Fig. 188.—General View of Buston Crannog, looking northwards. The water in foreground marks the position of Refuse-bed.
(From a Photograph by Mr. Lawrie.)
Structure of Island.—Notwithstanding the havoc committed on the wood-work of the crannog by long exposure to atmospheric agencies before it finally sunk under the protective influence of the muddy water, and subsequently, by the ruthless hands of the agriculturist, there still remained sufficient materials to give one not only a general, but particular and instructive notion of the mechanical principles on which the island was constructed. Its substance, as far as could be ascertained by digging holes here and there, was made up of layers of the stems of trees, chiefly birch; intermingled with which were occasionally found various other materials, such as brushwood, heather, moss, soil, and large stones. Penetrating deeply this heterogeneous mass, towards its margin, were numerous piles, forming a series of concentric and nearly circular stockades, which were separated from each other by an interval of 4 or 5 feet. On the south side there were four distinct circles to be seen, but on the north only three could be detected, as the third outermost appeared to have merged into the external one; and, in accordance with this diminished number of circles, the breadth of the stockaded zone also diminished. The piles in the inner circle, which were strongly made, and showed evidence of having been shaped and squared by sharp-cutting instruments, were uniformly arranged at a distance of from 4 to 5 feet, and enclosed an area more of the form of an ellipse than a circle (measuring 61 feet by 56), while those in the second and third circles were more irregularly, but generally more closely, set. All these uprights (except a few on the north side of the inner circle) were linked together by horizontal beams having square-cut holes, through which the upper ends of the piles passed. The horizontal beams were arranged in two ways. Some lay along the circumference and bound together all the uprights in the same circle to each other, while others took the radial position and connected each circle together. Some of the latter were long enough to embrace three circles, and when this was the case I have noticed that the upright in the middle circle was sometimes firmly caught in a deep cut in the transverse, instead of passing through a mortised hole (see Fig. 190). Although the uprights in the inner circle were not linked together circumferentially along the whole course of the horizontal beams, the particular construction of the log pavement on the north side rendering this unnecessary, every one of them had a radial beam, directed from within outwards, which kept it from yielding to lateral pressure. This purpose was equally well served in several ways; sometimes the inner end of the radial beams pressed tightly against the upright, at other times the former projected half-way into the log pavement, where its end was firmly fixed by a thick pin passing through it into the under structures of the island, and its middle contained either a notch or mortised hole for holding the latter in position. The external ends of these radial beams were occasionally observed to be continuous with additional strengthening materials, such as wooden props and large stones.
The main object of the whole of this elaborate structural system was to give stability to the island, afford fixed points on its surface, and prevent the superincumbent pressure of whatever buildings may have been erected over it from causing the general mass to bulge outwards—objects which appeared to have been most effectually attained.
The piles in the outer circle were merely round posts, smaller and more closely placed than those in the inner circles, being sometimes only a few inches apart, and appeared to have been bound together by a transverse rail, into which their tops were inserted after the manner of a hurdle. Portions of these rails, pierced with holes, were found at the south-east side, though none actually in position; so that the inference that the outer stockade was intended as a fence or bulwark seems quite legitimate. In support of this view I may state that nowhere along its course were the piles connected together by horizontal beams, either circumferentially or radially, nor did they penetrate deeply, so that for giving stability to the island the outer circle would be of little use.
Log Pavement.—Like the other crannogs examined by me, this one also had its central portion roughly paved with wooden beams like railway sleepers. On looking at these beams carefully it was observed that many of them, especially those made of oak, had also holes at their extremities, and that the plan of being linked and fixed together by stout wooden pins was by no means peculiar to the marginal portion of the crannog. Here, however, they lay mostly in a radial position, and on the south side; some were distinctly seen to be joined with the uprights in the inner circle with one end, while the outer, which pointed to the centre, was firmly pinned to the wood below. In several parts this general network of large beams was covered over by a pavement made of small round logs, mostly of birch, and placed close together, but, being soft and easily removed, I could not be certain whether or not it extended over the whole area. If so, it must have been a secondary pavement formed after the crannog was inhabited, as marks of fire, with slag and ashes, were found in two or three places lying immediately on the large oak beams below it.
On the north side of the crannog the uprights in the inner circle were not linked together circumferentially by horizontal beams, because (as I have already remarked) the different structure of the log pavement here rendered this plan unnecessary. The reason of this was, that on this side a considerable segment of the log pavement was built up, for a depth of 2 feet or so, of several layers of those round logs of soft wood, laid transversely to each other, and carefully arranged flush with the outer edge of the uprights, so that the only direction in which the latter were free was counteracted by the radial transverses alone (see Fig. 190).
The space between this portion of the log pavement and the next circle of stockades was filled up with layers of turf and moss, the depth of which corresponded with that of the built-up edge of the log pavement. After removing the turf and moss from this space in one or two places, we came on the wood of the island, which here consisted entirely of birch-trees with the bark on, and looking as fresh as if they had been recently cut. The heather and moss also looked quite fresh, but soon, after exposure to the air, everything turned black. (See Plate IV.)
Remains of Dwelling-house.—Over the area of the log pavement there were here and there the remains of large uprights, which appeared to have been used as supports for some sort of dwelling-house. On the north side, a few feet from the margin of the log pavement, there were three or four of these, as if forming another circle, one of which I extracted with difficulty and found it to be 8 feet long, 7 of which were imbedded in the structure of the island. It was neatly formed of a rectangular shape (10 inches by 6), and its downward end was cut and pointed as if for insertion into a mortised hole. The centre of the log pavement was occupied by a mass of ashes, charcoal, and stones, forming a bed about 21⁄2 feet thick, being nearly the entire depth of the mound above the wood-work, and a little to the west of this, and situated between two large square-shaped uprights, there was a thin bed of charcoal and burnt straw, together with some flat stones covered with a quantity of slag. On the east side, near the circle of piles, conclusive evidence of another fireplace was observed, but no well-formed hearths were anywhere met with.