32 THE UNDERGROUND WATERS OF NORTH-WEST YORKSHIRE. 
that at some more or less remote period there was a constant 
flow along the dry valley, and that the great recess at Malham 
Cove has been produced mainly by the agency of the plunging 
stream wliich has prevented the accumulation of protective talus 
at the Cove foot, besides drilling out a pool. Such pools are 
invariably found at the foot of waterfalls with a sheer drop, 
and by the recession of falls they commonly form ravines. That 
no such pool or ravine now exists at the foot of Malham Cove 
may be interpreted as proving that a sufficiently long interval 
has elapsed since the stream regularly flowed over the top to 
admit of the obliteration of the pool by the accumulation of 
fallen blocks, or by the action of the great spring which emerges 
there and still prevents the formation of any talus. 
It is worthy of note that springs emerging upon steep hill- 
sides commonly show the same feature of a scarp at the back, 
and for the same reason that the outflowing water prevents the 
accumulation of talus. 
The cause of the great sheer face at the Cove is one 
familiar to all geologists, viz. : the occurrence of a more durable 
bed to form the sill of the fall, as at Niagara, Hardraw Scar, &c. 
In this case the massive bed of limestone so well shown in the 
photograph has been a determining factor. (PI. X.). 
Upon the stage when the stream followed the surface channel 
only, and which may have been connected with the frozen state 
of the ground at the close of the Glacial Period, several phases 
of partial absorption and retreat must have supervened when 
successive sinks gradually developed. The surface flow would 
thus cease by a regular retrogression, and now it can be seen 
that the recession has reached yery nearly to the North Craven 
Fault. 
During the early stages of retreat the swallow-holes must, 
it would appear, have carried the water into the master-joints 
below the point named the Watlowes on the map, and so out 
at Cove ; the emergence being not necessarily all at the present 
outlet but sometimes at some height above it. There is a cave 
opening on the face of the Scar which may be one of these 
