The proximity of Buston and Lochlee, as well as the topographical similarity in the surrounding landscapes, would indicate that this great change took place about the same time in both these localities. Long before this, however, both canoes were finally sunk or abandoned, the interval of time being measured by no less than 4 feet of a deposit. Again the entire time that has elapsed since the gangway at Lochlee was laid down till the lake was drained, and tally ceased to be kept, is represented by 10 feet.
Subsidence of the Crannogs.—Before quitting this somewhat speculative line of research, one other subject remains to be discussed, viz., the submergence of the Lake-Dwellings. This phenomenon is a uniform feature in all those hitherto examined, and though the causes of it have not been much inquired into by previous writers, they will, I think, be found of sufficient importance to merit the attention of both the antiquary and the geologist. There are just two immediate causes to which this result can be assigned: viz., either a subsidence of the surface of the island, or a rising of the waters of the surrounding lake; or, as may happen sometimes, a combination of both causes. The physical agencies that are likely to operate in producing a subsidence of the island may be categorically stated thus:—
(1.) A compression of the island due to consolidation or decay of its structural materials.
(2.) A sinking of the whole mass of these materials in soft mud as a direct result of weight.
(3.) A general compressing and sinking of the sedimentary strata of the lake-bed.
When the deeper structures of the Lochlee crannog were examined, it was particularly noted that there was no flattening of any of the large logs, as if they had been subjected to great pressure. At Lochspouts and Buston, so far as the water permitted of similar explorations being made, this observation was equally applicable to them. Though quite soft, the logs always preserved their original shape and contour. One day, at Lochspouts, I was greatly puzzled by finding what was evidently portion of a birch-tree, from 6 to 9 inches in diameter, quite flat, and with scarcely any wood left inside the thick bark. In no instance previously had I seen the evidence of pressure on logs of this size; but after carefully considering the point, it was ascertained that such effects occurred only in the upper portion of the mound, and above the log pavement, where the wood had been exposed to atmospheric influences, so that when the woody fibres rotted away, the flattening of the bark was easily produced. All the logs found buried in water or mud retained their original dimensions, and showed no traces of having yielded to superincumbent pressure.
In calculating the pressure of the entire crannog on the lake bottom, it is only necessary to take into account the weight of the materials above the surface of the island, as the greater density of the displaced water would act as an upward pressure sufficient, before the mass attained its equilibrium, to allow the surface of the island to project a few feet above the level of the water, the amount varying according to the depth of the lake. After the island grounded, if constructed on the principles suggested at page 262, any additional structures would act as a direct weight on the bed of the lake; but in estimating the final and total submergence due to this element, we must consider the weight of rubbish gradually accumulated during the period of occupancy. As the base of the island at Lochlee was only 4 feet below the level of that of the gangway, it follows therefore that the maximum result due to the weight of the island could not exceed this amount.
But the most important cause of submergence is the gradual compression or consolidation of the lake-bed due to the increase of its sediment. The depth of this increase at Lochlee, at least since the gangway was laid, was found to be not less than 10 feet. Independent altogether of any chemical changes going on in the sediment, however gradually formed, its own weight must have acted very considerably in pressing the lower strata into less bulk. Its accumulated depth, however, is far from giving a correct indication of the rise in the bed of the lake; in fact, 10 feet of silt might not raise the latter to half this extent. Another thing, which must not be forgotten in this discussion, is the subsidence which takes place after bogs and marshes are drained. This is a fact well known to those conversant with the effects of drainage. In the Old Statistical Account of Scotland, vol. xi. p. 163, I find it stated that in three years after the drainage of Kinordy Moss, its surface sank 3 feet; and Sir George Grant Suttie, writing of a drained marsh at Balgone, says that after drainage its level sank from 3 to 4 feet (see footnote, page 249). Bogs are in fact like sponges saturated with water, swollen to such an extent that they occupy a much larger space than their solid materials would otherwise do. It will also be remembered (page 191) that, after the last and more careful drainage of Loch Buston, some five years ago, the subsidence of the Knowe was sufficiently noticeable to attract the attention of the farmer.
To assign more accurately to these agencies the respective amount of subsidence due to each is impossible, but that their combined effect is sufficient to account for the total submergence of the principal lake-dwellings hitherto examined is proved by the measurements and observations made at Lochspouts (page 168), which show a minimum result of 10 feet.
If the above reasoning be correct, little importance remains to be attached to the rising of the waters as a cause of submergence, even in the exceptional circumstances where the agencies that produce this effect, such as the destruction of the forests and the increase of peat bogs, are known to have been in operation. It occurs in localities where the outlet is level and the flow of water sluggish. Those of the Irish writers who have taken notice of the phenomenon generally assign it to this cause. Sir W. R. Wilde thus refers to it: "We likewise learn from their recent submerged condition how much water had accumulated on the face of the country since their construction, probably owing to the great decrease of forest timber and the increased growth of bog. From the additions made to the height of the stockades, and also from the traces of fire discovered at different elevations in the sections made of these islands, it may be inferred that the rise of the waters commenced during the period of their occupation."—(Wilde's Catalogue, p. 221.)
The observations made at Lochlee led me to ascribe an exceptionally large share to this element as a cause of the submergence of the crannog, but since then further investigations have proved that the phenomenon takes place to a similar, if not greater, extent in localities where no rising of the surface of the waters could have occurred.