HERTFORDSHIRE NATURAL HISTORY SOCIETY. 
XXXI 
Tertiaries into the Chalk, and so lessening the volume of water 
they convey above the surface, and these swallow-holes have been 
enlarging. It may he asked: Has not the Yer been doing the 
same ? It has not, for nowhere is it a stream flowing over clay; 
it is a Chalk stream from its source to its junction with the Colne, 
and it is not subject to floods as is the Colne. It is not intended 
that it should be inferred from this that the Yer is not now de¬ 
creasing in volume to some slight extent, for there is evidence that 
it is doing so, probably owing to the lowering of the plane of 
saturation of the Chalk, but instead of its waters being conveyed 
away through swallow-holes, its volume is increased by springs in 
its bed. 
“ But this is not all. Lower down the valley, just above its 
junction with the Yer, the Colne must at one time have had 
its waters considerably augmented by a stream flowing out of 
Elstree Reservoir. In this stream there are now swallow-holes 
which usually take all the water which drains the London Clay 
area between Shenley, Elstree, Stanmore Heath, Letchmoor Heath, 
and Radlett. This water is thus conveyed underground, and when 
it is required the swallow-holes have to be plugged. Moreover, 
along the margin where the Tertiaries cease to cover the Chalk, 
there are several other swallow-holes, not now in the bed of any 
stream, but, nevertheless, they convey away water which may once 
have formed streams flowing into the upper part of the Colne. 
We may thus add about 20 square miles to the 40 we had before, 
making an area of about 60 square miles in the upper basin of 
the Colne, the rain falling on which is now usually absorbed by 
swallow-holes and conveyed into the underground Chalk reservoir, 
but long ago formed streams which must have augmented the 
volume of the Colne above its junction with the Yer. Much of 
this water may now find its way into the Colne below the junction 
of the two streams; the springs at Otterspool, near Watford, which 
are known to many of the members of the two Societies here 
present, are probably partly fed by it. Some of it is doubtless 
pumped up from the Chalk at waterworks and in numerous wells in 
Watford and Rickmansworth and their vicinity; and some probably 
flows on in the Chalk towards the valley of the Thames, or, following 
the plane of dip in beds of flints in a south-easterly direction, is 
intercepted at the deep borings of the Hew River Company in 
the valley of the Lea. 
“ I think it will be evident that the Colne must at some former 
period have been, above its junction with the Yer, a much larger 
stream than it is now, while we have no reason to suppose that the 
volume of the Yer has very materially decreased; and to this fact it 
seems likely to be due that the name of the Colne pertains to that 
which is now much the smaller stream. We have seen that it was 
smaller in Chauncy’s time, nearly two centuries ago, but I think 
we have some evidence that the bed of the Colne was not, as it now 
is, usually dry through Mimms Park a century before then, when 
the fine mansion, known as Horth Mimms Place, was built. This 
