522 Dr. Mac Culloch on the Geology of Glen Tilt. 



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 perpendi- 

 cular, 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 



