514 L. C. Miall — Formation of Swallow-holes. 



once by a reference to some vague natural force, he will come to 

 perceive the somewhat complex conditions which a complete ex- 

 planation must satisfy. 



The feature which is most likely to impress him, from the number 

 of examples which he will meet with in a small area, as well as 

 from its rarity in non-calcareous strata, is the perpendicularity of the 

 sides of these cavities. The drift and alluvium at the surface may 

 have converging slopes, beds of shale intersected by the chasm will 

 only rest at lov,^ angles, but a swallow-hole in a thick-bedded lime- 

 stone has commonly vertical sides, and the weathering which it 

 undergoes daily does not tend to derange its general shape. We 

 cannot, I think, dispose of this peculiarity by a summary reference 

 to the action of carbonic acid dissolved in rain-water. It is necessary 

 to show further why that force, or any other concerned, should pro- 

 duce (but only under conditions which are rarely united) a kind of 

 vertical denudation. 



When we collect all the facts of observation which relate to 

 swallow-holes, we find that the significance of some of them is not 

 immediately obvious. We recollect many examples of these pits 

 which have fluted sides, and some have also fluted pillars standing 

 upright in the midst. We notice that they are very commonly sunk 

 through a layer of drift or alluvium, which forms a funnel-shaped 

 mouth, and often chokes up the bottom. They generally receive a 

 stream of water, and always have an outlet, so that they never fill 

 during rain. They commonly occur near the outcrop of a bed of 

 limestone, as might be expected from the greater exposure of the 

 rock at such points to denuding agents. 



It is not often that swallow-holes are sufficiently accessible to 

 render a minute examination easy. Some of the largest in Craven, 

 such as Thund Pot and Hellen Pot, are difficult to approach, owing 

 to the steepness of the drift slopes. There are easily-explored 

 swallow-holes, which are nearly filled with debris, and present only 

 a conical pit with grass-grown sides. But there is one spot which 

 offers every advantage to the inquirer. Near the mountain-road 

 which leads from Hawes at the head of Wensleydale to Muker in 

 Swaledale are several of these pits close together. Their fancied 

 resemblance to churns has acquired for them the local name of 

 " Buttertubs," and the adjoining road is called Buttertubs Pass. 

 Here we find a number of perpendicular excavations in the main 

 limestone of the Yoredale series. The limestone is here about 75 

 feet thick, and the pits pass downwards from near the top of the 

 bed to its base. They are striking examples of one form of the 

 swallow-hole, and present peculiar details which it is not easy to 

 interpret. Some of them receive runlets of water, others are dry. 

 The sides are vertical, and generally fluted. We observe fluted 

 pillars also, and remark that the surface of the ground is covered 

 with drift. On one side is a narrow ravine with a steep face of rock, 

 a few yards from the Buttertubs. The surface of the limestone near 

 the edge is bare, or nearly so, and we thus get a good view of the 

 cavities from level ground, and can approach or enter them from that 



