130 THE LowER NITH. 
index of the tremendous erosive power of the river in cutting 
this narrow passage through the hard greywacke rock, then 
we must admit that the width and depth of the valley will be 
graded according to the size of the stream and the length of 
time it has been at work. The hills of Pennyland are over 
1000 feet above sea level, while the river bed is 100 feet; this 
gives a gap goo feet deep. Assuming that the ancient river 
has been no larger than the present one, such a vast amount 
of rock cutting would require an enormous period of time. 
When the river encountered the soft sandstones of the New 
Red Basin its valley walls were corroded to a greater degree 
than the harder rock upstream. Hence at Duncow to Cow- 
hill the valley is two or three miles wide, with a flat bottom 
and flaring sides. The shape is different from that of the 
Blackwood gap, but the amount of work done bears the same 
proportion in both cases to the length of time at work. A 
cross section at both points gives profiles of a valley of a 
mature age. Down stream the Nith is augmented by the 
Cluden and other tributaries; yet at Castledykes it can only 
show an excavation of the breccia band to the extent of 15 
feet. Evidently the river has not had the necessary time to 
grade its channel; it is a creature of yesterday. The work 
of gradation is just in its infancy. 
The third type is best seen a little south of Glencaple. 
The hills behind Kenneth Bank are nearly 300 feet high. 
This gives a gap of about one-third of the Blackwood gorge 
of goo feet produced by a stream greatly enlarged by tribu- 
taries. The Glencaple gap is evidently deeper than it seems. 
The valley walls on both sides of the river are steep, and the 
flood plain of mud which lies in the centre is nearly a mile 
wide. If the same angle of slope of the valley sides be ex- 
tended downward below the flood plain until they meet in the 
centre, another 150 feet of valley presently beneath sea level 
would give a total valley excavation of 450 feet. It is evident 
that the Glencaple gap, although of great age, is very much 
younger than the matured Duncow to Cowhill trough. It is 
no true index of the age of the Nith, but it suggests a younger 
life having been engrafted into older surrounding. The 
valley is a misfit. 
