Civ PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I906, 



shatter-belt, but runs down a glaciated rock, in a wide groove only 

 about 3 feet deep. These two cases give us an indication of the 

 importance of shatter-belts in accelerating denudation. 



This, however, is not all. The groove in the latter case does 

 not possess a smooth, concave surface, but has angular sides, and 

 the bottom is also angular, with notches cut along the dominant 

 points. Abrasion of the general surface of the rock appears to 

 have little effect, but the water saws along the joints, gradually 

 detaching the blocks, which are then carried away in flood and 

 rounded into pebbles. This is the general character of the rocky 

 waterworn gorges situate in the volcanic region, and not only 

 shows the importance of planes of weakness in determining the 

 amount of denudation in the case of hard rocks, but also accounts 

 for the small amount of mud carried by streams coursing over these 

 rocks as compared with the muddy streams of the Skiddaw-Slate 

 tract. The comparative clearness of flooded streams when running 

 over hard rocks does not, therefore, necessarily indicate that these 

 streams have established their base-lines, but may be due to the 

 different nature of water-erosion acting upon well-jointed hard 

 rocks and on ill-jointed soft rocks respectively. In the action of 

 streams along these joints we have a forcible indication of the 

 work which is done along the shatter-belts which consist of blocks 

 of hard rocks separated by joints, in addition to which there is 

 much easily-eroded crush-material that has been developed along 

 the joint-planes. 



We will take one or two other examples of the formation of 

 gorges along shatter-belts. 



Of a similar nature to Black-Moss Dub, but on a larger scale, is 

 the gorge at Birks Bridge in the Duddon Valley. Above, as in 

 the case of Black Dub, is an alluvial flat, once occupied by a lake. 

 A dry valley lies to the east, while the Duddon now flows 

 through a deep gorge, headed by a fall. The gorge below the 

 bridge is 23 feet deep, of which 8 feet are under water at ordinary 

 times. The bridge is pierced with holes above the road, for the 

 flood- waters to drain through, these holes being but 3 feet below 

 the top of the rock, so the gorge must be filled to the brim in times 

 of flood. In this case, again, the shatter-belt structure may readily 

 he seen. 



The two cases noticed above are on the floors of important 

 valleys, but the greater number of examples of streams occupying 



