54 J. HOPKINSON HEETFORDSHIRE RAINFALL, 



might be carried down it) ; and when the level of the underground 

 water had been so raised by the winter's rain that a lake was 

 formed at this point, the water flowing out of it along the usually- 

 dry channel of the Colne through j^orth Mimms Park. In this 

 park there is a mansion overlooking this dry river-bed. As it faces 

 the north, I think it must have been built when water flowed 

 much more frequently from Potterells than it does now ; for surely 

 the mansion would not have been built with this aspect, to overlook 

 the bed of a river which is now dry except for a short time during 

 or after very wet winters. 



I have long been puzzled to account for the phenomena here 

 presented, until it occurred to me that the plane of saturation in 

 the Chalk may have been lowered as far as this spot by the 

 abstraction of water at the pumping-stations in the Yalley of the 

 Lea, 8 miles to the east, or E.S.E. I have since then ascertained 

 that the late Eobert AVilliam Mylne formed the opinion that the 

 Amwell Springs in the Yalley of the Lea are fed by water from 

 these swallow-holes. Whether this be the case or not, it will be 

 apparent that water getting into the Chalk here and meeting with 

 beds of flints, will be more likely to follow the dip of these beds 

 towards the S.E. than it would be to follow, underground, the 

 course of the Colne in a directly opposite direction to the dip of 

 the Chalk, for it could only follow this direction by percolating 

 through beds offering varying degrees of resistance to its passage, 

 which would not be the case if it followed the dip of the Chalk. 



If the water-level in the Chalk, in whichever direction from the 

 river these swallow-holes convey water away from it, were higher 

 than the bed of the river, the channels which now usually convey 

 water from the river would convey water from the saturated chalk 

 into the river. Even if they do not directly communicate by open 

 passages with higher strata, hydrostatic pressure would cause the 

 water in them to rise into the river. This may occasionally happen 

 now, and if the channels existed when the plane of saturation 

 inclined downwards from the higher ground around them perma- 

 nently, as it now only does occasionally, these channels, if then 

 existing, would have been the channels of springs. 



The water flowing down the Colne, where it is joined by its 

 tributary the Ver, is now usually only a small fraction of that 

 flowing down the Yer. If such had been the case when these 

 rivers were named, the Yer would have been considered the main 

 stream, with the name of Colne, or the river below the junction of 

 the two would have been called the Yer. 



The catchment-basin of the Colne, above its junction with the 

 Yer, has a greater area than the catchment-basin of the Yer, the 

 chief reason why the Colne is here so much smaller than the Yer 

 being that so much water is lost to it in swallow-holes. Another 

 reason is that the Yer drains a larger area of permeable beds. 



The Yer is much farther from any point of great abstraction 

 of water than is the iippcr part of the Colne, and its waters ai'e 

 considerably augmented by springs in its bed at various points. 



