XXX 
PROCEEDINGS OF THE 
pletely cut through the Tertiaries so as very nearly to cut off the 
Heading Beds from the main mass and form of them an ‘outlier’ 
between Hatfield and North Mimms; and the southern stream and 
one of its branches having nearly made an outlier of the London 
Clay and Beading Beds between North and South Mimms and 
Shenley. 
“ Let us consider one of the results of this. These rivers, at one 
time flowing throughout their entire course over impervious London 
Clay, have now cut down their beds to the Chalk for much the 
greater part of their course, and when this is the case we know 
that water no longer flows on the surface unless the plane of satura¬ 
tion in the Chalk comes to the surface. The Chalk, being permeable, 
absorbs the water and becomes saturated, the plane of saturation 
rising slightly from the valleys towards the hills. But the Chalk 
absorbs water very slowly, and therefore, after heavy rain, water 
may flow for a time in a Chalk river-bed when the plane of satu¬ 
ration does not come up to this bed. Some of the water is thus 
gradually, imperceptibly, sinking into the Chalk while there is 
sufficient to spare to form a river on the surface; but sooner or 
later the water will find a more rapid way into the Chalk than 
through mere absorption. A weak spot or fissure is met with 
and down sinks the water. Having once found a way, almost every 
drop enlarges the crevice by the mechanical removal and chemical 
dissolution of the chalk, and thus ‘ swallow-holes ’ appear on the 
surface, and ‘ pipes ’ form underground. At first there may be 
merely more rapid percolation, the pipe filling with gravel and 
sand, and the insoluble residue of the Chalk, such as flints, but in 
course of time a channel is sometimes formed, as in several swallow- 
holes here, down which the water rushes to the plane of saturation 
with ever-increasing volume. 
‘ ‘ At this spot there are channels which the water has gradually 
made for itself down which a man might be carried. I have seen 
these swallow-holes when no water reached them; I have seen 
them when a powerful stream came from the south and rushed 
down them in torrents with a roar which I heard long before 
I could see what was its cause; and I have seen them, or rather 
the spot where we know they are, when they were covered by a 
sheet of water forming a peaceful lake into which streams flowed 
from north and south, and out of which a stream flowed on through 
Mimms Park and into the permanent river of the Colne. Then 
doubtless the plane of saturation in the Chalk was raised to above 
the apertures of the swallow-holes, in fact to the level of the water 
in the lake. 
“ I do not know when these rivers received their names, but the 
Yer, or Yerlam, must have been so named when the Bomans in¬ 
vaded Britain and attacked the old British town of Yerulam, and 
the Colne was probably named before the Yer. Even if we can 
only date back the naming of these rivers about 2000 years, 
almost every day during that period the Colne and its tribu¬ 
taries above this point have been cutting their beds through the 
