AND THE AVON GORGE. 
191 
of the Downs. Instead of doing so, its cuts a notch through the 
Downs. The natural course of the Stoke Trym would seem to 
be to follow the line of the softer Old Red depression. Instead 
of doing so, it cuts a notch in the Dolomitic Conglomerate. In 
each case the explanation is the same ; not that a rent has been 
formed in the ridge to meet the special needs of the stream, but 
that the notch in the ridge, and the wider and more general 
depression above the ridge, are the different products of different 
modes of denudation. The one is due to special denudation; the 
other to general denudation. When the stream began to cut its 
notch into the ridge, the ridge as such had no existence. 
Above the point at which the stream is joined by the Stoke 
branch, the valley of the Trym is a notch of no very great depth, 
hut with the characteristic vertical cliff which is generally to be 
seen on one side or both of the valleys cut down into the 
Dolomitic Conglomerate. Here it calls for no further remark. 
But near the point of junction of the Henbury and Westbury 
branches, it is worth noting that it flows in a line of depression 
of Lower Limestone Shales between the Mountain Limestone 
ridge and the Old Red hill upon which Coomb House is situated. 
The Henbury Trym when it emerged from the Mountain Lime- 
stone took the course of least resistance through a depression 
long ago formed in the Lower Limestone Shales, and subse- 
quently filled in with Dolomitic Conglomerate. 
The Westbury branch rises in the Mountain Limestone, or 
rather in the Lias that rests upon it, N.E. of the village of that 
name. It does not present any features that call for notice. 
The Henbury branch, on the other hand, is of great interest. 
It again exemplifies the notching of a ridge by special denudation. 
Rising in the Rhoetic some two miles to the N. of Henbury, its 
natural course, on the present configuration of the land, would 
seem to be, to flow W., over comparatively low ground, to the 
Severn. But instead of doing this, it boldly makes for the 
