Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/331
marked by extensive ruin: the bare schistose foundation has also been in many places exposed. The alluvium in question consists of the materials of the surrounding country, but all of them water worn and mixed with sand and gravel as from considerable attrition. In this, as in numerous other examples, there is no assignable mode in which the materials could have been either formed, or deposited, by the slow action of the existing streams, while the total absence of a similar deposit in other parts of the valley equally subjected to similar causes, bespeak a different origin. The cause appears to have been of a diluvian nature, and the determination of the deposit to this point, may perhaps be found in the form of the valley at this place and the obstruction which it has offered to a mass of matter impelled through the upper parts where no such obstruction to its deposition existed. Such an obstruction as that which I have suggested might formerly perhaps have been rendered more complete by the continuity of the strata beyond this point, which has since been gradually destroyed by the slow action of the Tilt: in consequence of this the deposit of alluvial matter has itself gradually diminished, sliding successively down the hill as it has been undermined, and together with the more gradual abrasions of the land in the upper parts of the valley, hurried by the daily course of the river to the Garry.
The probability of this supposition is strengthened by the occurrence of a similar circumstance at no great distance. This may be seen near the junction of the Garry and the Tumel in the hills above Fascally; and the deposit is here also of enormous thickness and of similar materials. Like that in Glen Tilt, it occurs where the valley appears to have favoured the accumulation of a diluvian deposit, and where the subsequent action of the river in its rapid and corroding progress downwards as it runs through the narrow