Dr. C. Callaway — Cheddar Gorge. 69 



joints, whether open or concealed, and the zigzag course would 

 be maintained. The following figure illustrates my explanation. 



Diagrammatic plan of part of the Cheddar Gorge, showing the rectangular Jointing, 

 the open joints followed by the stream being indicated by the thick lines. 

 The arrow shows the dii'ection of the slope. 



The theory I have here expounded will apply still more aptly 

 to the second alternative — that the valley was excavated by an 

 underground river. Such a stream must have followed joints, for 

 they are the only conceivable channels, and a succession of zigzags 

 is the only form which the gorge could assume. 



But it may be objected that the course of the stream which carved 

 out the Cheddar glen was probably determined in an overlying 

 formation, Cretaceous or Jurassic. Let us inquire how this 

 hypothesis will work. Such a river flowed over a surface of Chalk 

 or other Mesozoic sediment. Vertically above the line of the present 

 glen it must ex hypothesi have described the same zigzags. It cut 

 its way down lower and lower until it flowed over a surface of 

 Carboniferous Limestone, and then it was found that the lines 

 of the zigzags were parallel with the joints in the limestone. Such 

 a coincidence is perhaps credible, but it demands the exercise of 

 greater faith than my nature is able to command. 



There is a second improbability in this hypothesis. Why do the 

 zigzags stop at the mouth of the gorge ? On my theory, this 

 ought to be the case, because it is here that the limestone ends 

 and the Trias comes in. But if the course of the river which 

 excavated the valley had been determined on a Mesozoic surface, 

 the rapid meanders would have been continued further on to the 

 south-west. We find, on the contrary, that when the present river 

 leaves the limestone, and passes on to the Trias, its course becomes 

 less serpentine. Yet in its journey of half a mile over the Trias 

 it falls only 25 feet. It follows from this that as the river cut its 

 way down to lower levels, its course south-west of the limestone 

 became less and less steep, and its flow less rapid. It ought, 

 therefore, according to the known laws of river action, to have 

 become more serpentine below the limestone. As the opposite effect 

 has resulted, we are driven to conclude that there was something 

 in the limestone itself, irrespective of the windings of the river 

 on its Cretaceous or Jurassic surface, which determined the present 

 zigzag shape of the gleu. 



Whether the river that formed the gorge originated on or in the 

 limestone on the one hand, or on a higher Mesozoic surface on. 

 the other, appears to be not very material. In the latter case, 

 it is to me incredible that the old curves should be maintained 

 in a rock traversed by joints, many of which the solvent action 

 of river- water must have quickly converted into fissures. 



