68 Dr. C. Callaway — Cheddar Gorge. 



Black Eock, the elevation is 511 feet, and, less than two miles due 

 north of this point, the highest summit of the Mendips reaches 

 1,068 feet, or a rise of about 304 feet per mile. The slope in the 

 lower half-mile of the gorge is therefore little less steep than the 

 upper half of the side of the mountain. A stream running down 

 this incline would be a torrent, and we should not expect it to 

 meander rapidly in homogeneous rocks, whether it flowed above 

 ground or below. A special cause of the curving or zigzagging 

 was therefore to be looked for. 



It is well known that the Cheddar gorge is excavated in the 

 Carboniferous Limestone, that the strata dip at about 20° to the 

 south or a little to the east of south, and that in a general way 

 the valley runs along the strike, so that on the south side the 

 outcropping strata overhang, and often form lofty cliffs, while on 

 the north the beds slope gently down to the floor of the valley. 

 A stream rushing rapidly along the strike of hard strata might be 

 expected to carve out a tolerably straight course. 



I ascertained that the serpentine course of the valley is caused by 

 the jointing in the limestone. The rock is traversed by two systems 

 of joints, which appear to be vertical or nearly so, intersecting 

 approximately at a right angle. This fact presents itself at almost 

 every turn in the glen. The joint-surfaces are seen to form salient 

 and re-entrant angles, the former opposite the latter, so that if the 

 two sides of the glen were brought together they would seem to 

 interlock. This correspondence has probably given rise to the 

 supposition that the gorge owes its origin to a zigzag rift, followed 

 by a movement asunder of its two sides. 



The joints are often closely approximated, so that, in quarrying, 

 the limestone readily breaks away in cubical blocks. But this is 

 not material to the problem. If the joints are not open ones, they 

 will not admit the passage of water, and a rock traversed by them 

 is practically impervious. 



That the jointing has caused the zigzags in the glen is apparent, 

 whether the stream flows above ground or below the surface. Take 

 the former alternative. The general direction of the movement would, 

 be along the diagonals of the parallelograms made by the joints, for 

 that is the direction of the present slope of the valley, and it is the 

 direction supposed to be taken by the present subterranean river.' 

 But the water would soak along open joints, and widen them into 

 fissures, and the flow would be diverted into these fissures. The 

 movement would be sometimes to the right, sometimes to the left ; 

 but it would always tend to return to the line of the diagonals. 

 The stream, after flowing for some distance along an open joint or 

 fissure (say) to the right, would be compelled by gravity to seek 

 a passage to the- left, and this would be afforded when the next 

 fissure in that direction was reached. The natural course of the 

 current would thus be determined by the open joints, and would 

 follow a zigzag line. The channel thus produced would be gradually 

 enlarged into a valley ; but the aerial weathering would act along 

 1 Biickland & Conybeare : Trans. Geol. Soc, ser. ii, vol. i, pt. 2, p. 223, note. 



