HERTFORD SHIRE NATURAL HISTORY SOCIETY. 
XXIX 
Field Meeting, 19th April, 1890. 
NOBTH MIMMS, HATFIELD. 
This was a combined meeting of the Hertfordshire Natural 
History Society and the Geologists’ Association of London. 
The party assembled at Potter’s Bar Station, and, under the 
guidance of Mr. Upheld Green, F.G.S., walked by way of Mimms 
Hall and "Warren Gate to Potterells Park. From Mimms Hall the 
walk was by the side of the bourne which drains the London Clay 
area on the south. In Potterells Park are some large cavities or 
“Swallow-holes,” through which the water generally descends into 
the Chalk, but upon this occasion the bourne was dry, and the 
stream did not how into the holes. Assembled on the slope of the 
dell in which the swallow-holes are situated, Mr. Upheld Green 
alluded to the loss the party sustained by the absence, through an 
accident, of Mr. Hopkinson, who was to have acted as conductor. 
He had, however, sent the following paper, which Mr. Green read, 
on 44 The Biver Colne and the Swallow-holes at Potterells.” 
44 It is a very general rule that the name of a river is retained to 
the head of its largest branch, which, bringing down the greatest 
quantity of water, drains the largest area. If there is a deviation 
from this rule, it is when the largest stream takes an abrupt turn, 
the name then sometimes being carried by a smaller stream which 
takes a more direct course. It has been pointed out that to comply 
with this rule the name Colne ought to have been carried up to the 
stream known as the Yer, for not only is the Yer a much larger 
river than the Colne at the junction of the two streams, but it also 
maintains the general direction of the united stream quite as much 
as does the Colne. That the Colne, above here, was a smaller river 
than its tributary the Yer, was remarked upon so far back as the 
year 1700, for Chauncy, in his 4 Historical Antiquities of Hertford¬ 
shire,’ says: 4 The Colne, which springs forth near Tittenhanger, 
then passeth the road near Colney Street, and, running above two 
miles in length, meets the Yerlume (Yer) near Park Street, but 
though the Yerlume is much the greater stream, yet the Colne 
usurps the glory of her own name, and floweth thence to Watford.’ 
44 1 am not aware that any explanation of this anomaly has yet 
been offered, but I think that we may possibly have a key to it at 
this spot. The swallow-holes here absorb most of the drainage of 
about 40 square miles, the stream from the south draining about 
35 square miles, and that from the north about five. The whole 
of the London Clay area between Bell Bar, Northaw, Potter’s Bar, 
Chipping Barnet, Barnet Gate, Green Street, Shenley, Badge Hill, 
and North Mimms, is drained by the southern stream; while the 
northern stream drains the area between North Mimms, Hulsham 
Green, Bell Bar, and Brockman Park. A glance at a geological map 
will show that these streams and their tributaries have materially 
altered the configuration of the country, the northern stream, 
which has several small tributaries flowing from under the glacial 
gravel between Hatfield Park and Bell Bar, having almost com- 
