In examining such a valley as that which is the subject of the present paper, the course of the river, as well as the ranges of hills which bound it, must necessarily enter into the history of its geological formation. Where these are of similar structure it is easy to comprehend the whole in one line of description, taking the course of the river for a guide. But in Glen Tilt a dissimilarity so complete is observed between the opposite sides of the valley, and the numerous and interesting phenomena which occur in the bed of the stream itself are so dissimilar to both, that it is impossible to convey an adequate idea of the whole structure without a distinct examination of the three several lines. A perpetual transference of the imaginary spectator from one point to another, would produce as much confusion in the narration, as such a proceeding in examining the ground would obscure the judgment and puzzle the investigation of the real observer. I have therefore chosen to detail the examination of the three lines separately, and have commenced with the river as the most natural if not the most useful method, describing in succession the ranges of hills, of which the history is required to render the phenomena which are to be seen in the river intelligible.
The Tilt has its rise in a small loch not far from that long valley which is the common division of the waters running east and west to join the Dee on one hand and the Tumel on the other. As it enters that valley it falls immediately into a line tending westward, from which it deviates towards the south-west before its termination in the Garry, the narrow bottom of the glen affording but little room for lateral deviations, and its direction being subject to no material variation. For two or three miles its course lies so much through alluvial matter that it offers nothing to the spectator; nor does any material feature occur before its junction with the Tarff.
