278 UNDERGROUND WATERS OF NORTH-WEST YORKSHIRE. 
and the water level in ordinary weather 25 feet lower. It will 
therefore be seen that the water at the bottom of Alum Pot is 
approximately on the same level as that in Footnaw's Hole 
and also in Turn Dub. Since the water is in motion through 
this underground passage, which must be constantly filled, the 
friction will probably be sufficient to account for the small 
difference of level. 
In dry weather Footnaw's Hole (PI. XXXV., Fig. 1) appears 
as a wide cleft in the limestone, with sloping banks of silt and 
sand round two sides, and precipitous limestone rocks on the 
other two. When the streams are in flood after heavy rain, or 
during the melting of snow, the water in Footnaw's Hole rises 
to the Hp and flows over down Footnaw's Beck into the Ribble. 
Turn Dub is rarely, if ever, dry, while it is only in exception- 
ally wet weather that water flows from Footnaw's Hole. 
Thus it would appear that Footnaw's Hole is a flood outlet, 
and onty comes into operation when the underground passage 
leading to Turn Dub is full and, therefore, unable to take the 
excess of water. As the lip of Footnaw's Hole is just below 
the 825 -foot contour, and Turn Dub just below that of 800 feet, 
there cannot be a fall of more than 25 feet from the former to 
the latter. 
Further, since in ordinary weather, when the stream is 
issuing from Turn Dub only, the water in Footnaw's Hole stands 
some 20 feet below the ground level, it will be seen that there 
must be a syphon -like passage below the river ; and since this 
passage must be constantly filled with water up to the level 
of the overflow of Turn Dub, it will account for the very slow 
passage of the fluorescein over at least this part of the journey. 
Since the water passes beneath the River Ribble it follows 
that there must be some impervious cover, because if this w ere 
not the case the water of the underground stream would find 
an escape at the lowest point, namely, in the bed of the river, 
and would not, as is the actual case, pass under that stream 
and rise some 10 or 12 feet above it on the opposite bank. 
With a view to ascertaining the nature of this impervious 
cover and its thickness, it w^as determined to carry out a series 
of boring operations in the alluvial flat between Turn Dub and 
