convey only a partial notion of a more comprehensive system, the meaning of which I was only able to perceive after my experience at Buston. Notwithstanding that I made the structural arrangements of the Lochlee crannog a particular point of study, I failed to adduce a satisfactory theory for the details recorded; and, beyond showing that the two inner circles of uprights, with their radial and circumferential transverses which immediately surrounded the log pavement, formed a kind of breastwork or wall some 3 feet high, I made no advance on Dr. Stuart's theory. I could offer no explanation of the other large mortised beams found external to this circle, nor of the network of oak beams--some with mortised holes, and others with tenons to fit into continuous beams--which became manifest on making the deep shaft, and appeared to permeate the whole structure of the island. The advantage of carefully recording every fact, however trivial or obscure, has never been better illustrated than in this very article (that on the Lochlee crannog), as, with the more recent light thrown upon the subject, there can be no doubt that these structures, as well as the mortised beams (one of which contained three holes) lying on the western margin of the crannog, were some of the radial beams of an encircling girdle, still _in situ_, which surrounded and knit together the island in a precisely similar manner to that at Buston, as described at page 197. At Buston also, the inner circle, as evidenced by its mural remains, formed part of the enclosure surrounding the log pavement, and thus corresponded with, and served the same purpose as, the breastwork at Lochlee. Again, on the south side of both crannogs, the circles were more numerous, and occupied a larger area than on the north side, but with this difference,--that at Lochlee the midden covered the greater portion of this space, which at Buston was converted into an open and partially paved promenade.

In the incidental notices of these islands, given in Chapter II., the narrators sometimes describe them as having been built on a framework of oak, as at Lochrutton, Loch Kinder, etc. At other times the only evidence of a crannog consists in fishing up oak beams from the bottom of a lake, or their discovery under its surface inextricably mortised into others, as at Loch Lomond, Lochmaben, Loch Lochy, etc., while in drained lakes and morasses the chief indications are the tops of upright piles surrounding a flat mound. But all these accounts, as well as the more recent notices of crannogs, are characterised by two prominent structural features, viz., (1) upright piles in the form of one or more circles; and (2) the remains of flat beams containing large square-cut holes at their extremities.

The quotations from the Irish writers, given at pp. 6-11, will also show that the same structural features characterise many of the descriptive notices of the crannogs of Ireland. Dr. Stuart also draws particular attention to similar beams at Dowalton, some of which were of "great size and length (one of them 12 feet long), with three mortised holes in the length, 7 inches square." These indications, as well as a careful study of the structural details recorded in my reports of the explorations made at Lochlee, Lochspouts, and Buston, have led me to believe that all the wooden islands were constructed after one uniform plan, and that this plan was actually the outcome of the highest mechanical principles that the circumstances could admit of. Indeed, I am prepared to maintain that were the same problem submitted to modern engineers, they could make no improvement either on the principle or mechanism displayed in these singular structures. Let me therefore, in the first place, note the conditions that called forth such high mechanical and engineering qualities. For defence and protection, which, I presume, no one will doubt were the primary objects of these islands, a small mossy lake, with its margin overgrown with reeds and grasses, and situated in a secluded locality amidst the thick meshes of the primeval forests of those days, would present the most desirable topographical conditions. Having fixed on such a locality, the next consideration would be the selection of materials for building the island. In a lake containing the soft and yielding sediment due to decomposed vegetable matter, it is manifest that any heavy substances, as stones and earth, would be totally inadmissible owing to their weight, so that solid logs of wood, provided there was an abundant supply at hand, would be the best and cheapest material that could be used. To construct in 10 or 12 feet of water, virtually floating over an unfathomable quagmire, a solid compact island, with a circular area of 100 feet or more, and capable of enduring for centuries as a retreat for men and animals, would, I daresay, be the means of eliciting from many an engineer of the present day a more frequent manifestation of the proverbial symptom of a puzzled Scotchman than from these early brothers of the craft--the crannog-builders.

This is how they worked:--

(1.) Immediately over the chosen site a circular raft of trunks of
trees, laid above branches and brushwood, was formed, and above it
additional layers of logs, together with stones, gravel, etc., were
heaped up till the whole mass grounded.

(2.) As this process went on, upright piles, made of oak, and of
the required length, were inserted into prepared holes in the
structure, and probably also a few were inserted into the bed of
the lake.

(3.) The rough logs forming the horizontal layers were made of
various kinds of wood, generally birch, it being the most abundant.
These were occasionally pinned together by thick oak pegs, and here
and there at various levels oak-beams mortised into one another
stretched across the substance of the island, and joined the
surrounding piles.

(4.) When a sufficient height above the water-line was attained, a
prepared pavement of oak-beams was constructed, and mortised beams
were laid over the tops of the encircling piles which bound them
firmly together as already described. The margin of the island was
also slantingly shaped by an intricate arrangement of beams and
stones, constituting in some cases, according to Dr. Stuart, a
well-formed breakwater.

(5.) When the skeleton of the island was thus finished, probably
turf would be laid over its margin where the pointed piles
protruded, and a superficial barrier of hurdles, or some such
fence, erected close to the edge of the water.

(6.) Frequently a wooden gangway, probably submerged, stretched to
the shore, by means of which secret access to the crannog could be
obtained without the use of a canoe. (For the mode of structure of
gangway, see page 101.)

Bearing in mind that all these structures were solidly put together without nails or bolts, and that the gangways, which have remained permanently fixed to the present time, had neither joint nor mortise, we may fearlessly challenge modern science to produce better results under these, or indeed any, circumstances.

That future discoveries may show slight deviations from the exactness of these details I am quite prepared to believe, as the mechanical knowledge of the age was too thorough not to be readily adapted to varied circumstances. What I do however maintain is, that the general principles of structure here laid down, being the outcome of a sound knowledge of mechanics, varied during the whole period of their practical application as little as the essential laws of architecture do in the structure of different styles of our modern houses.

Not only do these wooden islands evince much mechanical and technical skill on the part of their founders, but, what is still more singular, their area of distribution appears to have been co-extensive with the districts formerly occupied by Celtic races. Hence we have here another proof of the extraordinary vigour, intense individuality, and plastic character of early Celtic civilisation, in thus developing, from its own inherent resources, a unique form of stronghold, simple in structure, but yet admirably adapted to the unsettled conditions of life and military requirements of the period.