Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/330

 mark the course of the Tumel and the Tay, and to exclude the sea which once flowed high up into the present estuary of the latter river.

The actual corrosion produced by the mountain torrents is most strongly marked in those which fall from the skirts of Ben Gloe to join the Tilt. It is easy to perceive in these, that the fortuitous inclination of the strata has not produced the intervals now occupied by the streams, but that beds once continuous have been cut through often at right angles to their direction, leaving their sections fairly exposed in the deep beds of these powerful agents. The depth of these sections is always striking and frequently enormous, exceeding an hundred feet; with sides almost perpendicular, while the dark and turbulent water roars, seldom visible, and scarcely audible, beneath. In the progress of waste and ruin the falling of the upper parts gradually produces a more open chasm, destined perhaps in the progress of time to form a glen such as those narrow and prolonged ones which constitute a frequent feature throughout this district.

To what extent these agents actually operate in changing the present surface of the globe, is an enquiry beyond the objects of this brief notice, but we have, in this valley as in numerous other parts of this country, abundant proof, that many of the most conspicuous and extensive alluvial deposits have had their rise in causes of a much more general and extensive nature.

Of this a very remarkable example is to be seen near the farm of Auchgowall in the lower part of the valley, extending to Gilbert's bridge. It is most conspicuous on the left bank, where it is seen covering all the hills to a considerable height and a great depth, its thickness being distinctly shown by the sections of the streams which descend to join the Tilt, and of which the courses are