That broken planks and old mortised beams were mixed up with the trestle-work in various places, would support the idea of a prior structure; while evidence that the whole superstructure had at one or more times been destroyed by fire was quite conclusive. According to this theory, the elaborate mortised beams at the north-east corner might have been a landing-stage, but, in their present position, they are quite inexplicable.
3. Level of Lake.—Amongst the problems of a discursive character here referred to, perhaps there is none of greater interest than that which deals with the cause and effect of the change that has taken place in the level of the lake. From eye-witnesses we know that, before the first drainage was carried out, the mound used to be covered with water in the winter-time; and Mr. Charles Reid tells me that the line of level which he has adopted in measuring for the plan of the lake is 8 feet 7 inches above the log pavement. Now the area assigned to the lake by Mr. Reid is considerably less than what the old residenters of the district make it out to have been, as several of them have stated that they had seen its waters extend beyond the road on the west side (see Plan), and yet from his data the depth of water would just cover the highest part of the mound, which, it will be remembered, was about the same height above the log pavement. Originally the island must have been higher than the lake, but allowing that the log pavement was only 3 feet above the surface of the water, we have at least 11 feet of change of level to account for. This phenomenon could only be caused by a sinking of the whole island, or a rising of the water, or a combination of both causes. I do not think that the weight of the island and its superincumbent mass would press so heavily on the bottom of the lake as to cause it to sink much, since the enormous amount of wood-work, of which it consisted, being lighter than water, would have a corresponding buoyant effect, and so help to counteract the weight of the aërial portion. Nor has any great compression of its substance taken place from decay, because in the course of making the deep section under the log pavement, we found the contour of the large trees quite symmetrical and perfectly round; and although the wood was very soft it was not compressed, owing to its being completely saturated with water, which of course is virtually incompressible. Although I have often seen small brushwood flattened from pressure, yet I have never seen this effect produced on a branch larger than my wrist, and only in one instance did I notice it on a piece of wood of this size. Moreover, the gangway, which certainly could not sink from its weight, was deeply buried, its uppermost horizontal beams being not less than 7 feet below the surface of the field. We must therefore fall back on the only other alternatives, and assign this change in the relative position of the crannog and level of the lake either to a general compression and sinking of the bed of the lake or to the rising of the water. The latter result is somewhat unusual, because running water, having a tendency to deepen its channel, and the accumulation of sedimentary deposits, often produce an opposite effect, and cause the complete drainage of lake basins. I have therefore carefully examined the outlet of the lake to ascertain if possible the causes that led to this rise in its bed.
Its natural outflow was at the south-east corner, and the little stream, after running southwards for a few yards, quickly turned westwards into a narrow valley which wended towards Fail Loch. Just at this abrupt turning the background rose somewhat steeply to the south, so that the termination of the valley as it entered into the Lochlee basin was very liable to be obstructed by débris washed from the slopes above. Besides being thus favourably situated to catch washed-down materials, it is probable that during the summer the surplus water would be very scanty, and vegetation abundant, so that in the course of time the bed of the outlet would gradually be raised. A section cut across the outlet would readily disclose the sequence of the silted materials, had it not been that the soil was disturbed by a deep covered drain, which was made when the first drainage operations were executed, and ran along its whole course. Also, I understand that previous to this the lake was used as a mill-dam. We cannot therefore get rid of the elements of uncertainty in any calculations which might be based on the change of the level of the lake and the accumulation of silt in its bed and outlet.
I may however mention, on the grounds already stated, that since the foundations of the crannog were laid, the increase in the bed of the lake in its vicinity cannot have been less than 10 or 14 feet; and 11 feet is the lowest estimate that I can assign to the submergence of its surface. On the supposition that this apparent encroachment of the water was uniform, and since the last fireplace was about 6 feet above the lowest, and allowing 11⁄2 feet for the time the former was used, we have then the total period of occupancy of the crannog represented by 71⁄2 feet of total submergence. We have no means of comparing this period with its representation in so many feet of lake sediment, but I may state that since the canoe, found about 100 yards from the crannog, was abandoned, no less than 5 feet of this mossy lake sediment accumulated over it.
PLATE II.
PLAN & SECTIONS OF
Crannog at Lochlee.
The composition of the silt forming the bed of the lake, especially near the crannog, as already described at pp. 98, 99, points to the fact that for centuries the increase was due principally to the decomposition of vegetable matters, while latterly it was caused more by a deposition of fine clay; and when excavating along the line of the gangway we had an opportunity of verifying the regularity of this succession. A change so marked in the sediment can only be accounted for by a corresponding change in the surrounding scenery, and no explanation is more likely than that the primeval forests had given place to the inroads of agriculture, when some of the upturned virgin soil would be washed down, as it still is, by every trickling rill that finds its way into this lake basin.